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Com a sua missão inicial já completa (projetar armas termonucleares), Livermore tentou projetos radicais que falharam. Seus primeiros três testes nucleares foram falhas: em 1953, duas armas de hidreto de urânio e em 1954 uma arma bifásica teve seu secundário prematuramente aquecido pela radiação que a implosão não trabalhou de forma correta.
Mudando de assunto, o Livermore resolveu pegar ideias arquivadas de Los Alamos para o exército e a marinha. Isso levou Livermore a se especializar em armas táticas de pequeno diâmetro, particularmente algumas usando usando um sistema de implosão linear de dois pontos como por exemplo o [[Swan (arma nuclear|Swan]]. Armas táticas de pequeno diâmetro se tornaram primários para secundários de pequenos diâmetros. Por volta de 1960, quando a corrida armamentista entre as superpotências tornou-se uma corrida de mísseis balísticos, as ogivas nucleares de Livermore foram mais úteis que as grandes e pesadas ogivas nucleares do Laboratório Nacional de Los Alamos. As ogivas de Los Alamos foram utilizadas no primeiro [[míssil balístico de alcance intermediário]], IRBMs, mas as ogivas pequeninas de Livermore foram utilizadas no primeiro [[míssil balístico intercontinental]], ICBM, dos E.U.A e no [[míssil balístico lançado por submarino]], SLBM, dos E.U.A. Como também foram utilizadas nos primeiros [[MIRV]]s de ambos os mísseis.<ref>Sybil Francis, ''Warhead Politics: Livermore and the Competitive System of Nuclear Warhead Design'', UCRL-LR-124754, June 1995, Ph.D. Dissertation, Massachusetts Institute of Technology, available from National Technical Information Service. This 233-page thesis was written by a weapons-lab outsider for public distribution. The author had access to all the classified information at Livermore that was relevant to her research on warhead design; consequently, she was required to use non-descriptive code words for certain innovations.</ref>
Mudando de assunto, o Livermore resolveu pegar ideias arquivadas de Los Alamos para o exército e a marinha. Isso levou Livermore a se especializar em armas táticas de pequeno diâmetro, particularmente algumas usando usando um sistema de implosão linear de dois pontos.
Em 1957 e 1958, ambos os laboratórios construíram e testaram quase todos as configurações e conceitos possíveis, em antecipação a previsão de que o tratado de proibição de testes nucleares de 1958 fosse permanente. Mas os testes recomeçaram em 1961 em resposta aos testes multimegatons soviéticos, incluindo a Tsar Bomba. Neste tempo os dois laboratórios pareciam duplicatas criando projetos muito semelhantes e passaram a cooperar de forma a atingir maiores resultados a um menor custo. Muitos projetos foram trocados. Por exemplo, a ogive [[W38]] para o míssil [[Titan]] I começou no Livermore, continuou em Los Alamos quando se tornou a ogiva do míssil [[SM-65 Atlas]] e em 1959 foi dado novamente ao Livermore, que trocou com Los Alamos o projeto da [[W54]], esta que passou então de Livermore para Los Alamos.
O período de real inovação estava acabando até então, de qualquer forma. Quando depois da década de 1960 cada míssil novo requeria uma nova ogiva por razões de marketing. E uma nova forma de aumentar o rendimento foi descoberta envolvendo o primário e o secundário com uma jaqueta de urânio, pobre ou enriquecido, o urânio enriquecido provinha geralmente do desmantelamento das ogivas multimegatons antigas de alto rendimento.
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== Teste explosivo ==
Shifting gears, Livermore settled for taking ideas Los Alamos had shelved and developing them for the Army and Navy. This led Livermore to specialize in small-diameter tactical weapons, particularly ones using two-point implosion systems, such as the Swan. Small-diameter tactical weapons became primaries for small-diameter secondaries. Around 1960, when the superpower arms race became a ballistic missile race, Livermore warheads were more useful than the large, heavy Los Alamos warheads. Los Alamos warheads were used on the first [[intermediate-range ballistic missile]]s, IRBMs, but smaller Livermore warheads were used on the first [[intercontinental ballistic missile]]s, ICBMs, and [[submarine-launched ballistic missile]]s, SLBMs, as well as on the first [[multiple independently targetable reentry vehicle|multiple warhead]] systems on such missiles.<ref>Sybil Francis, ''Warhead Politics: Livermore and the Competitive System of Nuclear Warhead Design'', UCRL-LR-124754, June 1995, Ph.D. Dissertation, Massachusetts Institute of Technology, available from National Technical Information Service. This 233-page thesis was written by a weapons-lab outsider for public distribution. The author had access to all the classified information at Livermore that was relevant to her research on warhead design; consequently, she was required to use non-descriptive code words for certain innovations.</ref>
Armas nucleares são em geral desenhados e projetadas por tentativa e erro. A tentativa envolve a detonação de um protótipo.
Em uma explosão nuclear, um grande número de eventos discretos, com várias possibilidades, agregam-se em um evento caótico de curta duração no interior do dispositivo, Complexos modelos matemáticos são requeridos para o processo de aproximação, e na década de 1950 não haviam computadores potentes o suficiente para realizar estas tarefas. Mesmo hoje, a simulação em supercomputadores não é adequada e para uma grande confiabilidade é necessário um testes nuclear.<ref>Walter Goad, [http://www.fas.org/irp/ops/ci/goad.html Declaration for the Wen Ho Lee case], May 17, 2000. Goad began thermonuclear weapon design work at Los Alamos in 1950. In his Declaration, he mentions "basic scientific problems of computability which cannot be solved by more computing power alone. These are typified by the problem of long range predictions of weather and climate, and extend to predictions of nuclear weapons behavior. This accounts for the fact that, after the enormous investment of effort over many years, weapons codes can still not be relied on for significantly new designs."</ref>
In 1957 and 1958 both labs built and tested as many designs as possible, in anticipation that a planned 1958 test ban might become permanent. By the time testing resumed in 1961 the two labs had become duplicates of each other, and design jobs were assigned more on workload considerations than lab specialty. Some designs were horse-traded. For example, the [[W38]] warhead for the [[Titan (rocket family)|Titan]] I missile started out as a Livermore project, was given to Los Alamos when it became the [[SM-65 Atlas|Atlas]] missile warhead, and in 1959 was given back to Livermore, in trade for the [[W54]] [[Davy Crockett (nuclear device)|Davy Crockett]] warhead, which went from Livermore to Los Alamos.
Foi fácil o suficiente projetar armas lançáveis para o estoque. Se o protótipo funcionasse, ela poderia ser militarizado e produzido em massa.
The period of real innovation was ending by then, anyway. Warhead designs after 1960 took on the character of model changes, with every new missile getting a new warhead for marketing reasons. The chief substantive change involved packing more fissile uranium into the secondary, as it became available with continued [[uranium enrichment]] and the dismantlement of the large high-yield bombs.
O que foi muito difícil era entender como isso funcionava e porque eles falhavam. Projetistas reuniam todos os dados possíveis durante as detonações, antes da arma detonar, e usaram os dados para calibrar seus modelos, apenas pela inserção de fatore de correção nas equações para fazer simulações alcançarem os resultados. Eles também analisavam a precipitação radiativa para ver o quão bem os combustível foi consumido e quanto de seu potencial ele atingiu.
==Explosive testing==
=== Tubos de luz ==
Nuclear weapons are in large part designed by trial and error. The trial often involves test explosion of a prototype.
Uma importante ferramenta para a análise de testes foi o diagnóstico de tubo de luz. Uma sonda dentro do dispositivo em teste poderia transmitir informação aquecendo uma placa de metal pela incandescência, um evento que poderia ser gravado por um tubo de luz.
A foto abaixo mostra o dispositivo camarão, detonado em 1 de março de 1954, em Bikini, como o teste [[Castle Bravo]]. Sua explosão de 15 megatons foi a maior já conduzida pelos Estados Unidos. A silhueta de um homem é mostrada por comparação. O dispositivo é alimentado por baixo, nas extremidades. Os tubos que aparecem encima do dispositivo que parecem alimentadores são na verdade tubos de luz. Os oito tubos na direita (1) envia informações da detonação do primário. Dois no meio (2) marcaram o tempo em que os raios x do primário atingiram o canal de radiação do secundário. Os dois últimos tubos (3) notaram o tempo em que a radiação atingiu o fim do canal de radiação, a diferença (2) e (3) sendo o tempo de transição da radiação pelo canal.<ref>Chuck Hansen, ''The Swords of Armageddon'', Volume IV, pp. 211–212, 284.</ref>
In a nuclear explosion, a large number of discrete events, with various probabilities, aggregate into short-lived, chaotic energy flows inside the device casing. Complex mathematical models are required to approximate the processes, and in the 1950s there were no computers powerful enough to run them properly. Even today's computers and simulation software are not adequate.<ref>Walter Goad, [http://www.fas.org/irp/ops/ci/goad.html Declaration for the Wen Ho Lee case], May 17, 2000. Goad began thermonuclear weapon design work at Los Alamos in 1950. In his Declaration, he mentions "basic scientific problems of computability which cannot be solved by more computing power alone. These are typified by the problem of long range predictions of weather and climate, and extend to predictions of nuclear weapons behavior. This accounts for the fact that, after the enormous investment of effort over many years, weapons codes can still not be relied on for significantly new designs." </ref>
[[File:Castle Bravo Shrimp composite.png|600 px|centre]]
Do shot cab, os tubos tornam-se horizontais e viajam 2,3 km por uma via construída no atol de Bikini par um coletor de dados protegido por um bunker na Ilha Namu.
It was easy enough to design reliable weapons for the stockpile. If the prototype worked, it could be weaponized and mass produced.
Enquanto os raios-x normalmente viajam na velocidade da luz por um material de baixa densidade como a espuma de plástico entre (2) e (3), a intensidade da radiação do primário detonando criou uma radiação relativamente opaca que agiu como um impasse lento para retardar a passagem de radiação. Enquanto o secundário está sendo comprimido via radiação induzida pela ablação, nêutrons do primário junto com os raios x penetram o secundário abaixo e criam trítio com a terceira reação listada na primeira seção acima. A reação de Li-6 + n é exotérmica produzindo 5 MeV por evento. A vela de ignição ainda não está comprimida e por isso não está crítica, então não haverá fissão ou fusão suficiente. Mas se nêutrons suficientes chegam antes da implosão do secundário estar completa, a crucial diferença de temperatura será degradada. Essa foi a causa reportada da falha do primeiro projeto termonuclear de Livermore, o dispositivo Morgenstern, testado como [[Castle Koon]], em 7 de abril de 1954.
It was much more difficult to understand how it worked or why it failed. Designers gathered as much data as possible during the explosion, before the device destroyed itself, and used the data to calibrate their models, often by inserting [[fudge factor]]s into equations to make the simulations match experimental results. They also analyzed the weapon debris in fallout to see how much of a potential nuclear reaction had taken place.
Esses problemas relacionados ao tempo são medidos pelos tubos de luz. As simulações matemáticas que eles calibram são chamadas códigos hidrodinâmicos de fluxo de radiação, ou códigos de canal. Eles são usados para prever o efeito de modificações em projeto futuros.
===Light pipes===
An important tool for test analysis was the diagnostic light pipe. A probe inside a test device could transmit information by heating a plate of metal to incandescence, an event that could be recorded at the far end of a long, very straight pipe.
Não é claro para o público o quão bem sucedidos foram os tubos de luz do camarão. O bunker estava longe o suficiente para ficar fora da cratera de uma milha, mas a detonação de 15 megatons, 2,5 vezes o esperado, violou o bunker lançando a sua port de 20 toneladas dentro do bunker.(as pessoas mais próximas da detonação estavam a 32 km de distância em um bunker que sobreviveu intacto).<ref>Dr. John C. Clark, as told to Robert Cahn, "We Were Trapped by Radioactive Fallout," ''The Saturday Evening Post'', July 20, 1957, pp. 17–19, 69–71.</ref>
The picture below shows the Shrimp device, detonated on March 1, 1954 at Bikini, as the [[Castle Bravo]] test. Its 15-megaton explosion was the largest ever by the United States. The silhouette of a man is shown for scale. The device is supported from below, at the ends. The pipes going into the shot cab ceiling, which appear to be supports, are diagnostic light pipes. The eight pipes at the right end (1) sent information about the detonation of the primary. Two in the middle (2) marked the time when x-radiation from the primary reached the radiation channel around the secondary. The last two pipes (3) noted the time radiation reached the far end of the radiation channel, the difference between (2) and (3) being the radiation transit time for the channel.<ref>Chuck Hansen, ''The Swords of Armageddon'', Volume IV, pp. 211-212, 284.</ref>
[[Ficheiro:Castle Bravo Shrimp composite.png|600 px|centre]]
=== Análise de precipitação nuclear ===
From the shot cab, the pipes turned horizontal and traveled 7500 ft (2.3 km), along a causeway built on the Bikini reef, to a remote-controlled data collection bunker on Namu Island.
O mais interessante dado sobre o Castle Bravo veio de uma análise radio-química de cinzas nucleares na precipitação. Por causa da escassez do enriquecimento do lítio 6, 60% do lítio no secundário do camarão era lítio-7, que não se transforma em trítio com a facilidade que o lítio 6. Mas ele não faz como o lítio 6, em que é necessário um nêutron para que se crie dois, o lítio 7 havia uma proporção desconhecida, que provou se alta.
Como notado acima, a precipitação nuclear do Castle Bravo, contou para o Mundo exterior, pela primeira vez, que bombas termonucleares eram mais dispositivos de fissão que dispositivo s de fusão nuclear. Um barco de pesca japonês, navegou de volta para casa com radiação suficiente no convés para que cientistas do Japão e do Mundo todo determinassem e anunciassem que a maioria da precipitação nuclear veio da fissão rápida do [[Urânio-238]] por nêutron energéticos de 14 MeV.
While x-rays would normally travel at the speed of light through a low density material like the plastic foam channel filler between (2) and (3), the intensity of radiation from the exploding primary created a relatively opaque radiation front in the channel filler which acted like a slow-moving logjam to retard the passage of radiant energy. Behind this moving front was a fully-ionized, low-z (low atomic number) plasma heated to 20,000 °C, soaking up energy like a black box, and eventually driving the implosion of the secondary.<ref>The public literature mentions three different force mechanism for this implosion: radiation pressure, plasma pressure, and explosive ablation of the outer surface of the secondary pusher. All three forces are present; and the relative contribution of each is one of the things the computer simulations try to explain. See [[Teller-Ulam design]].</ref>
=== Teste subterrâneo ===
The radiation transit time, on the order of half a microsecond, is the time it takes the entire radiation channel to reach thermal equilibrium as the radiation front moves down its length. The implosion of the secondary is based on the temperature difference and resulting pressure difference between the hot channel and the cool interior of the secondary. Its timing is important because the interior of the secondary is subject to neutron preheat.
[[Ficheiro:Nevada Test Site craters.jpg|thumb|right|Crateras subterrâneas em Yucca Flat, Nevada Test Site.]]
O alarme global sobre a precipitação nuclear, que começou com o evento Castle Bravo, eventualmente fez com os testes nucleares tivessem de ocorrer literalmente nos subterrâneos. O último teste nuclear dos E.U.A sobre o solo foi na [[Ilha Johnston]] em 4 de novembro de 1962. Durante as próximas três décadas, até 23 de setembro de 1992, os E.U.A conduziram uma média de 2,4 detonações explosões nucleares por mês, mas alguns no [[Nevada Test Site]] (NTS) no noroeste de Las Vegas.
A [[Yucca Flat]], seção do NTS, é coberto por subsidentes crateras resultadas do colapso de terra sobre as cavernas radiativas criadas por detonações nucleares (veja a foto).
While the radiation channel is heating and starting the implosion, neutrons from the primary catch up with the x-rays, penetrate into the secondary and start breeding tritium with the third reaction noted in the first section above. This Li-6 + n reaction is exothermic, producing 5 MeV per event. The spark plug is not yet compressed and thus is not critical, so there won't be significant fission or fusion. But if enough neutrons arrive before implosion of the secondary is complete, the crucial temperature difference will be degraded. This is the reported cause of failure for Livermore's first thermonuclear design, the Morgenstern device, tested as [[Castle Koon]], April 7, 1954.
Depois de 1974, o [[Tratado de Proibição de Testes de Threshold]] (TTBT), que limitou os testes subterrâneos a 150 quilotons ou menos, ogivas como a [[W88]] com quase meio megaton de poder tiverem de ser testadas sem o seu potencial máximo para respeitar o tratado. Desde que o primário seja detonado em potencial máximo para recolher dados sobre a implosão do secundário, a redução do rendimento poderia vir do secundário. Substituindo o combustível para fissão lítio-6 e deutério com hidreto de lítio-7, e portanto o rendimento total. O funcionamento do dispositivo ode ser avaliado usando tubos de luz, outros dispositivos sensíveis, e análise do resto da arma. O rendimento total da arma que será produzida em massa pode ser calculada pela extrapolação.
These timing issues are measured by light-pipe data. The mathematical simulations which they calibrate are called radiation flow hydrodynamics codes, or channel codes. They are used to predict the effect of future design modifications.
== Unidades de produção ==
It is not clear from the public record how successful the Shrimp light pipes were. The data bunker was far enough back to remain outside the mile-wide crater, but the 15-megaton blast, two and a half times greater than expected, breached the bunker by blowing its 20-ton door off the hinges and across the inside of the bunker. (The nearest people were twenty miles (32 km) farther away, in a bunker that survived intact.)<ref>Dr. John C. Clark, as told to Robert Cahn, "We Were Trapped by Radioactive Fallout," ''The Saturday Evening Post'', July 20, 1957, pp. 17-19, 69-71.[http://www.aracnet.com/~pdxavets/wetokian/trap1.htm]</ref>
Quando as armas bifásicas se tornaram padrão no começo da década de 1950, os projetos de armas determinaram o layout de novas e dispersas unidades de produção pelos E.U.A.
Porque os primários tendem a serem volumosos, especialmente em diâmetro, plutônio é o material físsil escolhido para fossos, com refletores de berílio. Ele tem uma massa crítica menor que a do urânio. A unidade de [[Rock Flats]] próximo de Boulder, Colorado, foi construída em 1952 para a produção e consequentemente se tornou a fábrica de plutônio e berílio.
===Fallout analysis===
A usina Y-12 em [[Oak Ridge]], [[Tennessee]], onde [[espetrômetros de massa]] chamados de [[calutron]]s enriqueceram o urânio para o [[Projeto Manhattan]], foi redesignado para a criação de secundários. O físsil U-235 cria as melhores velas de ignição porque sua massa crítica é grande, especialmente em formato cilíndrico para secundários das primeiras armas nucleares bifásicas. Os primeiros experimentos usaram os dois materiais físseis em combinação, como um composto de U-Pu para fossos e velas de ignição, mas para a produção em massa, foi mais fácil especializar as fábricas: fossos de plutônio nos primários e urânio enriquecido nas velas de ignição e impulsionadores nos secundários.
The most interesting data from Castle Bravo came from radio-chemical analysis of weapon debris in fallout. Because of a shortage of enriched lithium-6, 60% of the lithium in the Shrimp secondary was ordinary lithium-7, which doesn't breed tritium as easily as lithium-6 does. But it does breed lithium-6 as the product of an (n, 2n) reaction (one neutron in, two neutrons out), a known fact, but with unknown probability. The probability turned out to be high.
Y-12 fez a liga de lítio-6 e deutério como combustível da fusão nuclear e partes de U-238, os outros dois compostos do secundário.
Fallout analysis revealed to designers that, with the (n, 2n) reaction, the Shrimp secondary effectively had two and half times as much lithium-6 as expected. The tritium, the fusion yield, the neutrons, and the fission yield were all increased accordingly.<ref>Richard Rhodes, ''Dark Sun; the Making of the Hydrogen Bomb'', Simon and Schuster, 1995, p. 541.</ref>
A usina de [[Savannah River]] em [[Aiken]], [[Carolina do Sul]], também construída em 1952, operou reatores nucleares que convertiam U-238 em Pu-239 para fossos, e converteu lítio-6 (produzido em Y-12) em trítio para o gás impulsionador. Desde que seus reatores são moderados com [[água pesada]], [[óxido de deutério]], ela também fez deutério para o gás impulsionador e para a Y-12 criar lítio-6 e deutério (a liga).
As noted above, Bravo's fallout analysis also told the outside world, for the first time, that thermonuclear bombs are more fission devices than fusion devices. A Japanese fishing boat, the ''[[Daigo Fukuryū Maru|Lucky Dragon]]'', sailed home with enough fallout on its decks to allow scientists in Japan and elsewhere to determine, and announce, that most of the fallout had come from the fission of U-238 by fusion-produced 14 MeV neutrons.
== Projeto de segurança de ogivas nucleares ==
===Underground testing===
Porque mesmo as ogivas nucleares de baixo rendimento têm um poder de destruição surpreendente, projetistas de armas sempre reconheceram a necessidade de incorporar mecanismos e associar procedimentos intentados para prevenir detonações acidentais.
[[File:Steel balls png.png|thumb|right|300px|Um diagrama da ogiva''[[Yellow Sun|Green Grass]]'' que usava bolas de aço que poderiam ser inseridas e retiradas do núcleo]]
[[Ficheiro:Nevada Test Site craters.jpg|thumb|right|Subsidence Craters at Yucca Flat, Nevada Test Site.]]
The global alarm over radioactive fallout, which began with the Castle Bravo event, eventually drove nuclear testing underground. The last U.S. above-ground test took place at [[Johnston Island]] on November 4, 1962. During the next three decades, until September 23, 1992, the United States conducted an average of 2.4 underground nuclear explosions per month, all but a few at the [[Nevada Test Site]] (NTS) northwest of Las Vegas.
The [[Yucca Flat]] section of the NTS is covered with subsidence craters resulting from the collapse of terrain over radioactive underground caverns created by nuclear explosions (see photo).
After the 1974 [[Threshold Test Ban Treaty]] (TTBT), which limited underground explosions to 150 kilotons or less, warheads like the half-megaton W88 had to be tested at less than full yield. Since the primary must be detonated at full yield in order to generate data about the implosion of the secondary, the reduction in yield had to come from the secondary. Replacing much of the lithium-6 deuteride fusion fuel with lithium-7 hydride limited the deuterium available for fusion, and thus the overall yield, without changing the dynamics of the implosion. The functioning of the device could be evaluated using light pipes, other sensing devices, and analysis of trapped weapon debris. The full yield of the stockpiled weapon could be calculated by extrapolation.
==Production facilities==
When two-stage weapons became standard in the early 1950s, weapon design determined the layout of the new, widely dispersed U.S. production facilities, and vice versa.
Because primaries tend to be bulky, especially in diameter, plutonium is the fissile material of choice for pits, with beryllium reflectors. It has a smaller critical mass than uranium. The [[Rocky Flats Plant|Rocky Flats]] plant in Boulder, Colorado, was built in 1952 for pit production and consequently became the plutonium and beryllium fabrication facility.
The Y-12 plant in [[Oak Ridge]], [[Tennessee]], where [[mass spectrometers]] called [[Calutrons]] had enriched uranium for the [[Manhattan Project]], was redesigned to make secondaries. Fissile U-235 makes the best spark plugs because its critical mass is larger, especially in the cylindrical shape of early thermonuclear secondaries. Early experiments used the two fissile materials in combination, as composite Pu-Oy pits and spark plugs, but for mass production, it was easier to let the factories specialize: plutonium pits in primaries, uranium spark plugs and pushers in secondaries.
Y-12 made lithium-6 deuteride fusion fuel and U-238 parts, the other two ingredients of secondaries.
The [[Savannah River]] plant in [[Aiken, South Carolina|Aiken]], [[South Carolina]], also built in 1952, operated [[nuclear reactors]] which converted U-238 into Pu-239 for pits, and converted lithium-6 (produced at Y-12) into tritium for booster gas. Since its reactors were moderated with heavy water, deuterium oxide, it also made deuterium for booster gas and for Y-12 to use in making lithium-6 deuteride.
==Warhead design safety==
[[Ficheiro:Steel balls png.png|thumb|right|300px|A diagram of the ''[[Yellow Sun|Green Grass]]'' warhead's steel ball-bearing safety device, shown left, filled (safe) and right, empty (live). The steel balls were emptied into a hopper underneath the aircraft before flight, and could be re-inserted using a funnel by rotating the bomb on its trolley and raising the hopper.]]
*'''Gun-type weapons'''
It is inherently dangerous to have a weapon containing a quantity and shape of fissile material which can form a critical mass through a relatively simple accident. Because of this danger, the high explosives in [[Little Boy]] (four bags of [[Cordite]]) were inserted into the bomb in flight, shortly after takeoff on August 6, 1945. It was the first time a gun-type nuclear weapon had ever been fully assembled.
Also, if the weapon falls into water, the [[neutron moderator|moderating]] effect of the [[Light water reactor|water]] can also cause a [[criticality accident]], even without the weapon being physically damaged.
Gun-type weapons have always been inherently unsafe.
*'''In-flight pit insertion'''
Neither of these effects is likely with implosion weapons since there is normally insufficient fissile material to form a critical mass without the correct detonation of the lenses. However, the earliest implosion weapons had pits so close to criticality that accidental detonation with some nuclear yield was a concern.
On August 9, 1945, [[Fat Man]] was loaded onto its airplane fully assembled, but later, when levitated pits made a space between the pit and the tamper, it was feasible to utilize in-flight pit insertion. The bomber would take off with no fissile material in the bomb. Some older implosion-type weapons, such as the US [[Mark 4 nuclear bomb|Mark 4]] and [[Mark 5 nuclear bomb|Mark 5]], used this system.
In-flight pit insertion will not work with a hollow pit in contact with its tamper.
*'''Steel ball safety method'''
As shown in the diagram, one method used to decrease the likelihood of accidental detonation used metal balls. The balls were emptied into the pit; this would prevent detonation by increasing density of the hollowed pit. This design was used in the Green Grass weapon, also known as the Interim Megaton Weapon, which was used in the [[Violet Club]] and [[Yellow Sun|Yellow Sun Mk.1]] bombs.
[[Ficheiro:Swan One Point Test.png|right]]
*'''Chain safety method'''
Alternatively, the pit can be "safed" by having its normally-hollow core filled with an inert material such as a fine metal chain, possibly made of [[cadmium]] to absorb neutrons. While the chain is in the center of the pit, the pit can not be compressed into an appropriate shape to fission; when the weapon is to be armed, the chain is removed. Similarly, although a serious fire could detonate the explosives, destroying the pit and spreading plutonium to contaminate the surroundings as has happened in [[List of nuclear accidents|several weapons accidents]], it could not however, cause a nuclear explosion.
*'''Wire safety method'''
The US ''[[W47]]'' warhead used in ''[[Polaris missile|Polaris A1]]'' and ''[[Polaris missile|Polaris A2]]'' had a safety device consisting of a [[boron]]-coated wire inserted into the hollow pit at manufacture. The warhead was armed by withdrawing the wire onto a spool driven by an electric motor. However, once withdrawn the wire could not be re-inserted.<ref>Chuck Hansen, ''The Swords of Armageddon'', Volume VII, pp. 396-397.</ref>
*'''One-point safety'''
While the firing of one detonator out of many will not cause a hollow pit to go critical, especially a low-mass hollow pit that requires boosting, the introduction of two-point implosion systems made that possibility a real concern.
In a two-point system, if one detonator fires, one entire hemisphere of the pit will implode as designed. The high-explosive charge surrounding the other hemisphere will explode progressively, from the equator toward the opposite pole. Ideally, this will pinch the equator and squeeze the second hemisphere away from the first, like toothpaste in a tube. By the time the explosion envelops it, its implosion will be separated both in time and space from the implosion of the first hemisphere. The resulting dumbbell shape, with each end reaching maximum density at a different time, may not become critical.
Unfortunately, it is not possible to tell on the drawing board how this will play out. Nor is it possible using a dummy pit of U-238 and high-speed x-ray cameras, although such tests are helpful. For final determination, a test needs to be made with real fissile material. Consequently, starting in 1957, a year after Swan, both labs began one-point safety tests.
Out of 25 one-point safety tests conducted in 1957 and 1958, seven had zero or slight nuclear yield (success), three had high yields of 300 t to 500 t (severe failure), and the rest had unacceptable yields between those extremes.
Of particular concern was Livermore's W47 warhead for the Polaris submarine missile. The last test before the 1958 moratorium was a one-point test of the W47 primary, which had an unacceptably high nuclear yield of {{convert|400|lb|abbr=on}} of TNT equivalent (Hardtack II Titania). With the test moratorium in force, there was no way to refine the design and make it inherently one-point safe. Los Alamos had a suitable primary that was one-point safe, but rather than share with Los Alamos the credit for designing the first SLBM warhead, Livermore chose to use mechanical safing on its own inherently unsafe primary. The wire safety scheme described above was the result.<ref name=dud>Sybil Francis, ''Warhead Politics'', pp. 141, 160.</ref>
It turns out that the [[W47]] may have been safer than anticipated. The wire-safety system may have rendered most of the warheads "duds," unable to fire when detonated.<ref name="dud"/>
When testing resumed in 1961, and continued for three decades, there was sufficient time to make all warhead designs inherently one-point safe, without need for mechanical safing.
*'''[[Permissive Action Link]]s'''
In addition to the above steps to reduce the probability of a nuclear detonation arrising from a single fault, locking mechanisms referred to by NATO states as Permissive Action Links are sometimes attached to the control mechanisms for nuclear warheads. Permissive Action Links act solely to prevent an unauthorised use of a nuclear weapon.
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== Ver também ==
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