View source for Castle Bravo

{{Short description|1954 U.S. thermonuclear weapon test in the Marshall Islands}}
{{Use dmy dates|date=December 2024}}{{Use American English|date=December 2024}}
{{Infobox nuclear weapons test
 |name                = Castle Bravo
 |picture             = File:CastleBravo1.gif
 |picture_description = Time-lapse of the ''Bravo'' detonation and subsequent [[mushroom cloud]]
 |country             = United States
 |test_series         = [[Operation Castle]]
 |test_site           = [[Bikini Atoll]]
 |coordinates         = {{Coord|11|41|50|N|165|16|19|E|type:event_type:region|display=title,inline}}
 |date                = {{end date and age|1954|03|01|df=y}}
 |test_type           = [[Nuclear weapons testing#Types|Atmospheric]]
 |yield               = {{Convert|15|MtonTNT|lk=on}}
 |previous_test       = [[Operation Upshot–Knothole|Upshot–Knothole Climax]]
 |next_test           = [[Castle Romeo]]
 |map_type            = Pacific Ocean
}}

'''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 1 March 1954, the device remains the most powerful [[nuclear weapon|nuclear device]] ever detonated by the United States and the first [[lithium deuteride]]-fueled thermonuclear weapon tested using the [[Teller–Ulam design]].<ref>{{Cite web |title=Operation Castle |url=https://nuclearweaponarchive.org/Usa/Tests/Castle.html |access-date=September 23, 2017 |website=nuclearweaponarchive.org}}</ref><ref>{{Cite web |last=Rowberry |first=Ariana |date=November 30, 2001 |title=Castle Bravo: The Largest U.S. Nuclear Explosion |url=https://www.brookings.edu/blog/up-front/2014/02/27/castle-bravo-the-largest-u-s-nuclear-explosion/ |access-date=September 23, 2017 |publisher=Brookings Institution}}</ref> Castle Bravo's [[Nuclear weapon yield|yield]] was {{convert|15|MtonTNT|lk=on|abbr=~}}, 2.5 times the predicted {{convert|6|MtonTNT|abbr=on}}, due to unforeseen additional reactions involving [[Isotopes of lithium|lithium-7]],<ref name="nuclearweaponarchive.org">{{Cite web |date=May 17, 2006 |title=Operation Castle |url=http://nuclearweaponarchive.org/Usa/Tests/Castle.html |access-date=May 20, 2016 |website=nuclearweaponarchive.org}}</ref> which led to [[radioactive contamination]] in the surrounding area.<ref>{{Cite journal |last1=Hughes EW |last2=Molina MR |last3=Abella MKIL |last4=Nikolić-Hughes I |last5=Ruderman MA |date=July 30, 2019 |title=Radiation maps of ocean sediment from the Castle Bravo crater |journal=[[Proceedings of the National Academy of Sciences]] |volume=116 |pages=15420–15424 |bibcode=2019PNAS..11615420H |doi=10.1073/pnas.1903478116 |pmc=6681739 |pmid=31308235 |doi-access=free |number=31}}</ref>

Radioactive [[nuclear fallout]], the heaviest of which was in the form of pulverized surface coral from the detonation, fell on residents of [[Rongelap Atoll|Rongelap]] and [[Utirik Atoll|Utirik]] atolls, while the more [[particulate]] and gaseous fallout spread around the world. The inhabitants of the islands were evacuated 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 the heavy fallout, experiencing [[acute radiation syndrome]], including the death six months later of Kuboyama Aikichi, the boat's chief radioman. The blast incited a strong international reaction over atmospheric thermonuclear testing.<ref>{{Cite book |last=Foster |first=John Bellamy |title=The Ecological Revolution: Making Peace with the Planet |date=2009 |publisher=Monthly Review Press |page=73}}</ref>

The Bravo Crater is located at {{Coord|11|41|50|N|165|16|19|E|}}. The remains of the Castle Bravo causeway are at {{Coord|11|42|6|N|165|17|7|E|}}.

== Bomb design ==
{{Infobox weapon
| name               = ''SHRIMP''
| image              = Castle Bravo Shrimp Device 002 - restoration1.jpg
| caption            = The ''SHRIMP'' device in its shot cab
| type               = [[Teller–Ulam design]] [[thermonuclear weapon]]
| is_explosive       = yes
| designer           = Ben Diven (project engineer)<ref>{{Cite web |last=Danneskiold |first=Jim |date=14 April 2005 |title=Operation Castle tests focus of 20 April panel discussion |url=http://www.lanl.gov/news/index.php/fuseaction/nb.story/story_id/6422 |url-status=dead |archive-url=https://web.archive.org/web/20090507083344/https://www.lanl.gov/news/index.php/fuseaction/nb.story/story_id/6422 |archive-date=7 May 2009 |website=Los Alamos National Laboratory}}</ref>
| design_date        = 24 February 1953
| manufacturer       = [[Los Alamos National Laboratory]]
| unit_cost          = $2.7 million (1954) (${{format price|{{Inflation|US-GDP|2.7e6|1954}}}} in {{Inflation-year|US-GDP}} dollars{{Inflation-fn|US-GDP}})
| production_date    = October 1953
| number             = 1
| variants           = TX-21C, TX-26
| weight             = {{cvt|10,659|kg}}
| length             = {{cvt|455.93|cm}}
| diameter           = {{cvt|136.90|cm}}
| filling            = Lithium-6 deuteride
| filling_weight     = {{cvt|400|kg}}
| yield              = {{Unbulleted list
  | Expected: {{convert|5|MtonTNT}}
  | Actual: {{convert|15|MtonTNT}}
  }}
}}

=== Primary system ===
The ''Castle Bravo'' device was housed in a cylinder that weighed {{convert|23,500|lb}} and measured {{convert|179.5|in|cm}} in length and {{convert|53.9|in|cm}} in diameter.<ref name="nuclearweaponarchive.org" />

The primary device was a ''COBRA'' [[boosted fission weapon|deuterium–tritium gas-boosted]] atomic bomb made by [[Los Alamos Scientific Laboratory]], a very compact MK 7 device. This boosted fission device had been tested in the ''[[Operation Upshot–Knothole|Upshot–Knothole]]'' ''Climax'' event and yielded {{convert|61|ktonTNT|abbr=~}} (out of 50–70&nbsp;kt expected yield range). It was considered successful enough that the planned operation series ''Domino'', designed to explore the same question about a suitable primary for thermonuclear bombs, could be canceled.<ref name="swordsoarIII" />{{refpage|197}} The implosion system was quite lightweight at {{convert|900|lb|abbr=on}}, because it eliminated the aluminum pusher shell around the [[Tamper (nuclear weapon)|tamper]]{{refn|group=Note|In the Mark 7 HE system, the irregularities in the implosion front were relatively small rendering the pusher component unnecessary.<ref name="weaponactivity">{{Cite book |last=Glasstone |first=Samuel |title=LA-1632: Weapons Activities of Los Alamos Scientific Laboratory |date=1954 |volume=Part I |author-link=Samuel Glasstone}}</ref>{{refpage|60}}}} and used the more compact ring lenses,{{refn|group=Note|Ring Lenses were used in conjunction with 1E23 type bridge-wire detonators. The ring lenses reduced weapon's external diameter by making the HE layer thinner, and their simultaneity of shock wave emergence was considerably higher compared to previous hyperboloid lenses, enabling better and more accurate compression (LA-1632, table 4.1). At the same time, since the [[high explosive]] layer was thinner it was less opaque for the [[X-ray]]s emitted by the pit.<ref name="weaponactivity" />{{refpage|86}}{{refpage|98}}}} 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 provided 13 percent more compressive energy than Comp B).<ref name="weaponactivity" />{{refpage|86}}{{refpage|91}} 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.<ref name="swordsoarIII">{{Cite book |last=Hansen |first=Chuck |url=http://www.uscoldwar.com/ |title=Swords of Armageddon |date=1995 |volume=III |author-link=Chuck Hansen |access-date=December 28, 2016}}</ref>{{refpage|196}} A copper pit liner encased within the weapon-grade plutonium inner capsule prevented DT gas diffusion into the plutonium, a technique first tested in ''[[Greenhouse Item]]''.<ref name="swordsoarIII" />{{refpage|258}} The assembled module weighed {{convert|1840|lb|abbr=on}}, measuring {{convert|30.5|in|abbr=on}} 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 into the secondary.<ref name="nucweaparch">{{Cite web |title=The Nuclear Weapon Archive&nbsp;– A Guide to Nuclear Weapons |url=https://nuclearweaponarchive.org/ |access-date=September 23, 2017 |website=nuclearweaponarchive.org}}</ref>

=== Deuterium and lithium ===
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 [[nuclear fusion|fusion]] fuel. ''SHRIMP'' used [[lithium deuteride]] (LiD), which is solid at room temperature; ''Ivy Mike'' used [[cryogenic]] liquid [[deuterium]] (D<sub>2</sub>), which required elaborate cooling equipment. ''Castle Bravo'' was the first test by the United States of a practical deliverable [[hydrogen bomb|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 [[Mark 16 nuclear bomb|''JUGHEAD'']], which was slated to be tested as the initial ''Castle Yankee''. It also used a {{Convert|9.5|cm|in|-thick|adj=mid|order=flip}} [[7075 aluminum alloy|7075 aluminum]] ballistic case. Aluminum was used to drastically reduce the 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 other weapon-projects at the time.<ref name="swordsoarIII" />{{refpage|54}}{{refpage|237}}<ref name="ThePhysicsFactbook">{{Cite book |last=Sutherland |first=Karen |url=https://hypertextbook.com/facts/2004/KarenSutherland.shtml |title=Density of Steel |date=2004 |author-link=Karen Sutherland |access-date=December 28, 2016}}</ref>

The ''SHRIMP'' was at least in theory and in many critical aspects identical in geometry to the [[Mark 17 nuclear bomb|''RUNT'']] and [[Mark 17 nuclear bomb|''RUNT II'']] devices later proof-fired in the later ''[[Castle Romeo|Romeo]]'' and ''[[Castle Yankee|Yankee]]'' shots. On paper it was a scaled-down version of these devices, and its origins can be traced back to 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'' [[Dimensional analysis|scaled down]] to a 3/4 scale radiation-implosion system called the ''SHRIMP''. The proposed weight reduction (from TX-17's {{convert|42000|lb}} to TX-21's {{convert|25000|lb}}) would provide the Air Force with a much more versatile deliverable [[gravity bomb]].<ref name="swordsoarIII" />{{refpage|237}} 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 of lithium|isotopes]] (with 7.5% of the former). The enriched lithium used in ''Bravo'' was nominally 40% lithium-6 (the remainder 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 {{sup|6}}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 [[Mark 14 nuclear bomb|''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 in late 1953.<ref name="swordsoarmIII">{{Cite book |last=Hansen |first=Chuck |url=http://www.uscoldwar.com/ |title=Swords of Armageddon |date=1995 |volume=III |author-link=Chuck Hansen |access-date=May 20, 2016}}</ref>{{refpage|208}} 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 {{convert|500|L|sp=us}},{{refn|group=Note|Both SAUSAGE and the two RUNTs (SAUSAGE's "lithiated" versions) had fusion fuel volumes of 840 [[liter]]s. SAUSAGE used an 840-liter version of a cryogenic vessel developed for the PANDA committee (PANDA was SAUSAGE's unclassified name) and in part by the [[National Bureau of Standards]] (see more information [https://nvlpubs.nist.gov/nistpubs/jres/58/jresv58n5p243_A1b.pdf here]). This vessel fits the description of Richard Rhodes in ''Dark Sun'' (p. 490) and Mike's fusion fuel volume assumed by Andre Gsponer and Jean-Pierre Hurni in their paper "The physical principles of thermonuclear explosives, inertial confinement fusion, and the quest for fourth generation nuclear weapons", p. 68.}} corresponding to about 390&nbsp;kg of lithium deuteride (as LiD has a density of 0.78201 g/cm<sup>3</sup>).<ref name="T4">{{Cite book |last=Holian |first=Kathleen S. |title=T-4 Handbook of Material Properties Data Bases |date=1984 |volume=Ic |author-link=Kathleen S. Holian}}</ref>{{refpage|281}} The mixture cost about 4.54&nbsp;[[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.<ref name="swordsoarIII" />{{refpage|39}} [[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 [[Electronvolt|MeV]] through Jetter's cycle.

<gallery class="center" widths="220">
File:Castle Bravo Declassified Film 46th second.png|The ''SHRIMP'' shortly before installation in its shot cab
File:Castle Bravo Declassified Film 49th second.png|''SHRIMP''{{'}}s parabolic projection
File:Castle Bravo Declassified Film 47th second.png|''SHRIMP''{{'}}s cylindrical end
File:BravoSHRIMPShotCab.jpg|Shot-cab installation of ''SHRIMP'' device, with humans for scale
</gallery>

==== Jetter's cycle ====
[[File:jetter.svg|right]]
The Jetter cycle is a combination of reactions involving [[lithium]], [[deuterium]], and [[tritium]]. It consumes [[lithium-6]] and deuterium, and in two reactions (with energies of 17.6&nbsp;MeV and 4.8&nbsp;MeV, mediated by a neutron and tritium) it produces two [[alpha particle]]s.<ref>{{Cite web|url=https://ieer.org/wp/wp-content/uploads/1998/07/DangerousThermonuclearQuest.pdf|title="Dangerous Thermonuclear Quest: The Potential of Explosive Fusion Research for the Development of Pure Fusion Weapons" |page=4}}</ref>

The reaction would produce high-energy neutrons with 14&nbsp;MeV, and its [[Nuclear fusion#Neutronicity, confinement requirement, and power density|neutronicity]] was estimated at ≈0.885 (for a [[Lawson criterion]] of ≈1.5).

==== Possible additional tritium for high-yield ====
As ''SHRIMP'', along with the ''RUNT I'' and ''ALARM CLOCK'', were to be high-yield shots required to assure the thermonuclear "[[Mark 16 nuclear bomb|emergency capability]]," their fusion fuel may have been spiked with additional tritium, in the form of {{sup|6}}LiT.<ref name="swordsoarmIII" />{{refpage|236}} 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'',<ref name="swordsoarmIII" /> 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.

=== ''SHRIMP''{{'}}s indirect drive ===
[[File:BravoShotCab.jpg|thumb|''Bravo'' SHRIMP device shot-cab]]
Attached to the cylindrical ballistic case was a natural-uranium liner, the radiation case, that was about 2.5&nbsp;cm thick. Its internal surface was lined with about 240 μm-thick [[copper]] foil to increase the overall albedo of the [[hohlraum]].<ref name="X-Ray Albedo">{{Cite journal |last=Pruitt |author-link=J. S. Pruitt |date=1963 |title=High Energy X-Ray Photon Albedo |journal=Nuclear Instruments and Methods |volume=27 |issue=1 |pages=23–28 |bibcode=1964NucIM..27...23P |doi=10.1016/0029-554X(64)90131-4}}</ref><ref name="γ-Ray Albedo">{{Cite book |last=Bulatov and Garusov |title={{sup|60}}Co and {{sup|198}}Au γ-ray albedo of various materials |date=1958 |author-link=B. P. Bulatov and E. A. Garusov}}</ref> 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&nbsp;g/cm{{sup|2}}, 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. Therefore, hohlraum liners require the use of copper (or, as in other devices, [[gold]] or [[aluminium]]), as the absorption probability increases with the value of ''Z''{{sub|eff}} 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''{{sub|r}}) plateau. The primary emits radiation in a manner similar to a [[Flash (photography)|flash bulb]], and the secondary needs constant ''T''{{sub|r}} to properly implode.<ref name="IC">{{Cite book |title=Current Trends in International Fusion Research Proceedings of the Third Symposium |date=2002}}</ref> 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){{refn|group=Note|This temperature range is compatible with a hohlraum filling made of a low-''Z'' material because the fission bomb's tamper, pusher and high-explosive lenses as well as interstage's plastic foam strongly [[attenuation|attenuate]] the radiation emitted by the core. Thus, [[X-ray]]s deposited into the hohlraum liner from primary's interface with the interstage (i.e. the primary's outer surface) were "cooler" than the maximum temperature of a fission device.<ref name="Gsponer">{{Cite book |title=The physical principles of thermonuclear explosives, inertial confinement fusion, and the quest for fourth generation nuclear weapons |date=2009}}</ref>{{refpage|25}}<ref name="nucl">https://nuclearweaponarchive.org/Nwfaq/Nfaq4-4.html. {{dead link|date=February 2022}}</ref>}}, corresponding to several million [[kelvin]]s. Wall temperature depended on the temperature of the primary's [[Pit (nuclear weapon)|core]] which peaked at about 5.4 keV during boosted-fission.<ref name="Pritz">{{Cite journal |last1=Pritzker |first1=Andreas |author-link=A. Pritzger and W. Halg |last2=Hälg |first2=Walter |date=1981 |title=Radiation dynamics of nuclear explosion |journal=Zeitschrift für Angewandte Mathematik und Physik |volume=32 |issue=1 |pages=1–11 |bibcode=1981ZaMP...32....1P |doi=10.1007/BF00953545 |s2cid=122035869}}</ref>{{refpage|1-11}}<ref name="Gsponer" />{{refpage|9}} 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.<ref name="X-Ray Albedo" />{{refn|group=Note|These losses were associated with material's properties like back-scattering, [[quantum tunneling]], [[Radiant exitance|exitance]] etc.<ref name="X-Ray Albedo" />}} [[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 [[Mark 15 nuclear bomb|''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'',{{refn|group=Note|Tamper is the metal cladding encasing the secondary, and it is also termed ''pusher''; both terms can be used interchangeably}} and the case formed a radiation channel to conduct [[X-ray]]s 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,<ref name="astroduct">{{Cite book |last=Benz |first=Arnold |title=Plasma Astrophysics; Kinetic Processes in Solar and Stellar Coronae |date=1992 |author-link=Arnold O. Benz}}</ref>{{refpage|279}} 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"{{refn|group=Note|Not to be confused with the function of the fusion tamper}} the [[sputtering]] effect that would otherwise "choke" radiation from compressing the secondary.{{refn|group=Note|Sputtering is the manifestation of the underdense plasma corona of the ablating hohlraum and the tamper surfaces.<ref name="ProgressIgnition">{{Cite journal |last=Lindl |first=John |author-link=John D. Lindl |date=1992 |title=Progress toward Ignition and Burn Propagation in Inertial Confinement Fusion |journal=Physics Today |volume=45 |issue=9 |pages=32–40 |bibcode=1992PhT....45i..32L |doi=10.1063/1.881318}}</ref> It is a problem also shared with fusion reactors such as [[tokamak]]s, that has to do with the ablated heavy particles; For a hydrogen weapon, these particles are blown-off high-''Z'' granular particles (made off uranium of Pb–Bi eutectic; the selected material depends on the "cocktail", or high-''Z'' element mixture, of the [[hohlraum]] design to tailor its opacity), which fly inside the radiation channel and absorb radiation or reflect it, hampering radiation "ducting".<ref name="astroduct" />{{refpage|279}}}} 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.<ref name="Rhodes">{{Cite Q | Q105755363 | last1 = Rhodes | first1 = Richard | author-link1 = Richard Rhodes | df = dmy-all | via = [[Internet Archive]] }}</ref>{{rp|pages=438–454}} (see [[Nuclear weapon design#Light pipes|Nuclear weapon design]]). This point is above the threshold where the fusion fuel would turn opaque to its emitting radiation, as determined from its [[Opacity (optics)|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''.<ref name="swordsII">{{Cite book |last=Hansen |first=Chuck |url=http://www.uscoldwar.com/ |title=Swords of Armageddon |date=1995 |volume=II |author-link=Chuck Hansen |access-date=May 20, 2016}}</ref>{{refpage|185}}

[[File:Operation Castle AW 1.jpg|thumb|''SHRIMP'' device delivered via truck awaiting installation]]
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&nbsp;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 {{sup|235}}U is primarily based on the final fission reaction fragments detected in the radiochemical analysis, which conclusively showed the presence of {{sup|237}}U, found by the Japanese in the shot debris.<ref name="swordsIV">{{Cite book |last=Hansen |first=Chuck |url=http://www.uscoldwar.com/ |title=Swords of Armageddon |date=1995 |volume=IV |author-link=Chuck Hansen |access-date=May 20, 2016}}</ref>{{refpage|282}} The first-generation thermonuclear weapons (MK-14, 16, 17, 21, 22 and 24) all used uranium tampers enriched to 37.5% {{sup|235}}U.<ref name="swordsIV" />{{refpage|16}} The exception to this was the [[Mark 15 nuclear bomb|MK-15 ''ZOMBIE'']] that used a 93.5% enriched fission jacket.

=== The secondary assembly ===
[[File:Bravo secondary fireball.jpg|thumb|alt=Bravo secondary fireball|In a similar manner to the earlier pipes filled with a [[partial pressure]] of helium, as used in the [[Ivy Mike]] test of 1952, the 1954 Castle Bravo test was likewise heavily instrumented with [[Nuclear weapon design#Light pipes|Line-of-Sight (LOS) pipes]], to better define and quantify the timing and energies of the x-rays and neutrons produced by these early thermonuclear devices.<ref>{{Cite web |date=May 12, 1954 |title=Operation CASTLE Commander's Report |url=https://archive.org/details/CastleCommandersReport1954 |via=Internet Archive}}</ref><ref>{{Cite web |title=Declassified U.S. Nuclear Test Film #34 0800034 – Project Gnome – 1961. 6:14 minutes |url=https://www.youtube.com/watch?v=DFJ2MyWlXgs |url-status=live |archive-url=https://ghostarchive.org/varchive/youtube/20211221/DFJ2MyWlXgs |archive-date=December 21, 2021 |website=[[YouTube]]| date=October 31, 2007 }}{{cbignore}}</ref>  One of the outcomes of this diagnostic work resulted in this graphic depiction of the transport of energetic x-ray and neutrons through a vacuum line, some 2.3 km long, whereupon it heated solid matter at the "station 1200" blockhouse and thus generated a secondary fireball.<ref>{{Cite web |title=How Archive Data Contribute to Certification. Fred N. Mortensen, John M. Scott, and Stirling A. Colgate |url=https://permalink.lanl.gov/object/tr?what=info%3Alanl-repo%2Flareport%2FLA-UR-03-5462 |url-status=live |archive-url=https://web.archive.org/web/20161223223806/http://permalink.lanl.gov/object/tr?what=info%3Alanl-repo%2Flareport%2FLA-UR-03-5462 |archive-date=December 23, 2016 |access-date=December 23, 2016}}</ref><ref>{{Cite web |date=June 12, 2007 |title=LANL: Los Alamos Science: LA Science No. 28 |url=http://la-science.lanl.gov/lascience28.shtml |url-status=dead |archive-url=https://web.archive.org/web/20070612184310/http://la-science.lanl.gov/lascience28.shtml |archive-date=June 12, 2007}}</ref>]]
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 produce interference with 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 with the primary, as they diminished bending and torsional loads on the secondary, another technique adopted from the ''SAUSAGE''.<ref name="Rhodes" />{{refpage|438-454}} 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 [[Mark 24 nuclear bomb|''RUNT'']], and the [[Mark 14 nuclear bomb|''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.{{refn|group=Note|Both the ballistic case and hohlraum were perforated in these points so that light emanating from the nuclear components could travel unobstructed to the recording station. A slight drop in yield was expected because of those apertures, much like in the ''Mike'' test.<ref name="Rhodes" /> The hot-spot openings, similar to the "starburst" diagnostics in hohlraums used in [[inertial confinement fusion]] (ICF) indirect drive experiments,<ref name="starb">{{Cite journal |last1=Cook |first1=R. C. |author-link=R. C. Cook |last2=Kozioziemski |first2=B. J. |last3=Nikroo |first3=A. |last4=Wilkens |first4=H. L. |last5=Bhandarkar |first5=S. |last6=Forsman |first6=A. C. |last7=Haan |first7=S. W. |last8=Hoppe |first8=M. L. |last9=Huang |first9=H. |last10=Mapoles |first10=E. |last11=Moody |first11=J. D. |last12=Sater |first12=J. D. |last13=Seugling |first13=R. M. |last14=Stephens |first14=R. B. |last15=Takagi |first15=M. |date=2008 |title=National Ignition Facility target design and fabrication |url=https://digital.library.unt.edu/ark:/67531/metadc900158/m2/1/high_res_d/924012.pdf |journal=Laser and Particle Beams |volume=26 |issue=3 |page=479 |bibcode=2008LPB....26..479C |doi=10.1017/S0263034608000499 |doi-access=free |last16=Xu |first16=H. W. |osti=924012 }}</ref> caused local radiation decoupling and hence poor radiation reflection by the hohlraum. Radiation decoupling in turn reduced locally the efficiency of the ablation process on the surface of secondary's tamper, destabilizing implosion by a small degree. Nevertheless, even minor instabilities during ablation amplified the already dreaded Taylor mixing.}} These pipe sections were {{convert|8+5/8|in}} in diameter and {{convert|40|ft}} 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 {{convert|1|acre|adj=on}} 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, and so that Los Alamos could determine both the [[Relativity of simultaneity|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.<ref name="swordsoarIII" />{{refpage|63}}{{refpage|229}}

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&nbsp;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&nbsp;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''{{sub|eff}}, 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&nbsp;kg, and its initial firing yielded {{convert|0.6|ktTNT|abbr=on}}. Then it would be completely fissioned by the fusion neutrons, contributing about {{convert|330|ktTNT|abbr=on}} 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.<ref name="Gsponer" /> The neutrons entered the assembly by a small hole{{refn|group=Note|The cylindrical hole was plugged with {{sup|10}}B-doped paraffin wax to time the neutrons' arrival.<ref name="swordsoarIII" />}} through the ≈28&nbsp;cm thick {{sup|238}}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 [[Cross section (geometry)|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 (nuclear weapons)|tamper]] with a standoff air-gap about 0.9&nbsp;cm wide that was to increase the tamper's momentum, a levitation technique used as early as ''[[Operation Sandstone]]'' and described by physicist [[Ted Taylor (physicist)|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 mitigate the unavoidable and undesirable [[Rayleigh–Taylor instability|''Taylor mixing'']].

=== Use of boron ===
Boron was used at many locations in this dry system; it has a high cross-section for the absorption of slow neutrons, which fission {{sup|235}}U and {{sup|239}}Pu, but a low cross-section for the absorption of fast neutrons, which fission {{sup|238}}U. Because of this characteristic, {{sup|10}}B deposited onto the surface of the secondary stage would prevent pre-detonation of the ''spark plug'' by stray neutrons from the primary without interfering with the subsequent fissioning of the {{sup|238}}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 {{sup|10}}B,<ref name="swordsoarIII" />{{refpage|179}} the secondary was compressed more highly, at a cost of some radiated neutrons. By contrast, the ''[[Castle Koon]]'' ''MORGENSTERN'' device did not use {{sup|10}}B 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.<ref name="swordsoarIII" />{{refpage|317}} 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.<ref name="swordsoarmIII" />

== Detonation ==
[[File:CastleBravo2.gif|thumb|''Bravo'' detonation and fireball]]
The device was mounted in a "shot cab" on an artificial island built on a reef off Namu Island, in [[Bikini Atoll]]. A sizable array of diagnostic instruments were 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 1 March 1954, local time (18:45 on 28 February [[GMT]]).<ref name="nuclearweaponarchive.org" />

When Bravo was detonated, within one second it formed a fireball almost {{convert|4.5|mi}} across. This fireball was visible on [[Kwajalein Atoll]] over {{convert|250|mi}} away. The explosion left a crater {{convert|6,500|ft}} in diameter and {{convert|250|ft}} in depth. The [[mushroom cloud]] reached a height of {{convert|47000|ft}} and a diameter of {{convert|7|mi}} in about a minute, a height of {{convert|130000|ft|km}} and {{convert|100|km|mi|order=flip|abbr=on}} in diameter in less than 10 minutes and was expanding at more than {{convert|160|m/s|km/h mph|sp=us}}. As a result of the blast, the cloud contaminated more than {{convert|7000|mi2}} of the surrounding Pacific Ocean, including some of the surrounding small islands like [[Rongerik Atoll|Rongerik]], [[Rongelap Atoll|Rongelap]], and [[Utirik Atoll|Utirik]].<ref>{{Cite book |last=Titus |first=A. Costandina |title=Bombs in the Backyard: Atomic Testing and American Politics |date=2001 |publisher=University of Nevada |location=Reno}}</ref>
[[File:Castle Bravo Blast.jpg| thumb|right | Castle Bravo mushroom cloud a few seconds after detonation]]

In terms of energy released (usually measured in [[TNT equivalent|TNT equivalence]]), ''Castle Bravo'' was about 1,000 times more powerful than the atomic [[Little Boy|bomb that was dropped on Hiroshima]] during [[World War II]]. ''Castle Bravo'' is the sixth largest nuclear explosion in history, exceeded by the Soviet tests of ''[[Tsar Bomba]]'' at approximately 50 Mt, ''[[Test 219]]'' at 24.2 Mt, and three other (''Test 147'', ''Test 173'' and ''Test 174'') ≈20 Mt Soviet tests in 1962 at [[Novaya Zemlya]].

=== High yield ===
[[File:Castlebravodiagram.svg|thumb|Diagram of [[Tritium]] bonus provided by Lithium-7 isotope]]
The yield of 15 (± 5) Mt<ref>{{Cite web |date=March 6, 1954 |title=Commander Task Group 7.1 Eniwetok to U.S. AEC |url=https://nsarchive.gwu.edu/document/31248-document-3-commander-task-group-71-eniwetok-us-aec-1-march-1954-attached-2-and-6 |access-date=March 1, 2024 |website=National Security Archive}}</ref> was triple that of the 5 Mt predicted by its designers.<ref name="nuclearweaponarchive.org" /><ref name="Rhodes" />{{refpage|541}} 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 participate in reactions that were too slow to contribute to the yield.<ref name="Rhodes" />{{refpage|541}} It was correctly 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.

It was assumed that the lithium-7 would absorb one neutron, producing lithium-8, which decays (through [[beta decay]] into [[Isotopes of beryllium|beryllium-8]]) to a pair of alpha particles on a timescale of nearly a second, vastly longer than the timescale of nuclear detonation.<ref name=":3">{{Cite book |last1=Parsons |first1=Keith M. |title=Bombing the Marshall Islands: A Cold War Tragedy |last2=Zaballa |first2=Robert A. |publisher=[[Cambridge University Press]] |year=2017 |isbn=978-1-108-50874-2 |pages=53–56}}</ref> However, when lithium-7 is bombarded with [[neutron temperature|energetic neutrons]] with an energy greater than 2.47 MeV, rather than simply absorbing a neutron, it undergoes nuclear fission into an alpha particle, a tritium [[atomic nucleus|nucleus]], and another neutron.<ref name=":3" /> 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.<ref name=":3" />

Summarizing, the reactions involving lithium-6 result in some combination of the two following net reactions:
:{{sup|1}}n + {{sup|6}}Li → {{sup|3}}H + {{sup|4}}He + 4.783 MeV
:{{sup|6}}Li + {{sup|2}}H → 2 {{sup|4}}He + 22.373 MeV
But when lithium-7 is present, one also has some amounts of the following two net reactions:
:{{sup|7}}Li + {{sup|1}}n → {{sup|3}}H + {{sup|4}}He + {{sup|1}}n
:{{sup|7}}Li + {{sup|2}}H → 2 {{sup|4}}He + {{sup|1}}n + 15.123 MeV

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.{{citation needed|date=August 2015}}

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.

In an additional unexpected event, albeit one of far less consequence, X-rays traveling through line-of-sight (LOS) pipes caused a small second fireball at Station 1200 with a yield of {{convert|1|ktonTNT}}.

== High levels of fallout ==
[[File:Bravo fallout2.png|right|thumb|The ''Bravo'' fallout plume spread dangerous levels of radioactivity over an area over {{convert|280|mi|km}} long, including inhabited islands. The contour lines show the cumulative radiation [[Absorbed dose|exposure]] in [[roentgen (unit)|roentgens]] (R) for the first 96 hours after the test.<ref>{{Cite book |last=Glasstone |first=Samuel |url=https://books.google.com/books?id=Ovu108BraNUC |title=The Effects of Nuclear Weapons |publisher=U.S. Department of Defense, U.S. Atomic Energy Commission |year=1962 |pages=462}} (Dose given in roentgens in the 1962 ed.)</ref><ref>{{Cite book |last1=Glasstone |first1=Samuel |url=https://books.google.com/books?id=65tBAAAAIAAJ |title=The Effects of Nuclear Weapons (3rd ed.) |last2=Dolan |first2=Philip J. |publisher=U.S. Department of Defense, U.S. Atomic Energy Commission |year=1977 |isbn=978-0-318-20369-0 |page=437}} (Dose given in [[Rad (unit)|rads]] in 1977 ed.)</ref> Although widely published, this fallout map is not perfectly correct<ref name="DTRIAC SR-12-001">{{Cite web |last1=Kunkel |first1=Thomas |last2=Ristvet |first2=Brian |date=January 25, 2013 |title=Castle Bravo: Fifty Years of Legend and Lore |url=https://blog.nuclearsecrecy.com/wp-content/uploads/2013/06/SR-12-001-CASTLE-BRAVO.pdf |url-status=live |archive-url=https://web.archive.org/web/20140310004623/http://blog.nuclearsecrecy.com/wp-content/uploads/2013/06/SR-12-001-CASTLE-BRAVO.pdf |archive-date=March 10, 2014 |access-date=May 20, 2016 |publisher=[[Defense Threat Reduction Agency]] |location=Albuquerque, NM}}</ref>]]

The fission reactions of the natural uranium tamper were quite dirty, producing a large amount of [[nuclear fallout|fallout]]. That, combined with the larger than expected yield and a major wind shift, produced some very serious consequences for those in the fallout range. 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.

=== Inhabited islands affected ===
[[Nuclear fallout|Radioactive fallout]] was spread eastward onto the inhabited [[Rongelap Atoll|Rongelap]] and [[Rongerik Atoll|Rongerik]] atolls, which were evacuated<ref>{{Cite news |date=June 22, 2009 |title=Les cobayes du Dr Folamour |url=https://www.lemonde.fr/planete/article/2009/06/22/les-cobayes-du-dr-folamour_1209927_3244.html |access-date=May 20, 2016 |work=Le Monde |language=fr}}</ref> 48 hours after the detonation.<ref name="Nuclear Issues">{{Cite web |title=Nuclear Issues |url=http://www.rmiembassyus.org/Nuclear%20Issues.htm |url-status=dead |archive-url=https://web.archive.org/web/20160424042410/http://www.rmiembassyus.org/Nuclear%20Issues.htm |archive-date=April 24, 2016 |access-date=May 20, 2016}}</ref> In 1957, the [[United States Atomic Energy Commission|Atomic Energy Commission]] deemed Rongelap safe to return, and allowed 82 inhabitants to move back to the island. Upon their return, they discovered that their previous [[staple food]]s, including [[arrowroot]], makmok, and fish, had either disappeared or gave residents various illnesses,<ref>{{Cite book |last=Smith-Norris |first=Martha |url=https://muse.jhu.edu/book/45709 |title=Domination and Resistance: The United States and the Marshall Islands during the Cold War |date=2016 |publisher=University of Hawai'i Press |isbn=978-0-8248-5814-8 |language=en}}</ref> and they were again removed.<ref name="ghostfleet">{{Cite episode |title=The Ghost Fleet of Bikini Atoll |url=http://www.history.co.uk/shows/ghost-fleet-of-the-bikini-atoll |access-date=May 20, 2016 |series=Mystery of Old World Cultures |network=A&E Television Networks |station=Military History Channel |date=October 11, 2009 |archive-date=April 2, 2019 |archive-url=https://web.archive.org/web/20190402123924/https://www.history.co.uk/shows/ghost-fleet-of-the-bikini-atoll |url-status=dead}}</ref> 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 defect]]s were reported.{{medical citation needed|date=October 2016}} 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.<ref name="Nuclear Issues" /> A medical study, named [[Project 4.1]], studied the effects of the fallout on the islanders.<ref name="Nuclear Issues" />

[[File:Castle Bravo fish contamination map.png|left|thumb|Map showing points (X) where contaminated fish were caught or where the sea was found to be excessively radioactive. '''B'''=original "danger zone" around Bikini announced by the U.S. government. '''W'''="danger zone" extended later. '''xF'''=position of the ''Lucky Dragon'' fishing boat. '''NE, EC, and SE'''&nbsp;are equatorial currents.]]

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.<ref>{{Cite journal |last=Sevitt |first=S. |date=July 23, 1955 |title=The Bombs |journal=The Lancet |volume=266 |issue=6882 |pages=199–201 |doi=10.1016/s0140-6736(55)92780-x |pmid=13243688}}</ref>

=== Fishing boats ===
A Japanese fishing boat, {{Transliteration|ja|Hepburn|[[Daigo Fukuryū Maru]]}} (Lucky Dragon No. 5), came in direct contact with the fallout, which caused many of the crew to grow ill due to radiation sickness. One member died of a secondary infection six months later after acute radiation exposure, and another had a child that was stillborn and deformed.<ref name="Oishi 2017 55–68">{{Citation |last1=Oishi |first1=Matashichi |title=The fisherman |date=2017 |work=Grappling with the Bomb |pages=55–68 |series=Britain's Pacific H-bomb tests |publisher=ANU Press |isbn=978-1-76046-137-9 |jstor=j.ctt1ws7w90.9 |last2=Maclellan |first2=Nic}}</ref> 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.<ref name="Rhodes" />{{refpage|542}} 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, and they denied that the crew was affected by radioactive fallout, despite data from the Japanese fishing vessel and other international sources showing the contrary.<ref name="Oishi 2017 55–68" />

[[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 deduced that the bomb had three stages and showed that the fission phase at the end of the explosion increased the amount of radioactivity a thousand-fold. 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".<ref name="honoluluweekly">{{Cite news |last=Keever |first=Beverly Deepe |date=February 25, 2004 |title=Shot in the Dark |url=http://www.honoluluweekly.com/archives/coverstory%202004/2-25-04%20Bravo%20shot/Bravo%20shot.html |url-status=dead |archive-url=https://web.archive.org/web/20110712220056/http://www.honoluluweekly.com/archives/coverstory%202004/2-25-04%20Bravo%20shot/Bravo%20shot.html |archive-date=July 12, 2011 |access-date=May 20, 2016 |work=Honolulu Weekly |quote=The Japanese government and people dubbed it "a second Hiroshima" and it nearly led to severing diplomatic relations}}</ref> Nevertheless, the Japanese and US governments quickly reached a political settlement, with the transfer to Japan of $15.3 million as compensation,<ref>{{Cite web |date=August 1996 |title=50 Facts About U.S. Nuclear Weapons |url=https://www.brookings.edu/50-facts-about-u-s-nuclear-weapons/ |url-status=live |archive-url=https://web.archive.org/web/20110719155737/http://www.brookings.edu/projects/archive/nucweapons/50.aspx |archive-date=July 19, 2011 |access-date=May 20, 2016 |website=The Brookings Institution}}</ref> with the surviving victims receiving about {{JPY|2}}&nbsp;million each ($5,550 in 1954, or about ${{formatnum:{{Inflation|US|5550|1954|r=-2}}}} in {{inflation/year|US}}).<ref name="chugoku">{{Cite news |last=Hirano |first=Keiji |date=February 29, 2004 |title=Bikini Atoll H-bomb damaged fisheries, created prejudice |url=http://www.chugoku-np.co.jp/abom/04e/kyodo/Ak04022901.html |url-status=dead |archive-url=https://web.archive.org/web/20130429144626/http://www.chugoku-np.co.jp/abom/04e/kyodo/Ak04022901.html |archive-date=April 29, 2013 |access-date=May 20, 2016 |publisher=[[Chugoku]]}}</ref> It was also agreed that the victims would not be given [[Hibakusha]] status.

In 2016, 45 Japanese fishermen from other ships sued their government for not disclosing records about their exposure to Operation Castle fallout. Records released in 2014 acknowledge that the crews of 10 ships were exposed but under health-damaging levels.<ref name="ABC">{{Cite news |date=May 10, 2016 |title=Fishermen Sue Japan for Hiding Records of Fallout From US Nuclear Tests |url=https://abcnews.go.com/International/fishermen-sue-japan-hiding-records-fallout-us-nuclear/story?id=39012795 |access-date=November 20, 2023 |work=ABC News |language=en}}</ref> In 2018 the suit was rejected by the Kochi District Court, who acknowledged the fishermen's radiation exposure but could not "conclude that the state persistently gave up providing support and conducting health surveys to hide the radiation exposure".<ref>{{Cite web |last= |date=July 21, 2018 |title=Former fishermen lose H-bomb damages suit linked to Bikini Atoll tests U.S. conducted in 1954 |url=https://www.japantimes.co.jp/news/2018/07/21/national/crime-legal/shikoku-court-rejects-damages-claim-former-fishermen-1954-u-s-h-bomb-tests/ |url-access=subscription |website=[[The Japan Times]] |language=en}}</ref>
[[File:Bikini Atoll Post BRAVO.png|right|thumb|The device's firing crew was located on Enyu island, variously spelled as Eneu island, as depicted here]]

=== Bomb test personnel take shelter ===
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.<ref name="trapped">{{Cite news |last=Clark |first=John C. |date=July 1957 |editor-last=Robert Cahn |title=Trapped by Radioactive Fallout |url=http://www.sonicbomb.com/content/atomic/docs/trapped_by_radioactive_fallout.pdf |access-date=May 20, 2016 |work=[[Saturday Evening Post]]}}</ref> In contrast to the crew of the ''{{nowrap|Lucky Dragon No. 5}}'', who did not anticipate the hazard and therefore did not take shelter in the [[Hold (ship)|hold]] of their ship, or refrain from inhaling the fallout dust,<ref>{{Cite news |last=Hoffman |first=Michael |date=August 28, 2011 |title=Forgotten atrocity of the atomic age |url=http://www.japantimes.co.jp/culture/2011/08/28/books/book-reviews/forgotten-atrocity-of-the-atomic-age |access-date=May 20, 2016 |work=[[Japan Times]] |page=11}}</ref> the firing crew that triggered the explosion safely sheltered 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 [[shelter in place|sheltering in place]] ("buttoning up") for several hours until outside radiation decayed to safer levels. "25 [[roentgen (unit)|roentgens]] per hour" was recorded above the bunker.<ref name="trapped" /><ref>{{Cite web |last=Ely |first=Dave |title=Operation Castle: Bravo Blast |url=http://www.dgely.com/Bikini/Nuclear%20Testing/Operation%20Castle/Operation%20Castle%20Bravo%20Blast.htm |url-status=usurped |archive-url=https://web.archive.org/web/20131022173734/http://www.dgely.com/Bikini/Nuclear%20Testing/Operation%20Castle/Operation%20Castle%20Bravo%20Blast.htm |archive-date=October 22, 2013 |access-date=August 25, 2013 |website=dgely.com}}</ref>

=== US Navy ships affected ===
The US Navy [[Tanker (ship)|tanker]] {{USS|Patapsco|AOG-1|6}} 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.<ref name="HRERNWI">{{Cite book |last1=Newton |first1=Richard G. |url=https://archive.org/details/humanradiationex00cudd |title=Human Radiation Exposures Related to Nuclear Weapons Industries |last2=Cuddihy |first2=George J. |date=September 1985 |publisher=[[Lovelace Respiratory Research Institute|Inhalation Toxicology Research Institute, Lovelace Biomedical & Environmental Research Institute]] |location=Albuquerque, New Mexico |page=[https://archive.org/details/humanradiationex00cudd/page/n124 109]}}</ref> 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.<ref name="HRERNWI" /> ''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 [[Roentgen (unit)|R/hr]].<ref name="HRERNWI" /> 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.<ref name="HRERNWI" />

=== International incident ===
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.<ref>{{Cite book |last=DeGroot |first=Gerard |title=The Bomb: A Life |date=2004 |publisher=Jonathan Cape |isbn=978-0-224-06232-9 |location=London |pages=196–198}}</ref>

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 [[tropics|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.<ref>{{Cite report |url=http://www.osti.gov/scitech/biblio/4279860 |title=World-Wide Fallout from Operation Castle |last=List |first=Robert J. |date=May 17, 1955 |doi=10.2172/4279860 |osti=4279860 |access-date=May 20, 2016}}</ref>

Stratospheric fallout particles of [[strontium-90]] from the test were later captured with balloon-borne [[air filter]]s used to sample the air at stratospheric altitudes; the research ([[Project Ashcan]]) was conducted to better understand the [[stratosphere]] and fallout times, and arrive at more accurate meteorological models after [[hindcast]]ing.<ref>{{Cite report |title=Analysis of Stratospheric Strontium{{sup|90}} Measurements |last1=Machta |first1=Lester |last2=List |first2=Robert J. |date=March 1, 1959 |osti=4225048 |author-link=Lester Machta |journal=[[Journal of Geophysical Research]]}}</ref>

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.<ref>{{Cite web |last=Winter |first=Mark |title=Caesium biological information |url=http://www.webelements.com/caesium/biology.html |access-date=May 20, 2016 |website=WebElements Periodic Table of the Elements}}</ref> 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.<ref>{{Cite book |last=Nash |first=Gary B. |title=The American People: Creating a Nation and a Society |date=2007 |publisher=Longman |isbn=978-0-205-80553-2 |edition=6th |location=New York |display-authors=etal}}{{page needed|date=November 2021}}</ref>

== Impact on US policy ==
The test caused a reassessment of US policies towards nuclear weapons and energy in order to contend with massive international backlash that declared the disaster "intolerable". The following year's [[Russell–Einstein Manifesto]] explicitly focused on the hydrogen bomb's threat to human existence demonstrated by the test:
{{Blockquote|text=No doubt in an [[Nuclear warfare|H-bomb war]] great cities would be obliterated ... If everybody in London, New York and Moscow were exterminated the world might, in the course of a few centuries, recover from the blow. But we now know, especially since the '''Bikini test''', that nuclear bombs can gradually spread destruction over a very much wider area than had been supposed.

It is stated on very good authority that a bomb can now be manufactured which will be 2,500 times as powerful (37.5 megatons) as that which destroyed Hiroshima (15 kilotons). Such a bomb, if exploded near the ground or under water, sends radio-active particles into the [[Stratosphere|upper air]]. They sink gradually and reach the surface of the earth in the form of a deadly dust or [[Black rain|rain]]. It was this dust which infected the Japanese fishermen and their catch of fish.|title=Russell-Einstein Manifesto, 9 July 1955}}
In May 1954 [[National Security Council (United States)|National Security Council]] meeting, President [[Dwight D. Eisenhower]] said "everybody seems to think that we are skunks, saber-rattlers, and warmongers." Secretary of State [[John Foster Dulles]] said "comparisons are now being made between ours and [[Adolf Hitler|Hitler]]'s military machine."<ref name="l317">{{cite web |last=Makhijani |first=Arjun |date=2024-12-13 |title=The entanglement of fusion energy research and bombs |url=https://thebulletin.org/premium/2024-11/the-entanglement-of-fusion-energy-research-and-bombs/ |access-date=2025-02-08 |website=Bulletin of the Atomic Scientists}}</ref>

Attempts to repair this image included the yield and fallout limiting of all future tests, and an emphasis on peaceful nuclear energy production, from both nascent fission reactor plants and speculative fusion implosion facilities.

=== Nuclear testing policies ===

==== Total yield and fission yield energy budgets ====
Following the disaster and higher-than-expected yields throughout Operation Castle (48.2 Mt total yield to the expected 23 Mt), US policy on thermonuclear testing in the Pacific changed. In 1956, [[Operation Redwing]] was conducted on an "energy budget", limiting the total testing yield to 20 Mt, and specifically limiting the fission yield, contentiously divided between [[Los Alamos Scientific Laboratory]] and [[University of California Radiation Laboratory at Livermore]].<ref name="t639">{{cite web |date=1997-10-22 |title=Operation Redwing |url=https://nuclearweaponarchive.org/Usa/Tests/Redwing.html |access-date=2025-01-17 |website=The Nuclear Weapon Archive}}</ref> While some very "dirty" fission weapons were tested, this also began the usage of the "materials substitution method", where the fission product fallout-producing uranium-238 [[Tamper (nuclear weapon)|tamper]] was replaced with a "clean" lead tamper, at the cost of halving the yield.

==== 15 megaton yield standard ====
In 1958, during preparation for [[Operation Hardtack I]], the second round of thermonuclear testing since Castle, President [[Dwight D. Eisenhower|Eisenhower]] established an unwritten rule that no single American test could exceed the 15 Mt yield of Castle Bravo. His successor, president [[John F. Kennedy]] adhered to this standard, even following the 50 Mt Soviet [[Tsar Bomba]] test in 1961 and pressure from the [[United States Department of Defense|Department of Defense]], [[United States Atomic Energy Commission|Atomic Energy Commission]], and the Livermore laboratory.<ref name="g657">{{cite journal |last=Grams |first=Jon |year=2021 |title=Ripple: An Investigation of the World's Most Advanced High-Yield Thermonuclear Weapon Design |journal=Journal of Cold War Studies |publisher=MIT Press - Journals |volume=23 |issue=2 |pages=133–161 |doi=10.1162/jcws_a_01011 |issn=1520-3972}}</ref> Following the 1963 [[Partial Nuclear Test Ban Treaty]] against non-underground tests, American testing continued underground, with the largest yield in the 1971 [[Cannikin|Grommet Cannikin]] test at 5 Mt.

=== Impact on scientific direction ===

==== Fission energy ====
{{Main|Atoms for Peace}}
The US government's attempts at [[public relations]] damage control focused around the existing language of the [[Atoms for Peace]] initiative launched four months prior, expanding to the concept of the "peaceful thermonuclear atom". Speaking at a conference six months later, [[Lewis Strauss]], the chair of the [[United States Atomic Energy Commission|Atomic Energy Commission]] and a primary influence in the crash program of hydrogen bomb development, famously promised a peaceful era of nuclear energy:
{{Blockquote|text=It is not too much to expect that our children will enjoy in their homes electrical energy [[too cheap to meter]], will know of great periodic regional famines in the world only as matter of history, will [[Nuclear marine propulsion|travel effortlessly over the seas and under them]] and [[Aircraft nuclear propulsion|through the air]] with a minimum of danger and at great speeds, and will experience a lifespan far longer than ours, as [[Nuclear medicine|disease yields]] and man comes to understand what causes him to age. This is the forecast for an age of peace.|author=[[Lewis Strauss]]|title=Speech to the [[National Association of Science Writers]], September 1954}}
The Soviet Union had already announced electrical grid connection of the [[Obninsk Nuclear Power Plant]] on 27 June. The United States would achieve this briefly with the [[BORAX-III]] reactor in 1955 and permanently with the [[Shippingport Atomic Power Station]] in 1957.

==== Fusion energy ====

===== Energy from conventional yields =====
{{Main|Project PACER}}
In 1957, hydrogen bomb architect [[Edward Teller]] and other Livermore weapons scientists proposed a gigawatt-level electrical plant, based on steam generation from 1 megaton bombs dropped every 12 hours into a 1,000&nbsp;ft cavity. American research on such plants continued throughout the Cold War.

===== Energy from minimal yields =====
{{Main|Inertial confinement fusion}}
Livermore scientist [[John Nuckolls]] began a "hectoton group" (100 tons) at the laboratory, investigating ways to remove the fission primary and inventing the concept of low-yield implosions of deuterium-tritium pellets i.e. [[inertial confinement fusion]].<ref name="l317" />

== Weapon history ==
{{more citations needed|section|date=February 2019}}
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 weapon|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&nbsp;[[TNT equivalent|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&nbsp;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 {{sup|238}}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]], [[Sakharov's Third Idea|developed]] and [[RDS-37|tested]] their first Teller–Ulam device in 1955.

The publication of the Bravo [[nuclear forensics|fallout analysis]] was a militarily sensitive issue, with [[Joseph Rotblat]] possibly deducing the [[Teller–Ulam design|staging nature]] of the Castle Bravo device by studying the ratio and presence of tell-tale isotopes, namely [[uranium-237]], present in the fallout.<ref>{{Cite book |last=Braun |first=Reiner |title=Joseph Rotblat: Visionary for Peace |date=2007 |publisher=Wiley-VCH |isbn=978-3-527-40690-6}}</ref> This information could potentially reveal the means by which megaton-yield nuclear devices achieve their yield.<ref name="scienceandglobalsecurity_Geer">{{Cite journal |last=Geer |first=Lars-Erik De |date=1991 |title=The Radioactive Signature of the Hydrogen Bomb |url=http://scienceandglobalsecurity.org/archive/sgs02degeer.pdf |journal=Science & Global Security |publisher=Gordon and Breach Science Publishers |volume=2 |issue=4 |pages=351–363 |bibcode=1991S&GS....2..351D |doi=10.1080/08929889108426372 |access-date=February 22, 2016}}</ref> Soviet scientist [[Andrei Sakharov]] hit upon what the Soviet Union regarded as "[[History of the Teller–Ulam design|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 {{convert|17600|lb}} and measuring {{convert|12.5|ft}} long and {{convert|58|in|m}} in diameter. This 18-megaton bomb was produced until July 1956.<ref>Strategic Air Command History Development of Atomic Weapons 1956, pp. 29, 39</ref> In 1957, it was converted into the [[Mark 36 nuclear bomb]] and entered into production again.
{{Clear}}

== Health impacts ==
[[File:Project 4.1 figures.png|thumb|Page 36 from the Project 4.1 final report, showing four photographs of exposed Marshallese. Faces blotted out for privacy reasons.]]
Following the test, the [[United States Department of Energy]] estimated that 253 inhabitants of the [[Marshall Islands]] were impacted by the radioactive fallout.<ref>{{Cite journal |last1=Lauerman |first1=John F. |last2=Reuther |first2=Christopher |date=September 1997 |title=Trouble in Paradise |journal=Environmental Health Perspectives |volume=105 |issue=9 |pages=914–917 |doi=10.2307/3433870 |jstor=3433870 |pmc=1470349 |pmid=9341101}}</ref> 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.<ref name=":2">{{Cite journal |date=January 1, 1966 |title=Fallout Radiation And Growth |journal=The British Medical Journal |volume=1 |issue=5496 |page=1132 |doi=10.1136/bmj.1.5496.1132-a |jstor=25407693 |pmc=1844058 |pmid=20790967}}</ref>{{failed verification|date=August 2018}} Populations neighboring the test site were exposed to high levels of radiation resulting in mild radiation sickness of many (nausea, vomiting, diarrhea). The unexpected strength of the detonation, combined with shifting wind patterns, sent some of the radioactive fallout over the inhabited atolls of [[Rongelap Atoll|Rongelap]] and [[Utrik Atoll|Utrik]]. Within 52 hours, the 86 people on Rongelap and 167 on Utrik were evacuated to [[Kwajalein Atoll|Kwajalein]] for medical care.<ref>{{cite web |title=The Legacy of U.S. Nuclear Testing and Radiation Exposure in the Marshall Islands |url=https://mh.usembassy.gov/the-legacy-of-u-s-nuclear-testing-and-radiation-exposure-in-the-marshall-islands/ |publisher=[[United States Embassy|U.S. Embassy]] in the Republic of the Marshall Islands |access-date=8 July 2024 |date=15 September 2012}}</ref> Several weeks later, many people began suffering from [[alopecia]] (hair loss) and skin lesions.<ref>{{Cite journal |date=January 1, 1955 |title=Radioactive Fallout in the Marshall Islands |journal=Science |volume=122 |issue=3181 |pages=1178–1179 |bibcode=1955Sci...122.1178. |doi=10.1126/science.122.3181.1178 |jstor=1749478 |pmid=17807268}}</ref>

The exposure to fallout has been linked to increase the likelihood of several types of cancer such as [[leukemia]] and [[thyroid cancer]].<ref name="StrangeGlow">{{Cite book |last=Jorgensen |first=Timothy J. |title=Strange Glow: The Story of Radiation |publisher=[[Princeton University Press]] |year=2017 |isbn=978-0-691-17834-9}}</ref><ref name=":0">{{Cite journal |last1=Simon |first1=Steven L. |last2=Bouville |first2=André |last3=Land |first3=Charles E. |date=January 1, 2006 |title=Fallout from Nuclear Weapons Tests and Cancer Risks: Exposures 50 years ago still have health implications today that will continue into the future |journal=[[American Scientist]] |volume=94 |issue=1 |pages=48–57 |url=https://www.americanscientist.org/article/fallout-from-nuclear-weapons-tests-and-cancer-risks |access-date=8 July 2024 |doi=10.1511/2006.57.982 |jstor=27858707|url-access=subscription }}</ref> The relationship between [[iodine-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.<ref name=":0" />

There is a presumed association between radiation levels and functioning of the female reproductive system.<ref>{{Cite journal |last1=Grossman |first1=Charles M. |last2=Morton |first2=William E. |last3=Nussbaum |first3=Rudi H. |last4=Goldberg |first4=Mark S. |last5=Mayo |first5=Nancy E. |last6=Levy |first6=Adrian R. |last7=Scott |first7=Susan C. |date=January 1, 1999 |title=Reproductive Outcomes after Radiation Exposure |journal=Epidemiology |volume=10 |issue=2 |pages=202–203 |doi=10.1097/00001648-199903000-00024 |jstor=3703102 |pmid=10069262 |doi-access=free}}</ref>

== In popular culture ==
The Castle Bravo detonation and the subsequent poisoning of the crew aboard ''[[Daigo Fukuryū Maru]]'' led to an increase in antinuclear protests in Japan. It was compared to the [[Atomic bombings of Hiroshima and Nagasaki|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]].<ref>{{Cite book |last=Brothers |first=Peter H. |title=Mushroom Clouds and Mushroom Men: The Fantastic Cinema of Ishiro Honda |date=2009 |publisher=[[AuthorHouse]]}}</ref> In the 2019 film ''[[Godzilla: King of the Monsters (2019 film)|Godzilla: King of the Monsters]]'', Castle Bravo becomes the call sign for Monarch Outpost 54 located in the Atlantic Ocean, near [[Bermuda]].{{Citation needed|date=June 2019}}

The [[Donald Fagen]] song "Memorabilia" from his 2012 album ''[[Sunken Condos]]'' mentions both the Castle Bravo and [[Ivy King]] nuclear tests.<ref>{{Citation |title=Donald Fagen – Memorabilia |url=https://genius.com/Donald-fagen-memorabilia-lyrics |access-date=October 31, 2018}}</ref>

In 2013, the [[Defense Threat Reduction Agency]] published ''Castle Bravo: Fifty Years of Legend and Lore''.<ref name="DTRIAC SR-12-001" /> The report is a guide to off-site radiation exposures, a narrative history, and a guide to primary historical references concerning the Castle Bravo test.<ref name="DTRIAC SR-12-001" />  The report focuses on the circumstances that resulted in radioactive exposure of the uninhabited atolls, and makes no attempt to address in detail the effects on or around Bikini Atoll.<ref name="DTRIAC SR-12-001" />

== Gallery ==
<gallery class="center" widths="180">
File:Military Effects Studies on Operation Castle (1954).webm|De-classified military effects-studies for Operation Castle (1954).
File:Castle Bravo Blast.jpg
File:Bikini Atoll 2001-01-14, Landsat 7 ETM+ bands 7-5-1-8.png|''Bravo'' crater, relative to [[Bikini Atoll]].
File:Castle Bravo nuclear test (cropped).jpg
File:HD.10.290 (10540757794).jpg
</gallery>

== See also ==
{{Portal|Nuclear technology}}
* [[History of nuclear weapons]]
* {{anl|Operation Ivy}}
* {{anl|Tsar Bomba}}
* {{anl|Moruroa#French nuclear weapons testing|Moruroa|wedge=nuclear weapons testing }}

== References ==
; Notes
{{reflist|group=Note}}

; Citations
{{Reflist|30em}}

; Bibliography
{{Refbegin|2}}
* Chuck Hansen, ''U. S. Nuclear Weapons: The Secret History'' (Arlington: AeroFax, 1988)
* Holly M. Barker, ''Bravo for the Marshallese: Regaining control in a Post-Nuclear, Post Colonial World'' (Belmont, CA: Wadsworth, 2004)
* {{usurped|1=[https://web.archive.org/web/20160424042410/http://www.rmiembassyus.org/Nuclear%20Issues.htm Republic of the Marshall Islands Embassy website]}}
* {{Cite journal |last1=Cronkite, E. P. |last2=Conard, R. A. |last3=Bond, V. P. |year=1997 |title=Historical events associated with fallout from Bravo Shot&nbsp;– Operation Castle and 25 Y of medical findings |url=http://www.health-physics.com/pt/re/healthphys/abstract.00004032-199707000-00014.htm |journal=Health Physics |volume=73 |issue=1 |pages=176–186 |doi=10.1097/00004032-199707000-00014 |pmid=9199227|bibcode=1997HeaPh..73..176C |url-access=subscription }}
{{Refend}}

== External links ==
{{Commons category|Castle Bravo}}
* {{Internet Archive short film|id=CastleCommandersReport1954|name=Operation Castle Commanders Report (1954)}}
* {{Internet Archive short film|id=MilitaryEffectsStudiesonOperationCastle1954|name=Military Effects Studies Operation Castle (1954)}}
* {{Internet Archive short film|id=gov.doe.0800013|name=Nuclear Test Film – Operation Castle (1954)}}
* [https://news.bbc.co.uk/onthisday/hi/dates/stories/march/1/newsid_2781000/2781419.stm US tests hydrogen bomb in Bikini] ([[BBC News]])
* {{usurped|1=[https://archive.today/20130105003623/http://www.sonicbomb.com/modules.php?name=News&file=article&sid=104 First-person article about conducting the test]}}
* [https://www.scribd.com/document/59537319 Strategic Air Command History – Development of Atomic Weapons 1956]

{{Nuclear weapons tests of the United States}}

[[Category:1950s in the Marshall Islands]]
[[Category:1954 disasters in Oceania]]
[[Category:1954 in military history]]
[[Category:1954 in the environment]]
[[Category:1954 in the Trust Territory of the Pacific Islands]]
[[Category:Explosions in 1954]]
[[Category:March 1954 in Oceania]]
[[Category:Nuclear accidents and incidents]]
[[Category:Nuclear testing at Bikini Atoll]]
[[Category:Ralik Chain]]