What was the last high-yield thermonuclear weapon design test conducted by the US at Bikini Atoll?

Castle Bravo was the first in a series of high-yield thermonuclear weapon design tests conducted by the United States at Bikini Atoll, Marshall Islands, as part of "Operation Castle". Detonated on March 1, 1954, the device was the most powerful nuclear device detonated by the United States and its first lithium deuteride fueled thermonuclear weapon. Castle Bravo's yield was 15 megatons of TNT, 2.5 times the predicted 6.0 megatons, due to unforeseen additional reactions involving Li, which led to the unexpected radioactive contamination of areas to the east of Bikini Atoll.

Castle Bravo

Fallout from the detonation fell on residents of Rongelap and Utirik atolls and spread around the world. The inhabitants of the islands were not evacuated until three days later and suffered radiation sickness. Twenty-three crew members of the Japanese fishing vessel "Daigo Fukuryū Maru" ("Lucky Dragon No. 5") were also contaminated by fallout, experiencing acute radiation syndrome. The blast incited international reaction over atmospheric thermonuclear testing.

The Bravo Crater is located at. The remains of the Castle Bravo causeway are at.

The "Castle Bravo" device was housed in a cylinder that weighed and measured in length and in diameter.

The primary device was a "COBRA" Deuterium-Tritium gas-boosted atomic bomb made by Los Alamos Scientific Laboratory, a very compact MK 7 device. This boosted fission device was tested in the "Upshot Knothole" "Climax" event and yielded (out of 50–70 kt expected yield range), successful enough that the planned operation series "Domino", designed to explore the same question about a suitable primary for thermonuclear bombs, could be cancelled. The implosion system was quite lightweight at, because it eliminated the aluminum pusher shell around the tamper and used the more compact ring lenses, a design feature shared with the Mark 5, 12, 13 and 18 designs. The explosive material of the inner charges in the MK 7 was changed to the more powerful Cyclotol 75/25, instead of the Composition B used in most stockpiled bombs at that time, as Cyclotol 75/25 was denser than Composition B and thus could generate the same amount of explosive force in a smaller volume (it actually provided 13 percent more compressive energy than Comp B). The composite uranium-plutonium "COBRA" core was levitated in a type-D pit. "COBRA" was Los Alamos' most recent product of design work on the "new principles" of the hollow core. A copper pit liner encased within the weapon-grade plutonium inner capsule prevented DT gas diffusion in plutonium, a technique first tested in "Greenhouse Item". The assembled module weighed, measuring across. It was located at the end of the device, which, as seen in the declassified film, shows a small cone projecting from the ballistic case. This cone is the part of the paraboloid that was used to focus the radiation emanating from the primary to the secondary.

The device was called SHRIMP and had the same basic configuration (radiation implosion) as the "Ivy Mike" wet device, except with a different type of fusion fuel. "SHRIMP" used lithium deuteride (LiD), which is solid at room temperature; "Ivy Mike" used cryogenic liquid deuterium (D), which required elaborate cooling equipment. "Castle Bravo" was the first test by the United States of a practical deliverable fusion bomb, even though the TX-21 as proof-tested in the Bravo event was not weaponized. The successful test rendered obsolete the cryogenic design used by "Ivy Mike" and its weaponized derivative, the "JUGHEAD", which was slated to be tested as the initial "Castle Yankee". It also used a 7075 aluminum 9.5 cm thick ballistic case. Aluminum was used to drastically reduce bomb's weight and simultaneously provided sufficient radiation confinement time to raise yield, a departure from the heavy stainless steel casing (304L or MIM 316L) employed by contemporary weapon-projects.

The "SHRIMP" was at least in theory and in many critical aspects identical in geometry to the "RUNT" and "RUNT II" devices later proof-fired in "Castle Romeo" and "Castle Yankee" respectively. On paper it was a scaled-down version of these devices, and its origins can be traced back to the spring and summer of 1953. The United States Air Force indicated the importance of lighter thermonuclear weapons for delivery by the B-47 Stratojet and B-58 Hustler. Los Alamos National Laboratory responded to this indication with a follow-up enriched version of the "RUNT" scaled down to a 3/4 scale radiation-implosion system called the "SHRIMP". The proposed weight reduction (from TX-17's to TX-21's ) would provide the Air Force with a much more versatile deliverable gravity bomb. The final version tested in "Castle" used partially enriched lithium as its fusion fuel. Natural lithium is a mixture of lithium-6 and lithium-7 isotopes (with 7.5% of the former). The enriched lithium used in "Bravo" was nominally 40% lithium-6 (the balance was the much more common lithium-7, which was incorrectly assumed to be inert). The fuel slugs varied in enrichment from 37 to 40% in Li, and the slugs with lower enrichment were positioned at the end of the fusion-fuel chamber, away from the primary. The lower levels of lithium enrichment in the fuel slugs, compared with the "ALARM CLOCK" and many later hydrogen weapons, were due to shortages in enriched lithium at that time, as the first of the "Alloy Development Plants" (ADP) started production by the fall of 1953. The volume of LiD fuel used was approximately 60% the volume of the fusion fuel filling used in the wet "SAUSAGE" and dry "RUNT I" and "II" devices, or about, corresponding to about 400 kg of lithium deuteride (as LiD has a density of 0.78201 g/cm). The mixture cost about 4.54 USD/g at that time. The fusion burn efficiency was close to 25.1%, the highest attained efficiency of the first thermonuclear weapon generation. This efficiency is well within the figures given in a November 1956 statement, when a DOD official disclosed that thermonuclear devices with efficiencies ranging from 15% to up about 40% had been tested. Hans Bethe reportedly stated independently that the first generation of thermonuclear weapons had (fusion) efficiencies varying from as low as 15% to up about 25%.

The thermonuclear burn would produce (like the fission fuel in the primary) pulsations (generations) of high-energy neutrons with an average temperature of 14 MeV through Jetter's cycle. The cycle is a combination of endothermic and exothermic neutronic reactions involving lithium and deuterium/tritium. The reaction's neutronicity was estimated at ~0.885 (for a Lawson criterion of ~1.5). These figures do not count the Li isotope, where the LiD has a neutronicity of ~0.835 and, along with the cross-sections, were given as group averages from about 2.40 to approximately 2.55 MeV and from 14.0 to 14.1 MeV; the small power group is not statistically present (see also Nuclear fusion).

As "SHRIMP", along with the "RUNT I" and "ALARM CLOCK" were to be high-yield shots required to assure the thermonuclear “emergency capability”, their fusion fuel may have been spiked with additional tritium, in the form of LiT. All of the high-energy 14 MeV neutrons would cause fission in the uranium fusion tamper wrapped around the secondary and the spark plug's plutonium rod. The ratio of deuterium (and tritium) atoms burned by 14 MeV neutrons spawned by the burning was expected to vary from 5:1 to 3:1, a standardization derived from "Mike", while for these estimations, the ratio of 3:1 was predominantly used in ISRINEX. The neutronicity of the fusion reactions harnessed by the fusion tamper would dramatically increase the yield of the device.

Attached to the cylindrical ballistic case was a natural-uranium liner, the radiation case, that was about 2.5 cm thick. Its internal surface was lined (glued) with a copper liner that was about 240 μm thick, and made from 0.08-μm thick copper foil, to increase the overall albedo of the hohlraum. Copper possesses excellent reflecting properties, and its low cost, compared to other reflecting materials like gold, made it useful for mass-produced hydrogen weapons. Hohlraum albedo is a very important design parameter for any inertial-confinement configuration. A relatively high albedo permits higher interstage coupling due to the more favorable azimuthal and latitudinal angles of reflected radiation. The limiting value of the albedo for high-"Z" materials is reached when the thickness is 5–10 g/cm, or 0.5–1.0 free paths. Thus, a hohlraum made of uranium much thicker than a free path of uranium would be needlessly heavy and costly. At the same time, the angular anisotropy increases as the atomic number of the scatterer material is reduced. Thus hohlraum liners require the use of copper (or, as in other devices, gold or aluminum), as the absorption probability increases with the value of "Z" of the scatterer. There are two sources of X-rays in the hohlraum: the primary's irradiance, which is dominant at the beginning and during the "pulse" rise; and the wall, which is important during the required radiation temperature's ("T") plateau. The primary emits radiation in a manner similar to a flash bulb, and the secondary needs constant "T" to properly implode. This constant wall temperature is dictated by the ablation pressure requirements to drive compression, which lie on average at about 0.4 keV (out of a range of 0.2 to 2 keV), corresponding to several million kelvins. Wall temperature depended on the temperature of the primary's core which peaked at about 5.4 keV during boosted-fission. The final wall-temperature, which corresponds to energy of the wall-reradiated X-rays to the secondary's pusher, also drops due to losses from the hohlraum material itself. Natural uranium nails, lined to the top of their head with copper, attached the radiation case to the ballistic case. The nails were bolted in vertical arrays in a double-shear configuration to better distribute the shear loads. This method of attaching the radiation case to the ballistic case was first used successfully in the Ivy "Mike" device. The radiation case had a parabolic end, which housed the "COBRA" primary that was employed to create the conditions needed to start the fusion reaction, and its other end was a cylinder, as also seen in Bravo's declassified film.

The space between the uranium "fusion tamper", and the case formed a radiation channel to conduct X-rays from the primary to the secondary assembly; the interstage. It is one of the most closely guarded secrets of a multistage thermonuclear weapon. Implosion of the secondary assembly is indirectly driven, and the techniques used in the interstage to smooth the spatial profile (i.e. reduce coherence and nonuniformities) of the primary's irradiance are of utmost importance. This was done with the introduction of the "channel filler" – an optical element used as a refractive medium, also encountered as "random-phase plate" in the ICF laser assemblies. This medium was a polystyrene plastic foam filling, extruded or impregnated with a low-molecular-weight hydrocarbon (possibly methane gas), which turned to a low-"Z" plasma from the X-rays, and along with channeling radiation it modulated the ablation front on the high-Z surfaces; it "tamped" the sputtering effect that would otherwise "choke" radiation from compressing the secondary. The reemitted X-rays from the radiation case must be deposited uniformly on the outer walls of the secondary’s tamper and ablate it externally, driving the thermonuclear fuel capsule (increasing the density and temperature of the fusion fuel) to the point needed to sustain a thermonuclear reaction. (see Nuclear weapon design). This point is above the threshold where the fusion fuel would turn opaque to its emitting radiation, as determined from its Rosseland opacity, meaning that the generated energy balances the energy lost to fuel's vicinity (as radiation, particle losses). After all, for any hydrogen weapon system to work, this energy equilibrium must be maintained through the compression equilibrium between the fusion tamper and the spark plug (see below), hence their name "equilibrium supers".

Since the ablative process takes place on both walls of the radiation channel, a numerical estimate made with ISRINEX (a thermonuclear explosion simulation program) suggested that the uranium tamper also had a thickness of 2.5 cm, so that an equal pressure would be applied to both walls of the hohlraum. The rocket effect on the surface of tamper's wall created by the ablation of its several superficial layers would force an equal mass of uranium that rested in the remainder of the tamper to speed inwards, thus imploding the thermonuclear core. At the same time, the rocket effect on the surface of the hohlraum would force the radiation case to speed outwards. The ballistic case would confine the exploding radiation case for as long as necessary. The fact that the tamper material was uranium enriched in U is primarily based on the final fission reaction fragments detected in the radiochemical analysis, which conclusively showed the presence of U, found by the Japanese in the shot debris. The first-generation thermonuclear weapons (MK-14, 16, 17, 21, 22 and 24) all used uranium tampers enriched to 37.5% U. The exception to this was the MK-15 "ZOMBIE" that used a 93.5% enriched fission jacket.

The secondary assembly was the actual "SHRIMP" component of the weapon. The weapon, like most contemporary thermonuclear weapons at that time, bore the same codename as the secondary component. The secondary was situated in the cylindrical end of the device, where its end was locked to the radiation case by a type of mortise and tenon joint. The hohlraum at its cylindrical end had an internal projection, which nested the secondary and had better structural strength to support the secondary's assembly, which had most of the device's mass. A visualization to this is that the joint looked much like a cap (the secondary) fitted in a cone (the projection of the radiation case). Any other major supporting structure would interfere to radiation transfer from the primary to the secondary and complex vibrational behavior. With this form of joint bearing most of the structural loads of the secondary, the latter and the hohlraum-ballistic case ensemble behaved as a single mass sharing common eigenmodes. To reduce excessive loading of the joint, especially during deployment of the weapon, the forward section of the secondary (i.e. the thermal blast/heat shield) was anchored to the radiation case by a set of thin wires, which also aligned the center line of the secondary the primary, as they diminished bending and torsional loads on the secondary, another technique adopted from the "SAUSAGE". The secondary assembly was an elongated truncated cone. From its front part (excluding the blast-heat shield) to its aft section it was steeply tapered. Tapering was used for two reasons. First, radiation drops by the square of the distance, hence radiation coupling is relatively poor in the aftermost sections of the secondary. This made the use of a higher mass of the then scarce fusion fuel in the rear end of the secondary assembly ineffective and the overall design wasteful. This was also the reason why the lower-enriched slugs of fusion fuel were placed far aft of the fuel capsule. Second, as the primary could not illuminate the whole surface of the hohlraum, in part due to the large axial length of the secondary, relatively small solid angles would be effective to compress the secondary, leading to poor radiation focusing. By tapering the secondary, the hohlraum could be shaped as a cylinder in its aft section obviating the need to machine the radiation case to a parabola at both ends. This optimized radiation focusing and enabled a streamlined production line, as it was cheaper, faster and easier to manufacture a radiation case with only one parabolic end. The tapering in this design was much steeper than its cousins, the "RUNT", and the "ALARM CLOCK" devices. "SHRIMP's" tapering and its mounting to the hohlraum apparently made the whole secondary assembly resemble the body of a shrimp. The secondary's length is defined by the two pairs of dark-colored diagnostic "hot spot" pipes attached to the middle and left section of the device. These pipe sections were in diameter and long and were butt-welded end-to-end to the ballistic case leading out to the top of the shot cab. They would carry the initial reaction's light up to the array of 12 mirror towers built in an arc on the artificial shot island created for the event. From those pipes, mirrors would reflect early bomb light from the bomb casing to a series of remote high-speed cameras, so that Los Alamos could determine both the simultaneity of the design (i.e. the time interval between primary's firing and secondary's ignition) and the thermonuclear burn rate in these two crucial areas of the secondary device.

This secondary assembly device contained the lithium deuteride fusion fuel in a stainless-steel canister. Running down to the center of the secondary was a 1.3 cm thick hollow cylindrical rod of plutonium, nested in the steel canister. This was the "spark plug", a tritium-boosted fission device. It was assembled by plutonium rings and had a hollow volume inside that measured about 0.5 cm in diameter. This central volume was lined with copper, which like the liner in the primary's fissile core prevented DT gas diffusion in plutonium. The spark plug's boosting charge contained about 4 grams of tritium and, imploding together with the secondary's compression, was timed to detonate by the first generations of neutrons that arrived from the primary. Timing was defined by the geometric characteristics of the sparkplug (its uncompressed annular radius), which detonated when its criticality, or "k", transcended 1. Its purpose was to compress the fusion material around it from its inside, equally applying pressure with the tamper. The compression factor of the fusion fuel and its adiabatic compression energy determined the minimal energy required for the spark plug to counteract the compression of the fusion fuel and the tamper's momentum. The spark plug weighed about 18 kg, and its initial firing yielded. Then it would be completely fissioned by the fusion neutrons, contributing about to the total yield. The energy required by the spark plug to counteract the compression of the fusion fuel was lower than the primary's yield because coupling of the primary’s energy in the hohlraum is accompanied by losses due to the difference between the X-ray fireball and the hohlraum temperatures. The neutrons entered the assembly by a small hole through the ~28 cm thick U blast-heat shield. It was positioned in front of the secondary assembly facing the primary. Similar to the tamper-fusion capsule assembly, the shield was shaped as a circular frustum, with its small diameter facing the primary's side, and with its large diameter locked by a type of mortise and tenon joint to the rest of the secondary assembly. The shield-tamper ensemble can be visualized as a circular bifrustum. All parts of the tamper were similarly locked together to provide structural support and rigidity to the secondary assembly. Surrounding the fusion-fuel–spark-plug assembly was the uranium tamper with a standoff air-gap about 0.9 cm wide that was to increase the tamper's momentum, a levitation technique used as early as "Operation Sandstone" and famously quoted by Ted Taylor as "hammer-on-the-nail-impact". Since there were also technical concerns that high-"Z" tamper material would mix rapidly with the relatively low-density fusion fuel, leading to unacceptably large radiation losses the stand-off gap also acted as a buffer to hinder to some extent the unavoidable and undesirable "Taylor mixing".

Boron was used at many locations in this dry system; it has a high cross-section for the absorption of slow neutrons, which fission U and Pu, but a low cross-section for the absorption of fast neutrons, which fission U. Because of this characteristic, B deposited onto the surface of the secondary stage would prevent predetonation of the "spark plug" by stray neutrons from the primary without interfering with the subsequent fissioning of the U of the fusion tamper wrapping the secondary. Boron also played a role in increasing the compressive plasma pressure around the secondary by blocking the sputtering effect, leading to higher thermonuclear efficiency. Because the structural foam holding the secondary in place within the casing was doped with B, the secondary was compressed more highly, at a cost of some radiated neutrons. An example of the usefulness of B can be seen by the fact that the fizzled "Castle Koon" "MORGENSTERN" device didn't use it in its design. As a result, the intense neutron flux from its "RACER IV" primary predetonated the spherical fission spark plug, which in turn "cooked" the fusion fuel, leading to an overall poor compression. It should be noted, that the plastic's low molecular weight is unable to implode the secondary's mass. Its plasma-pressure is confined in the boiled-off sections of the tamper and the radiation case so that material from neither of these two "walls" can enter the radiation channel that has to be open for the radiation transit.

The device was mounted in a "shot cab" on an artificial island built on a reef off Namu Island, in the Bikini Atoll. A sizable array of diagnostic instruments was trained on it, including high-speed cameras trained through an arc of mirror towers around the shot cab.

The detonation took place at 06:45 on March 1, 1954, local time (18:45 on February 28 GMT).
When Bravo was detonated, within one second it formed a fireball almost across. This fireball was visible on Kwajalein atoll over away. The explosion left a crater in diameter and in depth. The mushroom cloud reached a height of and a diameter of in about a minute, a height of and in diameter in less than 10 minutes and was expanding at more than. As a result of the blast, the cloud contaminated more than of the surrounding Pacific Ocean, including some of the surrounding small islands like Rongerik, Rongelap, and Utirik.

In terms of TNT equivalence, "Castle Bravo" was about 1,000 times more powerful than each of the atomic bombs that were dropped on Hiroshima and Nagasaki during World War II. "Castle Bravo" is the fifth largest nuclear explosion in history, exceeded by the Soviet tests of "Tsar Bomba" at approximately 50 Mt, "Test 219" at 24.2 Mt, and two other ~20 Mt Soviet tests in 1962 at Novaya Zemlya.

The yield of 15 megatons was 3 times that of the 5 Mt predicted by its designers. The cause of the higher yield was an error made by designers of the device at Los Alamos National Laboratory. They considered only the lithium-6 isotope in the lithium-deuteride secondary to be reactive; the lithium-7 isotope, accounting for 60% of the lithium content, was assumed to be inert. It was expected that the lithium-6 isotope would absorb a neutron from the fissioning plutonium and emit an alpha particle and tritium in the process, of which the latter would then fuse with the deuterium and increase the yield in a predicted manner. Lithium-6 indeed reacted in this manner.

It was assumed that the lithium-7 would absorb one neutron, producing lithium-8, which decays (through beryllium-8) to a pair of alpha particles on a timescale of seconds, vastly longer than the timescale of nuclear detonation. However, when lithium-7 is bombarded with energetic neutrons, rather than simply absorbing a neutron, it captures the neutron and decays almost instantly into an alpha particle, a tritium nucleus, and another neutron. As a result, much more tritium was produced than expected, the extra tritium fusing with deuterium and producing an extra neutron. The extra neutron produced by fusion and the extra neutron released directly by lithium-7 decay produced a much larger neutron flux. The result was greatly increased fissioning of the uranium tamper and increased yield.

This resultant extra fuel (both lithium-6 and lithium-7) contributed greatly to the fusion reactions and neutron production and in this manner greatly increased the device's explosive output. The test used lithium with a high percentage of lithium-7 only because lithium-6 was then scarce and expensive; the later "Castle Union" test used almost pure lithium-6. Had sufficient lithium-6 been available, the usability of the common lithium-7 might not have been discovered.

The unexpectedly high yield of the device severely damaged many of the permanent buildings on the control site island on the far side of the atoll. Little of the desired diagnostic data on the shot was collected; many instruments designed to transmit their data back before being destroyed by the blast were instead vaporized instantly, while most of the instruments that were expected to be recovered for data retrieval were destroyed by the blast.

The fission reactions of the natural uranium tamper were quite dirty, producing a large amount of fallout. That, combined with the much-larger-than-expected yield and a major wind shift, produced some very serious consequences. In the declassified film "Operation Castle", the task force commander Major General Percy Clarkson pointed to a diagram indicating that the wind shift was still in the range of "acceptable fallout", although just barely.

The decision to carry out the Bravo test under the prevailing winds was made by Dr. Alvin C. Graves, the Scientific Director of Operation Castle. Graves had total authority over detonating the weapon, above that of the military commander of Operation Castle. Graves appears in the widely available film of the earlier 1952 test "Ivy Mike", which examines the last-minute fallout decisions. The narrator, the western actor Reed Hadley, is filmed aboard the control ship in that film, showing the final conference. Hadley points out that 20,000 people live in the potential area of the fallout. He asks the control panel scientist if the test can be aborted and is told "yes", but it would ruin all their preparations in setting up timed measuring instruments. In Mike, the fallout correctly landed north of the inhabited area but, in the 1954 Bravo test, there was a large amount of wind shear, and the wind that was blowing north the day before the test steadily veered towards the east.

Radioactive fallout was spread eastward onto the inhabited Rongelap and Rongerik atolls, which were evacuated 48 hours after the detonation. They were returned to the islands three years later but were removed again when their island was found to be unsafe. Ultimately, 15 islands and atolls were contaminated, and by 1963 Marshall Islands natives began to suffer from thyroid tumors, including 20 of 29 Rongelap children at the time of Bravo, and many birth defects were reported. The islanders received compensation from the U.S. government, relative to how much contamination they received, beginning in 1956; by 1995 the Nuclear Claims Tribunal reported that it had awarded $43.2 million, nearly its entire fund, to 1,196 claimants for 1,311 illnesses. A medical study, named Project 4.1, studied the effects of the fallout on the islanders.

Although the atmospheric fallout plume drifted eastward, once fallout landed in the water it was carried in several directions by ocean currents, including northwest and southwest.

A Japanese fishing boat, "Daigo Fukuryū Maru" (Lucky Dragon No.5), came in direct contact with the fallout, which caused many of the crew to grow ill; one died of a secondary infection. This resulted in an international incident and reignited Japanese concerns about radiation, especially as Japanese citizens were once more adversely affected by US nuclear weapons. The official US position had been that the growth in the strength of atomic bombs was not accompanied by an equivalent growth in radioactivity released. Japanese scientists who had collected data from the fishing vessel disagreed with this.

Sir Joseph Rotblat, working at St Bartholomew's Hospital, London, demonstrated that the contamination caused by the fallout from the test was far greater than that stated officially. Rotblat was able to deduce that the bomb had three stages and showed that the fission phase at the end of the explosion increased the amount of radioactivity a thousandfold. Rotblat's paper was taken up by the media, and the outcry in Japan reached such a level that diplomatic relations became strained and the incident was even dubbed by some as a "second Hiroshima". Nevertheless, the Japanese and US governments quickly reached a political settlement, with the transfer to Japan of $15.3 million as compensation, with the surviving victims receiving about 2 million each ($5,550 in 1954, or about $ in 2022). It was also agreed that the victims would not be given Hibakusha status.

Unanticipated fallout and the radiation emitted by it also affected many of the vessels and personnel involved in the test, in some cases forcing them into bunkers for several hours. In contrast to the crew of the "Lucky Dragon No. 5", who did not anticipate the hazard and therefore did not take shelter in the hold of their ship, or refrain from inhaling the fallout dust, the firing crew that triggered the explosion was able to safely shelter in their firing station when they noticed the wind was carrying the fallout in the unanticipated direction towards the island of Enyu on the Bikini Atoll where they were located, with the fire crew sheltering in place ("buttoning up") for several hours until outside radiation decayed to safer levels. "25 Roentgens per hour" was recorded above the bunker.

The US Navy tanker was at Enewetak Atoll in late February 1954. "Patapsco" lacked a decontamination washdown system, and was therefore ordered on 27 February, to return to Pearl Harbor at the highest possible speed. A breakdown in her engine systems, namely a cracked cylinder liner, slowed "Patapsco" to one-third of her full speed, and when the Castle Bravo detonation took place, she was still about 180 to 195 nautical miles east of Bikini. "Patapsco" was in the range of nuclear fallout, which began landing on the ship in the mid-afternoon of 2 March. By this time "Patapsco" was 565 to 586 nautical miles from ground zero. The fallout was at first thought to be harmless and there were no radiation detectors aboard, so no decontamination measures were taken. Measurements taken after "Patapsco" had returned to Pearl Harbor suggested an exposure range of 0.18 to 0.62 R/hr. Total exposure estimates range from 3.3 R to 18 R of whole-body radiation, taking into account the effects of natural washdown from rain, and variations between above- and below-deck exposure.

Sixteen crew members of the aircraft carrier USS "Bairoko" received beta burns, and there was an increased cancer rate among its crew. Radioactive contamination also affected many of the testing facilities built on other islands of the Bikini atoll system.

The fallout spread traces of radioactive material as far as Australia, India and Japan, and even the United States and parts of Europe. Though organized as a secret test, Castle Bravo quickly became an international incident, prompting calls for a ban on the atmospheric testing of thermonuclear devices.

A worldwide network of gummed film stations was established to monitor fallout following Operation Castle. Although meteorological data was poor, a general connection of tropospheric flow patterns with observed fallout was evident. There was a tendency for fallout/debris to remain in tropical latitudes, with incursions into the temperate regions associated with meteorological disturbances of the predominantly zonal flow. Outside of the tropics, the Southwestern United States received the greatest total fallout, about five times that received in Japan.

Stratospheric fallout particles of strontium-90 from the test were later captured with balloon-borne air filters used to sample the air at stratospheric altitudes, the research (Project Ashcan) was conducted to better understand the stratosphere, fallout times and arrive at more accurate meteorological models after hindcasting.

The fallout from "Castle Bravo" and other testing on the atoll also affected islanders who had previously inhabited the atoll, and who returned there some time after the tests. This was due to the presence of radioactive caesium-137 in locally grown coconut milk. Plants and trees absorb potassium as part of the normal biological process, but will also readily absorb caesium if present, being of the same group on the periodic table, and therefore very similar chemically. Islanders consuming contaminated coconut milk were found to have abnormally high concentrations of caesium in their bodies and so had to be evacuated from the atoll a second time.

The American magazine "Consumer Reports" warned of the contamination of milk with strontium-90.

The Soviet Union had previously used lithium deuteride in its Sloika design (known as the "Joe-4" in the U.S.), in 1953. It was not a true hydrogen bomb; fusion provided only 15–20% of its yield, most coming from boosted fission reactions. Its yield was 400 kilotons, and it could not be infinitely scaled, as with a true thermonuclear device.

The Teller–Ulam-based "Ivy Mike" device had a much greater yield of 10.4 Mt, but most of this also came from fission: 77% of the total came from fast fission of its natural-uranium tamper.

Castle Bravo had the greatest yield of any U.S. nuclear test, 15 Mt, though again, a substantial fraction came from fission. In the Teller–Ulam design, the fission and fusion stages were kept physically separate in a reflective cavity. The radiation from the exploding fission primary brought the fuel in the fusion secondary to critical density and pressure, setting off thermonuclear (fusion) chain reactions, which in turn set off a tertiary fissioning of the bomb's U fusion tamper and casing. Consequently, this type of bomb is also known as a "fission-fusion-fission" device. The Soviet researchers, led by Andrei Sakharov, developed and tested their first Teller–Ulam device in 1955.

The publication of the Bravo fallout analysis was a militarily sensitive issue, with Joseph Rotblat possibly deducing the staging nature of the Castle Bravo device by studying the ratio and presence of tell-tale isotopes, namely uranium-237, present in the fallout. This information could potentially reveal the means by which megaton-yield nuclear devices achieve their yield. Soviet scientist Andrei Sakharov hit upon what the Soviet Union regarded as "Sakharov's third idea" during the month after the Castle Bravo test, the final piece of the puzzle being the idea that the compression of the secondary can be accomplished by the primary's X-rays before fusion began.

The Shrimp device design later evolved into the Mark 21 nuclear bomb, of which 275 units were produced, weighing and measuring long and in diameter. This 4-megaton bomb was produced until July 1956. In 1957, it was converted into the Mark 36 nuclear bomb and entered into production again.

The inhabitants of the Marshall Islands were impacted by the detonation of Castle Bravo. This single test exposed the surrounding populations to varying levels of radiation. The fallout levels attributed to the Castle Bravo test are the highest in history. Populations neighboring the test site were exposed to high levels of radiation resulting in mild radiation sickness of many (nausea, vomiting, diarrhea). Several weeks later, many people began suffering from alopecia (hair loss) and skin lesions as well.

The exposure to fallout has been linked to increase the likelihood of several types of cancer such as leukemia and thyroid cancer. The relationship between I-131 levels and thyroid cancer is still being researched. There are also correlations between fallout exposure levels and diseases such as thyroid disease like hypothyroidism. Populations of the Marshall Islands that received significant exposure to radionuclides have a much greater risk of developing cancer.

The female population of the Marshall Islands have a sixty times greater mortality rate from cervical cancer than a comparable mainland United States population. The Islands populations also have a five time greater likelihood of breast or gastrointestinal mortality, and lung cancer mortality is three times higher than the mainland population. The mortality rate of the male population on the Marshall Islands from lung cancer is four times greater than the overall United States rates, and the oral cancer rates are ten times greater.

There is a presumed association between radiation levels and female reproductive system functioning.

The 1957 novel "On the Beach" by Nevil Shute is about a nuclear war that released so much radioactive fallout that all the life in the Northern Hemisphere disappeared, while the Southern Hemisphere awaited a similar fate. However, the American government voiced a criticism of this premise—a threat of extinction from nuclear war—because they did not have enough nuclear weapons to cause human extinction.

The Castle Bravo detonation and the subsequent poisoning of the crew aboard Lucky Dragon 5 led to an increase in antinuclear protests in Japan. It was compared to the bombings of Hiroshima and Nagasaki and the Castle Bravo test was frequently part of the plots of numerous Japanese media, especially in relation to Japan's most widely recognized media icon, Godzilla. During the opening scene of the 2014 American "Godzilla" reboot and in the prequel comic "Godzilla: Awakening", the Castle Bravo nuclear test was an attempt to kill Godzilla and a prehistoric parasite called "Shinomura". Godzilla, however, survived the "test", despite humanity believing otherwise, while the Shinomura was incinerated.

The Donald Fagen song "Memorabilia" off his 2012 album "Sunken Condos" mentions both the Castle Bravo and Ivy King nuclear tests.

In 2013, the Defense Threat Reduction Agency released "Castle Bravo: Fifty Years of Legend and Lore".

Much of the stimulus for the treaty was increasing public unease about radioactive fallout as a result of above-ground or underwater nuclear testing, particularly given the increasing power of nuclear devices, as well as concern about the general environmental damage caused by testing. In 1952–53, the US and Soviet Union detonated their first thermonuclear weapons (hydrogen bombs), far more powerful than the atomic bombs tested and deployed since 1945. In 1954, the US Castle Bravo test at Bikini Atoll (part of Operation Castle) had a yield of 15 megatons of TNT, more than doubling the expected yield. The Castle Bravo test resulted in the worst radiological event in US history as radioactive particles spread over more than, affected inhabited areas (including Rongelap Atoll and Utirik Atoll), and sickened Japanese fishermen aboard the "Lucky Dragon" upon whom "ashes of death" had rained. In the same year, a Soviet test sent radioactive particles over Japan. Around the same time, victims of the atomic bombing of Hiroshima visited the US for medical care, which attracted significant public attention. In 1961, the Soviet Union tested the Tsar Bomba, which had a yield of 50 megatons and remains the most powerful man-made explosion in history, though due to a highly efficient detonation fallout was relatively limited. Between 1951 and 1958, the US conducted 166 atmospheric tests, the Soviet Union conducted 82, and Britain conducted 21; only 22 underground tests were conducted in this period (all by the US).

Partial Nuclear Test Ban Treaty

The Partial Test Ban Treaty (PTBT) is the abbreviated name of the 1963 Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water, which prohibited all test detonations of nuclear weapons except for those conducted underground. It is also abbreviated as the Limited Test Ban Treaty (LTBT) and Nuclear Test Ban Treaty (NTBT), though the latter may also refer to the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which succeeded the PTBT for ratifying parties.

Negotiations initially focused on a comprehensive ban, but this was abandoned due to technical questions surrounding the detection of underground tests and Soviet concerns over the intrusiveness of proposed verification methods. The impetus for the test ban was provided by rising public anxiety over the magnitude of nuclear tests, particularly tests of new thermonuclear weapons (hydrogen bombs), and the resulting nuclear fallout. A test ban was also seen as a means of slowing nuclear proliferation and the nuclear arms race. Though the PTBT did not halt proliferation or the arms race, its enactment did coincide with a substantial decline in the concentration of radioactive particles in the atmosphere.

The PTBT was signed by the governments of the Soviet Union, United Kingdom, and United States in Moscow on 5 August 1963 before being opened for signature by other countries. The treaty formally went into effect on 10 October 1963. Since then, 123 other states have become party to the treaty. Ten states have signed but not ratified the treaty.

In 1945, Britain and Canada made an early call for an international discussion on controlling atomic power. At the time, the US had yet to formulate a cohesive policy or strategy on nuclear weapons. Taking advantage of this was Vannevar Bush, who had initiated and administered the Manhattan Project, but nevertheless had a long-term policy goal of banning on nuclear weapons production. As a first step in this direction, Bush proposed an international agency dedicated to nuclear control. Bush unsuccessfully argued in 1952 that the US pursue a test ban agreement with the Soviet Union before testing its first thermonuclear weapon, but his interest in international controls was echoed in the 1946 Acheson–Lilienthal Report, which had been commissioned by President Harry S. Truman to help construct US nuclear weapons policy. J. Robert Oppenheimer, who had led Los Alamos National Laboratory during the Manhattan Project, exerted significant influence over the report, particularly in its recommendation of an international body that would control production of and research on the world's supply of uranium and thorium. A version of the Acheson-Lilienthal plan was presented to the United Nations Atomic Energy Commission as the Baruch Plan in June 1946. The Baruch Plan proposed that an International Atomic Development Authority would control all research on and material and equipment involved in the production of atomic energy. Though Dwight D. Eisenhower, then the Chief of Staff of the United States Army, was not a significant figure in the Truman administration on nuclear questions, he did support Truman's nuclear control policy, including the Baruch Plan's provision for an international control agency, provided that the control system was accompanied by "a system of free and complete inspection."

In 1954, just weeks after the Castle Bravo test, Indian prime minister Jawaharlal Nehru made the first call for a "standstill agreement" on nuclear testing, who saw a testing moratorium as a stepping stone to more comprehensive arms control agreements. In the same year, the British Labour Party, then led by Clement Attlee, called on the UN to ban testing of thermonuclear weapons. 1955 marks the beginning of test-ban negotiations, as Soviet leader Nikita Khrushchev first proposed talks on the subject in February 1955. On 10 May 1955, the Soviet Union proposed a test ban before the UN Disarmament Commission's "Committee of Five" (Britain, Canada, France, the Soviet Union, and the US). This proposal, which closely reflected a prior Anglo-French proposal, was initially part of a comprehensive disarmament proposal meant to reduce conventional arms levels and eliminate nuclear weapons. Despite the closeness of the Soviet proposal to earlier Western proposals, the US reversed its position on the provisions and rejected the Soviet offer "in the absence of more general control agreements," including limits on the production of fissionable material and protections against a surprise nuclear strike. The May 1955 proposal is now seen as evidence of Khrushchev's "new approach" to foreign policy, as Khrushchev sought to mend relations with the West. The proposal would serve as the basis of the Soviet negotiating position through 1957.

Eisenhower had supported nuclear testing after World War II. In 1947, he rejected arguments by Stafford L. Warren, the Manhattan Project's chief physician, concerning the detrimental health effects of atmospheric testing, agreeing instead with James Bryant Conant, a chemist and participant in the Manhattan Project, who was skeptical of Warren's then-theoretical claims. Warren's arguments were lent credence in the scientific community and public by the Castle Bravo test of 1954. Eisenhower, as president, first explicitly expressed interest in a comprehensive test ban that year, arguing before the National Security Council, "We could put [the Russians] on the spot if we accepted a moratorium ... Everybody seems to think that we're skunks, saber-rattlers and warmongers. We ought not miss any chance to make clear our peaceful objectives." Then-Secretary of State John Foster Dulles had responded skeptically to the limited arms-control suggestion of Nehru, whose proposal for a test ban was discarded by the National Security Council for being "not practical." Harold Stassen, Eisenhower's special assistant for disarmament, argued that the US should prioritize a test ban as a first step towards comprehensive arms control, conditional on the Soviet Union accepting on-site inspections, over full disarmament. Stassen's suggestion was dismissed by others in the administration over fears that the Soviet Union would be able to conduct secret tests. On the advice of Dulles, Atomic Energy Commission (AEC) chairman Lewis Strauss, and Secretary of Defense Charles Erwin Wilson, Eisenhower rejected the idea of considering a test ban outside general disarmament efforts. During the 1952 and 1956 presidential elections, Eisenhower fended off challenger Adlai Stevenson, who ran in large part on support for a test ban.
The British governments of 1954–58 (under Conservatives Winston Churchill, Anthony Eden, and Harold Macmillan) also quietly resisted a test ban, despite the British public favoring a deal, until the US Congress approved expanded nuclear collaboration in 1958 and until after Britain had tested its first hydrogen bombs. In their view, testing was necessary if the UK nuclear program were to continue to develop. This opposition was tempered by concern that resistance to a test ban might lead the US and Soviet Union to pursue an agreement without Britain having any say in the matter.

Members of the Soviet military–industrial complex also opposed a test ban, though some scientists, including Igor Kurchatov, were supportive of antinuclear efforts. France, which was in the midst of developing its own nuclear weapon, also firmly opposed a test ban in the late 1950s.

The proliferation of thermonuclear weapons coincided with a rise in public concern about nuclear fallout debris contaminating food sources, particularly the threat of high levels of strontium-90 in milk (see the Baby Tooth Survey). This survey was a "campaign [that] effectively employed a variety of media advocacy strategies" to alarm the public and "galvanized" support against atmospheric nuclear testing. Lewis Strauss and Edward Teller, dubbed the "father of the hydrogen bomb," both sought to tamp down on these fears, arguing that fallout [at the dose levels of US exposure] were fairly harmless and that a test ban would enable the Soviet Union to surpass the US in nuclear capabilities. Teller also suggested that testing was necessary to develop nuclear weapons that produced less fallout. While Teller would largely win the contemporary televised debates of 1958 with Linus Pauling, on technical grounds, the emotive "media advocacy strategies", of which the Tooth survey is a prominent example, were to cause support in the US public for a test ban to continue to grow from 20% in 1954 to 63% by 1957. Moreover, widespread antinuclear protests were organized and led by theologian and Nobel Peace Prize laureate Albert Schweitzer, whose appeals were endorsed by Pope Pius XII, and Linus Pauling, the latter of whom organized an anti-test petition signed by more than 9,000 scientists across 43 countries (including the infirm and elderly Albert Einstein). Although notably, many health scientists that were asked to sign were critical of Pauling's "thoroughly unscientific" fallout assumptions, despite most wishing for a test-ban for other reasons, they could not bring themselves to sign a document that promoted such exaggerations of the dangers, as it would result in eventual public distrust of scientists in the long term.

The AEC would eventually concede, as well, that even low levels of radiation were harmful. It was a combination of rising public support for a test ban and the shock of the 1957 Soviet "Sputnik" launch that encouraged Eisenhower to take steps towards a test ban in 1958.

There was also increased environmental concern in the Soviet Union. In the mid-1950s, Soviet scientists began taking regular radiation readings near Leningrad, Moscow, and Odessa and collected data on the prevalence of strontium-90, which indicated that strontium-90 levels in western Russia approximately matched those in the eastern US. Rising Soviet concern was punctuated in September 1957 by the Kyshtym disaster, which forced the evacuation of 10,000 people after an explosion at a nuclear plant. Around the same time, 219 Soviet scientists signed Pauling's antinuclear petition. Soviet political elites did not share the concerns of others in the Soviet Union. However; Kurchatov unsuccessfully called on Khrushchev to halt testing in 1958.

On 14 June 1957, following Eisenhower's suggestion that existing detection measures were inadequate to ensure compliance, the Soviet Union put forth a plan for a two-to-three-year testing moratorium. The moratorium would be overseen by an international commission reliant on national monitoring stations, but, importantly, would involve no on-the-ground inspections. Eisenhower initially saw the deal as favorable, but eventually came to see otherwise. In particular, Strauss and Teller, as well as Ernest Lawrence and Mark Muir Mills, protested the offer. At a meeting with Eisenhower in the White House, the group argued that testing was necessary for the US to eventually develop bombs that produced no fallout ("clean bombs"). The group repeated the oft-cited fact, which was supported by Freeman Dyson, that the Soviet Union could conduct secret nuclear tests. In 1958, at the request of Igor Kurchatov, Soviet nuclear physicist and weapons designer Andrei Sakharov published a pair of widely circulated academic papers challenging the claim of Teller and others that a clean, fallout-free nuclear bomb could be developed, due to the formation of carbon-14 when nuclear devices are detonated in the air. A one-megaton clean bomb, Sakharov estimated, would cause 6,600 deaths over 8,000 years, figures derived largely from estimates on the quantity of carbon-14 generated from atmospheric nitrogen and the contemporary risk models at the time, along with the assumption that the world population is "thirty billion persons" in a few thousand years. In 1961, Sakharov was part of the design team for a 50 megaton "clean bomb", which has become known as the Tsar Bomba, detonated over the island of Novaya Zemlya.
In the spring of 1957, the US National Security Council had explored including a one-year test moratorium and a "cut-off" of fissionable-material production in a "partial" disarmament plan. The British government, then led by Macmillan, had yet to fully endorse a test ban. Accordingly, it pushed the US to demand that the production cut-off be closely timed with the testing moratorium, betting that the Soviet Union would reject this. London also encouraged the US to delay its disarmament plan, in part by moving the start of the moratorium back to November 1958. At the same time, Macmillan linked British support for a test ban to a revision of the Atomic Energy Act of 1946 (McMahon Act), which prohibited sharing of nuclear information with foreign governments. Eisenhower, eager to mend ties with Britain following the Suez Crisis of 1956, was receptive to Macmillan's conditions, but the AEC and the congressional Joint Committee on Atomic Energy were firmly opposed. It was not until after "Sputnik" in late 1957 that Eisenhower quickly moved to expand nuclear collaboration with the UK via presidential directives and the establishment of bilateral committees on nuclear matters. In early 1958, Eisenhower publicly stated that amendments to the McMahon Act were a necessary condition of a test ban, framing the policy shift in the context of US commitment to its NATO allies.

In August 1957, the US assented to a two-year testing moratorium proposed by the Soviet Union, but required that it be linked to restrictions on the production of fissionable material with military uses, a condition that the Soviet Union rejected. While Eisenhower insisted on linking a test ban to a broader disarmament effort (e.g., the production cut-off), Moscow insisted on independent consideration of a test ban.

On 19 September 1957, the US conducted the first contained underground test at the Nevada Test Site, codenamed "Rainier". The "Rainier" shot complicated the push for a comprehensive test ban, as underground tests could not be as easily identified as atmospheric tests.

Despite Eisenhower's interest in a deal, his administration was hamstrung by discord among US scientists, technicians, and politicians. At one point, Eisenhower complained that "statecraft was becoming a prisoner of scientists." Until 1957, Strauss's AEC (including its Los Alamos and Livermore laboratories) was the dominant voice in the administration on nuclear affairs, with Teller's concerns over detection mechanisms also influencing Eisenhower. Unlike some others within the US scientific community, Strauss fervently advocated against a test ban, arguing that the US must maintain a clear nuclear advantage via regular testing and that the negative environmental impacts of such tests were overstated. Furthermore, Strauss repeatedly emphasized the risk of the Soviet Union violating a ban, a fear Eisenhower shared.

On 7 November 1957, after "Sputnik" and under pressure to bring on a dedicated science advisor, Eisenhower created the President's Science Advisory Committee (PSAC), which had the effect of eroding the AEC's monopoly over scientific advice. In stark contrast to the AEC, PSAC promoted a test ban and argued against Strauss's claims concerning its strategic implications and technical feasibility. In late 1957, the Soviet Union made a second offer of a three-year moratorium without inspections, but lacking any consensus within his administration, Eisenhower rejected it. In early 1958, the discord within American circles, particularly among scientists, was made clear in hearings before the Senate Subcommittee on Nuclear Disarmament, chaired by Senator Hubert Humphrey. The hearings featured conflicting testimony from the likes of Teller and Linus Pauling, as well as from Harold Stassen, who argued that a test ban could safely be separated from broader disarmament, and AEC members, who argued that a cutoff in nuclear production should precede a test ban.

In the summer of 1957, Khrushchev was at acute risk of losing power, as the Anti-Party Group composed of former Stalin allies Lazar Kaganovich, Georgy Malenkov, and Vyacheslav Molotov launched an attempt to replace Khrushchev as General Secretary of the Communist Party (effectively the leader of the Soviet Union) with Nikolai Bulganin, then the Premier of the Soviet Union. The attempted ouster, which was foiled in June, was followed by a series of actions by Khrushchev to consolidate power. In October 1957, still feeling vulnerable from Anti-Party Group's ploy, Khrushchev forced out defense minister Georgy Zhukov, cited as "the nation's most powerful military man." On 27 March 1958, Khrushchev forced Bulganin to resign and succeeded him as Premier. Between 1957 and 1960, Khrushchev had his firmest grip on power, with little real opposition.

Khrushchev was personally troubled by the power of nuclear weapons and would later recount that he believed the weapons could never be used. In the mid-1950s, Khrushchev took a keen interest in defense policy and sought to inaugurate an era of détente with the West. Initial efforts to reach accords, such as on disarmament at the 1955 Geneva Summit, proved fruitless, and Khrushchev saw test-ban negotiations as an opportunity to present the Soviet Union as "both powerful and responsible." At the 20th Communist Party Congress in 1956, Khrushchev declared that nuclear war should no longer be seen as "fatalistically inevitable." Simultaneously, however, Khrushchev expanded and advanced the Soviet nuclear arsenal at a cost to conventional Soviet forces (e.g., in early 1960, Khrushchev announced demobilization of 1.2 million troops).

On 31 March 1958, the Supreme Soviet of the Soviet Union approved a decision to halt nuclear testing, conditional on other nuclear powers doing the same. Khrushchev then called on Eisenhower and Macmillan to join the moratorium. Despite the action being met with widespread praise and an argument from Dulles that the US should reciprocate, Eisenhower dismissed the plan as a "gimmick"; the Soviet Union had just completed a testing series and the US was about to begin Operation Hardtack I, a series of atmospheric, surface-level, and underwater nuclear tests. Eisenhower instead insisted that any moratorium be linked to reduced production of nuclear weapons. In April 1958, the US began Operation Hardtack I as planned. The Soviet declaration concerned the British government, which feared that the moratorium might lead to a test ban before its own testing program was completed. Following the Soviet declaration, Eisenhower called for an international meeting of experts to determine proper control and verification measures—an idea first proposed by British Foreign Secretary Selwyn Lloyd.

The advocacy of PSAC, including that of its chairmen James Rhyne Killian and George Kistiakowsky, was a key factor in Eisenhower's eventual decision to initiate test-ban negotiations in 1958. In the spring of 1958, chairman Killian and the PSAC staff (namely Hans Bethe and Isidor Isaac Rabi) undertook a review of US test-ban policy, determining that a successful system for detecting underground tests could be created. At the recommendation of Dulles (who had recently come to support a test ban), the review prompted Eisenhower to propose technical negotiations with the Soviet Union, effectively detaching test-ban negotiations from negotiations over a halt to nuclear weapons production (the one-time US demand). In explaining the policy shift, Eisenhower privately said that continued resistance to a test ban would leave the US in a state of "moral isolation."

On 8 April 1958, still resisting Khrushchev's call for a moratorium, Eisenhower invited the Soviet Union to join these technical negotiations in the form of a conference on the technical aspects of a test-ban, specifically the technical details of ensuring compliance with a ban. The proposal was, to a degree, a concession to the Soviet Union, as a test ban would be explored independent of the previously demanded cutoff in fissionable-material production. Khrushchev initially declined the invitation, but eventually agreed "in spite of the serious doubts" he had after Eisenhower suggested a technical agreement on verification would be a precursor to a test ban.

On 1 July 1958, responding to Eisenhower's call, the nuclear powers convened the Conference of Experts in Geneva, aimed at studying means of detecting nuclear tests. The conference included scientists from the US, Britain, the Soviet Union, Canada, Czechoslovakia, France, Poland, and Romania. The US delegation was led by James Fisk, a member of PSAC, the Soviets by Evgenii Fedorov, and the British delegation by William Penney, who had led the British delegation to the Manhattan Project. Whereas the US approached the conference solely from a technical perspective, Penney was specifically instructed by Macmillan to attempt to achieve a political agreement. This difference in approach was reflected in the broader composition of the US and UK teams. US experts were primarily drawn from academia and industry. Fisk was a vice president at Bell Telephone Laboratories and was joined by Robert Bacher and Ernest Lawrence, both physicists who had worked on the Manhattan Project. Conversely, British delegates largely held government positions. The Soviet delegation was composed primarily of academics, though virtually all of them had some link to the Soviet government. The Soviets shared the British goal of achieving an agreement at the conference.

At particular issue was the ability of sensors to differentiate an underground test from an earthquake. There were four techniques examined: measurement of acoustic waves, seismic signals, radio waves, and inspection of radioactive debris. The Soviet delegation expressed confidence in each method, while Western experts argued that a more comprehensive compliance system would be necessary.

The Conference of Experts was characterized as "highly professional" and productive. By the end of August 1958, the experts devised an extensive control program, known as the "Geneva System," involving 160–170 land-based monitoring posts, plus 10 additional sea-based monitors and occasional flights over land following a suspicious event (with the inspection plane being provided and controlled by the state under inspection). The experts determined that such a scheme would be able to detect 90% of underground detonations, accurate to 5 kilotons, and atmospheric tests with a minimum yield of 1 kiloton. The US had initially advocated for 650 posts, versus a Soviet proposal of 100–110. The final recommendation was a compromise forged by the British delegation. In a widely publicized and well-received communiqué dated 21 August 1958, the conference declared that it "reached the conclusion that it is technically feasible to set up ... a workable and effective control system for the detection of violations of a possible agreement on the worldwide cessation of nuclear weapons tests."
The technical findings, released on 30 August 1958 in a report drafted by the Soviet delegation, were endorsed by the US and UK, which proposed that they serve as the basis for test-ban and international-control negotiations. However, the experts' report failed to address precisely who would do the monitoring and when on-site inspections—a US demand and Soviet concern—would be permitted. The experts also deemed detection of outer-space tests (tests more than above the earth's surface) to be impractical. Additionally, the size of the Geneva System may have rendered it too expensive to be put into effect. The 30 August report, which contained details on these limitations, received significantly less public attention than the 21 August communiqué.

Nevertheless, pleased by the findings, the Eisenhower administration proposed negotiations on a permanent test ban and announced it would self-impose a year-long testing moratorium if Britain and the Soviet Union did the same. This decision amounted to a victory for John Foster Dulles, Allen Dulles (then the Director of Central Intelligence), and PSAC, who had argued within the Eisenhower administration for separating a test ban from larger disarmament efforts, and a defeat for the Department of Defense and AEC, which had argued to the contrary.

In May 1958, Britain had informed that US that it would be willing to join a testing moratorium on 31 October 1958, by which point it would have finished its hydrogen-bomb testing, conditional on the US providing Britain with nuclear information following amendment of the McMahon Act. The US Congress approved amendments permitting greater collaboration in late June. Following Soviet assent on 30 August 1958 to the one-year moratorium, the three countries conducted a series of tests in September and October. At least 54 tests were conducted by the US and 14 by the Soviet Union in this period. On 31 October 1958 the three countries initiated test-ban negotiations (the Conference on the Discontinuance of Nuclear Tests) and agreed to a temporary moratorium (the Soviet Union joined the moratorium shortly after this date). The moratorium would last for close to three years.

The Conference on the Discontinuance of Nuclear Tests convened in Geneva at Moscow's request (the Western participants had proposed New York City). The US delegation was led by James Jeremiah Wadsworth, an envoy to the UN, the British by David Ormsby-Gore, the Minister of State for Foreign Affairs, and the Soviets by Semyon K. Tsarapkin, a disarmament expert with experience dating back to the 1946 Baruch Plan. The Geneva Conference began with a Soviet draft treaty grounded in the Geneva System. The three nuclear weapons states (the "original parties") would abide by a test ban, verified by the Geneva System, and work to prevent testing by potential nuclear states (such as France). This was rejected by Anglo-American negotiators due to fears that the verification provisions were too vague and the Geneva System too weak.

Shortly after the Geneva Conference began in the fall of 1958, Eisenhower faced renewed domestic opposition to a comprehensive test ban as Senator Albert Gore Sr. argued in a widely circulated letter that a partial ban would be preferable due to Soviet opposition to strong verification measures.

The Gore letter did spur some progress in negotiations, as the Soviet Union allowed in late November 1958 for explicit control measures to be included in the text of the drafted treaty. By March 1959, the negotiators had agreed upon seven treaty articles, but they primarily concerned uncontroversial issues and a number of disputes over verification persisted. First, the Soviet verification proposal was deemed by the West to be too reliant on self-inspection, with control posts primarily staffed by citizens of the country housing the posts and a minimal role for officials from the international supervisory body. The West insisted that half of a control post staff be drawn from another nuclear state and half from neutral parties. Second, the Soviet Union required that the international supervisory body, the Control Commission, require unanimity before acting; the West rejected the idea of giving Moscow a veto over the Commission's proceedings. Finally, the Soviet Union preferred temporary inspection teams drawn from citizens of the country under inspection, while the West insisted on permanent teams composed of inspectors from the Control Commission.

Additionally, despite the initial positive response to the Geneva experts' report, data gathered from Hardtack operations of 1958 (namely the underground "Rainier" shot) would further complication verification provisions as US scientists, including Hans Bethe (who backed a ban), became convinced that the Geneva findings were too optimistic regarding detection of underground tests, though Macmillan warned that using the data to block progress on a test ban might be perceived in the public as a political ploy. In early 1959, Wadsworth told Tsarapkin of new US skepticism towards the Geneva System. While the Geneva experts believed the system could detect underground tests down to five kilotons, the US now believed that it could only detect tests down to 20 kilotons (in comparison, the Little Boy bomb dropped on Hiroshima had an official yield of 13 kilotons). As a result, the Geneva detection regime and the number of control posts would have to be significantly expanded, including new posts within the Soviet Union. The Soviets dismissed the US argument as a ruse, suggesting that the Hardtack data had been falsified.

In early 1959, a roadblock to an agreement was removed as Macmillan and Eisenhower, over opposition from the Department of Defense, agreed to consider a test ban separately from broader disarmament endeavors.

On 13 April 1959, facing Soviet opposition to on-site detection systems for underground tests, Eisenhower proposed moving from a single, comprehensive test ban to a graduated agreement where atmospheric tests—those up to 50 km (31 mi) high, a limit Eisenhower would revise upward in May 1959—would be banned first, with negotiations on underground and outer-space tests continuing. This proposal was turned down on 23 April 1959 by Khrushchev, calling it a "dishonest deal." On 26 August 1959, the US announced it would extend its year-long testing moratorium to the end of 1959, and would not conduct tests after that point without prior warning. The Soviet Union reaffirmed that it would not conduct tests if the US and UK continued to observe a moratorium.

To break the deadlock over verification, Macmillan proposed a compromise in February 1959 whereby each of the original parties would be subject to a set number of on-site inspections each year. In May 1959, Khrushchev and Eisenhower agreed to explore Macmillan's quota proposal, though Eisenhower made further test-ban negotiations conditional on the Soviet Union dropping its Control Commission veto demand and participating in technical discussions on identification of high-altitude nuclear explosions. Khrushchev agreed to the latter and was noncommittal on the former. A working group in Geneva would eventually devise a costly system of 5–6 satellites orbiting at least above the earth, though it could not say with certainty that such a system would be able to determine the origin of a high-altitude test. US negotiators also questioned whether high-altitude tests could evade detection via radiation shielding. Concerning Macmillan's compromise, the Soviet Union privately suggested it would accept a quota of three inspections per year. The US argued that the quota should be set according to scientific necessity (i.e., be set according to the frequency of seismic events).

In June 1959, a report of a panel headed by Lloyd Berkner, a physicist, was introduced into discussions by Wadsworth. The report specifically concerned whether the Geneva System could be improved without increasing the number of control posts. Berkner's proposed measures were seen as highly costly and the technical findings themselves were accompanied by a caveat about the panel's high degree of uncertainty given limited data. Around the same time, analysis conducted by the Livermore National Laboratory and RAND Corporation at Teller's instruction found that the seismic effect of an underground test could be artificially dampened (referred to as "decoupling") to the point that a 300-kiloton detonation would appear in seismic readings as a one-kiloton detonation. These findings were largely affirmed by pro-ban scientists, including Bethe. The third blow to the verification negotiations was provided by a panel chaired by Robert Bacher, which found that even on-site inspections would have serious difficulty determining whether an underground test had been conducted.

In September 1959, Khrushchev visited the US While the test ban was not a focus on conversations, a positive meeting with Eisenhower at Camp David eventually led Tsarapkin to propose a technical working group in November 1959 that would consider the issues of on-site inspections and seismic decoupling in the "spirit of Camp David." Within the working group, Soviet delegates allowed for the timing of on-site inspections to be grounded in seismic data, but insisted on conditions that were seen as excessively strict. The Soviets also recognized the theory behind decoupling, but dismissed its practical applications. The working group closed in December with no progress and significant hostility. Eisenhower issued a statement blaming "the recent unwillingness of the politically guided Soviet experts to give serious scientific consideration to the effectiveness of seismic techniques for the detection of underground nuclear explosions." Eisenhower simultaneously declared that the US would not be held to its testing moratorium when it expired on 31 December 1959, though pledged to not test if Geneva talks progressed. The Soviet Union followed by reiterating its decision to not test as long as Western states did not test.

In early 1960, Eisenhower indicated his support for a comprehensive test ban conditional on proper monitoring of underground tests. On 11 February 1960, Wadsworth announced a new US proposal by which only tests deemed verifiable by the Geneva System would be banned, including all atmospheric, underwater, and outer-space tests within detection range. Underground tests measuring more than 4.75 on the Richter scale would also be barred, subject to revision as research on detection continued. Adopting Macmillan's quota compromise, the US proposed each nuclear state be subject to roughly 20 on-site inspections per year (the precise figure based on the frequency of seismic events).

Tsarapkin responded positively to the US proposal, though was wary of the prospect of allowing underground tests registering below magnitude 4.75. In its own proposal offered 19 March 1960 the Soviet Union accepted most US provisions, with certain amendments. First, the Soviet Union asked that underground tests under magnitude 4.75 be banned for a period of four-to-five years, subject to extension. Second, it sought to prohibit all outer-space tests, whether within detection range or not. Finally, the Soviet Union insisted that the inspection quota be determined on a political basis, not a scientific one. The Soviet offer faced a mixed reception. In the US, Senator Hubert Humphrey and the Federation of American Scientists (which was typically seen as supportive of a test ban) saw it as a clear step towards an agreement. Conversely, AEC chairman John A. McCone and Senator Clinton Presba Anderson, chair of the Joint Committee on Atomic Energy, argued that the Soviet system would be unable to prevent secret tests. That year, the AEC published a report arguing that the continuing testing moratorium risked "free world supremacy in nuclear weapons," and that renewed testing was critical for further weapons development. The joint committee also held hearings in April which cast doubt on the technical feasibility and cost of the proposed verification measures. Additionally, Teller continued to warn of the dangerous consequences of a test ban and the Department of Defense (including Neil H. McElroy and Donald A. Quarles, until recently its top two officials) pushed to continue testing and expand missile stockpiles.

Shortly after the Soviet proposal, Macmillan met with Eisenhower at Camp David to devise a response. The Anglo-American counterproposal agreed to ban small underground tests (those under magnitude 4.75) on a temporary basis (a duration of roughly 1 year, versus the Soviet proposal of 4–5 years), but this could only happen after verifiable tests had been banned and a seismic research group (the Seismic Research Program Advisory Group) convened. The Soviet Union responded positively to the counterproposal and the research group convened on 11 May 1960. The Soviet Union also offered to keep an underground ban out of the treaty under negotiation. In May 1960, there were high hopes that an agreement would be reached at an upcoming summit of Eisenhower, Khrushchev, Macmillan, and Charles de Gaulle of France in Paris.

A test ban seemed particularly close in 1960, with Britain and France in accord with the US (though France conducted its first nuclear test in February) and the Soviet Union having largely accepted the Macmillan-Eisenhower proposal. But US-Soviet relations soured after an American U-2 spy plane was shot down in Soviet airspace in May 1960. The Paris summit was abruptly cancelled and the Soviet Union withdrew from the seismic research group, which subsequently dissolved. Meetings of the Geneva Conference continued until December, but little progress was made as Western-Soviet relations continued to grow more antagonistic through the summer, punctuated by the Congo Crisis in July and angry exchanges at the UN in September. Macmillan would later claim to President John F. Kennedy that the failure to achieve a test ban in 1960 "was all the fault of the American 'big hole' obsession and the consequent insistence on a wantonly large number of on-site inspections."

Eisenhower would leave office with an agreement out of reach, as Eisenhower's technical advisors, upon whom he relied heavily, became mired in the complex technical questions of a test ban, driven in part by a strong interest among American experts to lower the error rate of seismic test detection technology. Some, including Kistiakowsky, would eventually raise concerns about the ability of inspections and monitors to successfully detect tests. The primary product of negotiations under Eisenhower was the testing moratorium without any enforcement mechanism. Ultimately, the goal of a comprehensive test ban would be abandoned in favor of a partial ban due to questions over seismic detection of underground tests.

Political scientist Robert Gilpin later argued that Eisenhower faced three camps in the push for a test ban. The first was the "control" camp, led by figures like Linus Pauling and astronomer Harlow Shapley, which believed that both testing and possession of nuclear weapons was dangerous. Second, there was the "finite containment" camp, populated by scientists like Hans Bethe, which was concerned by perceived Soviet aggression but still believed that a test ban would be workable with adequate verification measures. Third, the "infinite containment" camp, of which Strauss, Teller, and members of the defense establishment were members, believed that any test ban would grant the Soviet Union the ability to conduct secret tests and move ahead in the arms race.

The degree of Eisenhower's interest in a test ban is a matter of some historical dispute. Stephen E. Ambrose writes that by early 1960, a test ban had become "the major goal of his President, indeed of his entire career," and would be "his final and most lasting gift to his country." Conversely, John Lewis Gaddis characterizes negotiations of the 1950s as "an embarrassing series of American reversals," suggesting a lack of real US commitment to arms control efforts. The historian Robert Divine also attributed the failure to achieve a deal to Eisenhower's "lack of leadership," evidenced by his inability to overcome paralyzing differences among US diplomats, military leaders, national security experts, and scientists on the subject. Paul Nitze would similarly suggest that Eisenhower never formulated a cohesive test ban policy, noting his ability to "believe in two mutually contradictory and inconsistent propositions at the same time."

Upon assuming the presidency in January 1961, John F. Kennedy was committed to pursuing a comprehensive test ban and ordered a review of the American negotiating position in an effort to accelerate languishing talks, believing Eisenhower's approach to have been "insufficient." In making his case for a test ban, Kennedy drew a direct link between continued testing and nuclear proliferation, calling it the "'Nth-country' problem." While a candidate, Kennedy had argued, "For once China, or France, or Sweden, or half a dozen other nations successfully test an atomic bomb, the security of both Russians and Americans is dangerously weakened." He had also claimed that renewed testing would be "damaging to the American image" and might threaten the "existence of human life." On the campaign trail, Kennedy's test-ban proposal consisted of a continued US testing moratorium, expanded efforts to reach a comprehensive agreement, limit any future tests to those minimizing fallout, and expand research on fallout. Notably, early in his term, Kennedy also presided over a significant increase in defense spending, which was reciprocated by the Soviet Union shortly thereafter, thus placing the test-ban negotiations in the context of an accelerating arms race.

On 21 March 1961, test-ban negotiations resumed in Geneva and Arthur Dean, a lead US envoy, offered a new proposal in an attempt to bridge the gap between the two sides. The early Kennedy proposal largely grew out of later Eisenhower efforts, with a ban on all tests but low-yield underground ones (below magnitude 4.75), which would be subject to a three-year moratorium. The US and UK proposed 20 on-site inspections per annum, while the Soviet Union proposed three. The verification procedures included in the Anglo-American plan were unacceptable to Tsarapkin, who responded with separate proposals rejected by the Western powers. Specifically, the Soviet Union proposed a "troika" mechanism: a monitoring board composed of representatives of the West, the Soviet Union, and nonaligned states that would require unanimity before acting (effectively giving the Soviet Union veto authority). In May 1961, Kennedy attempted via secret contact between Attorney General Robert F. Kennedy and a Soviet intelligence officer to settle on 15 inspections per year. This was rejected by Khrushchev.

Ahead of the June 1961 Vienna summit between Kennedy and Khrushchev, Robert F. Kennedy spoke with the Soviet ambassador to the US, who suggested that progress on a test ban was possible in a direct meeting between the leaders. President Kennedy subsequently announced to the press that he had "strong hopes" for progress on a test ban. In Vienna, Khrushchev suggested that three inspections per year would have to be the limit, as anything more frequent would constitute espionage. Khrushchev privately believed allowing three inspections to be a significant concession to the West, as other Soviet officials preferred an even less intrusive system, and was angered by US resistance. Khrushchev later told his son, "hold out a finger to them—they chop off your whole hand."

Additionally, the Soviet Union had once been ready to support an control commission under the aegis of the UN, Khrushchev explained, but it could not longer do so given perceived bias in recent UN action in the Congo. Instead, Khrushchev reiterated the troika proposal. Furthermore, Khrushchev insisted that the test ban be considered in the context of "general and complete disarmament," arguing that a test ban on its own was unimportant; Kennedy said the US could only agree with a guarantee that a disarmament agreement would be reached quickly (the Vienna demands thus amounted to a reversal of both sides' earlier positions). Kennedy also disagreed that a test ban was itself insignificant; the world could expect many more countries in the coming years to cross the nuclear threshold without a test ban. Ultimately, the two leaders left Vienna without clear progress on the subject. The Soviet Union would drop the general-disarmament demand in November 1961.

Following the setback in Vienna and Berlin Crisis of 1961, as well as the Soviet decision to resume testing in August (attributed by Moscow to a changed international situation and French nuclear tests), Kennedy faced mounting pressure from the Department of Defense and nuclear laboratories to set aside the dream of a test ban. In June 1961, following stalled talks in Geneva, Kennedy had argued that Soviet negotiating behavior raised "a serious question about how long we can safely continue on a voluntary basis a refusal to undertake tests in this country without any assurance that the Russians are not testing." Whether or not the Soviet Union had actually conducted secret tests was a matter of debate within the Kennedy administration. A team led by physicist Wolfgang K. H. Panofsky reported that while the Soviet Union could have secretly tested weapons, there was no evidence indicating that it actually had. Panofsky's findings were dismissed by the Joint Chiefs of Staff as "assertive, ambiguous, semiliterate and generally unimpressive."

Two weeks after the lifting of the Soviet moratorium in August 1961, and after another failed Anglo-American attempt to have the Soviet Union agree to an atmospheric-test ban, the US restarted testing on 15 September 1961. Kennedy specifically limited such testing to underground and laboratory tests, but under mounting pressure as Soviet tests continued — during the time period of the Soviet "Tsar Bomba" 50 Mt+ test detonation on 30 October over Novaya Zemlya — Kennedy announced and dedicated funds to a renewed atmospheric testing program in November 1961.

A report on the 1961 Soviet tests, published by a group of American scientists led by Hans Bethe, determined "that [Soviet] laboratories had probably been working full speed during the whole moratorium on the assumption that tests would at some time be resume," with preparations likely having begun prior to the resumption of talks in Geneva in March 1961. In January 1962, Bethe, who had once supported a test ban, publicly argued that a ban was "no longer a desirable goal" and the US should test weapons developed by its laboratories. In contrast to Soviet laboratories, US laboratories had been relatively inactive on nuclear weapons issues during the moratorium.
In December 1961, Macmillan met with Kennedy in Bermuda, appealing for a final and permanent halt to tests. Kennedy, conversely, used the meeting to request permission to test on Christmas Island, with US testing grounds in the Pacific having largely been exhausted. Macmillan agreed to seek to give US permission "if the situation did not change." Christmas Island was ultimately opened to US use by February 1962.

On this matter of resumed atmospheric tests, Kennedy lacked the full backing of his administration and allies. In particularly, Macmillan, Adlai Stevenson (then the UN ambassador), the State Department, the United States Information Agency, and Jerome Wiesner, the PSAC chairman, opposed resuming atmospheric tests. On the side advocating resumption were the AEC, Joint Committee on Atomic Energy, Joint Chiefs of Staff (which had called for renewed atmospheric tests in October 1961), and Department of Defense, though then-Secretary of Defense Robert McNamara privately acknowledged that such tests were "not really necessary." Teller continued to advocate for atmospheric tests, as well, arguing in early 1962 that nuclear fallout was nothing be concerned about. Teller also argued that testing was necessary to continued advancement of US nuclear capabilities, particularly in terms of the mobility of its weapons and, accordingly, its second-strike capability.

Despite Teller's reassurances, Kennedy himself "hated the idea of reopening the race" and was uneasy with continued production of fallout, a negative consequence of resumed testing that its opponents within the administration stressed. Opponents of the tests also argued that renewed atmospheric tests would come at a significant moral cost to the US, given broad public opposition to the plan, and claimed that further tests were largely unnecessary, with the US already having an adequate nuclear arsenal. Arthur Dean believed that public opposition to atmospheric testing was so great that the US would have to halt such tests within four years even without an agreement. John Kenneth Galbraith, then the ambassador to India, had advised Kennedy in June 1961 that resumed testing "would cause us the gravest difficulties in Asia, Africa and elsewhere." Similarly, Hubert Humphrey described the moratorium as "a ray of hope to millions of worried people." Its termination, Humphrey warned, "might very well turn the political tides in the world in behalf of the Soviets."

Ultimately, Kennedy sided with those arguing for resumed testing. In particular, an argument by William C. Foster, the head of the Arms Control and Disarmament Agency, swayed Kennedy. Foster argued that if the US failed to respond to the Soviet test series, Moscow could order a second test series, which could give the Soviet Union a significant advantage. Furthermore, a second test series, without US reciprocation, could damage the push for a test ban and make Senate ratification of any agreement less likely. On 2 March 1962, building on the November 1961 announcement, Kennedy promised to resume atmospheric testing by the end of April 1962 if Moscow continued to resist the Anglo-American test-ban proposal. To an extent, the announcement was a compromise, as Kennedy restricted atmospheric tests to those tests which were "absolutely necessary," not feasible underground, and minimized fallout. The condition that testing would resume only if the Soviet Union continued to oppose the Anglo-American proposal also served as a concession to dissenting voices within his administration and to Macmillan.

Kennedy portrayed resumed testing as a necessary for the image of US resolve. If the US failed to respond to the Soviet test series, Kennedy explained, Moscow would "chalk it up, not to goodwill, but to a failure of will—not to our confidence in Western superiority, but to our fear of world opinion." Keeping the US in a position of strength, Kennedy argued, would be necessary for a test ban to ever come about.

The US suspension of atmospheric tests was lifted on 25 April 1962.

By March 1962, the trilateral talks in Geneva had shifted to 18-party talks at the UN Disarmament Conference. On 27 August 1962, within that conference, the US and UK offered two draft treaties to the Soviet Union. The primary proposal included a comprehensive ban verified by control posts under national command, but international supervision, and required on-site inspections. This was rejected by the Soviet Union due to the inspection requirement. The alternative proposal included a partial test ban—underground tests would be excluded—to be verified by national detection mechanisms, without supervision by a supranational body.

In October 1962, the US and Soviet Union experienced the Cuban Missile Crisis, which brought the two superpowers to the edge of nuclear war and prompted both Kennedy and Khrushchev to seek accelerated rapprochement. After years of dormant or lethargic negotiations, American and British negotiators subsequently forged a strong working relationship and with Soviet negotiators found common ground on test restrictions later in 1962. After years of pursuing a comprehensive ban, Khrushchev was convinced to accept a partial ban, partly due to the efforts of Soviet nuclear scientists, including Kurchatov, Sakharov, and Yulii Borisovich Khariton, who argued that atmospheric testing had severe consequences for human health. Khrushchev had been concerned by a partial ban due to the greater US experience in underground tests; by 1962, the US had conducted 89 such tests and the Soviet Union just two (the Soviet focus had been on cheaper, larger-yield atmospheric tests). For this reason, many in the Soviet weapons industry argued that a partial ban would give the US the advantage in nuclear capabilities. Khrushchev would later recount that he saw test-ban negotiations as a prime venue for ameliorating tensions after the crisis in Cuba.

Shocked by how close the world had come to thermonuclear war, Khrushchev proposed easing of tensions with the US. In a letter to President Kennedy dated 30 October 1962, Kurshchev outlined a range of bold initiatives to forestall the possibility of nuclear war, including proposing a non-aggression treaty between the North Atlantic Treaty Organization (NATO) and the Warsaw Pact or even the disbanding these military blocs, a treaty to cease all nuclear weapons testing and even the elimination of all nuclear weapons, resolution of the hot-button issue of Germany by both East and West formally accepting the existence of West Germany and East Germany, and US recognition of the government of mainland China. The letter invited counter-proposals and further exploration of these and other issues through peaceful negotiations. Khrushschev invited Norman Cousins, the editor of a major US periodical and an anti-nuclear weapons activist, to serve as liaison with President Kennedy, and Cousins met with Khrushchev for four hours in December 1962. Kennedy's response to Khrushchev's proposals was lukewarm but Kennedy expressed to Cousins that he felt constrained in exploring these issues due to pressure from hardliners in the US national security apparatus. However Kennedy pursued negotiations for a partial nuclear test ban.

On 13 November 1962, Tsarapkin indicated that the Soviet Union would accept a proposal drafted by US and Soviet experts involving automated test detection stations ("black boxes") and a limited number of on-site inspections. The two sides disagreed over the number of black boxes, however, as the US sought 12–20 such stations and the Soviet Union rejected any more than three. On 28 December 1962, Kennedy lowered the US demand to 8–10 stations. On 19 February 1963, the number was lowered further to seven, as Khrushchev continued to insist on no more than three. Kennedy was willing to reduce the number to six, though this was not clearly communicated to the Soviet Union. On 20 April 1963, Khrushchev withdrew support for three inspections entirely.

Progress was further complicated in early 1963, as a group in the US Congress called for the Soviet proposal to be discarded in favor of the Geneva System. On 27 May 1963, 34 US Senators, led by Humphrey and Thomas J. Dodd, introduced a resolution calling for Kennedy to propose another partial ban to the Soviet Union involving national monitoring and no on-site inspections. Absent Soviet agreement, the resolution called for Kennedy to continue to "pursue it with vigor, seeking the widest possible international support" while suspending all atmospheric and underwater tests. The effect of the resolution was to bolster the general push for a test ban, though Kennedy initially was concerned that it would damage attempts to secure a comprehensive ban, and had administration figures (including the Joint Chiefs of Staff) reiterate a call for a comprehensive ban. That same spring of 1963, however, Kennedy had sent antinuclear activist Norman Cousins to Moscow to meet with Khrushchev, where he explained that the political situation in the US made it very difficult for Kennedy agree to a comprehensive ban with Khrushchev's required terms. Cousins also assured Khrushchev that though Kennedy had rejected Khrushchev's offer of three yearly inspections, he still was set on achieving a test ban. In March 1963, Kennedy had also held a press conference in which he re-committed to negotiations with the Soviet Union as a means of preventing rapid nuclear proliferation, which he characterized as "the greatest possible danger and hazard."
One of Kennedy's advisors, Walt Whitman Rostow, advised the President to make a test ban conditional on the Soviet Union withdrawing troops from Cuba and abiding by a 1962 agreement on Laos, but Kennedy opted instead for test-ban negotiations without preconditions. On 10 June 1963, in an effort to reinvigorate and recontextualize a test ban, President Kennedy dedicated his commencement address at American University to "the most important topic on earth: world peace" and proceeded to make his case for the treaty. Kennedy first called on Americans to dispel the idea that peace is unattainable. "Let us focus instead on a more practical, more attainable peace," Kennedy said, "based not on a sudden revolution in human nature but on a gradual evolution in human institutions—on a series of concrete actions and effective agreements which are in the interest of all concerned." Second, Kennedy appealed for a new attitude towards the Soviet Union, calling Americans to not "see only a distorted and desperate view of the other side, not to see conflict as inevitable, accommodations as impossible and communication as nothing more than an exchange of threats." Finally, Kennedy argued for a reduction in Cold War tensions, with a test ban serving as a first step towards complete disarmament:

Kennedy proceeded to announce an agreement with Khrushchev and Macmillan to promptly resume comprehensive test-ban negotiations in Moscow and a US decision to unilaterally halt atmospheric tests. The speech was well received by Khrushchev, who later called it "the greatest speech by any American President since Roosevelt," though was met with some skepticism within the US. The speech was endorsed by Humphrey and other Democrats, but labeled a "dreadful mistake" by Republican Senator Barry Goldwater and "another case of concession" by Everett Dirksen, the leader of the Senate Republicans. Dirksen and Charles A. Halleck, the second-ranking House Republican, warned that the renewed negotiations might end in "virtual surrender."

Due to prior experience in arms control and his personal relationship with Khrushchev, former Assistant Secretary of War John J. McCloy was first considered the likely choice for chief US negotiator in Moscow, but his name was withdrawn after he turned out to be unavailable over the summer. W. Averell Harriman, a former ambassador to the Soviet Union well respected in Moscow, was chosen instead. The US delegation would also include Adrian S. Fisher, Carl Kaysen, John McNaughton, and William R. Tyler. In Britain, Macmillan initially wanted David Ormsby-Gore, who had just completed a term as foreign minister, to lead his delegation, but there were concerns that Ormsby-Gore would appear to be a US "stooge" (Kennedy described him as "the brightest man he ever knew"). Instead, Macmillan chose Quintin Hogg. Arthur M. Schlesinger Jr., a special advisor to Kennedy, believed that Hogg was "ill prepared on the technicalities of the problem and was consumed by a desire to get a treaty at almost any cost." Andrei Gromyko, the Soviet Minister of Foreign Affairs, served as Moscow's emissary.

Heading into the negotiations, there was still no resolution within the Kennedy Administration of the question of whether to pursue a comprehensive or partial ban. In an effort to achieve the former, Britain proposed reducing the number of mandated inspections to allay Soviet concerns, but Harriman believed such a reduction would have to be paired with other concessions that Khrushchev would be able to show off within the Soviet Union and to China. Withdrawing PGM-19 Jupiter missiles from Italy and Turkey would have been an option, had they not already been removed in the wake of the Cuban Missile Crisis. In meetings prior to the negotiations, Kennedy informed Harriman that he would be willing to make concessions on the Berlin question.

On 2 July 1963, Khrushchev proposed a partial ban on tests in the atmosphere, outer space, and underwater, which would avoid the contentious issue of detecting underground tests. Notably, Khrushchev did not link this proposal to a moratorium on underground tests (as had been proposed earlier), but said it should be followed by a non-aggression pact between NATO and the Warsaw Pact. "A test ban agreement combined with the signing of a non-aggression pact between the two groups of state will create a fresh international climate more favorable for a solution of the major problems of our time, including disarmament," Khrushchev said.

As the nuclear powers pursued a test ban agreement, they also sought to contend with a rising communist China, which at the time was pursuing its own nuclear program. In 1955, Mao Zedong expressed to the Soviet Union his belief that China could withstand a first nuclear strike and more than 100 million casualties. In the 1950s, the Soviet Union assisted the Chinese nuclear program, but stopped short of providing China with an actual nuclear bomb, which was followed by increasingly tense relations in the late 1950s and early 1960s. Khrushchev began the test-ban talks of 1958 with minimal prior discussion with China, and the two countries' agreement on military-technology cooperation was terminated in June 1959. Prior to the Moscow negotiations of the summer of 1963, Kennedy granted Harriman significant latitude in reaching a "Soviet-American understanding" vis-à-vis China. Secret Sino-Soviet talks in July 1963 revealed further discord between the two communist powers, as the Soviet Union released a statement that it did not "share the views of the Chinese leadership about creating 'a thousand times higher civilization' on the corpses of hundreds of millions of people." The Soviet Union also issued an ideological critique of China's nuclear policy, declaring that China's apparent openness to nuclear war was "in crying contradiction to the idea of Marxism-Leninism," as a nuclear war would "not distinguish between imperialists and working people."

The negotiations were inaugurated on 15 July 1963 at the Kremlin with Khrushchev in attendance. Khrushchev reiterated that the Anglo-American inspection plan would amount to espionage, effectively dismissing the possibility of a comprehensive ban. Following the script of his 3 July 1963 speech, Khrushchev did not demand a simultaneous moratorium on underground testing and instead proposed a non-aggression pact. Under instruction from Washington, Harriman replied that the US would explore the possibility of a non-aggression pact in good faith, but indicated that while a test ban could be quickly completed, a non-aggression pact would require lengthy discussions. Additionally, such a pact would complicate the issue of Western access to West Berlin. Harriman also took the opportunity to propose a non-proliferation agreement with would bar the transfer of nuclear weapons between countries. Khrushchev said that such an agreement should be considered in the future, but in the interim, a test ban would have the same effect on limiting proliferation.

Following initial discussions, Gromyko and Harriman began examining drafts of a test-ban agreement. First, language in the drafted preamble appeared to Harriman to prohibit the use of nuclear weapons in self-defense, which Harriman insisted be clarified. Harriman additionally demanded that an explicit clause concerning withdrawal from the agreement be added to the treaty; Khrushchev believed that each state had a sovereign right to withdraw, which should simply be assumed. Harriman informed Gromyko that without a clause governing withdrawal, which he believed the US Senate would demand, the US could not assent. Ultimately, the two sides settled upon compromise language:

Each Party shall in exercising its national sovereignty have the right to withdraw from the Treaty if it decides that extraordinary events, related to the subject matter of this Treaty, have jeopardized the supreme interests of its country.

Gromyko and Harriman debated how states not universally recognized (e.g., East Germany and China) could join the agreement. The US proposed asserting that accession to the treaty would not indicate international recognition. This was rejected by the Soviet Union. Eventually, with Kennedy's approval, US envoys Fisher and McNaughton devised a system whereby multiple government would serve as depositaries for the treaty, allowing individual states to sign only the agreement held by the government of their choice in association with other like-minded states. This solution, which overcame one of the more challenging roadblocks in the negotiations, also served to allay mounting concerns from Macmillan, which were relayed to Washington, that an agreement would once again be derailed. Finally, in an original Soviet draft, the signature of France would have been required for the treaty to come into effect. At Harriman's insistence, this requirement was removed.
The agreement was initialed on 25 July 1963, just 10 days after negotiations commenced. The following day, Kennedy delivered a 26-minute televised address on the agreement, declaring that since the invention of nuclear weapons, "all mankind has been struggling to escape from the darkening prospect of mass destruction on earth ... Yesterday a shaft of light cut into the darkness." Kennedy expressed hope that the test ban would be the first step towards broader rapprochement, limit nuclear fallout, restrict nuclear proliferation, and slow the arms race in such a way that fortifies US security. Kennedy concluded his address in reference to a Chinese proverb that he had used with Khrushchev in Vienna two years prior. "'A journey of a thousand miles must begin with a single step,'" Kennedy said. "And if that journey is a thousand miles, or even more, let history record that we, in this land, at this time, took the first step."

In a speech in Moscow following the agreement, Khrushchev declared that the treaty would not end the arms race and by itself could not "avert the danger of war," and reiterated his proposal of a NATO-Warsaw Pact non-aggression accord. For Khrushchev, the test ban negotiations had long been a means of improving the Soviet Union's global image and reducing strain in relations with the West. There are also some indications that military experts within the Soviet Union saw a test ban as a way to restrict US development of tactical nuclear weapons, which could have increased US willingness to deploy small nuclear weapons on battlefields while circumventing the Soviet nuclear deterrent. Concern that a comprehensive ban would retard modernization of the Soviet arsenal may have pushed Khrushchev towards a partial ban. Counteracting the move towards a partial ban was Khrushchev's interest in reducing spending on testing, as underground testing was more expensive than the atmospheric tests the Soviet Union had been conducting; Khrushchev preferred a comprehensive ban as it would have eliminated the cost of testing entirely. Furthermore, there was internal concern about nuclear proliferation, particularly regarding the prospect of France and China crossing the threshold and the possibility of a multilateral NATO nuclear force, which was seen as a step towards West Germany acquiring nuclear weapons (the first Soviet test ban proposal in 1955 was made in the same month than West Germany joined NATO).

It was not until after the agreement was reached that the negotiators broached the question of France and China joining the treaty. Harriman proposed to Khrushchev that the US lobby France while the Soviet Union pursued a Chinese signature. "That's your problem," Khrushchev said in reply. Earlier, the Soviet ambassador to the US, Mikhail A. Menshikov, reportedly asked whether the US could "deliver the French." Both Kennedy and Macmillan personally called on de Gaulle to join, offering assistance to the French nuclear program in return. Nevertheless, on 29 July 1963, France announced it would not join the treaty. It was followed by China two days later.

On 5 August 1963, British Foreign Secretary Alec Douglas-Home, Soviet foreign minister Gromyko, and US Secretary of State Dean Rusk signed the final agreement.

Between 8 and 27 August 1963, the United States Senate Committee on Foreign Relations held hearings on the treaty. The Kennedy administration largely presented a united front in favor of the deal. Leaders of the once-opposed Joint Chiefs of Staff (JCS) and AEC acknowledged that the treaty would be of net benefit, though Teller, former members of the JCS and AEC, and the commander of the Strategic Air Command made clear their firm opposition. The opponents' argument centered on four themes. First, banning atmospheric tests would prevent the US from ensuring the hardness of its LGM-30 Minuteman missile silos and, second, from developing a capable missile defense system. Third, it was argued that the Soviet Union led the US in high-yield weapons (recall the Soviet Tsar Bomba test of 1961), which required atmospheric testing banned by the treaty, while the US led the Soviet Union in low-yield weapons, which were tested underground and would be permitted by the treaty. Fourth, the ban would prevent peaceful, civilian uses of nuclear detonations. Teller declared that the treaty would be a "step away from safety and possibly ... toward war."

Administration testimony sought to counteract these arguments. Defense Secretary Robert McNamara announced his "unequivocal support" for the treaty before the Foreign Relations Committee, arguing that US nuclear forces were secure and clearly superior to those of the Soviet Union, and that any major Soviet tests would be detected. Glenn T. Seaborg, the chairman of the AEC, also gave his support to the treaty in testimony, as did Harold Brown, the Department of Defense's lead scientist, and Norris Bradbury, the longtime director of the Los Alamos Laboratory. Maxwell D. Taylor, the Chairman of the Joint Chiefs of Staff, also testified in favor of the deal. Taylor and other members of the JCS, including Curtis LeMay, had made their support for the treaty conditional on four "safeguards": (1) a continued, aggressive underground testing program, (2) continued nuclear research programs, (3) continued readiness to resume atmospheric tests, and (4) improved verification equipment. Kennedy emphasized that the US would retain the ability to use nuclear weapons in war, would not be bound by the treaty if the Soviets violated it, and would continue an aggressive underground testing program. Kennedy also stressed that a ban would be a key step in preventing nuclear war.

The testimonies of the Joint Chiefs were seen as particularly effective in allaying concerns, as were the reassurances issued by Kennedy, who had acquired a reputation for resoluteness against the Soviet Union in the wake of the Cuban Missile Crisis. Additionally, a number of prominent Republicans came out in support of the deal, including Eisenhower, Eisenhower's vice president Richard Nixon, and Senator Everett Dirksen, who had initially been skeptical of the treaty. Eisenhower's science advisor and former PSAC head, George Kistiakowsky, endorsed the treaty. Former President Harry S. Truman also lent his support. Supporters of the deal mounted a significant pressure campaign, with active lobbying in favor by a range of civilian groups, including the United Automobile Workers/AFL-CIO, the National Committee for a Sane Nuclear Policy, Women Strike for Peace, and Methodist, Unitarian Universalist, and Reform Jewish organizations. Jerome Wiesner, the chairman of PSAC, later said that this public advocacy was a primary motivation for Kennedy's push for a test ban. Civil opposition to the deal was less prominent, though the Veterans of Foreign Wars announced opposition to the deal along with the International Council of Christian Churches, which rejected a "covenant with a godless power." Polling in late August 1963 indicated that more than 60% of Americans supported the deal while less than 20% opposed it.

On 3 September 1963, the Foreign Relations Committee approved the treaty by a 16–1 vote. On 24 September 1963, the US Senate voted 80–14 to approve ratification of the treaty, exceeding the necessary two-thirds majority by 14 votes. The Soviet Union ratified the treaty the following day with a unanimous vote of the Presidium of the Supreme Soviet. On 10 October 1963, the treaty entered into effect.

The treaty declares as its "principal aim the speediest possible achievement of an agreement on general and complete disarmament under strict international control," and explicitly states the goal of achieving a comprehensive test ban (i.e., one which bans underground tests). The treaty permanently forbids the parties to the treaty from conducting, permitting, or encouraging any nuclear explosion in the atmosphere, outer space, or underwater, as well as "any other nuclear explosion" that threatens to send nuclear debris into another state's territory. The wording "any other nuclear explosion" prohibited peaceful nuclear explosions, due to the difficulty in differentiating those from military tests without expanded verification measures.

Per the compromise forged by US delegates Adrian S. Fisher and John McNaughton in Moscow, the third article of the treaty allows states to deposit instruments of ratification or accession with the government of Britain, the Soviet Union, or US, thereby avoiding the issue of the treaty appearing to legitimize governments lacking universal recognition. The fourth article reflects the compromise struck by Gromyko and Harriman in Moscow on departure from the treaty. It recognizes the sovereign right of states to withdraw from treaties, as Khrushchev argued, while explicitly granting parties the right to withdraw if "extraordinary events ... have jeopardized the supreme interests of its country," per the US demand.

By 15 April 1964, 6 months after the PTBT went into effect, more than 100 states had joined the treaty as signatories and 39 had ratified or acceded to it. The most recent party to the PTBT is Montenegro, which succeeded to the treaty in 2006., 126 states were party to the treaty, with 10 other states having signed but not deposited instruments of ratification. Sixty states have not signed the PTBT, including the nuclear states of China, France, and North Korea. Albania, an ideological ally of China at the time of the PTBT's enactment, also has not signed.

The PTBT's ratification coincided with the beginning of a steep decline in the presence of radioactive particles in the atmosphere (following a "bomb spike" in the early 1960s), but did not halt nuclear proliferation. One year after the PTBT's entry into force, China (a non-signatory) conducted the 596 test and became the world's fifth nuclear power. Since China, four other states are known or believed to have acquired nuclear weapons. However, the PTBT has been credited with "slowing" proliferation due to the greater expense associated with underground tests. Kennedy had warned in 1963 that without a test ban there could be 10 nuclear states by 1970 and 15–20 by 1975.

The decade following ratification of the PTBT (1963–72) featured more US nuclear tests than the decade prior (1953–62). In the following decade, the US conducted 385 nuclear tests and 23 peaceful nuclear explosions (PNEs), versus 268 tests and three PNEs in the preceding decade. In contrast, the number of Soviet detonations fell from 218 in the preceding decade to 157 in the following decade, as the Soviet Union was never able to meet the pace of US underground explosions. China and France, both non-signatories, conducted 53 tests between 1963 and 1973. In all, 436 tests were conducted between the signing of the PTBT and 1 July 1973, compared to 499 tests between 16 July 1945 and the signing of the PTBT. In the 1960s and 1970s, China conducted 22 atmospheric tests and France conducted 50. The last atmospheric test was conducted by China in 1980, after French atmospheric testing stopped in 1974. Public opposition to nuclear testing continued following the treaty's enactment. Greenpeace was founded in 1971 in opposition to a planned underground test on the Alaskan island of Amchitka. In 1982, a Greenpeace ship docked at Leningrad without permission demanding that the Soviet Union stop testing.

The PTBT was a first of a series of nuclear arms control treaties in the second half of 20th century. The PTBT has been considered the stepping stone to the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) of 1968, which made explicit reference to the progress provided by the PTBT. In addition to the NPT, the PTBT was followed within ten years by the Outer Space Treaty and Treaty of Tlatelolco in 1967, Seabed Arms Control Treaty in 1971, and the Anti-Ballistic Missile Treaty in 1972. In 1974, the Threshold Test Ban Treaty prohibited underground tests with yields above 150 kilotons.

In October 1977, the original parties to the PTBT renewed discussion of a comprehensive test ban in Geneva. Through the end of the 1970s, the US, UK, and Soviet Union reached agreement on draft provisions prohibiting all testing, temporarily banning PNEs, and establishing a verification system including on-site inspections. However, the sides remained at odds over the precise details of verification and talks would permanently disband with the departure of President Jimmy Carter in 1981.

Momentum towards a comprehensive ban reemerged under Mikhail Gorbachev and President Ronald Reagan, with Gorbachev initiating a testing moratorium in 1985. In December 1986, the US indicated support for the "long-term objective" of a comprehensive ban, followed by the commencement of testing negotiations between the US and Soviet Union in November 1987. In December 1987, the US and Soviet Union agreed to a joint program of experiments on detecting underground tests. In August 1988, Indonesia, Mexico, Peru, Sri Lanka, Venezuela, and Yugoslavia petitioned to transform the PTBT into a comprehensive ban by extending the treaty to underground tests. At a conference on the plan in January 1991, the US indicated it would not permit efforts to achieve a comprehensive ban by consensus with amendments to the PTBT.

Throughout the 1990s, progress accelerated towards a comprehensive test ban treaty (CTBT). Following a series of international meetings on the subject, the UN General Assembly approved resolution 50/64, which appealed for states to follow the PTBT and called for conclusion of the CTBT talks. In September 1996, the Comprehensive Nuclear-Test-Ban Treaty was signed and superseded the PTBT, though the PTBT is still in effect for states not party to the CTBT. The CTBT has yet to enter into force, as 8 required states have not ratified the treaty, including the US and China. France, Russia, and the UK have ratified the CTBT. The technology for detecting underground tests has significantly improved since the 1950s and 1960s, with monitors detecting tests down to 1 kiloton with a high degree of confidence.

Early compliance with the PTBT was believed to be good, though there have been a number of accidental releases of nuclear debris into the atmosphere by parties to the treaty. Additionally, "venting" of underground tests by the US and Soviet Union also continued to release radioactive debris into the atmosphere. Fully contained underground tests were not wholly "clean" either. Underground testing reduced the risk caused by radionuclides with short half-lives, such as iodine-131, and is generally safer than other forms of testing. However, underground testing may also cause long-lived radionuclides, including caesium-135, iodine-129, and plutonium, to seep into the ground.

A notable atmospheric release of radioactive gas followed the Soviet Chagan test of 15 January 1965 in present-day Kazakhstan. Roughly 20% of the radioactive debris produced by the 140-kiloton detonation was released into the atmosphere, with some fallout occurring over Japan. The US complained to Moscow, but no subsequent action was taken. On 25 April 1966, the "Pin Stripe" underground test in Nevada (part of Operation Flintlock) experienced a venting malfunction and produced a radioactive plume headed towards the Midwestern US; the AEC determined that the test did not threaten human health.

Another accidental release occurred following the "Baneberry" shot at the Nevada Test Site on 18 December 1970 (part of Operation Emery). The 10-kiloton underground detonation produced a fissure in the ground, which subsequently allowed radioactive gas to escape into the atmosphere. Radioactive material released by the fissure reached an altitude of and exposed 86 workers to radiation, though none at excessive levels. The incident has since been described as one of the "world's worst nuclear disasters."

Declassified US documents indicate that the US may have violated the PTBT's ban on atmospheric testing in 1972 by, at the instruction of Henry Kissinger, monitoring and collecting data on French atmospheric tests over the Pacific Ocean, which may have amounted to cooperation with the French program. Declassified documents also indicate that the US and UK circumvented the prescribed verification system in 1964–65 by establishing a series of additional control posts in Australia, Fiji, Mauritius, Pakistan, and South Africa.

The 1979 Vela Incident in the southern Atlantic Ocean may have been an atmospheric nuclear test in contravention of the PTBT by Israel and South Africa, both of which were parties to the treaty.

On March 1, 1954, the U.S. detonated its first practical thermonuclear weapon (which used isotopes of lithium as its fusion fuel), known as the "Shrimp" device of the Castle Bravo test, at Bikini Atoll, Marshall Islands. The device yielded 15 megatons, more than twice its expected yield, and became the worst radiological disaster in U.S. history. The combination of the unexpectedly large blast and poor weather conditions caused a cloud of radioactive nuclear fallout to contaminate over. 239 Marshall Island natives and 28 Americans were exposed to significant amounts of radiation, resulting in elevated levels of cancer and birth defects in the years to come.

History of nuclear weapons

Nuclear weapons possess enormous destructive power from nuclear fission or combined fission and fusion reactions. Building on scientific breakthroughs made during the 1930s, the United States, the United Kingdom and Canada collaborated during World War II, in what was called the Manhattan Project, to counter the suspected Nazi German atomic bomb project. In August 1945, two fission bombs were dropped on Japan, standing to date as the only use of nuclear weapons in combat. The Soviet Union started development shortly thereafter with their own atomic bomb project, and not long after that both countries developed even more powerful fusion weapons known as "hydrogen bombs".

In the first decades of the 20th century, physics was revolutionised with developments in the understanding of the nature of atoms. In 1898, Pierre and Marie Curie discovered that pitchblende, an ore of uranium, contained a substance—which they named radium—that emitted large amounts of radioactivity. Ernest Rutherford and Frederick Soddy identified that atoms were breaking down and turning into different elements. Hopes were raised among scientists and laymen that the elements around us could contain tremendous amounts of unseen energy, waiting to be harnessed.

H. G. Wells was inspired to write about atomic weapons in a 1914 novel, "The World Set Free", which appeared shortly before the First World War.
In a 1924 article, Winston Churchill speculated about the possible military implications: "Might not a bomb no bigger than an orange be found to possess a secret power to destroy a whole block of buildings—nay to concentrate the force of a thousand tons of cordite and blast a township at a stroke?"

In January 1933, Adolf Hitler was appointed Chancellor of Germany and it quickly became unsafe for Jewish scientists to remain in the country. Leó Szilárd fled to London where he proposed, and in 1934 patented, the idea of a nuclear chain reaction via neutrons. The patent also introduced the term critical mass to describe the minimum amount of material required to sustain the chain reaction and its potential to cause an explosion. (British patent 630,726.) He subsequently assigned the patent to the British Admiralty so that it could be covered by the Official Secrets Act. In a very real sense, Szilárd was the father of the atomic bomb academically.
Also in 1934, Irène and Frédéric Joliot-Curie discovered that artificial radioactivity could be induced in stable elements by bombarding them with alpha particles; Enrico Fermi reported similar results when bombarding uranium with neutrons.

In December 1938, Otto Hahn and Fritz Strassmann sent a manuscript to "Naturwissenschaften" reporting that they had detected the element barium after bombarding uranium with neutrons. Lise Meitner and her nephew Otto Robert Frisch correctly interpreted these results as being due to the splitting of the uranium atom. (Frisch confirmed this experimentally on January 13, 1939.) They gave the process the name "fission" because of its similarity to the splitting of a cell into two new cells.
Even before it was published, news of Meitner’s and Frisch’s interpretation crossed the Atlantic.
Scientists at Columbia University decided to replicate the experiment and on January 25, 1939, conducted the first nuclear fission experiment in the United States in the basement of Pupin Hall. The following year, they identified the active component of uranium as being the rare isotope uranium-235.

Uranium appears in nature primarily in two isotopes: uranium-238 and uranium-235. When the nucleus of uranium-235 absorbs a neutron, it undergoes nuclear fission, releasing energy and, on average, 2.5 neutrons. Because uranium-235 releases more neutrons than it absorbs, it can support a chain reaction and so is described as fissile. Uranium-238, on the other hand, is not fissile as it does not normally undergo fission when it absorbs a neutron.

By the time Nazi Germany invaded Poland in 1939, beginning World War II, many of Europe's top scientists had already fled the imminent conflict. Physicists on both sides were well aware of the possibility of utilizing nuclear fission as a weapon, but no one was quite sure how it could be done.
In August 1939, concerned that Germany might have its own project to develop fission-based weapons, Albert Einstein signed a letter to U.S. President Franklin D. Roosevelt warning him of the threat. Roosevelt responded by setting up the Uranium Committee under Lyman James Briggs but, with little initial funding ($6,000), progress was slow. It was not until the Japanese attack on Pearl Harbor in December, 1941, that the U.S. decided to commit the necessary resources.

Organized research first began in Britain as part of the Tube Alloys project. The Maud Committee was set up following the work of Frisch and Rudolf Peierls who calculated uranium-235's critical mass and found it to be much smaller than previously thought which meant that a deliverable bomb should be possible. In the February 1940 Frisch–Peierls memorandum they stated that: "The energy liberated in the explosion of such a super-bomb...will, for an instant, produce a temperature comparable to that of the interior of the sun. The blast from such an explosion would destroy life in a wide area. The size of this area is difficult to estimate, but it will probably cover the centre of a big city."

Edgar Sengier, a director of Shinkolobwe Mine in the Congo which produced by far the highest quality uranium ore in the world, had become aware of uranium's possible use in a bomb. In late 1940, fearing that it might be seized by the Germans, he shipped the mine's entire stockpile of ore to a warehouse on Staten Island.

For 18 months British research outpaced the American but by mid-1942, it became apparent that the industrial effort required was beyond Britain's already stretched wartime economy. In September 1942, General Leslie Groves was appointed to lead the U.S. project which became known as the Manhattan Project. Two of his first acts were to obtain authorization to assign the highest priority AAA rating on necessary procurements, and to order the purchase of all 1,250 tons of the Shinkolobwe ore. The Tube Alloys project was quickly overtaken by the U.S. effort and after Roosevelt and Churchill signed the Quebec Agreement in 1943, it was relocated and amalgamated into the Manhattan Project.

With a scientific team led by J. Robert Oppenheimer, the Manhattan project brought together some of the top scientific minds of the day, including many exiles from Europe, with the production power of American industry for the goal of producing fission-based explosive devices before Germany. Britain and the U.S. agreed to pool their resources and information for the project, but the other Allied power, the Soviet Union (USSR), was not informed. The U.S. made an unprecedented investment in the project which at the time was the largest industrial enterprise ever seen, spread across more than 30 sites in the U.S. and Canada. Scientific development was centralized in a secret laboratory at Los Alamos.

For a fission weapon to operate, there must be sufficient fissile material to support a chain reaction, a critical mass. To separate the fissile uranium-235 isotope from the non-fissile uranium-238, two methods were developed which took advantage of the fact that uranium-238 has a slightly greater atomic mass: electromagnetic separation and gaseous diffusion. Another secret site was erected at rural Oak Ridge, Tennessee, for the large-scale production and purification of the rare isotope, which required considerable investment. At the time, K-25, one of the Oak Ridge facilities, was the world's largest factory under one roof. The Oak Ridge site employed tens of thousands of people at its peak, most of whom had no idea what they were working on.

Although uranium-238 cannot be used for the initial stage of an atomic bomb, when it absorbs a neutron, it becomes uranium-239 which decays into neptunium-239, and finally the relatively stable plutonium-239, an element that does not exist naturally on Earth, but is fissile like uranium-235. After Fermi achieved the world's first sustained and controlled nuclear chain reaction with the creation of the first atomic pile, massive reactors were secretly constructed at what is now known as Hanford Site to transform uranium-238 into plutonium for a bomb.

The simplest form of nuclear weapon is a gun-type fission weapon, where a sub-critical mass would be shot at another sub-critical mass. The result would be a super-critical mass and an uncontrolled chain reaction that would create the desired explosion. The weapons envisaged in 1942 were the two gun-type weapons, Little Boy (uranium) and Thin Man (plutonium), and the Fat Man plutonium implosion bomb.

In early 1943 Oppenheimer determined that two projects should proceed forwards: the Thin Man project (plutonium gun) and the Fat Man project (plutonium implosion). The plutonium gun was to receive the bulk of the research effort, as it was the project with the most uncertainty involved. It was assumed that the uranium gun-type bomb could then be adapted from it.

In December 1943 the British mission of 19 scientists arrived in Los Alamos. Hans Bethe became head of the Theoretical Division.
In April 1944 it was found by Emilio Segrè that the plutonium-239 produced by the Hanford reactors had too high a level of background neutron radiation, and underwent spontaneous fission to a very small extent, due to the unexpected presence of plutonium-240 impurities. If such plutonium were used in a gun-type design, the chain reaction would start in the split second before the critical mass was fully assembled, blowing the weapon apart with a much lower yield than expected, in what is known as a fizzle.

As a result, development of Fat Man was given high priority. Chemical explosives were used to implode a sub-critical sphere of plutonium, thus increasing its density and making it into a critical mass. The difficulties with implosion centered on the problem of making the chemical explosives deliver a perfectly uniform shock wave upon the plutonium sphere— if it were even slightly asymmetric, the weapon would fizzle. This problem was solved by the use of explosive lenses which would focus the blast waves inside the imploding sphere, akin to the way in which an optical lens focuses light rays.

After D-Day, General Groves ordered a team of scientists to follow eastward-moving victorious Allied troops into Europe to assess the status of the German nuclear program (and to prevent the westward-moving Soviets from gaining any materials or scientific manpower). They concluded that, while Germany had an atomic bomb program headed by Werner Heisenberg, the government had not made a significant investment in the project, and it had been nowhere near success. Similarly, Japan's efforts at developing a nuclear weapon were starved of resources. The Japanese navy lost interest when a committee led by Yoshio Nishina concluded in 1943 that "it would probably be difficult even for the United States to realize the application of atomic power during the war".

Historians claim to have found a rough schematic showing a Nazi nuclear bomb. In March 1945, a German scientific team was directed by the physicist Kurt Diebner to develop a primitive nuclear device in Ohrdruf, Thuringia. Last ditch research was conducted in an experimental nuclear reactor at Haigerloch.

On April 12, after Roosevelt's death, Vice-President Harry S. Truman assumed the presidency. At the time of the unconditional surrender of Germany on May 8, 1945, the Manhattan Project was still months away from producing a working weapon.

Because of the difficulties in making a working plutonium bomb, it was decided that there should be a test of the weapon. On July 16, 1945, in the desert north of Alamogordo, New Mexico, the first nuclear test took place, code-named "Trinity", using a device nicknamed "the gadget." The test, a plutonium implosion-type device, released energy equivalent to 22 kilotons of TNT, far more powerful than any weapon ever used before. The news of the test's success was rushed to Truman at the Potsdam Conference, where Churchill was briefed and Soviet Premier Joseph Stalin was informed of the new weapon. On July 26, the Potsdam Declaration was issued containing an ultimatum for Japan: either surrender or suffer "complete and utter destruction", although nuclear weapons were not mentioned.
After hearing arguments from scientists and military officers over the possible use of nuclear weapons against Japan (though some recommended using them as demonstrations in unpopulated areas, most recommended using them against built up targets, a euphemistic term for populated cities), Truman ordered the use of the weapons on Japanese cities, hoping it would send a strong message that would end in the capitulation of the Japanese leadership, and avoid a lengthy invasion of the islands. Truman and his Secretary of State James F. Byrnes were also intent on ending the Pacific war before the Soviets could enter it, given that Roosevelt had promised Stalin control of Manchuria if he joined the invasion. On May 10–11, 1945, the Target Committee at Los Alamos, led by Oppenheimer, recommended Kyoto, Hiroshima, Yokohama, and Kokura as possible targets. Concerns about Kyoto's cultural heritage led to it being replaced by Nagasaki.
On August 6, 1945, a uranium-based weapon, Little Boy, was detonated above the Japanese city of Hiroshima, and three days later, a plutonium-based weapon, Fat Man, was detonated above the Japanese city of Nagasaki. To date, Hiroshima and Nagasaki remain the only two instances of nuclear weapons being used in combat. The atomic raids killed at least one hundred thousand Japanese civilians and military personnel outright, with the heat, radiation, and blast effects.
Many tens of thousands would later die of radiation sickness and related cancers. Truman promised a "rain of ruin" if Japan did not surrender immediately, threatening to systematically eliminate their ability to wage war. On August 15, Emperor Hirohito announced Japan's surrender.

The Soviet Union was not invited to share in the new weapons developed by the United States and the other Allies. During the war, information had been pouring in from a number of volunteer spies involved with the Manhattan Project (known in Soviet cables under the code-name of "Enormoz"), and the Soviet nuclear physicist Igor Kurchatov was carefully watching the Allied weapons development. It came as no surprise to Stalin when Truman had informed him at the Potsdam conference that he had a "powerful new weapon." Truman was shocked at Stalin's lack of interest. Stalin was nonetheless outraged by the situation, more by the Americans' guarded monopoly of the bomb than the weapon itself. Some historians share the assessment that Truman immediately authorized nuclear weapons as a "negotiating tool" in the early Cold War. In alarm at this monopoly, the Soviets urgently undertook their own atomic program.

The Soviet spies in the U.S. project were all volunteers and none were Soviet citizens. One of the most valuable, Klaus Fuchs, was a German émigré theoretical physicist who had been part of the early British nuclear efforts and the UK mission to Los Alamos. Fuchs had been intimately involved in the development of the implosion weapon, and passed on detailed cross-sections of the Trinity device to his Soviet contacts. Other Los Alamos spies—none of whom knew each other—included Theodore Hall and David Greenglass. The information was kept but not acted upon, as the Soviet Union was still too busy fighting the war in Europe to devote resources to this new project.

In the years immediately after World War II, the issue of who should control atomic weapons became a major international point of contention. Many of the Los Alamos scientists who had built the bomb began to call for "international control of atomic energy," often calling for either control by transnational organizations or the purposeful distribution of weapons information to all superpowers, but due to a deep distrust of the intentions of the Soviet Union, both in postwar Europe and in general, the policy-makers of the United States worked to maintain the American nuclear monopoly.

A half-hearted plan for international control was proposed at the newly formed United Nations by Bernard Baruch (The Baruch Plan), but it was clear both to American commentators—and to the Soviets—that it was an attempt primarily to stymie Soviet nuclear efforts. The Soviets vetoed the plan, effectively ending any immediate postwar negotiations on atomic energy, and made overtures towards banning the use of atomic weapons in general.

The Soviets had put their full industrial might and manpower into the development of their own atomic weapons. The initial problem for the Soviets was primarily one of resources—they had not scouted out uranium resources in the Soviet Union and the U.S. had made deals to monopolise the largest known (and high purity) reserves in the Belgian Congo. The USSR used penal labour to mine the old deposits in Czechoslovakia—now an area under their control—and searched for other domestic deposits (which were eventually found).

Two days after the bombing of Nagasaki, the U.S. government released an official technical history of the Manhattan Project, authored by Princeton physicist Henry DeWolf Smyth, known colloquially as the Smyth Report. The sanitized summary of the wartime effort focused primarily on the production facilities and scale of investment, written in part to justify the wartime expenditure to the American public.

The Soviet program, under the suspicious watch of former NKVD chief Lavrenty Beria (a participant and victor in Stalin's Great Purge of the 1930s), would use the Report as a blueprint, seeking to duplicate as much as possible the American effort. The "secret cities" used for the Soviet equivalents of Hanford and Oak Ridge literally vanished from the maps for decades to come.

At the Soviet equivalent of Los Alamos, Arzamas-16, physicist Yuli Khariton led the scientific effort to develop the weapon. Beria distrusted his scientists, however, and he distrusted the carefully collected espionage information. As such, Beria assigned multiple teams of scientists to the same task without informing each team of the other's existence. If they arrived at different conclusions, Beria would bring them together for the first time and have them debate with their newfound counterparts. Beria used the espionage information as a way to double-check the progress of his scientists, and in his effort for duplication of the American project even rejected more efficient bomb designs in favor of ones that more closely mimicked the tried-and-true Fat Man bomb used by the U.S. against Nagasaki.

On August 29, 1949, the effort brought its results, when the USSR tested its first fission bomb, dubbed "Joe-1" by the U.S., years ahead of American predictions. The news of the first Soviet bomb was announced to the world first by the United States, which had detected the nuclear fallout it generated from its test site in Kazakhstan.

The loss of the American monopoly on nuclear weapons marked the first tit-for-tat of the nuclear arms race. The response in the U.S. was one of apprehension, fear, and scapegoating, which would lead eventually into the Red-baiting tactics of McCarthyism. Yet recent information from declassified Venona intercepts and the opening of the KGB archives after the fall of the Soviet Union show that the USSR had useful spies that helped their program, although none were identified by McCarthy. Before this, though, President Truman announced a decision to begin a crash program that would develop a far more powerful weapon than those the U.S. used against Japan: the hydrogen bomb.

In 1946 Congress established the civilian Atomic Energy Commission (AEC) to take over the development of nuclear weapons from the military, and to develop nuclear power. The AEC made use of many private companies in processing uranium and thorium and in other urgent tasks related to the development of bombs. Many of these companies had very lax safety measures and employees were sometimes exposed to radiation levels far above what was allowed then or now. In 1974, the Formerly Utilized Sites Remedial Action Program (FUSRAP) of the Army Corps of Engineers was set up to deal with contaminated sites left over from these operations.

The notion of using a fission weapon to ignite a process of nuclear fusion can be dated back to 1942. At the first major theoretical conference on the development of an atomic bomb hosted by J. Robert Oppenheimer at the University of California, Berkeley, participant Edward Teller directed the majority of the discussion towards Enrico Fermi's idea of a "Super" bomb that would use the same reactions that powered the Sun itself.

It was thought at the time that a fission weapon would be quite simple to develop and that perhaps work on a hydrogen bomb (thermonuclear weapon) would be possible to complete before the end of the Second World War. However, in reality the problem of a regular atomic bomb was large enough to preoccupy the scientists for the next few years, much less the more speculative "Super" bomb. Only Teller continued working on the project—against the will of project leaders Oppenheimer and Hans Bethe.

After the atomic bombings of Japan, many scientists at Los Alamos rebelled against the notion of creating a weapon thousands of times more powerful than the first atomic bombs. For the scientists the question was in part technical—the weapon design was still quite uncertain and unworkable—and in part moral: such a weapon, they argued, could only be used against large civilian populations, and could thus only be used as a weapon of genocide.

Many scientists, such as Bethe, urged that the United States should not develop such weapons and set an example towards the Soviet Union. Promoters of the weapon, including Teller, Ernest Lawrence, and Luis Alvarez, argued that such a development was inevitable, and to deny such protection to the people of the United States—especially when the Soviet Union was likely to create such a weapon themselves—was itself an immoral and unwise act.

Oppenheimer, who was now head of the General Advisory Committee of the successor to the Manhattan Project, the Atomic Energy Commission, presided over a recommendation against the development of the weapon. The reasons were in part because the success of the technology seemed limited at the time (and not worth the investment of resources to confirm whether this was so), and because Oppenheimer believed that the atomic forces of the United States would be more effective if they consisted of many large fission weapons (of which multiple bombs could be dropped on the same targets) rather than the large and unwieldy super bombs, for which there was a relatively limited number of targets of sufficient size to warrant such a development.

What is more, if such weapons were developed by both superpowers, they would be more effective against the U.S. than against the USSR, as the U.S. had far more regions of dense industrial and civilian activity as targets for large weapons than the Soviet Union.

In the end, President Truman made the final decision, looking for a proper response to the first Soviet atomic bomb test in 1949. On January 31, 1950, Truman announced a crash program to develop the hydrogen (fusion) bomb. At this point, however, the exact mechanism was still not known: the classical hydrogen bomb, whereby the "heat" of the fission bomb would be used to ignite the fusion material, seemed highly unworkable. However, an insight by Los Alamos mathematician Stanislaw Ulam showed that the fission bomb and the fusion fuel could be in separate parts of the bomb, and that "radiation" of the fission bomb could first work in a way to "compress" the fusion material before igniting it.

Teller pushed the notion further, and used the results of the boosted-fission "George" test (a boosted-fission device using a small amount of fusion fuel to boost the yield of a fission bomb) to confirm the fusion of heavy hydrogen elements before preparing for their first true multi-stage, Teller-Ulam hydrogen bomb test. Many scientists, initially against the weapon, such as Oppenheimer and Bethe, changed their previous opinions, seeing the development as being unstoppable.

The first fusion bomb was tested by the United States in "Operation Ivy" on November 1, 1952, on Elugelab Island in the Enewetak (or Eniwetok) Atoll of the Marshall Islands, code-named "Mike." Mike used liquid deuterium as its fusion fuel and a large fission weapon as its trigger. The device was a prototype design and not a deliverable weapon: standing over 20 ft (6 m) high and weighing at least 140,000 lb (64 t) (its refrigeration equipment added an additional as well), it could not have been dropped from even the largest planes.

Its explosion yielded energy equivalent to 10.4 megatons of TNT—over 450 times the power of the bomb dropped onto Nagasaki— and obliterated Elugelab, leaving an underwater crater 6240 ft (1.9 km) wide and 164 ft (50 m) deep where the island had once been. Truman had initially tried to create a media blackout about the test—hoping it would not become an issue in the upcoming presidential election—but on January 7, 1953, Truman announced the development of the hydrogen bomb to the world as hints and speculations of it were already beginning to emerge in the press.

Not to be outdone, the Soviet Union exploded its first thermonuclear device, designed by the physicist Andrei Sakharov, on August 12, 1953, labeled "Joe-4" by the West. This created concern within the U.S. government and military, because, unlike Mike, the Soviet device was a deliverable weapon, which the U.S. did not yet have. This first device though was arguably not a true hydrogen bomb, and could only reach explosive yields in the hundreds of kilotons (never reaching the megaton range of a staged weapon). Still, it was a powerful propaganda tool for the Soviet Union, and the technical differences were fairly oblique to the American public and politicians.

Following the Mike blast by less than a year, Joe-4 seemed to validate claims that the bombs were inevitable and vindicate those who had supported the development of the fusion program. Coming during the height of McCarthyism, the effect was pronounced on the security hearings in early 1954, which revoked former Los Alamos director Robert Oppenheimer's security clearance on the grounds that he was unreliable, had not supported the American hydrogen bomb program, and had made long-standing left-wing ties in the 1930s. Edward Teller participated in the hearing as the only major scientist to testify against Oppenheimer, resulting in his virtual expulsion from the physics community.

On March 1, 1954, the U.S. detonated its first practical thermonuclear weapon (which used isotopes of lithium as its fusion fuel), known as the "Shrimp" device of the Castle Bravo test, at Bikini Atoll, Marshall Islands. The device yielded 15 megatons, more than twice its expected yield, and became the worst radiological disaster in U.S. history. The combination of the unexpectedly large blast and poor weather conditions caused a cloud of radioactive nuclear fallout to contaminate over. 239 Marshall Island natives and 28 Americans were exposed to significant amounts of radiation, resulting in elevated levels of cancer and birth defects in the years to come.

The crew of the Japanese tuna-fishing boat "Lucky Dragon 5", who had been fishing just outside the exclusion zone, returned to port suffering from radiation sickness and skin burns; one crew member was terminally ill. Efforts were made to recover the cargo of contaminated fish but at least two large tuna were probably sold and eaten. A further 75 tons of tuna caught between March and December were found to be unfit for human consumption. When the crew member died and the full results of the contamination were made public by the U.S., Japanese concerns were reignited about the hazards of radiation.

The hydrogen bomb age had a profound effect on the thoughts of nuclear war in the popular and military mind. With only fission bombs, nuclear war was something that possibly could be limited. Dropped by planes and only able to destroy the most built up areas of major cities, it was possible for many to look at fission bombs as a technological extension of large-scale conventional bombing—such as the extensive firebombing of German and Japanese cities during World War II. Proponents brushed aside as grave exaggeration claims that such weapons could lead to worldwide death or harm.

Even in the decades before fission weapons, there had been speculation about the possibility for human beings to end all life on the planet, either by accident or purposeful maliciousness—but technology had not provided the capacity for such action. The great power of hydrogen bombs made worldwide annihilation possible.

The Castle Bravo incident itself raised a number of questions about the survivability of a nuclear war. Government scientists in both the U.S. and the USSR had insisted that fusion weapons, unlike fission weapons, were cleaner, as fusion reactions did not produce the dangerously radioactive by-products of fission reactions. While technically true, this hid a more gruesome point: the last stage of a multi-staged hydrogen bomb often used the neutrons produced by the fusion reactions to induce fissioning in a jacket of natural uranium, and provided around half of the yield of the device itself.

This fission stage made fusion weapons considerably more dirty than they were made out to be. This was evident in the towering cloud of deadly fallout that followed the "Bravo" test. When the Soviet Union tested its first megaton device in 1955, the possibility of a "limited" nuclear war seemed even more remote in the public and political mind. Even cities and countries that were not direct targets would suffer fallout contamination. Extremely harmful fission products would disperse via normal weather patterns and embed in soil and water around the planet.

Speculation began to run towards what fallout and dust from a full-scale nuclear exchange would do to the world as a whole, rather than just cities and countries directly involved. In this way, the fate of the world was now tied to the fate of the bomb-wielding superpowers.

Throughout the 1950s and the early 1960s the U.S. and the USSR both endeavored, in a tit-for-tat approach, to prevent the other power from acquiring nuclear supremacy. This had massive political and cultural effects during the Cold War.

The first atomic bombs dropped on Hiroshima and Nagasaki were large, custom-made devices, requiring highly trained personnel for their arming and deployment. They could be dropped only from the largest bomber planes—at the time the B-29 Superfortress—and each plane could only carry a single bomb in its hold.

The first hydrogen bombs were similarly massive and complicated. This ratio of one plane to one bomb was still fairly impressive in comparison with conventional, non-nuclear weapons, but against other nuclear-armed countries it was considered a grave danger. In the immediate postwar years, the U.S. expended much effort on making the bombs "G.I.-proof"—capable of being used and deployed by members of the U.S. Army, rather than Nobel Prize–winning scientists. In the 1950s, the U.S. undertook a nuclear testing program to improve the nuclear arsenal.

Starting in 1951, the Nevada Test Site (in the Nevada desert) became the primary location for all U.S. nuclear testing (in the USSR, Semipalatinsk Test Site in Kazakhstan served a similar role). Tests were divided into two primary categories: "weapons related" (verifying that a new weapon worked or looking at exactly how it worked) and "weapons effects" (looking at how weapons behaved under various conditions or how structures behaved when subjected to weapons).

In the beginning, almost all nuclear tests were either atmospheric (conducted above ground, in the atmosphere) or underwater (such as some of the tests done in the Marshall Islands). Testing was used as a sign of both national and technological strength, but also raised questions about the safety of the tests, which released nuclear fallout into the atmosphere (most dramatically with the Castle Bravo test in 1954, but in more limited amounts with almost all atmospheric nuclear testing).

Because testing was seen as a sign of technological development (the ability to design usable weapons without some form of testing was considered dubious), halts on testing were often called for as stand-ins for halts in the nuclear arms race itself, and many prominent scientists and statesmen lobbied for a ban on nuclear testing. In 1958, the U.S., USSR, and the United Kingdom (a new nuclear power) declared a temporary testing moratorium for both political and health reasons, but by 1961 the Soviet Union had broken the moratorium and both the USSR and the U.S. began testing with great frequency.

As a show of political strength, the Soviet Union tested the largest-ever nuclear weapon in October 1961, the massive Tsar Bomba, which was tested in a reduced state with a yield of around 50 megatons—in its full state it was estimated to have been around 100 Mt. The weapon was largely impractical for actual military use, but was hot enough to induce third-degree burns at a distance of 62 mi (100 km) away. In its full, dirty, design it would have increased the amount of worldwide fallout since 1945 by 25%.

In 1963, all nuclear and many non-nuclear states signed the Limited Test Ban Treaty, pledging to refrain from testing nuclear weapons in the atmosphere, underwater, or in outer space. The treaty permitted underground tests.

Most tests were considerably more modest, and worked for direct technical purposes as well as their potential political overtones. Weapons improvements took on two primary forms. One was an increase in efficiency and power, and within only a few years fission bombs were developed that were many times more powerful than the ones created during World War II. The other was a program of miniaturization, reducing the size of the nuclear weapons.

Smaller bombs meant that bombers could carry more of them, and also that they could be carried on the new generation of rockets in development in the 1950s and 1960s. U.S. rocket science received a large boost in the postwar years, largely with the help of engineers acquired from the Nazi rocketry program. These included scientists such as Wernher von Braun, who had helped design the V-2 rockets the Nazis launched across the English Channel. An American program, Project Paperclip, had endeavored to move German scientists into American hands (and away from Soviet hands) and put them to work for the U.S.

Early nuclear-tipped rockets—such as the MGR-1 Honest John, first deployed by the U.S. in 1953—were surface-to-surface missiles with relatively short ranges (around 15 mi/25 km maximum) and yields around twice the size of the first fission weapons. The limited range meant they could only be used in certain types of military situations. U.S. rockets could not, for example, threaten Moscow with an immediate strike, and could only be used as tactical weapons (that is, for small-scale military situations).

Strategic weapons—weapons that could threaten an entire country—relied, for the time being, on long-range bombers that could penetrate deep into enemy territory. In the U.S., this requirement led, in 1946, to creation of the Strategic Air Command—a system of bombers headed by General Curtis LeMay (who previously presided over the firebombing of Japan during WWII). In operations like Chrome Dome, SAC kept nuclear-armed planes in the air 24 hours a day, ready for an order to attack Moscow.

These technological possibilities enabled nuclear strategy to develop a logic considerably different from previous military thinking. Because the threat of nuclear warfare was so awful, it was first thought that it might make any war of the future impossible. President Dwight D. Eisenhower's doctrine of "massive retaliation" in the early years of the Cold War was a message to the USSR, saying that if the Red Army attempted to invade the parts of Europe not given to the Eastern bloc during the Potsdam Conference (such as West Germany), nuclear weapons would be used against the Soviet troops and potentially the Soviet leaders.

With the development of more rapid-response technologies (such as rockets and long-range bombers), this policy began to shift. If the Soviet Union also had nuclear weapons and a policy of "massive retaliation" was carried out, it was reasoned, then any Soviet forces not killed in the initial attack, or launched while the attack was ongoing, would be able to serve their own form of nuclear retaliation against the U.S. Recognizing that this was an undesirable outcome, military officers and game theorists at the RAND think tank developed a nuclear warfare strategy that was eventually called Mutually Assured Destruction (MAD).

MAD divided potential nuclear war into two stages: "first strike" and "second strike". First strike meant the first use of nuclear weapons by one nuclear-equipped nation against another nuclear-equipped nation. If the attacking nation did not prevent the attacked nation from a nuclear response, the attacked nation would respond with a second strike against the attacking nation. In this situation, whether the U.S. first attacked the USSR or the USSR first attacked the U.S., the end result would be that both nations would be damaged to the point of utter social collapse.

According to game theory, because starting a nuclear war was suicidal, no logical country would shoot first. However, if a country could launch a first strike that utterly destroyed the target country's ability to respond, that might give that country the confidence to initiate a nuclear war. The object of a country operating by the MAD doctrine is to deny the opposing country this first strike capability.

MAD played on two seemingly opposed modes of thought: cold logic and emotional fear. The English phrase MAD was often known by, "nuclear deterrence," was translated by the French as "dissuasion," and "terrorization" by the Soviets. This apparent paradox of nuclear war was summed up by British Prime Minister Winston Churchill as "the worse things get, the better they are"—the greater the threat of mutual destruction, the safer the world would be.

This philosophy made a number of technological and political demands on participating nations. For one thing, it said that it should always be assumed that an enemy nation may be trying to acquire first strike capability, which must always be avoided. In American politics this translated into demands to avoid "bomber gaps" and "missile gaps" where the Soviet Union could potentially outshoot the Americans. It also encouraged the production of thousands of nuclear weapons by both the U.S. and the USSR, far more than needed to simply destroy the major civilian and military infrastructures of the opposing country. These policies and strategies were satirized in the 1964 Stanley Kubrick film Dr. Strangelove, in which the Soviets, unable to keep up with the US's first strike capability, instead plan for MAD by building a Doomsday Machine, and thus, after a (literally) mad US General orders a nuclear attack on the USSR, the end of the world is brought about.
The policy also encouraged the development of the first early warning systems. Conventional war, even at its fastest, was fought over days and weeks. With long-range bombers, from the start of a nuclear attack to its conclusion was mere hours. Rockets could reduce a conflict to minutes. Planners reasoned that conventional command and control systems could not adequately react to a nuclear attack, so great lengths were taken to develop computer systems that could look for enemy attacks and direct rapid responses.

The U.S. poured massive funding into development of SAGE, a system that could track and intercept enemy bomber aircraft using information from remote radar stations. It was the first computer system to feature real-time processing, multiplexing, and display devices. It was the first general computing machine, and a direct predecessor of modern computers.

The atomic bombings of Hiroshima and Nagasaki and the end of World War II quickly followed the 1945 Trinity nuclear test, and the Little Boy device was detonated over the Japanese city of Hiroshima on 6 August 1945. Exploding with a yield equivalent to 12,500 tonnes of TNT, the blast and thermal wave of the bomb destroyed nearly 50,000 buildings and killed approximately 75,000 people. Subsequently, the world’s nuclear weapons stockpiles grew.

Operation Crossroads was a series of nuclear weapon tests conducted by the United States at Bikini Atoll in the Pacific Ocean in the summer of 1946. Its purpose was to test the effect of nuclear weapons on naval ships. To prepare the Bikini atoll for the nuclear tests, Bikini's native residents were evicted from their homes and resettled on smaller, uninhabited islands where they were unable to sustain themselves.

National leaders debated the impact of nuclear weapons on domestic and foreign policy. Also involved in the debate about nuclear weapons policy was the scientific community, through professional associations such as the Federation of Atomic Scientists and the Pugwash Conference on Science and World Affairs. Radioactive fallout from nuclear weapons testing was first drawn to public attention in 1954 when a Hydrogen bomb test in the Pacific contaminated the crew of the Japanese fishing boat "Lucky Dragon". One of the fishermen died in Japan seven months later. The incident caused widespread concern around the world and "provided a decisive impetus for the emergence of the anti-nuclear weapons movement in many countries". The anti-nuclear weapons movement grew rapidly because for many people the atomic bomb "encapsulated the very worst direction in which society was moving".

Peace movements emerged in Japan and in 1954 they converged to form a unified "Japanese Council Against Atomic and Hydrogen Bombs". Japanese opposition to the Pacific nuclear weapons tests was widespread, and "an estimated 35 million signatures were collected on petitions calling for bans on nuclear weapons". The Russell–Einstein Manifesto was issued in London on July 9, 1955 by Bertrand Russell in the midst of the Cold War. It highlighted the dangers posed by nuclear weapons and called for world leaders to seek peaceful resolutions to international conflict. The signatories included eleven pre-eminent intellectuals and scientists, including Albert Einstein, who signed it just days before his death on April 18, 1955. A few days after the release, philanthropist Cyrus S. Eaton offered to sponsor a conference—called for in the manifesto—in Pugwash, Nova Scotia, Eaton's birthplace. This conference was to be the first of the Pugwash Conferences on Science and World Affairs, held in July 1957.

In the United Kingdom, the first Aldermaston March organised by the Campaign for Nuclear Disarmament took place at Easter 1958, when several thousand people marched for four days from Trafalgar Square, London, to the Atomic Weapons Research Establishment close to Aldermaston in Berkshire, England, to demonstrate their opposition to nuclear weapons. The Aldermaston marches continued into the late 1960s when tens of thousands of people took part in the four-day marches.

In 1959, a letter in the "Bulletin of the Atomic Scientists" was the start of a successful campaign to stop the Atomic Energy Commission dumping radioactive waste in the sea 19 kilometres from Boston. On November 1, 1961, at the height of the Cold War, about 50,000 women brought together by Women Strike for Peace marched in 60 cities in the United States to demonstrate against nuclear weapons. It was the largest national women's peace protest of the 20th century.

In 1958, Linus Pauling and his wife presented the United Nations with the petition signed by more than 11,000 scientists calling for an end to nuclear-weapon testing. The "Baby Tooth Survey," headed by Dr Louise Reiss, demonstrated conclusively in 1961 that above-ground nuclear testing posed significant public health risks in the form of radioactive fallout spread primarily via milk from cows that had ingested contaminated grass. Public pressure and the research results subsequently led to a moratorium on above-ground nuclear weapons testing, followed by the Partial Test Ban Treaty, signed in 1963 by John F. Kennedy and Nikita Khrushchev.

Bombers and short-range rockets were not reliable: planes could be shot down, and earlier nuclear missiles could cover only a limited range— for example, the first Soviet rockets' range limited them to targets in Europe. However, by the 1960s, both the United States and the Soviet Union had developed intercontinental ballistic missiles, which could be launched from extremely remote areas far away from their target. They had also developed submarine-launched ballistic missiles, which had less range but could be launched from submarines very close to the target without any radar warning. This made any national protection from nuclear missiles increasingly impractical.

The military realities made for a precarious diplomatic situation. The international politics of brinkmanship led leaders to exclaim their willingness to participate in a nuclear war rather than concede any advantage to their opponents, feeding public fears that their generation may be the last. Civil defense programs undertaken by both superpowers, exemplified by the construction of fallout shelters and urging civilians about the survivability of nuclear war, did little to ease public concerns.

The climax of brinksmanship came in early 1962, when an American U-2 spy plane photographed a series of launch sites for medium-range ballistic missiles being constructed on the island of Cuba, just off the coast of the southern United States, beginning what became known as the Cuban Missile Crisis. The U.S. administration of John F. Kennedy concluded that the Soviet Union, then led by Nikita Khrushchev, was planning to station Soviet nuclear missiles on the island (as a response to placing US Jupiter MRBMs in Italy and Turkey), which was under the control of communist Fidel Castro. On October 22, Kennedy announced the discoveries in a televised address. He announced a naval blockade around Cuba that would turn back Soviet nuclear shipments, and warned that the military was prepared "for any eventualities." The missiles had 2,400 mile (4,000 km) range, and would allow the Soviet Union to quickly destroy many major American cities on the Eastern Seaboard if a nuclear war began.

The leaders of the two superpowers stood nose to nose, seemingly poised over the beginnings of a third world war. Khrushchev's ambitions for putting the weapons on the island were motivated in part by the fact that the U.S. had stationed similar weapons in Britain, Italy, and nearby Turkey, and had previously attempted to sponsor an invasion of Cuba the year before in the failed Bay of Pigs Invasion. On October 26, Khrushchev sent a message to Kennedy offering to withdraw all missiles if Kennedy committed to a policy of no future invasions of Cuba. Khrushchev worded the threat of assured destruction eloquently:
"You and I should not now pull on the ends of the rope in which you have tied a knot of war, because the harder you and I pull, the tighter the knot will become. And a time may come when this knot is tied so tight that the person who tied it is no longer capable of untying it, and then the knot will have to be cut. What that would mean I need not explain to you, because you yourself understand perfectly what dreaded forces our two countries possess."
A day later, however, the Soviets sent another message, this time demanding that the U.S. remove its missiles from Turkey before any missiles were withdrawn from Cuba. On the same day, a U-2 plane was shot down over Cuba and another almost intercepted over the Soviet Union, as Soviet merchant ships neared the quarantine zone. Kennedy responded by accepting the first deal publicly, and sending his brother Robert to the Soviet embassy to accept the second deal privately. On October 28, the Soviet ships stopped at the quarantine line and, after some hesitation, turned back towards the Soviet Union. Khrushchev announced that he had ordered the removal of all missiles in Cuba, and U.S. Secretary of State Dean Rusk was moved to comment, "We went eyeball to eyeball, and the other fellow just blinked."

The Crisis was later seen as the closest the U.S. and the USSR ever came to nuclear war and had been narrowly averted by last-minute compromise by both superpowers. Fears of communication difficulties led to the installment of the first hotline, a direct link between the superpowers that allowed them to more easily discuss future military activities and political maneuverings. It had been made clear that missiles, bombers, submarines, and computerized firing systems made escalating any situation to Armageddon far more easy than anybody desired.

After stepping so close to the brink, both the U.S. and the USSR worked to reduce their nuclear tensions in the years immediately following. The most immediate culmination of this work was the signing of the Partial Test Ban Treaty in 1963, in which the U.S. and USSR agreed to no longer test nuclear weapons in the atmosphere, underwater, or in outer space. Testing underground continued, allowing for further weapons development, but the worldwide fallout risks were purposefully reduced, and the era of using massive nuclear tests as a form of saber rattling ended.

In December 1979, NATO decided to deploy cruise and Pershing II missiles in Western Europe in response to Soviet deployment of intermediate range mobile missiles, and in the early 1980s, a "dangerous Soviet-US nuclear confrontation" arose. In New York on June 12, 1982, one million people gathered to protest about nuclear weapons, and to support the second UN Special Session on Disarmament. As the nuclear abolitionist movement grew, there were many protests at the Nevada Test Site. For example, on February 6, 1987, nearly 2,000 demonstrators, including six members of Congress, protested against nuclear weapons testing and more than 400 people were arrested. Four of the significant groups organizing this renewal of anti-nuclear activism were Greenpeace, The American Peace Test, The Western Shoshone, and Nevada Desert Experience.

There have been at least four major false alarms, the most recent in 1995, that resulted in the activation of nuclear attack early warning protocols. They include the accidental loading of a training tape into the American early-warning computers; a computer chip failure that appeared to show a random number of attacking missiles; a rare alignment of the Sun, the U.S. missile fields and a Soviet early-warning satellite that caused it to confuse high-altitude clouds with missile launches; the launch of a Norwegian research rocket resulted in President Yeltsin activating his nuclear briefcase for the first time.

In the fifties and sixties, three more countries joined the "nuclear club." The United Kingdom had been an integral part of the Manhattan Project following the Quebec Agreement in 1943. The passing of the McMahon Act by the United States in 1946 unilaterally broke this partnership and prevented the passage of any further information to the United Kingdom. The British Government, under Clement Attlee, determined that a British Bomb was essential. Because of British involvement in the Manhattan Project, Britain had extensive knowledge in some areas, but not in others.

An improved version of 'Fat Man' was developed, and on 26 February 1952, Prime Minister Winston Churchill announced that the United Kingdom also had an atomic bomb and a successful test took place on 3 October 1952. At first these were free-fall bombs, intended for use by the V Force of jet bombers. A Vickers Valiant dropped the first UK nuclear weapon on 11 October 1956 at Maralinga, South Australia. Later came a missile, Blue Steel, intended for carriage by the V Force bombers, and then the Blue Streak medium-range ballistic missile (later canceled). Anglo-American cooperation on nuclear weapons was restored by the 1958 US-UK Mutual Defence Agreement. As a result of this and the Polaris Sales Agreement, the United Kingdom has bought United States designs for submarine missiles and fitted its own warheads. It retains full independent control over the use of the missiles. It no longer possesses any free-fall bombs.

France had been heavily involved in nuclear research before World War II through the work of the Joliot-Curies. This was discontinued after the war because of the instability of the Fourth Republic and lack of finances. However, in the 1950s, France launched a civil nuclear research program, which produced plutonium as a byproduct.

In 1956, France formed a secret Committee for the Military Applications of Atomic Energy and a development program for delivery vehicles. With the return of Charles de Gaulle to the French presidency in 1958, final decisions to build a bomb were made, which led to a successful test in 1960. Since then, France has developed and maintained its own nuclear deterrent independent of NATO.

In 1951, China and the Soviet Union signed an agreement whereby China supplied uranium ore in exchange for technical assistance in producing nuclear weapons. In 1953, China established a research program under the guise of civilian nuclear energy. Throughout the 1950s the Soviet Union provided large amounts of equipment. But as the relations between the two countries worsened the Soviets reduced the amount of assistance and, in 1959, refused to donate a bomb for copying purposes. Despite this, the Chinese made rapid progress and tested an atomic bomb on October 16, 1964, at Lop Nur. They tested a nuclear missile on October 25, 1966, and a hydrogen bomb on June 14, 1967.

Chinese nuclear warheads were produced from 1968 and thermonuclear warheads from 1974. It is also thought that Chinese warheads have been successfully miniaturised from 2200 kg to 700 kg through the use of designs obtained by espionage from the United States. The current number of warheads is unknown, but as of 2017 it is thought to be in the low hundreds. China is the only nuclear weapons state to have guaranteed the non-first use of nuclear weapons.

After World War II, the balance of power between the Eastern and Western blocs and the fear of global destruction prevented the further military use of atomic bombs. This fear was even a central part of Cold War strategy, referred to as the doctrine of Mutually Assured Destruction. So important was this balance to international political stability that a treaty, the Anti-Ballistic Missile Treaty (or ABM treaty), was signed by the U.S. and the USSR in 1972 to curtail the development of defenses against nuclear weapons and the ballistic missiles that carry them. This doctrine resulted in a large increase in the number of nuclear weapons, as each side sought to ensure it possessed the firepower to destroy the opposition in all possible scenarios.

Early delivery systems for nuclear devices were primarily bombers like the United States B-29 Superfortress and Convair B-36, and later the B-52 Stratofortress. Ballistic missile systems, based on Wernher von Braun's World War II designs (specifically the V-2 rocket), were developed by both United States and Soviet Union teams (in the case of the U.S., effort was directed by the German scientists and engineers although the Soviet Union also made extensive use of captured German scientists, engineers, and technical data).

These systems were used to launch satellites, such as Sputnik, and to propel the Space Race, but they were primarily developed to create Intercontinental Ballistic Missiles (ICBMs) that could deliver nuclear weapons anywhere on the globe. Development of these systems continued throughout the Cold War—though plans and treaties, beginning with the Strategic Arms Limitation Treaty (SALT I), restricted deployment of these systems until, after the fall of the Soviet Union, system development essentially halted, and many weapons were disabled and destroyed. On January 27, 1967, more than 60 nations signed the Outer Space Treaty, banning nuclear weapons in space.

There have been a number of potential nuclear disasters. Following air accidents U.S. nuclear weapons have been lost near Atlantic City, New Jersey (1957); Savannah, Georgia (1958) (see Tybee Bomb); Goldsboro, North Carolina (1961); off the coast of Okinawa (1965); in the sea near Palomares, Spain (1966) (see 1966 Palomares B-52 crash); and near Thule, Greenland (1968) (see 1968 Thule Air Base B-52 crash). Most of the lost weapons were recovered, the Spanish device after three months' effort by the DSV Alvin and DSV Aluminaut.

The Soviet Union was less forthcoming about such incidents, but the environmental group Greenpeace believes that there are around forty non-U.S. nuclear devices that have been lost and not recovered, compared to eleven lost by America, mostly in submarine disasters. The U.S. has tried to recover Soviet devices, notably in the 1974 Project Azorian using the specialist salvage vessel "Hughes Glomar Explorer" to raise a Soviet submarine. After news leaked out about this boondoggle, the CIA would coin a favorite phrase for refusing to disclose sensitive information, called glomarization: "We can neither confirm nor deny the existence of the information requested but, hypothetically, if such data were to exist, the subject matter would be classified, and could not be disclosed."

The collapse of the Soviet Union in 1991 essentially ended the Cold War. However, the end of the Cold War failed to end the threat of nuclear weapon use, although global fears of nuclear war reduced substantially. In a major move of symbolic de-escalation, Boris Yeltsin, on January 26, 1992, announced that Russia planned to stop targeting United States cities with nuclear weapons.

The designing, testing, producing, deploying, and defending against nuclear weapons is one of the largest expenditures for the nations which possess nuclear weapons. In the United States during the Cold War years, between "one quarter to one third of all military spending since World War II [was] devoted to nuclear weapons and their infrastructure."
According to a retrospective Brookings Institution study published in 1998 by the Nuclear Weapons Cost Study Committee (formed in 1993 by the W. Alton Jones Foundation), the total expenditures for U.S. nuclear weapons from 1940 to 1998 was $5.5 trillion in 1996 Dollars.

For comparison, the total public debt at the end of fiscal year 1998 was $5,478,189,000,000 in 1998 Dollars or $5.3 trillion in 1996 Dollars. The "entire public debt" in 1998 was therefore equal to the cost of research, development, and deployment of U.S. nuclear weapons and nuclear weapons-related programs during the Cold War.

The "second nuclear age" can be regarded as proliferation of nuclear weapons among lesser powers and for reasons other than the American-Soviet-Chinese rivalry.

India embarked relatively early on a program aimed at nuclear weapons capability, but apparently accelerated this after the Sino-Indian War of 1962. India's first atomic-test explosion was in 1974 with "Smiling Buddha", which it described as a "peaceful nuclear explosion."

After the collapse of Eastern Military High Command and the disintegration of Pakistan as a result of the 1971 Winter war, Bhutto of Pakistan launched scientific research on nuclear weapons. The Indian test caused Pakistan to spur its programme, and the ISI conducted successful espionage operations in the Netherlands, while also developing the programme indigenously. India tested fission and perhaps fusion devices in 1998, and Pakistan successfully tested fission devices that same year, raising concerns that they would use nuclear weapons on each other.

All of the former Soviet bloc countries with nuclear weapons (Belarus, Ukraine, and Kazakhstan) returned their warheads to Russia by 1996.

South Africa also had an active program to develop uranium-based nuclear weapons, but dismantled its nuclear weapon program in the 1990s. Experts do not believe it actually tested such a weapon, though it later claimed it constructed several crude devices that it eventually dismantled. In the late 1970s American spy satellites detected a "brief, intense, double flash of light near the southern tip of Africa." Known as the Vela Incident, it was speculated to have been a South African or possibly Israeli nuclear weapons test, though some feel that it may have been caused by natural events or a detector malfunction.

Israel is widely believed to possess an arsenal of up to several hundred nuclear warheads, but this has never been officially confirmed or denied (though the existence of their Dimona nuclear facility was confirmed by Mordechai Vanunu in 1986).

In January 2004, Dr A. Q. Khan of Pakistan's programme confessed to having been a key mover in "proliferation activities", seen as part of an international proliferation network of materials, knowledge, and machines from Pakistan to Libya, Iran, and North Korea.

North Korea announced in 2003 that it had several nuclear explosives. The first claimed detonation was the 2006 North Korean nuclear test, conducted on October 9, 2006. On May 25, 2009, North Korea continued nuclear testing, violating United Nations Security Council Resolution 1718. A third test was conducted on, two tests were conducted in 2016 in January and September, followed by test a year later in September 2017.

The United States was in a Cold War Nuclear arms race with the Soviet Union to build bigger and better bombs. The next series of tests over Bikini Atoll was code named Operation Castle. The first test of that series was Castle Bravo, a new design utilizing a dry fuel thermonuclear hydrogen bomb. It was detonated at dawn on March 1, 1954.

Nuclear testing at Bikini Atoll

The nuclear testing at Bikini Atoll program was a series of 23 nuclear devices detonated by the United States between 1946 and 1958 at seven test sites on the reef itself, on the sea, in the air and underwater. The test weapons produced a combined fission yield of 42.2 Mt of explosive power.

The United States was engaged in a Cold War Nuclear arms race with the Soviet Union to build more advanced bombs from 1947 until 1991. The first series of tests over Bikini Atoll in July 1946 was code named Operation Crossroads. The first test was dropped from an aircraft and detonated above the target fleet. The second, "Baker", was suspending under a barge. It produced a large Wilson cloud and contaminated all of the target ships. Chemist Glenn T. Seaborg, the longest-serving chairman of the Atomic Energy Commission, called the second test "the world's first nuclear disaster."

The second series of tests in 1954 was codenamed Operation Castle. The first detonation, Castle Bravo, was a new design utilizing a dry fuel thermonuclear hydrogen bomb. It was detonated at dawn on March 1, 1954. Scientists miscalculated and the 15 megaton (Mt) nuclear explosion far exceeded the expected yield of 4 to 8 Mt (6 Mt predicted), and was about 1,000 times more powerful than each of the atomic bombs dropped on Hiroshima and Nagasaki during World War II. The scientists and military authorities were shocked by the size of the explosion and many of the instruments they had put in place to evaluate the effectiveness of the device were destroyed.

The military authorities and scientists had promised the Bikini Atoll's native residents that they would be able to return home after the nuclear tests. A majority of the island's family heads agreed to leave the island, and most of the residents were moved to the Rongerik Atoll and later to Kili Island. Both locations proved unsuitable to sustaining life, resulting in starvation and requiring the residents to receive ongoing aid.

Despite the promises made by authorities, this and further nuclear tests (Redwing in 1956 and Hardtack in 1958) rendered Bikini unfit for habitation, contaminating the soil and water, making subsistence farming and fishing too dangerous. The United States later paid the islanders and their descendants $125 million in compensation for damage caused by the nuclear testing program and their displacement from their home island, but had originally promised $2 billion to the native peoples. The committee behind this decision has since disbanded and the peoples have not been compensated. As of 2014, it may be technically possible for the former residents and their descendants to live on the atoll's islands, but virtually none of those alive today have ever lived on the atoll and very few want to move there. A 2016 investigation found radiation levels on Bikini Atoll as high as 639 mrem yr, well above the established safety standard threshold for habitation of 100 mrem yr. However, in 2017, Stanford University scientists reported "an abundance of marine life apparently thriving in the crater of Bikini Atoll." Research is being undertaken on how marine organisms are surviving in a radiation-filled environment, which could lead to improved understanding of cancer and increased human longevity.

In February 1946, the United States government asked the 167 Micronesian inhabitants of the atoll to "temporarily" relocate, so the United States government could begin testing atomic bombs for "the good of mankind and to end all world wars." After "confused and sorrowful deliberation" among the Bikinians, their leader, King Juda, agreed to the U.S. relocation request, announcing "We will go believing that everything is in the hands of God." Nine of the eleven family heads, or "alaps", chose Rongerik as their new home. United States Navy Seabees helped them to disassemble their church and community house and prepare to relocate them to their new home. On March 7, 1946, the residents gathered their personal belongings and saved building supplies. They were transported eastward on U.S. Navy landing craft 1108 and LST 861 to the uninhabited Rongerik Atoll, which was one-sixth the size of Bikini Atoll. No one lived on Rongerik because it had an inadequate water and food supply and due to deep-rooted traditional beliefs that the island was haunted by the "Demon Girls of Ujae" along with high radiation levels which were hazardous to the population due to the fallout of the Castle Bravo test. The Navy left them with a few weeks of food and water which soon proved to be inadequate.

To conduct the tests, the United States assembled a support fleet of more than 242 ships that provided quarters, experimental stations, and workshops for more than 42,000 personnel. The islands were primarily used as recreation and instrumentation sites. To support the nuclear bomb testing program, Seabees built bunkers, floating dry docks, steel towers for cameras and recording instruments, and other facilities on the island to support the servicemen. These included the "Up and Atom Officer's Club" and the "Cross Spikes Club", a bar and hangout created by servicemen on Bikini Island between June and September 1946 during the preparation for Operation Crossroads. The "club" was little more than a small open-air building that served alcohol to servicemen and provided outdoor entertainment, including a ping pong table. The "Cross Spikes Club" was the only entertainment the enlisted servicemen had access to during their June to September stay at Bikini.

The Bikini Atoll lagoon was designated a ship graveyard by the United States Navy. The United States brought in 95 ships including carriers, battleships, cruisers, destroyers, submarines, attack transports, landing ships, and auxiliary the island, along with 150 airplanes on the island's airstrip, to test the effect of a nuclear weapon. The proxy fleet if active would have comprised the sixth largest naval fleet in the world. All carried varying amounts of fuel and some carried live ordnance.

Crossroads consisted of two detonations, each with a yield of. Able was detonated over Bikini on July 1, 1946 and exploded at an altitude of but was dropped by aircraft about off target. It sank only five of the ships in the lagoon. The second, Baker, was detonated underwater at a depth of on July 25, sinking eight ships. The second underwater blast created a large condensation cloud and contaminated the ships with more radioactive water than was expected. Many of the surviving ships were too contaminated to be used again for testing and were sunk. The air-borne nuclear detonation raised the surface seawater temperature by, created blast waves with speeds of up to, and shock and surface waves up to high. Blast columns reached the floor of the lagoon which is approximately deep. The Bikini Island King visited Bikini Atoll in July after the second atomic bomb test code-named Baker and found it apparently in good condition.

A third burst, "Charlie", planned for 1947, was canceled primarily because of the United States Navy's inability to decontaminate the target ships after the "Baker" test. "Charlie" was rescheduled as Operation Wigwam, a deep water shot conducted in 1955 off the California coast.

The 15 megaton (Mt) nuclear explosion far exceeded the expected yield of 4 to 8 Mt (6 Mt predicted), and was about 1,000 times more powerful than each of the atomic bombs dropped on Hiroshima and Nagasaki during World War II. The nuclear weapon was the most powerful device ever detonated by the United States (and just under one-third the energy of the Tsar Bomba, the largest ever tested). The scientists and military authorities were shocked by the size of the explosion and many of the instruments they had put in place to evaluate the effectiveness of the device were destroyed.

The unexpectedly large yield, combined with other factors, led to the most significant accidental radiological contamination caused by the United States. A few minutes after the detonation, blast debris began to fall on Eneu/Enyu Island on Bikini Atoll where the U.S crew that triggered the device were located. When their geiger counters detected the unexpected fallout, they were forced to shelter in place indoors for a number of hours before it was safe enough for an airlift rescue operation to be carried out.

After impacting Eneu, the fallout continued to spread across the inhabited islands of the Rongelap, Rongerik, and Utrik Atolls. The inhabitants of Rongelap and Rongerik Atolls were evacuated by US servicemen two days after the detonation, but the residents of the more distant Utrik Atoll weren't evacuated for three days. Many of the inhabitants soon began to show symptoms of acute radiation syndrome. They returned to the islands three years later but were forced to relocate again when their islands were found to be unsafe.

The fallout gradually dispersed around the globe, depositing traces of radioactive material in Australia, India and Japan, and parts of the United States and Europe. Though organized as a secret test, Castle Bravo quickly became an international incident, prompting calls for a ban on the atmospheric testing of thermonuclear devices.

The Rongelap Atoll was coated with up to of snow-like irradiated calcium debris and ash over the entire island. Virtually all the inhabitants experienced severe radiation sickness, including itchiness, sore skin, vomiting, diarrhea, and fatigue. Their symptoms also included burning eyes and swelling of the neck, arms, and legs. The inhabitants were forced to abandon the islands, leaving all their belongings, three days after the test. They were relocated to Kwajalein for medical treatment.

Six days after the Castle Bravo test, the U.S government set up a secret project to study the medical effects of the weapon on the residents of the Marshall Islands.

The United States was subsequently accused of using the inhabitants as medical research subjects, without obtaining their consent, to study the effects of nuclear exposure. Until that time, the United States Atomic Energy Commission had given little thought to the potential impact of widespread fallout contamination and health and ecological impacts beyond the formally designated boundary of the test site.

Ninety minutes after the detonation, 23 crew members of the Japanese fishing boat the "Daigo Fukuryū Maru" ("Lucky Dragon No. 5") also were contaminated by the snow-like irradiated debris and ash. They had no idea what the explosion they'd seen meant nor any inkling of the nature of the deadly debris that rained down on them like snow. But they all soon became ill with the effects of acute radiation poisoning. One fisherman died almost a half year later while under doctor supervision in late September, with his cause of death regarded to have been due to underlying liver cirrhosis compounded by an infection. The majority of medical experts believe that the crew members were infected with hepatitis C through blood transfusions during part of their Acute radiation syndrome treatment.

Edward Teller, one of the driving minds behind the development of the hydrogen bomb, and architect of the Marshall Island tests, upon learning of the death of the fisherman through the mass media which painted his death as an anti-nuclear casus belli, commented, "It's unreasonable to make such a big deal over the death of a fisherman."

The seventeen-shot Redwing series followed—eleven tests at Enewetak Atoll and six at Bikini. The island residents who had been promised they would be able to return home to Bikini was thwarted indefinitely by the U.S. decision to resume nuclear testing at Bikini in 1954. During 1954, 1956, and 1958, twenty-one more nuclear bombs were detonated at Bikini, yielding a total of, equivalent to more than three thousand "Baker" bombs. Only one was an air burst, the 3.8 Mt Redwing "Cherokee" test. Air bursts distribute fallout in a large area, but surface bursts produce intense local fallout. These tests were followed by the 33-shot Hardtack tests which began in late April 1958. The last of ten tests were detonated on Bikini Atoll on 22 July 1958.

Shipwrecks in the lagoon include:The following above-ground nuclear device tests were conducted on or near Bikini Atoll from 1946 to 1958, comprising 15.1% of total test yield worldwide. These dates are given in US Eastern time zone The days of the week are a day earlier than they were at Bikini

In 1947, the United States convinced the United Nations to designate the islands of Micronesia a United Nations Strategic Trust Territory. This was the only trust ever granted by the U.N. The United States Navy controlled the Trust from a headquarters in Guam until 1951, when the United States Department of the Interior took over control, administering the territory from a base in Saipan. The directive to which the United States agreed stipulated that it would "promote the economic advancement and self-sufficiency of the inhabitants, and to this end shall... protect the inhabitants against the loss of their lands and resources..."

Despite the promise to "protect the inhabitants," from July 1946 through July 1947 the residents of Bikini Atoll were left alone on Rongerik Atoll. In January 1948, Dr. Leonard E. Mason, an anthropologist from the University of Hawaii, visited the temporary home of the relocated islanders on Rongerik Atoll and was horrified when he found the people were starving. A team of U.S. investigators concluded in late 1947 that the islanders must be moved immediately. Press from around the world harshly criticized the U.S. Navy for ignoring the people. Harold Ickes, a syndicated columnist, wrote "The natives are actually and literally starving to death."

The Navy then selected Ujelang Atoll for their temporary home and some young men from the Bikini Atoll population went ahead to begin constructing living accommodations. But U.S. Trust Authorities changed their mind. They decided to use Enewetak Atoll as a second nuclear weapons test site and relocated that atoll's residents to Ujelang Atoll instead and to the homes built for the Bikini Islanders.

In March 1948, 184 malnourished Bikini islanders were temporarily relocated again to Kwajalein Atoll. In June 1948 the Bikini residents chose Kili Island as a long-term home. The small, () island, one of the smallest islands in the Marshall Island chain, was uninhabited and wasn't ruled by a paramount "iroij", or king. In November 1948, the residents, now totaling 184 individuals, moved there. Living on Kili Island effectively destroyed their culture that had been based on fishing and island-hopping canoe voyages to various islets around the Bikini Atoll. Kili does not provide enough food for the transplanted residents.

In June 1968, based on scientific advice that the radiation levels were sufficiently reduced, President Lyndon B. Johnson promised the 540 Bikini Atoll families living on Kili and other islands that they would be able to return to their home. But the Atomic Energy Commission learned that the coconut crabs, an essential food source, retained high levels of radioactivity and could not be eaten. The Bikini Council voted to delay a return the island as a result.

In 1987, a few Bikini elders returned to the island to reestablish old property lines. Construction crews began building a hotel on Bikini, and installed generators, desalinators, and power lines. A packed coral and sand runway still exists on Enyu Island. Three extended families, eventually totaling about 100 people, moved back to their home island in 1972 despite the risk. But 10 years later a team of French scientists found some wells were too radioactive for use and determined that the pandanus and breadfruit were also dangerous for human consumption. Women were experiencing miscarriages, stillbirths, and genetic abnormalities in their children. The U.S.-administered Strategic Trust Territory decided that the islanders had to be evacuated from the atoll a second time.

Following their evacuation from the island, an 11-year-old boy, born on Bikini in 1971, died from cancer that was linked to radiation exposure he received while living on Bikini. The records obtained by the Marshallese Nuclear Claims Tribunal later revealed that Dr. Robert Conard, head of Brookhaven National Laboratory's medical team in the Marshall Islands, understated the risk of returning to the atoll. Dr. Konrad Kotrady was contracted by Brookhaven National Laboratory (BNL) to treat the Marshall Island residents. In 1977, he wrote a 14-page report to BNL that raised serious questions about the residents' return to Bikini and questioned the accuracy of Brookhaven's prior work on the islands. After they were promised their home was safe, and then being removed after this was found to be wrong, the Bikini Atoll islanders grew to distrust the official reports of the U.S. scientists.

The special International Atomic Energy Agency (IAEA) Bikini Advisory Group determined in 1997 that "It is safe to walk on all of the islands... although the residual radioactivity on islands in Bikini Atoll is still higher than on other atolls in the Marshall Islands, it is not hazardous to health at the levels measured... The main radiation risk would be from the food: eating locally grown produce, such as fruit, could add significant radioactivity to the body...Eating coconuts or breadfruit from Bikini Island occasionally would be no cause for concern. Eating many over a long period of time without having taken remedial measures, however, might result in radiation doses higher than internationally agreed safety levels." IAEA estimated that living in the atoll and consuming local food would result in an effective dose of about 15 mSv/year.

After the aborted resettlement in 1978, the leaders of the Bikini community have insisted since the early 1980s that the top of soil be excavated from the entire island. Scientists reply that while removing the soil would rid the island of cesium-137, it would also severely damage the environment, turning the atoll into a virtual wasteland of windswept sand. The Bikini Council has repeatedly contended that removing the topsoil is the only way to guarantee future generations safe living conditions.

In 1997, researchers found that the dose received from background radiation on the island was between 2.4 mSv/year—the same as natural background radiation—and 4.5 mSv/year, assuming that residents consumed a diet of imported foods. Because the local food supply is still irradiated, the group did not recommend resettling the island. A 1998 International Atomic Energy Agency report found that Bikini is still not safe for habitation because of dangerous levels of radiation.

A 2002 survey found that the coral inside the Bravo Crater has partially recovered. Zoe Richards of the ARC Centre of Excellence for Coral Reef Studies and James Cook University observed matrices of branching Porites coral up to 8 meters high.

In 1975 the islanders, who had returned to Bikini Atoll and later learned that it wasn't safe, sued the United States for the first time. They demanded a radiological study of the northern islands. In 1975, the United States set up "The Hawaiian Trust Fund for the People of Bikini", totaling $3 million. When the islanders were removed from the island in 1978, the U.S. added $3 million to the fund. The U.S. created a second trust fund, "The Resettlement Trust Fund for the People of Bikini", containing $20 million in 1982. The U.S. added another $90 million to that fund to pay to clean up, reconstruct homes and facilities, and resettle the islanders on Bikini and Eneu islands.

In 1983, the U.S. and the Marshall islanders signed the Compact of Free Association, which gave the Marshall Islands independence. The Compact became effective in 1986 and was subsequently modified by the Amended Compact that became effective in 2004. It also established the Nuclear Claims Tribunal, which was given the task of adjudicating compensation for victims and families affected by the nuclear testing program. Section 177 of the compact provided for reparations to the Bikini islanders and other northern atolls for damages. It included $75 million to be paid over 15 years. On March 5, 2001, the Nuclear Claims Tribunal ruled against the United States for damages done to the islands and its people.

The payments began in 1987 with $2.4 million paid annually to entire Bikini population, while the remaining $2.6 million is paid into "The Bikini Claims Trust Fund". This trust is intended to exist in perpetuity and to provide the islanders a 5% payment from the trust annually.

The United States provided $150 million in compensation for damage caused by the nuclear testing program and their displacement from their home island.

By 2001, of the original 167 residents who were relocated, 70 were still alive, and the entire population has grown to 2800. Most of the islanders and their descendants live on Kili, in Majuro, or in the United States. The opportunity for some Bikini islanders to potentially relocate back to their home island creates a dilemma. Only a few living islanders were born there. Most of the younger generation have never lived there or even visited and do not have a desire to return. Unemployment in the Marshall Islands was as of 2013 at about 40 percent. The population is growing at a four percent growth rate, so increasing numbers are taking advantage of terms in the Marshall Islands' Compact of Free Association that allow them to obtain jobs in the United States.

A 2017 study led by Steve Palumbi, a Stanford University professor of marine sciences, reported ocean life that seems highly resilient to the effects of radiation poisoning. The team described substantial diversity in the marine ecosystem, with animals appearing healthy to the naked eye, and according to Palumbi, the atoll's "lagoon is full of schools of fish all swirling around the living coral. In a strange way they are protected by the history of this place, the fish populations are better than in some other places because they have been left alone, the sharks are more abundant and the coral are big. It is a remarkable environment, quite odd." Both corals and long-lived animals such as coconut crabs should be vulnerable to radiation-induced cancers, and understanding how they have thrived in might lead to discoveries about preserving DNA – making Bikini Atoll, according to Pambuli, "an ironic setting for research that might help people live longer" by improving the scientific understanding of cancer. PBS documented field work undertaken by Palumbi and his graduate student Elora López on Bikini Atoll for the second episode ("Violent") of their series "Big Pacific." The episode explored "species, natural phenomena and behaviors of the Pacific Ocean" and the way the team is using DNA sequencing to study the rate and pattern of any mutations. López suggested possible explanations for the health of the marine life to "The Stanford Daily", such as a mechanism for DNA repair which is superior to that possessed by humans, or a method of maintaining a genome in the face of nuclear radiation.

The area has become, in effect, something of an unplanned sanctuary – as has also occurred in Europe in the Chernobyl exclusion zone – where the effects of radiation on animal life is being tested. Making an observation similar to that following the 1986 Chernobyl disaster, where serious animal deformities and mutations were abundant only in the immediate aftermath, Palumbi suggested that, as "fish have relatively short life-spans, it is possible the worst-affected fish died off many decades ago... and the fish living in Bikini Atoll today are only subject to low-levels of radiation exposure as they frequently swim in and out of the atoll." Nurse sharks have two dorsal fins but possibly-mutated individuals with only a single fin were observed. Pambuli and his team have focussed on the hubcap-sized crabs as their coconut diet is contaminated with radioactive caesium-137 from groundwater and on the corals because both have longer life-spans that allow the scientists "to delve into what effect the radiation exposure has had on the animals' DNA after building up in their systems for many years."

Bikini Atoll remains uninhabitable for humans due to what United Nations special rapporteur Călin Georgescu reported in 2012 as "near-irreversible environmental contamination." Gamma radiation levels in 2016 averaged 184 mrem yr, well above the maximum allowed for human habitation of 100 mrem yr, thereby rendering the water, seafood, and plants as all unsafe for human consumption. In his book "Strange Glow: The Story of Radiation", Timothy Jorgensen reported on the increased cancer risk, especially for leukemia and thyroid cancers, amongst inhabitants of nearby islands. Given this broader context, the resilience and apparent cancer-resistance of marine organism is particularly interesting. As Palumbi stated: "The fact there is life there and the life there is trying to come back from the most violent thing we've ever done to it is pretty hopeful."

The inhabitants of the Marshall Islands, particularly those closest to Bikini Atoll, were exposed to high levels of radiation. The highest levels of radiation exposure were found in the areas of local fallout. The fallout produced from nuclear tests can affect the human populations internally or externally. External irradiation is from penetrating gamma rays that come from particles on the ground. The levels of external radiation exposure can be reduced if one was indoors because buildings act as a shield. Inhalation of radioactive fallout and epidermal absorption are the primary means of irradiation. However most exposure is from consumption of food that has been contaminated through fallout. The people of the islands would consume meat or products from animals that had been irradiated, therefore irradiating the consumer. Food shipped into the islands was also affected by contamination through contaminated cooking utensils. Many dairy products, such as milk and yogurt, were contaminated as a result of radionuclides landing on pastures. Iodine-131, a highly radioactive isotope, was ingested or inhaled by many through various forms. The iodine-131 consumed would become concentrated in one's thyroid.

On the Marshall Islands, the detonation of Castle Bravo was the cause of most of the radiation exposure to the surrounding populations. The fallout levels attributed to the Castle Bravo test are the highest in history. The exposure to fallout has been linked to increase the likelihood of several types of cancer such as leukemia and thyroid cancer. The relationship between I-131 levels and thyroid cancer is continuing to be researched. There are also correlations between fallout exposure levels and diseases such as thyroid disease like hypothyroidism. Populations of the Marshall Islands that received significant exposure to radionuclides have a much greater risk of developing cancer. The Castle Bravo test detonation produced an explosion of approximately 15 megatons, which is about three times its predicted value. Populations neighboring the test site were exposed to high levels of radiation resulting in mild radiation sickness of many (nausea, vomiting, diarrhea). Several weeks later, many people began suffering from alopecia (hair loss) and skin lesions as well. The female population of the Marshall Islands have a sixty times greater cervical cancer mortality than a comparable mainland United States population. The Islands populations also have a five time greater likelihood of breast or gastrointestinal mortality, and lung cancer mortality is three times higher than the mainland population. The male population on the Marshall Islands lung cancer mortality is four times greater than the overall United States rates, and the oral cancer rates are ten times greater.

There is a presumed association between radiation levels and female reproductive system functioning.