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{{Short description|Process by which nuclear WMDs are designed and produced}}
[[File:The gadget in the Trinity Test Site tower (1945).jpg|right|thumb|300px|The first nuclear explosive devices provided the basic building blocks of future weapons. Pictured is the ''Gadget'' device being prepared for the [[Trinity (nuclear test)|Trinity nuclear test]].]]

'''Nuclear weapons design''' are physical, chemical, and engineering arrangements that cause the physics package<ref>The physics package is the nuclear explosive module inside the bomb casing, missile warhead, or artillery shell, etc., which delivers the weapon to its target. While photographs of weapon casings are common, photographs of the physics package are quite rare, even for the oldest and crudest nuclear weapons. For a photograph of a modern physics package see [[W80 (nuclear warhead)|W80]].</ref> of a [[nuclear weapon]] to detonate. There are three existing basic design types:
# '''Pure fission weapons''' are the simplest, least technically demanding, were the first nuclear weapons built, and so far the only type ever used in warfare, by the United States on [[Empire of Japan|Japan]] in [[World War II]].
# '''[[Boosted fission weapon]]s''' increase yield beyond that of the implosion design, by using small quantities of fusion fuel to enhance the fission chain reaction. Boosting can more than double the weapon's fission energy yield.
# '''Staged [[thermonuclear weapon]]s''' are arrangements of two or more "stages", most usually two. The first stage is typically a boosted fission weapon (except for the earliest thermonuclear weapons, which used a pure fission weapon). Its detonation causes it to shine intensely with [[X-ray]]s, which illuminate and implode the second stage filled with fusion fuel. This initiates a sequence of events which results in a thermonuclear, or fusion, burn. This process affords potential yields up to hundreds of times those of fission weapons.<ref>{{citation |author= |year=1961 |title=To the Outside World, a Superbomb more Bluff than Bang |magazine=[[Life (magazine)|Life]] |location=New York |volume=51|issue=19, November 10, 1961 |pages=34–37 |url=https://books.google.com/books?id=4VMEAAAAMBAJ&pg=PA34 |access-date=2010-06-28 |url-status=live |archive-url=https://web.archive.org/web/20210904154852/https://books.google.com/books?id=4VMEAAAAMBAJ&pg=PA34 |archive-date=2021-09-04}}. Article on the Soviet [[Tsar Bomba]] test. Because explosions are spherical in shape and targets are spread out on the relatively flat surface of the earth, numerous smaller weapons cause more destruction. From page 35: "...&nbsp;five five-megaton weapons would demolish a greater area than a single 50-megatonner."</ref>

Pure fission weapons have been the first type to be built by new nuclear powers. Large industrial states with well-developed nuclear arsenals have two-stage thermonuclear weapons, which are the most compact, scalable, and cost effective option, once the necessary technical base and industrial infrastructure are built.

Most known innovations in nuclear weapon design originated in the United States, though some were later developed independently by other states.<ref>The United States and the Soviet Union were the only nations to build large nuclear arsenals with every possible type of nuclear weapon. The U.S. had a four-year head start and was the first to produce fissile material and fission weapons, all in 1945. The only Soviet claim for a design first was the [[Joe 4]] detonation on August 12, 1953, said to be the first deliverable hydrogen bomb. However, as Herbert York revealed in ''The Advisors: Oppenheimer, Teller and the Superbomb'' (W.H. Freeman, 1976), it was not a true hydrogen bomb (it was a boosted fission weapon of the Sloika/Alarm Clock type, not a two-stage thermonuclear). Soviet dates for the essential elements of warhead miniaturization – boosted, hollow-pit, two-point, air lens primaries – are not available in the open literature, but the larger size of Soviet ballistic missiles is often explained as evidence of an initial Soviet difficulty in miniaturizing warheads.</ref>

In early news accounts, pure fission weapons were called atomic bombs or '''A-bombs''' and weapons involving fusion were called '''hydrogen bombs''' or '''H-bombs'''. Practitioners of nuclear policy, however, favor the terms nuclear and thermonuclear, respectively.
{{Nuclear weapons}}

==Nuclear reactions==
Nuclear fission separates or splits heavier atoms to form lighter atoms. Nuclear fusion combines lighter atoms to form heavier atoms. Both reactions generate roughly a million times more energy than comparable chemical reactions, making nuclear bombs a million times more powerful than non-nuclear bombs, which a French patent claimed in May 1939.<ref>{{cite patent |title=Perfectionnements aux charges explosives (Improvements to explosive charges) |inventor1-last=[[Centre national de la recherche scientifique|Caisse Nationale de la Recherche Scientifique (National Fund for Scientific Research)]] |pubdate=1951-01-16 |country=FR |number=971324}}.</ref>

In some ways, fission and fusion are opposite and complementary reactions, but the particulars are unique for each. To understand how nuclear weapons are designed, it is useful to know the important similarities and differences between fission and fusion. The following explanation uses rounded numbers and approximations.<ref>The main source for this section is Samuel Glasstone and Philip Dolan, ''The Effects of Nuclear Weapons'', Third Edition, 1977, U.S. Dept of Defense and U.S. Dept of Energy (see links in General References, below), with the same information in more detail in Samuel Glasstone, ''Sourcebook on Atomic Energy'', Third Edition, 1979, U.S. Atomic Energy Commission, Krieger Publishing.</ref>

===Fission===
{{Main|Nuclear fission}}
When a free neutron hits the nucleus of a fissile atom like [[uranium-235]] (<sup>235</sup>U), the uranium nucleus splits into two smaller nuclei called fission fragments, plus more neutrons (for <sup>235</sup>U three about as often as two; an average of just under 2.5 per fission). The fission chain reaction in a supercritical mass of fuel can be self-sustaining because it produces enough surplus neutrons to offset losses of neutrons escaping the supercritical assembly. Most of these have the speed (kinetic energy) required to cause new fissions in neighboring uranium nuclei.<ref>{{cite web |title=nuclear fission {{!}} Examples & Process {{!}} Britannica |website=britannica.com |url=https://www.britannica.com/science/nuclear-fission |access-date=2022-05-30}}</ref>

The uranium-235 nucleus can split in many ways, provided the atomic numbers add up to 92 and the mass numbers add up to 236 (uranium-235 plus the neutron that caused the split). The following equation shows one possible split, namely into [[strontium|strontium-95]] (<sup>95</sup>Sr), [[xenon|xenon-139]] (<sup>139</sup>Xe), and two neutrons (n), plus energy:<ref>Glasstone and Dolan, ''Effects'', p. 12.</ref>
:::<math>\ {}^{235}\mathrm{U} + \mathrm{n} \longrightarrow {}^{95}\mathrm{Sr} + {}^{139}\mathrm{Xe} + 2\ \mathrm{n} + 180\ \mathrm{MeV}</math>

The immediate energy release per atom is about 180 million [[electron volt]]s (MeV); i.e., 74 TJ/kg. Only 7% of this is gamma radiation and kinetic energy of fission neutrons. The remaining 93% is kinetic energy (or energy of motion) of the charged fission fragments, flying away from each other mutually repelled by the positive charge of their protons (38 for strontium, 54 for xenon). This initial kinetic energy is 67 TJ/kg, imparting an initial speed of about 12,000 kilometers per second (i.e. 1.2&nbsp;cm per nanosecond). The charged fragments' high electric charge causes many inelastic [[coulomb collision]]s with nearby nuclei, and these fragments remain trapped inside the bomb's fissile [[pit (nuclear weapon)|pit]] and [[tamper (nuclear weapons)|tamper]] until their kinetic energy is converted into [[heat]]. Given the speed of the fragments and the [[mean free path]] between nuclei in the compressed fuel assembly (for the implosion design), this takes about a millionth of a second (a microsecond), by which time the core and tamper of the bomb have expanded to a ball of [[Plasma (physics)|plasma]] several meters in diameter with a temperature of tens of millions of degrees Celsius.

This is hot enough to emit [[black-body radiation]] in the X-ray spectrum. These X-rays are absorbed by the surrounding air, producing the fireball and blast of a nuclear explosion.

Most fission products have too many neutrons to be stable so they are radioactive by [[beta decay]], converting neutrons into protons by throwing off beta particles (electrons), neutrinos and gamma rays. Their half-lives range from milliseconds to about 200,000 years. Many decay into isotopes that are themselves radioactive, so from 1 to 6 (average 3) decays may be required to reach stability.<ref>Glasstone, ''Sourcebook'', p. 503.</ref> In reactors, the radioactive products are the nuclear waste in [[spent fuel]]. In bombs, they become radioactive fallout, both local and global.<ref>{{cite web |title=Nuclear explained – U.S. Energy Information Administration (EIA) |website=eia.gov |url=https://www.eia.gov/energyexplained/nuclear/ |access-date=2022-05-30}}</ref>

Meanwhile, inside the exploding bomb, the free neutrons released by fission carry away about 3% of the initial fission energy. Neutron kinetic energy adds to the blast energy of a bomb, but not as effectively as the energy from charged fragments, since neutrons do not give up their kinetic energy as quickly in collisions with charged nuclei or electrons. The dominant contribution of fission neutrons to the bomb's power is the initiation of subsequent fissions. Over half of the neutrons escape the bomb core, but the rest strike <sup>235</sup>U nuclei causing them to fission in an exponentially growing chain reaction (1, 2, 4, 8, 16, etc.). Starting from one atom, the number of fissions can theoretically double a hundred times in a microsecond, which could consume all uranium or plutonium up to hundreds of tons by the hundredth link in the chain. Typically in a modern weapon, the weapon's pit contains {{convert|3.5|to|4.5|kg}} of plutonium and at detonation produces approximately {{convert|5|to|10|ktTNT}} yield, representing the fissioning of approximately {{convert|0.5|kg}} of plutonium.<ref>{{cite web |title=NWFAQ: 4.2.5 Special Purpose Applications |last=Sublette |first=Carey |website=Nuclearweaponarchive.org |url=https://nuclearweaponarchive.org/Nwfaq/Nfaq4-2.html#Nfaq4.2.5 |access-date=11 August 2021 |quote=Modern boosted fission triggers take this evolution towards higher yield to weight, smaller volume, and greater ease of radiation escape to an extreme. Comparable explosive yields are produced by a core consisting of 3.5–4.5 kg of plutonium, 5–6 kg of beryllium reflector, and some 20 kilograms of high explosive containing essentially no high-Z material.}}</ref><ref>{{cite web |title=NWFAQ: 4.4.3.4 Principles of Compression |last=Sublette |first=Carey |website=nuclearweaponarchive.org |url=https://nuclearweaponarchive.org/Nwfaq/Nfaq4-4.html#Nfaq4.4.3.4 |access-date=11 August 2021 |quote=A simplistic computation of the work done in imploding a 10 liter secondary in the "W-80" ... the primary actually produced (5 kt)...}}</ref>

Materials which can sustain a chain reaction are called [[fissile]]. The two fissile materials used in nuclear weapons are: <sup>235</sup>U, also known as [[highly enriched uranium#Highly enriched uranium (HEU)|highly enriched uranium]] (HEU), "oralloy" meaning "Oak Ridge alloy",<ref>{{cite web |title=Atomic Glossary  |publisher= Nuclear Museum |url=https://ahf.nuclearmuseum.org/ahf/history/atomic-glossary/ |access-date=24 July 2023}}</ref> or "25" (a combination of the last digit of the atomic number of uranium-235, which is 92, and the last digit of its mass number, which is 235); and <sup>239</sup>Pu, also known as plutonium-239, or "49" (from "94" and "239").<ref>{{cite book |last=Rhodes |first=Richard |author-link=Richard Rhodes |title=[[The Making of the Atomic Bomb]] |location=New York |publisher=Simon & Schuster |year=1986 |isbn=0-671-44133-7|oclc=13793436|page=563}}</ref>

Uranium's most common [[isotope]], <sup>238</sup>U, is fissionable but not fissile, meaning that it cannot sustain a chain reaction because its daughter fission neutrons are not (on average) energetic enough to cause follow-on <sup>238</sup>U fissions. However, the neutrons released by fusion of the heavy hydrogen isotopes [[deuterium]] and [[tritium]] will fission <sup>238</sup>U. This <sup>238</sup>U fission reaction in the outer jacket of the secondary assembly of a two-stage thermonuclear bomb produces by far the greatest fraction of the bomb's energy yield, as well as most of its radioactive debris.

For national powers engaged in a nuclear arms race, this fact of <sup>238</sup>U's ability to fast-fission from thermonuclear neutron bombardment is of central importance. The plenitude and cheapness of both bulk dry fusion fuel (lithium deuteride) and <sup>238</sup>U (a byproduct of uranium enrichment) permit the economical production of very large nuclear arsenals, in comparison to pure fission weapons requiring the expensive <sup>235</sup>U or <sup>239</sup>Pu fuels.

===Fusion===
{{Main|Nuclear fusion}}
Fusion produces neutrons which dissipate energy from the reaction.<ref>"neutrons carry off most of the reaction energy", Glasstone and Dolan, ''Effects'', p. 21.</ref> In weapons, the most important fusion reaction is called the D-T reaction. Using the heat and pressure of fission, hydrogen-2, or deuterium (<sup>2</sup>D), fuses with hydrogen-3, or tritium (<sup>3</sup>T), to form helium-4 (<sup>4</sup>He) plus one neutron (n) and energy:<ref name="fusionmath"/>
:::<math>{}^2\mathrm{D} + {}^3\mathrm{T} \longrightarrow {}^4\mathrm{He} + n + 17.6\ \mathrm{MeV} </math>

[[File:Deuterium-tritium fusion.svg|right|200 px]]
The total energy output, 17.6 MeV, is one tenth of that with fission, but the ingredients are only one-fiftieth as massive, so the energy output per unit mass is approximately five times as great. In this fusion reaction, 14 of the 17.6 MeV (80% of the energy released in the reaction) shows up as the kinetic energy of the neutron, which, having no electric charge and being almost as massive as the hydrogen nuclei that created it, can escape the scene without leaving its energy behind to help sustain the reaction – or to generate x-rays for blast and fire.{{Citation needed|date=June 2021}}

The only practical way to capture most of the fusion energy is to trap the neutrons inside a massive bottle of heavy material such as lead, uranium, or plutonium. If the 14 MeV neutron is captured by uranium (of either isotope; 14 MeV is high enough to fission both <sup>235</sup>U and <sup>238</sup>U) or plutonium, the result is fission and the release of 180 MeV of fission energy, multiplying the energy output tenfold.{{Citation needed|date=June 2021}}

For weapon use, fission is necessary to start fusion, helps to sustain fusion, and captures and multiplies the energy carried by the fusion neutrons. In the case of a neutron bomb (see below), the last-mentioned factor does not apply, since the objective is to facilitate the escape of neutrons, rather than to use them to increase the weapon's raw power.{{Citation needed|date=June 2021}}

===Tritium production===
An essential nuclear reaction is the one that creates [[tritium]], or hydrogen-3. Tritium is employed in two ways. First, pure tritium gas is produced for placement inside the cores of boosted fission devices in order to increase their energy yields. This is especially so for the fission primaries of thermonuclear weapons. The second way is indirect, and takes advantage of the fact that the neutrons emitted by a supercritical fission "spark plug" in the secondary assembly of a two-stage thermonuclear bomb will produce tritium ''[[in situ]]'' when these neutrons collide with the lithium nuclei in the bomb's lithium deuteride fuel supply.

Elemental gaseous tritium for fission primaries is also made by bombarding [[isotopes of lithium|lithium-6]] (<sup>6</sup>Li) with [[neutron]]s (n), only in a nuclear reactor. This neutron bombardment will cause the lithium-6 nucleus to split, producing an alpha particle, or [[helium]]-4 (<sup>4</sup>He), plus a triton (<sup>3</sup>T) and energy:<ref name="fusionmath"/>
:::<math>{}^6\mathrm{Li} + n \longrightarrow {}^4\mathrm{He} + {}^3\mathrm{T} + 5\ \mathrm{MeV} </math>

But as was discovered in the first test of this type of device, [[Castle Bravo]], when [[Isotopes_of_lithium|lithium-7]] is present, one also has some amounts of the following two net reactions:
:{{sup|7}}Li + {{sup|1}}n → {{sup|3}}T + {{sup|4}}He + {{sup|1}}n
:{{sup|7}}Li + {{sup|2}}H → 2 {{sup|4}}He + {{sup|1}}n + 15.123 MeV

Most lithium is <sup>7</sup>Li, and this gave Castle Bravo a yield 2.5 times larger than expected.<ref>Parsons, Keith M.; Zaballa, Robert A. (2017). Bombing the Marshall Islands: A Cold War Tragedy. Cambridge University Press. pp. 53–56. ISBN 978-1-108-50874-2</ref>

The neutrons are supplied by the nuclear reactor in a way similar to production of plutonium <sup>239</sup>Pu from <sup>238</sup>U feedstock: target rods of the <sup>6</sup>Li feedstock are arranged around a uranium-fueled core, and are removed for processing once it has been calculated that most of the lithium nuclei have been transmuted to tritium.

Of the four basic types of nuclear weapon, the first, pure fission, uses the first of the three nuclear reactions above. The second, fusion-boosted fission, uses the first two. The third, two-stage thermonuclear, uses all three.

==Pure fission weapons==
{{Unreferenced section|date=October 2022}}
[[File:TrinityDetonation1945GIF.gif|thumb|Trinity-''Gadget'' was the first ever pure-fission nuclear device to be detonated, with an estimated yield of 25&nbsp;kilotons.]]

The first task of a nuclear weapon design is to rapidly assemble a [[critical mass|supercritical mass]] of fissile (weapon grade) uranium or plutonium. A supercritical mass is one in which the percentage of fission-produced neutrons captured by other neighboring fissile nuclei is large enough that each fission event, on average, causes more than one follow-on fission event. Neutrons released by the first fission events induce subsequent fission events at an exponentially accelerating rate. Each follow-on fissioning continues a sequence of these reactions that works its way throughout the supercritical mass of fuel nuclei. This process is conceived and described colloquially as the [[nuclear chain reaction]].

To start the chain reaction in a supercritical assembly, at least one free neutron must be injected and collide with a fissile fuel nucleus. The neutron joins with the nucleus (technically a fusion event) and destabilizes the nucleus, which explodes into two middleweight nuclear fragments (from the severing of the [[strong nuclear force]] holding the mutually-repulsive protons together), plus two or three free neutrons. These race away and collide with neighboring fuel nuclei. This process repeats over and over until the fuel assembly goes sub-critical (from thermal expansion), after which the chain reaction shuts down because the daughter neutrons can no longer find new fuel nuclei to hit before escaping the less-dense fuel mass. Each following fission event in the chain approximately doubles the neutron population (net, after losses due to some neutrons escaping the fuel mass, and others that collide with any non-fuel impurity nuclei present).

For the gun assembly method (see below) of supercritical mass formation, the fuel itself can be relied upon to initiate the chain reaction. This is because even the best weapon-grade uranium contains a significant number of <sup>238</sup>U nuclei. These are susceptible to [[spontaneous fission]] events, which occur randomly (it is a quantum mechanical phenomenon). Because the fissile material in a gun-assembled critical mass is not compressed, the design need only ensure the two sub-critical masses remain close enough to each other long enough that a <sup>238</sup>U spontaneous fission will occur while the weapon is in the vicinity of the target. This is not difficult to arrange as it takes but a second or two in a typical-size fuel mass for this to occur. (Still, many such bombs meant for delivery by air (gravity bomb, artillery shell or rocket) use injected neutrons to gain finer control over the exact detonation altitude, important for the destructive effectiveness of airbursts.)

This condition of spontaneous fission highlights the necessity to assemble the supercritical mass of fuel very rapidly. The time required to accomplish this is called the weapon's [[insertion time|critical insertion time]]. If spontaneous fission were to occur when the supercritical mass was only partially assembled, the chain reaction would begin prematurely. Neutron losses through the void between the two subcritical masses (gun assembly) or the voids between not-fully-compressed fuel nuclei (implosion assembly) would sap the bomb of the number of fission events needed to attain the full design yield. Additionally, heat resulting from the fissions that do occur would work against the continued assembly of the supercritical mass, from thermal expansion of the fuel. This failure is called [[predetonation]]. The resulting explosion would be called a "fizzle" by bomb engineers and weapon users. Plutonium's high rate of spontaneous fission makes uranium fuel a necessity for gun-assembled bombs, with their much greater insertion time and much greater mass of fuel required (because of the lack of fuel compression).

There is another source of free neutrons that can spoil a fission explosion. All uranium and plutonium nuclei have a decay mode that results in energetic [[alpha particle]]s. If the fuel mass contains impurity elements of low atomic number (Z), these charged alphas can penetrate the coulomb barrier of these impurity nuclei and undergo a reaction that yields a free neutron. The rate of alpha emission of fissile nuclei is one to two million times that of spontaneous fission, so weapon engineers are careful to use fuel of high purity.

Fission weapons used in the vicinity of other nuclear explosions must be protected from the intrusion of free neutrons from outside. Such shielding material will almost always be penetrated, however, if the outside neutron flux is intense enough. When a weapon misfires or fizzles because of the effects of other nuclear detonations, it is called [[nuclear fratricide]].

For the implosion-assembled design, once the critical mass is assembled to maximum density, a burst of neutrons must be supplied to start the chain reaction. Early weapons used a modulated neutron generator code named "[[Urchin (detonator)|Urchin]]" inside the pit containing [[polonium]]-210 and [[beryllium]] separated by a thin barrier. Implosion of the pit crushes the neutron generator, mixing the two metals, thereby allowing alpha particles from the polonium to interact with beryllium to produce free neutrons. In modern weapons, the [[neutron generator]] is a high-voltage vacuum tube containing a [[particle accelerator]] which bombards a deuterium/tritium-metal hydride target with deuterium and tritium [[ion]]s. The resulting small-scale fusion produces neutrons at a protected location outside the physics package, from which they penetrate the pit. This method allows better timing of the first fission events in the chain reaction, which optimally should occur at the point of maximum compression/supercriticality. Timing of the neutron injection is a more important parameter than the number of neutrons injected: the first generations of the chain reaction are vastly more effective due to the exponential function by which neutron multiplication evolves.

The critical mass of an uncompressed sphere of bare metal is {{convert|50|kg|lb|abbr=on}} for uranium-235 and {{convert|16|kg|lb|abbr=on}} for delta-phase plutonium-239. In practical applications, the amount of material required for criticality is modified by shape, purity, density, and the proximity to [[neutron reflector|neutron-reflecting material]], all of which affect the escape or capture of neutrons.

To avoid a premature chain reaction during handling, the fissile material in the weapon must be kept subcritical. It may consist of one or more components containing less than one uncompressed critical mass each. A thin hollow shell can have more than the bare-sphere critical mass, as can a cylinder, which can be arbitrarily long without ever reaching criticality. Another method of reducing criticality risk is to incorporate material with a large cross-section for neutron capture, such as boron (specifically <sup>10</sup>B comprising 20% of natural boron). Naturally this neutron absorber must be removed before the weapon is detonated. This is easy for a gun-assembled bomb: the projectile mass simply shoves the absorber out of the void between the two subcritical masses by the force of its motion.

The use of plutonium affects weapon design due to its high rate of alpha emission. This results in Pu metal spontaneously producing significant heat; a 5&nbsp;kilogram mass produces 9.68&nbsp;watts of thermal power. Such a piece would feel warm to the touch, which is no problem if that heat is dissipated promptly and not allowed to build up the temperature. But this is a problem inside a nuclear bomb. For this reason bombs using Pu fuel use aluminum parts to wick away the excess heat, and this complicates bomb design because Al plays no active role in the explosion processes.

A tamper is an optional layer of dense material surrounding the fissile material. Due to its [[inertia]] it delays the thermal expansion of the fissioning fuel mass, keeping it supercritical for longer. Often{{when|date=October 2023}} the same layer serves both as tamper and as neutron reflector.

===Gun-type assembly===
[[File:Gun-type fission weapon en-labels thin lines.svg|thumb|350px|Diagram of a gun-type fission weapon]]
{{Main|Gun-type fission weapon}}
[[Little Boy]], the Hiroshima bomb, used {{convert|64|kg|lb|abbr=on}} of uranium with an average enrichment of around 80%, or {{convert|51|kg|lb|abbr=on}} of uranium-235, just about the bare-metal critical mass {{xref|(see [[Little Boy#Assembly details|Little Boy]] article for a detailed drawing)}}. When assembled inside its tamper/reflector of [[tungsten carbide]], the {{convert|64|kg|lb|abbr=on}} was more than twice critical mass. Before the detonation, the uranium-235 was formed into two sub-critical pieces, one of which was later fired down a gun barrel to join the other, starting the nuclear explosion. Analysis shows that less than 2% of the uranium mass underwent fission;<ref>Glasstone and Dolan, ''Effects'', pp. 12–13. When 454 g (one pound) of <sup>235</sup>U undergoes complete fission, the yield is 8 kilotons. The 13 to 16-kiloton yield of the Little Boy bomb was therefore produced by the fission of no more than {{convert|2|lb|g}} of <sup>235</sup>U, out of the {{convert|141|lb|g}} in the pit. Thus, the remaining {{convert|139|lb|kg}}, 98.5% of the total, contributed nothing to the energy yield.</ref> the remainder, representing most of the entire wartime output of the [[Y-12 National Security Complex|giant Y-12 factories]] at Oak Ridge, scattered uselessly.<ref>Compere, A.L., and Griffith, W.L. 1991. "The U.S. Calutron Program for Uranium Enrichment: History,. Technology, Operations, and Production. Report", ORNL-5928, as cited in John Coster-Mullen, "Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man", 2003, footnote 28, p. 18. The total wartime output of Oralloy produced at Oak Ridge by July 28, 1945, was {{convert|165|lb|kg}}. Of this amount, 84% was scattered over Hiroshima (see previous footnote).</ref>

The inefficiency was caused by the speed with which the uncompressed fissioning uranium expanded and became sub-critical by virtue of decreased density. Despite its inefficiency, this design, because of its shape, was adapted for use in small-diameter, cylindrical artillery shells (a [[gun-type fission weapon#US nuclear artillery|gun-type warhead]] fired from the barrel of a much larger gun).{{Citation needed|date=October 2023}} Such warheads were deployed by the United States until 1992, accounting for a significant fraction of the <sup>235</sup>U in the arsenal{{Citation needed|date=June 2021}}, and were some of the first weapons dismantled to comply with treaties limiting warhead numbers.{{Citation needed|date=June 2021|reason=Very doubtful given the only treaty dealing with tactical weapons was the intermediate-ranged nuclear forces treaty}} The rationale for this decision was undoubtedly a combination of the lower yield and grave safety issues associated with the gun-type design.{{Citation needed|date=June 2021|reason=W33s were stored disassembled}}

===Implosion-type{{anchor|Implosion-type_weapon}}===
[[File:Implosion Nuclear weapon.svg|right|350px]]
For both the [[Trinity (nuclear test)|Trinity device]] and the [[Fat Man]] (Nagasaki) bomb, nearly identical plutonium fission through implosion designs were used. The Fat Man device specifically used {{convert|6.2|kg|lb|abbr=on}}, about {{convert|350|ml|usoz|abbr=on|disp=or}} in volume, of [[Pu-239]], which is only 41% of bare-sphere critical mass {{xref|(see [[Fat Man#Interior|Fat Man]] article for a detailed drawing)}}. Surrounded by a [[uranium-238|U-238]] reflector/tamper, the Fat Man's pit was brought close to critical mass by the neutron-reflecting properties of the U-238. During detonation, criticality was achieved by implosion. The plutonium pit was squeezed to increase its density by simultaneous detonation, as with the "Trinity" test detonation three weeks earlier, of the conventional explosives placed uniformly around the pit. The explosives were detonated by multiple [[exploding-bridgewire detonator]]s. It is estimated that only about 20% of the plutonium underwent fission; the rest, about {{convert|5|kg|lb|abbr=on}}, was scattered.

[[File:Implosion bomb animated.gif|left|175px]]

An implosion shock wave might be of such short duration that only part of the pit is compressed at any instant as the wave passes through it. To prevent this, a pusher shell may be needed. The pusher is located between the explosive lens and the tamper. It works by reflecting some of the shock wave backward, thereby having the effect of lengthening its duration. It is made out of a low [[density]] [[metal]] – such as [[aluminium]], [[beryllium]], or an [[alloy]] of the two metals (aluminium is easier and safer to shape, and is two orders of magnitude cheaper; beryllium has high neutron-reflective capability). Fat Man used an aluminium pusher.

The series of [[RaLa Experiment]] tests of implosion-type fission weapon design concepts, carried out from July 1944 through February 1945 at the [[Los Alamos Laboratory]] and a remote site {{convert|14.3|km|mi|abbr=on}} east of it in Bayo Canyon, proved the practicality of the implosion design for a fission device, with the February 1945 tests positively determining its usability for the final Trinity/Fat Man plutonium implosion design.<ref>{{cite book |last=Hoddeson |first=Lillian |display-authors=etal |title=Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943–1945 |date=2004 |publisher=Cambridge University Press |page=271 |isbn=978-0-521-54117-6}}</ref>

The key to Fat Man's greater efficiency was the inward momentum of the massive U-238 tamper. (The natural uranium tamper did not undergo fission from thermal neutrons, but did contribute perhaps 20% of the total yield from fission by fast neutrons). After the chain reaction started in the plutonium, it continued until the explosion reversed the momentum of the implosion and expanded enough to stop the chain reaction. By holding everything together for a few hundred nanoseconds more, the tamper increased the efficiency.

====Plutonium pit====
{{Main|Pit (nuclear weapon)}}
[[File:X-Ray-Image-HE-Lens-Test-Shot.gif|thumb|right|Flash X-Ray images of the converging shock waves formed during a test of the high explosive lens system.]]

The core of an implosion weapon – the fissile material and any reflector or tamper bonded to it – is known as the ''pit''. Some weapons tested during the 1950s used pits made with [[uranium-235|U-235]] alone, or in [[composite material|composite]] with [[plutonium]],<ref>[https://fas.org/sgp/othergov/doe/rdd-7.html "Restricted Data Declassification Decisions from 1945 until Present"] {{webarchive |url=https://web.archive.org/web/20160423121258/https://fas.org/sgp/othergov/doe/rdd-7.html |date=April 23, 2016}} – "Fact that plutonium and uranium may be bonded to each other in unspecified pits or weapons."</ref> but all-plutonium pits are the smallest in diameter and have been the standard since the early 1960s.{{Citation needed|date=June 2021}}

Casting and then machining plutonium is difficult not only because of its toxicity, but also because plutonium has many different [[allotropes of plutonium|metallic phases]]. As plutonium cools, changes in phase result in distortion and cracking. This distortion is normally overcome by alloying it with 30–35 mMol (0.9–1.0% by weight) [[gallium]], forming a [[plutonium-gallium alloy]], which causes it to take up its delta phase over a wide temperature range.<ref name="RDD-7"/> When cooling from molten it then has only a single phase change, from epsilon to delta, instead of the four changes it would otherwise pass through. Other [[valence (chemistry)|trivalent]] [[metal]]s would also work, but gallium has a small neutron [[absorption cross section]] and helps protect the plutonium against [[corrosion]]. A drawback is that gallium compounds are corrosive and so if the plutonium is recovered from dismantled weapons for conversion to [[plutonium dioxide]] for [[nuclear reactor|power reactors]], there is the difficulty of removing the gallium.{{Citation needed|date=June 2021}}

Because plutonium is chemically reactive it is common to plate the completed pit with a thin layer of inert metal, which also reduces the toxic hazard.<ref name="NWFAQ-6.2"/> [[The gadget]] used galvanic silver plating; afterward, [[nickel]] deposited from [[nickel tetracarbonyl]] vapors was used,<ref name="NWFAQ-6.2"/> but thereafter and since, [[gold]] became the preferred material.{{Citation needed|date=May 2009|reason=not found in nuclearweaponarchive.org cite}} Recent designs improve safety by plating pits with [[vanadium]] to make the pits more fire-resistant.{{Citation needed|date=June 2021|reason=Modern pits are sealed in a fire resistant shell, vanadium was an innovation in the never produced W89}}

===Levitated-pit implosion===
[[File:SandstoneYoke.gif|thumb|The ''Sandstone'' series of nuclear-weapons tests in 1948 proved the feasibility of increased yield efficiency via the levitated-pit design method.]]
The first improvement on the Fat Man design was to put an air space between the tamper and the pit to create a hammer-on-nail impact. The pit, supported on a hollow cone inside the tamper cavity, was said to be "levitated". The three tests of [[Operation Sandstone]], in 1948, used Fat Man designs with levitated pits. The largest yield was 49 kilotons, more than twice the yield of the unlevitated Fat Man.<ref>All information on nuclear weapon tests comes from Chuck Hansen, ''The Swords of Armageddon: U.S. Nuclear Weapons Development since 1945'', October 1995, Chucklea Productions, Volume VIII, p. 154, Table A-1, "U.S. Nuclear Detonations and Tests, 1945–1962".</ref>

It was immediately clear{{according to whom|date=October 2023}} that implosion was the best design for a fission weapon. Its only drawback seemed to be its diameter. Fat Man was {{convert|1.5|m|ft|0}} wide vs {{convert|61|cm|ft|0}} for Little Boy.

The Pu-239 pit of Fat Man was only {{convert|9.1|cm|in}} in diameter, the size of a softball. The bulk of Fat Man's girth was the implosion mechanism, namely concentric layers of U-238, aluminium, and high explosives. The key to reducing that girth was the two-point implosion design.{{Citation needed|date=June 2021|reason=Doubtful given Swan would be challenging to harden for laydown and ground penetration delivery.}}

===Two-point linear implosion===
[[File:Linear implosion schematic.svg|right|350 px]]
In the two-point linear implosion, the nuclear fuel is cast into a solid shape and placed within the center of a cylinder of high explosive. Detonators are placed at either end of the explosive cylinder, and a plate-like insert, or ''shaper'', is placed in the explosive just inside the detonators. When the detonators are fired, the initial detonation is trapped between the shaper and the end of the cylinder, causing it to travel out to the edges of the shaper where it is diffracted around the edges into the main mass of explosive. This causes the detonation to form into a ring that proceeds inward from the shaper.<ref>[https://nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html#Nfaq4.1.6.3 Nuclear Weapons FAQ: 4.1.6.3 Hybrid Assembly Techniques] {{webarchive |url=https://web.archive.org/web/20160419071500/https://nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html#Nfaq4.1.6.3 |date=April 19, 2016}}, accessed December 1, 2007. Drawing adapted from the same source.</ref>

Due to the lack of a tamper or lenses to shape the progression, the detonation does not reach the pit in a spherical shape. To produce the desired spherical implosion, the fissile material itself is shaped to produce the same effect. Due to the physics of the shock wave propagation within the explosive mass, this requires the pit to be a [[prolate spheroid]], that is, roughly egg shaped. The shock wave first reaches the pit at its tips, driving them inward and causing the mass to become spherical. The shock may also change plutonium from delta to alpha phase, increasing its density by 23%, but without the inward momentum of a true implosion.{{Citation needed|date=June 2021}}

The lack of compression makes such designs inefficient, but the simplicity and small diameter make it suitable for use in artillery shells and atomic demolition munitions – ADMs – also known as backpack or [[suitcase nuke]]s; an example is the [[W48]] artillery shell, the smallest nuclear weapon ever built or deployed. All such low-yield battlefield weapons, whether gun-type U-235 designs or linear implosion Pu-239 designs, pay a high price in fissile material in order to achieve diameters between six and ten inches (15 and 25&nbsp;cm).{{Citation needed|date=June 2021}}

===Hollow-pit implosion===
{{Unreferenced section|date=October 2022}}
A more efficient implosion system uses a hollow pit.{{Citation needed|date=June 2021}}

A hollow plutonium pit was the original plan for the 1945 Fat Man bomb, but there was not enough time to develop and test the implosion system for it. A simpler solid-pit design was considered more reliable, given the time constraints, but it required a heavy U-238 tamper, a thick aluminium pusher, and three tons of high explosives.{{Citation needed|date=June 2021}}

After the war, interest in the hollow pit design was revived. Its obvious advantage is that a hollow shell of plutonium, shock-deformed and driven inward toward its empty center, would carry momentum into its violent assembly as a solid sphere. It would be self-tamping, requiring a smaller U-238 tamper, no aluminium pusher, and less high explosive.{{Citation needed|date=June 2021}}

==Fusion-boosted fission==
{{Main|Boosted fission weapon}}
[[File:Greenhouse - Item Device 3.png|thumb|''Item'' of the Greenhouse-series of tests was the first nuclear weapon device to achieve yield utilizing boosting-principles.]]
The next step in miniaturization was to speed up the fissioning of the pit to reduce the minimum inertial confinement time. This would allow the efficient fission of the fuel with less mass in the form of tamper or the fuel itself. The key to achieving faster fission would be to introduce more neutrons, and among the many ways to do this, adding a fusion reaction was relatively easy in the case of a hollow pit.{{Citation needed|date=June 2021}}

The easiest fusion reaction to achieve is found in a 50–50 mixture of tritium and deuterium.<ref name="Fission-Fusion Hybrid Weapons">{{cite web |last1=Sublette |first1=Carey |title=Fission-Fusion Hybrid Weapons |website=nuclearweaponarchive |url=https://nuclearweaponarchive.org/}}</ref> For [[fusion power]] experiments this mixture must be held at high temperatures for relatively lengthy times in order to have an efficient reaction. For explosive use, however, the goal is not to produce efficient fusion, but simply provide extra neutrons early in the process.{{Citation needed|date=June 2021|reason=extremely doubtful. The fusion fuel needs to fuse rapidly to provide said neutrons.}} Since a nuclear explosion is supercritical, any extra neutrons will be multiplied by the chain reaction, so even tiny quantities introduced early can have a large effect on the outcome. For this reason, even the relatively low compression pressures and times (in fusion terms) found in the center of a hollow pit warhead are enough to create the desired effect.{{Citation needed|date=June 2021|reason=Even a modest ~0.1kt provides enormous pressures and temperatures in a pit suitable for fusion.}}

In the boosted design, the fusion fuel in gas form is pumped into the pit during arming. This will fuse into helium and release free neutrons soon after fission begins.{{citation needed|date=August 2023}} The neutrons will start a large number of new chain reactions while the pit is still critical or nearly critical. Once the hollow pit is perfected, there is little reason not to boost; deuterium and tritium are easily produced in the small quantities needed, and the technical aspects are trivial.<ref name="Fission-Fusion Hybrid Weapons"/>

The concept of fusion-boosted fission was first tested on May 25, 1951, in the [[Greenhouse Item|Item]] shot of [[Operation Greenhouse]], [[Enewetak Atoll|Eniwetok]], yield 45.5 kilotons.{{Citation needed|date=June 2021}}

Boosting reduces diameter in three ways, all the result of faster fission:
* Since the compressed pit does not need to be held together as long, the massive U-238 tamper can be replaced by a light-weight beryllium shell (to reflect escaping neutrons back into the pit). The diameter is reduced.{{Citation needed|date=June 2021}}
* The mass of the pit can be reduced by half, without reducing yield. Diameter is reduced again.{{Citation needed|date=June 2021}}
* Since the mass of the metal being imploded (tamper plus pit) is reduced, a smaller charge of high explosive is needed, reducing diameter even further.{{Citation needed|date=June 2021}}

[[File:U.S. Swan Device.svg|center]]{{Citation needed|date=June 2021|reason=image is also uncited}}

The first device whose dimensions suggest employment of all these features (two-point, hollow-pit, fusion-boosted implosion) was the [[Swan (nuclear primary)|Swan]] device. It had a cylindrical shape with a diameter of {{convert|11.6|in|cm|abbr=on|order=flip}} and a length of {{convert|22.8|in|cm|abbr=on|order=flip}}.{{Citation needed|date=June 2021|reason=The B28 predates Swan and was a compact (sub 20") boosted weapon}}

It was first tested standalone and then as the primary of a two-stage thermonuclear device during [[Operation Redwing]]. It was weaponized as the [[Robin primary]] and became the first off-the-shelf, multi-use primary, and the prototype for all that followed.{{Citation needed|date=June 2021}}

[[File:Nuclear Weapon Miniaturization.png|400px|right]]

After the success of Swan, {{convert|11|or|12|in|cm|order=flip}} seemed to become the standard diameter of boosted single-stage devices tested during the 1950s.{{Citation needed|date=June 2021|reason=The W81 suggests the B61 has a spherical primary}} Length was usually twice the diameter, but one such device, which became the [[W54]] warhead, was closer to a sphere, only {{convert|15|in|cm|order=flip}} long.

One of the applications of the W54 was the [[Davy Crockett (nuclear device)|Davy Crockett XM-388 recoilless rifle projectile]]. It had a dimension of just {{convert|11|in|cm|order=flip}}, and is shown here in comparison to its Fat Man predecessor ({{convert|60|in|cm|order=flip|disp=or}}).

Another benefit of boosting, in addition to making weapons smaller, lighter, and with less fissile material for a given yield, is that it renders weapons immune to predetonation.{{Citation needed|date=June 2021|reason=hardens the weapon to predetonation, does not make it immune. terminology also seems to be made up, it was "predetonation", not "radiation interference"}} It was discovered in the mid-1950s that plutonium pits would be particularly susceptible to partial [[predetonation]] if exposed to the intense radiation of a nearby nuclear explosion (electronics might also be damaged, but this was a separate problem).{{Citation needed|date=June 2021|reason=doubtful, delayed neutrons are a thing in U235 too}} RI was a particular problem before effective [[early warning radar]] systems because a first strike attack might make retaliatory weapons useless. Boosting reduces the amount of plutonium needed in a weapon to below the quantity which would be vulnerable to this effect.{{Citation needed|date=June 2021|reason=The concern was with fraticide or defensive warhead detonations, not weapons in ICBMs or on the ground. Any launcher close enough to a weapon burst to worry about neutrons is shattered by the blast wave}}

==Two-stage thermonuclear==
{{Main|Thermonuclear weapon}}
[[File:IvyMikeGIFColorCorrected.gif|thumb|right|[[Ivy Mike]], the first two-stage thermonuclear detonation, 10.4&nbsp;megatons, November 1, 1952.]]

Pure fission or fusion-boosted fission weapons can be made to yield hundreds of kilotons, at great expense in fissile material and tritium, but by far the most efficient way to increase nuclear weapon yield beyond ten or so kilotons is to add a second independent stage, called a secondary.{{Citation needed|date=June 2021}}

In the 1940s, bomb designers at [[Los Alamos National Laboratory|Los Alamos]] thought the secondary would be a canister of deuterium in liquefied or hydride form. The fusion reaction would be D-D, harder to achieve than D-T, but more affordable. A fission bomb at one end would shock-compress and heat the near end, and fusion would propagate through the canister to the far end. Mathematical simulations showed it would not work, even with large amounts of expensive tritium added.{{Citation needed|date=June 2021}}

The entire fusion fuel canister would need to be enveloped by fission energy, to both compress and heat it, as with the booster charge in a boosted primary. The design breakthrough came in January 1951, when [[Edward Teller]] and [[Stanislaw Ulam]] invented radiation implosion – for nearly three decades known publicly only as the [[Teller-Ulam]] H-bomb secret.<ref>[https://www.aip.org/history-programs/niels-bohr-library/oral-histories/35680 So I pieced together from Edward's testament and from his memoir that Stan had come to him in February of 1951] {{Webarchive|url=https://web.archive.org/web/20180213135308/https://www.aip.org/history-programs/niels-bohr-library/oral-histories/35680 |date=2018-02-13}} American Institute of Physics interview with Richard Garwin by Ken Ford, dated December 2012</ref><ref>[https://www.aip.org/history-programs/niels-bohr-library/oral-histories/28636-1 he was going to use first hydrodynamics and just the shockwaves and then neutron heating, which would have been a disaster. It would have blown it up before it got going. It was Teller who came up with the radiation.] {{Webarchive|url=https://web.archive.org/web/20210223052546/https://www.aip.org/history-programs/niels-bohr-library/oral-histories/28636-1 |date=2021-02-23}}, American Institute of Physics interview with Marshall Rosenbluth by Kai-Henrik Barth, dated August 2003</ref>

The concept of radiation implosion was first tested on May 9, 1951, in the George shot of [[Operation Greenhouse]], Eniwetok, yield 225&nbsp;kilotons. The first full test was on November 1, 1952, the [[Ivy Mike|Mike]] shot of [[Operation Ivy]], Eniwetok, yield 10.4&nbsp;megatons.{{Citation needed|date=June 2021}}

In radiation implosion, the burst of X-ray energy coming from an exploding primary is captured and contained within an opaque-walled radiation channel which surrounds the nuclear energy components of the secondary. The radiation quickly turns the plastic foam that had been filling the channel into a plasma which is mostly transparent to X-rays, and the radiation is absorbed in the outermost layers of the pusher/tamper surrounding the secondary, which ablates and applies a massive force<ref>[https://nuclearweaponarchive.org/Nwfaq/Nfaq4-4.html#Nfaq4.4.3.3 4.4 Elements of Thermonuclear Weapon Design] {{webarchive |url=https://web.archive.org/web/20160311152031/https://nuclearweaponarchive.org/Nwfaq/Nfaq4-4.html#Nfaq4.4.3.3 |date=March 11, 2016}}. Nuclearweaponarchive.org. Retrieved on 2011-05-01.</ref> (much like an inside out rocket engine) causing the fusion fuel capsule to implode much like the pit of the primary. As the secondary implodes a fissile "spark plug" at its center ignites and provides neutrons and heat which enable the lithium deuteride fusion fuel to produce tritium and ignite as well. The fission and fusion chain reactions exchange neutrons with each other and boost the efficiency of both reactions. The greater implosive force, enhanced efficiency of the fissile "spark plug" due to boosting via fusion neutrons, and the fusion explosion itself provide significantly greater explosive yield from the secondary despite often not being much larger than the primary.{{Citation needed|date=June 2021}}

[[File:TellerUlamAblation.png|center|thumb|700px|Ablation mechanism firing sequence.
{{Ordered list
|Warhead before firing. The nested spheres at the top are the fission primary; the cylinders below are the fusion secondary device.|Fission primary's explosives have detonated and collapsed the primary's [[plutonium pit|fissile pit]].
|The primary's fission reaction has run to completion, and the primary is now at several million degrees and radiating gamma and hard X-rays, heating up the inside of the [[hohlraum]], the shield, and the secondary's tamper.
|The primary's reaction is over and it has expanded. The surface of the pusher for the secondary is now so hot that it is also ablating or expanding away, pushing the rest of the secondary (tamper, fusion fuel, and fissile spark plug) inward. The spark plug starts to fission. Not depicted: the radiation case is also ablating and expanding outward (omitted for clarity of diagram).
|The secondary's fuel has started the fusion reaction and shortly will burn up. A fireball starts to form.
}}]]

For example, for the Redwing Mohawk test on July 3, 1956, a secondary called the Flute was attached to the Swan primary. The Flute was {{convert|15|in|cm|order=flip}} in diameter and {{convert|23.4|in|cm|order=flip}} long, about the size of the Swan. But it weighed ten times as much and yielded 24 times as much energy (355&nbsp;kilotons vs 15&nbsp;kilotons).{{Citation needed|date=June 2021}}

Equally important, the active ingredients in the Flute probably cost no more than those in the Swan. Most of the fission came from cheap U-238, and the tritium was manufactured in place during the explosion. Only the spark plug at the axis of the secondary needed to be fissile.{{Citation needed|date=June 2021}}

A spherical secondary can achieve higher implosion densities than a cylindrical secondary, because spherical implosion pushes in from all directions toward the same spot. However, in warheads yielding more than one megaton, the diameter of a spherical secondary would be too large for most applications. A cylindrical secondary is necessary in such cases. The small, cone-shaped re-entry vehicles in multiple-warhead ballistic missiles after 1970 tended to have warheads with spherical secondaries, and yields of a few hundred kilotons.{{Citation needed|date=June 2021}}

In engineering terms, radiation implosion allows for the exploitation of several known features of nuclear bomb materials which heretofore had eluded practical application. For example:
* The optimal way to store deuterium in a reasonably dense state is to chemically bond it with lithium, as lithium deuteride. But the lithium-6 isotope is also the raw material for tritium production, and an exploding bomb is a nuclear reactor. Radiation implosion will hold everything together long enough to permit the complete conversion of lithium-6 into tritium, while the bomb explodes. So the bonding agent for deuterium permits use of the D-T fusion reaction without any pre-manufactured tritium being stored in the secondary. The tritium production constraint disappears.{{Citation needed|date=June 2021}}
* For the secondary to be imploded by the hot, radiation-induced plasma surrounding it, it must remain cool for the first microsecond, i.e., it must be encased in a massive radiation (heat) shield. The shield's massiveness allows it to double as a tamper, adding momentum and duration to the implosion. No material is better suited for both of these jobs than ordinary, cheap uranium-238, which also happens to undergo fission when struck by the neutrons produced by D-T fusion. This casing, called the pusher, thus has three jobs: to keep the secondary cool; to hold it, inertially, in a highly compressed state; and, finally, to serve as the chief energy source for the entire bomb. The consumable pusher makes the bomb more a uranium fission bomb than a hydrogen fusion bomb. Insiders never used the term "hydrogen bomb".<ref>Until a reliable design was worked out in the early 1950s, the hydrogen bomb (public name) was called the superbomb by insiders. After that, insiders used a more descriptive name: two-stage thermonuclear. Two examples. From Herb York, ''The Advisors'', 1976, "This book is about ... the development of the H-bomb, or the superbomb as it was then called." p. ix, and "The rapid and successful development of the superbomb (or super as it came to be called) ..." p. 5. From National Public Radio Talk of the Nation, November 8, 2005, Siegfried Hecker of Los Alamos, "the hydrogen bomb – that is, a two-stage thermonuclear device, as we referred to it – is indeed the principal part of the US arsenal, as it is of the Russian arsenal."</ref>
* Finally, the heat for fusion ignition comes not from the primary but from a second fission bomb called the spark plug, embedded in the heart of the secondary. The implosion of the secondary implodes this spark plug, detonating it and igniting fusion in the material around it, but the spark plug then continues to fission in the neutron-rich environment until it is fully consumed, adding significantly to the yield.<ref name="CLR"/>

In the ensuing fifty years, no one has come up with a more efficient way to build a thermonuclear bomb. It is the design of choice for the United States, Russia, the United Kingdom, China, and France, the five thermonuclear powers. On 3 September 2017 [[2017 North Korean nuclear test|North Korea carried out]] what it reported as its first "two-stage thermo-nuclear weapon" test.<ref name=cnbc-20170903>{{cite news |title=North Korea hydrogen bomb: Read the full announcement from Pyongyang |last=Kemp |first=Ted |publisher=CNBC News |date=3 September 2017 |url=https://www.cnbc.com/2017/09/03/north-korea-hydrogen-bomb-read-the-full-announcement-from-pyongyang.html |access-date=5 September 2017 |url-status=live |archive-url=https://web.archive.org/web/20170904051152/https://www.cnbc.com/2017/09/03/north-korea-hydrogen-bomb-read-the-full-announcement-from-pyongyang.html |archive-date=4 September 2017}}</ref> According to [[Ted Taylor (physicist)|Dr. Theodore Taylor]], after reviewing leaked [[Mordechai Vanunu#Negev Nuclear Research Center|photographs]] of disassembled weapons components taken before 1986, Israel possessed boosted weapons and would require supercomputers of that era to advance further toward full two-stage weapons in the megaton range without nuclear test detonations.<ref>{{cite web |title=Israel's Nuclear Weapon Capability: An Overview |website=wisconsinproject.org |url=https://www.wisconsinproject.org/israels-nuclear-weapon-capability-an-overview/ |access-date=2016-10-03 |url-status=dead |archive-url=https://web.archive.org/web/20150429192508/http://www.wisconsinproject.org/countries/israel/nuke.html |archive-date=2015-04-29}}</ref> The other nuclear-armed nations, India and Pakistan, probably have single-stage weapons, possibly boosted.<ref name="CLR"/>

===Interstage===
In a two-stage thermonuclear weapon the energy from the primary impacts the secondary. An essential{{Citation needed|date=June 2021|reason=Details released about Ivy Mike suggest an interstage is not needed for larger weapons}} energy transfer modulator called the interstage, between the primary and the secondary, protects the secondary's fusion fuel from heating too quickly, which could cause it to explode in a conventional (and small) heat explosion before the fusion and fission reactions get a chance to start.{{Citation needed|date=June 2021|reason=While some might modulate, the important part is filling the radiation channels with low-Z plasma that is not opaque to radiation like high-Z plasma}}

There is very little information in the open literature about the mechanism of the interstage.{{Citation needed|date=June 2021|reason=details of plasma opacity can be found in ICF literature}} Its first mention in a U.S. government document formally released to the public appears to be a caption in a graphic promoting the Reliable Replacement Warhead Program in 2007. If built, this new design would replace "toxic, brittle material" and "expensive 'special' material" in the interstage.<ref>[[commons:File:Reliable Replacement Warhead Features.jpg|"Improved Security, Safety & Manufacturability of the Reliable Replacement Warhead"]], NNSA March 2007.</ref> This statement suggests the interstage may contain beryllium to moderate the flux of neutrons from the primary, and perhaps something to absorb and re-radiate the x-rays in a particular manner.<ref>[https://fas.org/sgp/eprint/morland_image026.gif A 1976 drawing] {{webarchive |url=https://web.archive.org/web/20160403132417/https://fas.org/sgp/eprint/morland_image026.gif |date=April 3, 2016}} which depicts an interstage that absorbs and re-radiates x-rays. From Howard Morland, [https://fas.org/sgp/eprint/cardozo.html "The Article"], {{webarchive |url=https://web.archive.org/web/20160322014302/https://fas.org/sgp/eprint/cardozo.html |date=March 22, 2016}} ''Cardozo Law Review'', March 2005, p. 1374.</ref> There is also some speculation that this interstage material, which may be code-named [[Fogbank]], might be an [[aerogel]], possibly doped with beryllium and/or other substances.<ref>{{cite news |title=Technical hitch delays renewal of nuclear warheads for Trident |author=Ian Sample |newspaper=[[The Guardian]] |date=6 March 2008 |url=https://www.theguardian.com/uk/2008/mar/06/military.greenpolitics?gusrc=rss&feed=politics |access-date=15 December 2016 |url-status=live |archive-url=https://web.archive.org/web/20160305035909/http://www.theguardian.com/uk/2008/mar/06/military.greenpolitics?gusrc=rss&feed=politics |archive-date=5 March 2016}}</ref><ref>[https://www.armscontrolwonk.com/archive/201814/fogbank/ "ArmsControlWonk: FOGBANK"] {{webarchive |url=https://web.archive.org/web/20100114172137/http://www.armscontrolwonk.com/1814/fogbank |date=January 14, 2010}}, March 7, 2008. (Accessed 2010-04-06)</ref>

The interstage and the secondary are encased together inside a stainless steel membrane to form the canned subassembly (CSA), an arrangement which has never been depicted in any open-source drawing.<ref>[https://fas.org/sgp/eprint/w-88sand.htm "SAND8.8 – 1151 Nuclear Weapon Data – Sigma I"], {{webarchive |url=https://web.archive.org/web/20160423004514/https://fas.org/sgp/eprint/w-88sand.htm |date=April 23, 2016}} Sandia Laboratories, September 1988.</ref> The most detailed illustration of an interstage shows a British thermonuclear weapon with a cluster of items between its primary and a cylindrical secondary. They are labeled "end-cap and neutron focus lens", "reflector/neutron gun carriage", and "reflector wrap". The origin of the drawing, posted on the internet by Greenpeace, is uncertain, and there is no accompanying explanation.<ref>[https://fas.org/sgp/eprint/morland_image037.gif The Greenpeace drawing.] {{webarchive |url=https://web.archive.org/web/20160315104941/https://fas.org/sgp/eprint/morland_image037.gif |date=March 15, 2016}} From Morland, ''Cardozo Law Review'', March 2005, p. 1378.</ref>

==Specific designs==
While every nuclear weapon design falls into one of these categories, specific designs have occasionally become the subject of news accounts and public discussion, often with incorrect descriptions about how they work and what they do. Examples:

==={{Anchor|Alarm Clock}}Alarm Clock/Sloika===
[[File:Castle Union.gif|thumb|Castle-''Union'', 6.9&nbsp;megatons.]]
The first effort to exploit the symbiotic relationship between fission and fusion was a 1940s design that mixed fission and fusion fuel in alternating thin layers. As a single-stage device, it would have been a cumbersome application of boosted fission. It first became practical when incorporated into the secondary of a two-stage thermonuclear weapon.<ref>"The 'Alarm Clock' ... became practical only by the inclusion of Li6 (in 1950) and its combination with the radiation implosion." Hans A. Bethe, [https://fas.org/nuke/guide/usa/nuclear/bethe-52.htm Memorandum on the History of Thermonuclear Program] {{webarchive |url=https://web.archive.org/web/20160304030002/https://fas.org/nuke/guide/usa/nuclear/bethe-52.htm |date=March 4, 2016}}, May 28, 1952.</ref>

The U.S. name, Alarm Clock, came from Teller: he called it that because it might "wake up the world" to the possibility of the potential of the Super.{{sfn|Rhodes|1995|p=256}} The Russian name for the same design was more descriptive: Sloika ({{langx|ru|Слойка}}), a layered pastry cake. A single-stage Soviet Sloika was tested as [[RDS-6s]] on August 12, 1953. No single-stage U.S. version was tested, but the code named [[Castle Union]] shot of [[Operation Castle]], April 26, 1954, was a two-stage thermonuclear device code-named Alarm Clock. Its yield, at [[Bikini Atoll|Bikini]], was 6.9 megatons.{{Citation needed|date=June 2021}}

Because the Soviet Sloika test used dry lithium-6 deuteride eight months before the first U.S. test to use it (Castle Bravo, March 1, 1954), it was sometimes claimed that the USSR won the H-bomb race, even though the United States tested and developed the first hydrogen bomb: the Ivy Mike H-bomb test. The 1952 U.S. Ivy Mike test used cryogenically cooled liquid deuterium as the fusion fuel in the secondary, and employed the D-D fusion reaction. However, the first Soviet test to use a radiation-imploded secondary, the essential feature of a true H-bomb, was on November 23, 1955, three years after Ivy Mike. In fact, real work on the implosion scheme in the Soviet Union only commenced in the very early part of 1953, several months after the successful testing of Sloika.{{Citation needed|date=June 2021}}

===Clean bombs===
[[File:Bassoon Prime.jpg|right|thumb|upright|Bassoon, the prototype for a 9.3-megaton clean bomb or a 25-megaton dirty bomb. Dirty version shown here, before its 1956 test. The two attachments on the left are ''[[#Light pipes|light pipes]]''; see below for elaboration.]]
On March 1, 1954, the largest-ever U.S. nuclear test explosion, the 15-megaton [[Castle Bravo]] shot of [[Operation Castle]] at Bikini Atoll, delivered a promptly lethal dose of fission-product fallout to more than {{convert|6000|sqmi|km2}} of Pacific Ocean surface.<ref>See [[:File:Bravo fallout2.png|map]].</ref> Radiation injuries to [[Castle Bravo#Inhabited islands affected|Marshall Islanders]] and [[Daigo Fukuryū Maru|Japanese fishermen]] made that fact public and revealed the role of fission in hydrogen bombs.

In response to the public alarm over fallout, an effort was made to design a clean multi-megaton weapon, relying almost entirely on fusion. The energy produced by the fissioning of [[uranium-238|unenriched natural uranium]], when used as the tamper material in the secondary and subsequent stages in the Teller-Ulam design, can far exceed the energy released by fusion, as was the case in the Castle Bravo test. Replacing the [[fissionable]] material in the tamper with another material is essential to producing a "clean" bomb. In such a device, the tamper no longer contributes energy, so for any given weight, a clean bomb will have less yield. The earliest known incidence of a three-stage device being tested, with the third stage, called the tertiary, being ignited by the secondary, was May 27, 1956, in the Bassoon device. This device was tested in the Zuni shot of [[Operation Redwing]]. This shot used non-fissionable tampers; an inert substitute material such as tungsten or lead was used. Its yield was 3.5 megatons, 85% fusion and only 15% fission.{{Citation needed|date=June 2021}}

The Ripple concept, which used ablation to achieve fusion using very little fission, was and still is by far the cleanest design. Unlike previous clean bombs, which were clean simply by replacing fission fuel with inert substance, Ripple was by design clean. Ripple was also extremely efficient; plans for a 15 kt/kg were made during [[Operation Dominic]]. Shot Androscoggin featured a proof-of-concept Ripple design, resulting in a 63-kiloton fizzle (significantly lower than the predicted 15 megatons). It was repeated in shot Housatonic, which featured a 9.96 megaton explosion that was reportedly >99.9% fusion.<ref> {https://direct.mit.edu/jcws/article-abstract/23/2/133/101892/Ripple-An-Investigation-of-the-World-s-Most?redirectedFrom=fulltext} </ref>

The public records for devices that produced the highest proportion of their yield via fusion reactions are the [[peaceful nuclear explosion]]s of the 1970s. Others include the 10 megaton Dominic Housatonic at over 99.9% fusion, 50-megaton [[Tsar Bomba]] at 97% fusion,<ref>[https://nuclearweaponarchive.org/Nwfaq/Nfaq4-5.html 4.5 Thermonuclear Weapon Designs and Later Subsections] {{webarchive |url=https://web.archive.org/web/20160303170957/https://nuclearweaponarchive.org/Nwfaq/Nfaq4-5.html |date=March 3, 2016}}. Nuclearweaponarchive.org. Retrieved on 2011-05-01.</ref> the 9.3-megaton [[Operation Hardtack I|Hardtack Poplar]] test at 95%,<ref>[https://nuclearweaponarchive.org/Usa/Tests/Hardtack1.html Operation Hardtack I] {{webarchive |url=https://web.archive.org/web/20160910232153/https://nuclearweaponarchive.org/Usa/Tests/Hardtack1.html |date=September 10, 2016}}. Nuclearweaponarchive.org. Retrieved on 2011-05-01.</ref> and the 4.5-megaton [[Operation Redwing|Redwing Navajo]] test at 95% fusion.<ref>[https://nuclearweaponarchive.org/Usa/Tests/Redwing.html Operation Redwing] {{webarchive |url=https://web.archive.org/web/20160910232205/https://nuclearweaponarchive.org/Usa/Tests/Redwing.html |date=September 10, 2016}}. Nuclearweaponarchive.org. Retrieved on 2011-05-01.</ref>

The most ambitious peaceful application of nuclear explosions was pursued by the USSR with the aim of creating a [[Pechora–Kama Canal|{{cvt|112|km|mi|adj=on}} long canal between the Pechora river basin and the Kama river basin]], about half of which was to be constructed through a series of underground nuclear explosions. It was reported that about 250 nuclear devices might be used to get the final goal. The ''Taiga'' test was to demonstrate the feasibility of the project. Three of these "clean" devices of 15&nbsp;kiloton yield each were placed in separate boreholes spaced about {{convert|165|m|ft|sigfig=2}} apart at depths of {{convert|127|m|ft}}. They were simultaneously detonated on March 23, 1971, catapulting a radioactive plume into the air that was carried eastward by wind. The resulting trench was around {{convert|700|m|ft}} long and {{convert|340|m|ft}} wide, with an unimpressive depth of just {{convert|10|to|15|m|ft|sigfig=1}}.<ref>{{cite journal |last1=Ramzaev |first1=V. |last2=Repin |first2=V. |last3=Medvedev |first3=A. |last4=Khramtsov |first4=E. |last5=Timofeeva |first5=M. |last6=Yakovlev |first6=V. |title=Radiological investigations at the "Taiga" nuclear explosion site: Site description and in situ measurements |journal=Journal of Environmental Radioactivity |language=en |volume=102 |issue=7 |pages=672–680 |pmid=21524834 |doi=10.1016/j.jenvrad.2011.04.003 |date=July 2011 |bibcode=2011JEnvR.102..672R |url=https://linkinghub.elsevier.com/retrieve/pii/S0265931X11000750}}</ref> Despite their "clean" nature, the area still exhibits a noticeably higher (albeit mostly harmless) concentration of fission products, the intense neutron bombardment of the soil, the device itself and the support structures also activated their stable elements to create a significant amount of man-made radioactive elements like <sup>60</sup>Co. The overall danger posed by the concentration of radioactive elements present at the site created by these three devices is still negligible, but a larger scale project as was envisioned would have had significant consequences both from the fallout of radioactive plume and the radioactive elements created by the neutron bombardment.<ref>{{cite journal |last1=Ramzaev |first1=V. |last2=Repin |first2=V. |last3=Medvedev |first3=A. |last4=Khramtsov |first4=E. |last5=Timofeeva |first5=M. |last6=Yakovlev |first6=V. |title=Radiological investigations at the "Taiga" nuclear explosion site, part II: man-made γ-ray emitting radionuclides in the ground and the resultant kerma rate in air |journal=Journal of Environmental Radioactivity |language=en |volume=109 |pages=1–12 |pmid=22541991 |doi=10.1016/j.jenvrad.2011.12.009 |date=July 2012 |bibcode=2012JEnvR.109....1R |url=https://linkinghub.elsevier.com/retrieve/pii/S0265931X11003043}}</ref>

On July 19, 1956, AEC Chairman Lewis Strauss said that the [[Operation Redwing|Redwing Zuni]] shot clean bomb test "produced much of importance ... from a humanitarian aspect." However, less than two days after this announcement, the dirty version of Bassoon, called Bassoon Prime, with a [[uranium-238]] tamper in place, was tested on a barge off the coast of Bikini Atoll as the [[Operation Redwing|Redwing Tewa]] shot. The Bassoon Prime produced a 5-megaton yield, of which 87% came from fission. Data obtained from this test, and others, culminated in the eventual deployment of the highest-yielding US nuclear weapon known, and the highest [[nuclear weapon yield|yield-to-weight weapon ever made]], a three-stage thermonuclear weapon with a maximum "dirty" yield of 25 megatons, designated as the [[B41 nuclear bomb]], which was to be carried by U.S. Air Force bombers until it was decommissioned; this weapon was never fully tested.{{Citation needed|date=June 2021|reason=also relevancy}}

===Third generation===
First and second generation nuclear weapons release energy as omnidirectional blasts. Third generation<ref>{{cite book |title=The Role and Control of Weapons in the 1990s |last1=Barnaby |first1=Frank |date=2012 |publisher=Routledge |isbn=978-1134901913 |url=https://books.google.com/books?id=H8wwRGrD6V4C&q=third+generation+nuclear+weapons+project+excalibur+prometheus&pg=PT148 |access-date=2020-11-02 |url-status=live |archive-url=https://web.archive.org/web/20210904154853/https://books.google.com/books?id=H8wwRGrD6V4C&q=third+generation+nuclear+weapons+project+excalibur+prometheus&pg=PT148 |archive-date=2021-09-04}}</ref><ref>{{cite web |title=Bulletin of the Atomic Scientists |publisher=Educational Foundation for Nuclear Science, Inc |date=March 1991 |url=https://books.google.com/books?id=rwwAAAAAMBAJ&q=shaped+nuclear+charge+third+generation+nuclear+weapons&pg=PA31 |access-date=2020-11-02 |url-status=live |archive-url=https://web.archive.org/web/20210904154853/https://books.google.com/books?id=rwwAAAAAMBAJ&q=shaped+nuclear+charge+third+generation+nuclear+weapons&pg=PA31 |archive-date=2021-09-04}}</ref><ref>{{cite book |title=SDI: Technology, survivability, and software |publisher=DIANE |isbn=978-1428922679 |url=https://books.google.com/books?id=XDTo_35uQcUC&q=sdi+nuclear+shotgun&pg=PA122 |access-date=2020-11-02 |url-status=live |archive-url=https://web.archive.org/web/20210904154853/https://books.google.com/books?id=XDTo_35uQcUC&q=sdi+nuclear+shotgun&pg=PA122 |archive-date=2021-09-04}}</ref> nuclear weapons are experimental special effect warheads and devices that can release energy in a directed manner, some of which were tested during the [[Cold War]] but were never deployed. These include:
* Project Prometheus, also known as "Nuclear Shotgun", which would have used a nuclear explosion to accelerate kinetic penetrators against ICBMs.<ref>{{cite book |title=The Role and Control of Weapons in the 1990s |isbn=978-1134901913 |last1=Barnaby |first1=Frank |date=2012 |publisher=Routledge |url=https://books.google.com/books?id=H8wwRGrD6V4C&q=prometheus+nuclear+shotgun&pg=PT148 |access-date=2020-11-02 |url-status=live |archive-url=https://web.archive.org/web/20210904154854/https://books.google.com/books?id=H8wwRGrD6V4C&q=prometheus+nuclear+shotgun&pg=PT148 |archive-date=2021-09-04}}</ref>
* [[Project Excalibur]], a nuclear-pumped X-ray laser to [[ballistic missile defense|destroy ballistic missiles]].
* [[Nuclear shaped charge]]s that focus their energy in particular directions.
* [[Project Orion (nuclear propulsion)|Project Orion]] explored the use of nuclear explosives for rocket propulsion.

===Fourth generation===
The idea of "4th-generation" nuclear weapons has been proposed as a possible successor to the examples of weapons designs listed above. These methods tend to revolve around using non-nuclear primaries to set off further fission or fusion reactions. For example, if [[antimatter]] were usable and controllable in macroscopic quantities, a reaction between a small amount of antimatter and an equivalent amount of matter could release energy comparable to a small fission weapon, and could in turn be used as the first stage of a very compact thermonuclear weapon. Extremely-powerful  lasers could also potentially be used this way, if they could be made powerful-enough, and compact-enough, to be viable as a weapon. Most of these ideas are versions of [[pure fusion weapon]]s, and share the common property that they involve hitherto unrealized technologies as their "primary" stages.<ref>{{cite arXiv |eprint=physics/0510071 |last1=Gsponer |first1=Andre |title=Fourth Generation Nuclear Weapons: Military effectiveness and collateral effects |year=2005}}</ref>

While many nations have invested significantly in [[inertial confinement fusion]] research programs, since the 1970s it has not been considered promising for direct weapons use, but rather as a tool for weapons- and energy-related research that can be used in the absence of full-scale nuclear testing. Whether any nations are aggressively pursuing "4th-generation" weapons is not clear. In many case (as with antimatter) the underlying technology is presently thought to be very far from being viable, and if it was viable would be a powerful weapon in and of itself, outside of a nuclear weapons context, and without providing any significant advantages above existing nuclear weapons designs<ref>[http://whyfiles.org/167new_nukes/4.html Never say "never"] {{webarchive |url=https://web.archive.org/web/20160418234450/http://whyfiles.org/167new_nukes/4.html |date=April 18, 2016}}. Whyfiles.org. Retrieved on 2011-05-01.</ref>

===Pure fusion weapons===
{{Main|Pure fusion weapon}}

Since the 1950s, the United States and Soviet Union investigated the possibility of releasing significant amounts of nuclear fusion energy without the use of a fission primary. Such "pure fusion weapons" were primarily imagined as low-yield, tactical nuclear weapons whose advantage would be their ability to be used without producing fallout on the scale of weapons that release fission products. In 1998, the [[United States Department of Energy]] declassified the following:
{{blockquote|
(1) Fact that the DOE made a substantial investment in the past to develop a pure fusion weapon

(2)	That the U.S. does not have and is not developing a pure fusion weapon; and
(3)	That no credible design for a pure fusion weapon resulted from the DOE investment.<ref>{{cite web|title=Restricted Data Declassification Decisions, 1946 to the Present (RDD-7)|url=https://sgp.fas.org/othergov/doe/rdd-7.html|date=1 January 2001}}</ref>}}

[[Red mercury]], a likely hoax substance, has been hyped as a catalyst for a pure fusion weapon.{{Citation needed|date=April 2024}}

===Cobalt bombs===
{{Main|Cobalt bomb}} {{See also|Salted bomb}}
A doomsday bomb, made popular by [[Nevil Shute]]'s 1957 [[On the Beach (novel)|novel]], and subsequent 1959 movie, ''[[On the Beach (1959 film)|On the Beach]]'', the cobalt bomb is a hydrogen bomb with a jacket of cobalt. The neutron-activated cobalt would have maximized the environmental damage from radioactive fallout. These bombs were popularized in the 1964 film ''[[Dr. Strangelove|Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb]]''; the material added to the bombs is referred to in the film as 'cobalt-thorium G'.{{Citation needed|date=June 2021}}

Such "salted" weapons were investigated by U.S. Department of Defense.<ref>{{cite book |first1=Samuel |last1=Glasstone |title=The Effects of Nuclear Weapons |year=1962 |publisher=U.S. Department of Defense, U.S. Atomic Energy Commission |pages=464–466 |url=https://books.google.com/books?id=Ovu108BraNUC}}</ref> Fission products are as deadly as neutron-activated cobalt.

Initially, gamma radiation from the fission products of an equivalent size fission-fusion-fission bomb are much more intense than [[Cobalt-60]] ({{SimpleNuclide|cobalt|60}}): 15,000 times more intense at 1 hour; 35 times more intense at 1 week; 5 times more intense at 1 month; and about equal at 6 months. Thereafter fission drops off rapidly so that {{SimpleNuclide|cobalt|60}} fallout is 8 times more intense than fission at 1 year and 150 times more intense at 5 years. The very long-lived isotopes produced by fission would overtake the {{SimpleNuclide|cobalt|60}} again after about 75 years.<ref name="Nuclear Weapons FAQ: 1.6"/>

The triple "taiga" nuclear [[salvo]] test, as part of the preliminary March 1971 [[Pechora–Kama Canal]] project, produced a small amount of fission products and therefore a comparatively large amount of case material activated products are responsible for most of the residual activity at the site today, namely {{SimpleNuclide|cobalt|60}}. {{As of|2011|post=,}} [[neutron activation|fusion generated neutron activation]] was responsible for about half of the gamma dose at the test site. That dose is too small to cause deleterious effects, and normal green vegetation exists all around the lake that was formed.<ref>{{cite journal |title=Radiological investigations at the 'Taiga' nuclear explosion site: Site description and in situ measurements |pmid=21524834 |volume=102 |issue=7 |journal=Journal of Environmental Radioactivity |pages=672–680 |year=2011 |last1=Ramzaev |first1=V |last2=Repin |first2=V |last3=Medvedev |first3=A |last4=Khramtsov |first4=E |last5=Timofeeva |first5=M |last6=Yakovlev |first6=V |doi=10.1016/j.jenvrad.2011.04.003|bibcode=2011JEnvR.102..672R }}</ref><ref>{{cite journal |title=Radiological investigations at the 'Taiga' nuclear explosion site, part II: man-made γ-ray emitting radionuclides in the ground and the resultant kerma rate in air |pmid=22541991 |volume=109 |journal=Journal of Environmental Radioactivity |pages=1–12 |year=2012 |last1=Ramzaev |first1=V |last2=Repin |first2=V |last3=Medvedev |first3=A |last4=Khramtsov |first4=E |last5=Timofeeva |first5=M |last6=Yakovlev |first6=V |doi=10.1016/j.jenvrad.2011.12.009|bibcode=2012JEnvR.109....1R }}</ref>

===Arbitrarily large multi-staged devices===
The idea of a device which has an arbitrarily large number of Teller-Ulam stages, with each driving a larger radiation-driven implosion than the preceding stage, is frequently suggested,<ref>{{cite book
|last1=Winterberg
|first1=Friedwardt
|title=The Release of Thermonuclear Energy by Inertial Confinement: Ways Towards Ignition
|publisher=World Scientific
|date=2010
|pages=192–193
|isbn=978-9814295918
|url=https://books.google.com/books?id=B7RV_vASz0EC&q=arbitrarily+large+gains%22staged+Teller-Ulam&pg=PA192
|access-date=2020-11-02
|url-status=live
|archive-url=https://web.archive.org/web/20210805053441/https://books.google.com/books?id=B7RV_vASz0EC&q=arbitrarily+large+gains%22staged+Teller-Ulam&pg=PA192
|archive-date=2021-08-05
}}</ref><ref>{{cite book
|last1=Croddy
|first1=Eric A.
|last2=Wirtz
|first2=James J.
|last3=Larsen
|first3=Jeffrey, Eds.
|title=Weapons of Mass Destruction: An Encyclopedia of Worldwide Policy, Technology, and History
|publisher=ABC-CLIO, Inc.
|date=2005
|page=376
|isbn=978-1-85109-490-5
|url=https://books.google.com/books?id=ZzlNgS70OHAC&q=almost+unlimited+yield&pg=RA1-PA376
|access-date=2020-11-02
|url-status=live
|archive-url=https://web.archive.org/web/20210904154854/https://books.google.com/books?id=ZzlNgS70OHAC&q=almost+unlimited+yield&pg=RA1-PA376
|archive-date=2021-09-04
}}</ref> but technically disputed.<ref name=ieri>{{cite web |title=Fission, Fusion and Staging |website=[[IERI]] |url=https://www.ieri.be/fr/publications/ierinews/2011/juillet/fission-fusion-and-staging |access-date=2013-05-22 |url-status=live |archive-url=https://web.archive.org/web/20160305053224/http://ieri.be/fr/publications/ierinews/2011/juillet/fission-fusion-and-staging |archive-date=2016-03-05}}.</ref> There are "well-known sketches and some reasonable-looking calculations in the open literature about two-stage weapons, but no similarly accurate descriptions of true three stage concepts."<ref name=ieri/>

During the mid-1950s through early 1960s, scientists working in the weapons laboratories of the United States investigated weapons concepts as large as 1,000&nbsp;megatons,<ref>[https://nsarchive2.gwu.edu/nukevault/ebb249/doc09.pdf The Air Force and Strategic Deterrence 1951–1960. USAF historical division Liaison Office by George F. Lemmer 1967, p. 13. Formerly restricted data] {{webarchive |url=https://web.archive.org/web/20140617080527/http://www2.gwu.edu/~nsarchiv/nukevault/ebb249/doc09.pdf |date=June 17, 2014}}.</ref> and [[Edward Teller]] announced the design of a 10,000-megaton weapon code-named [[Sundial (weapon)|SUNDIAL]] at a meeting of the General Advisory Committee of the Atomic Energy Commission.<ref>{{cite web|title=In Search of a Bigger Boom|url=https://blog.nuclearsecrecy.com/2012/09/12/in-search-of-a-bigger-boom/|last=Wellerstein|first=Alex|date=12 September 2012}}</ref> Much of the information about these efforts remains classified,<ref>{{cite web |title=2013 FOIA Log |url=https://documents.theblackvault.com/documents/foia/FOIA%2014-00108-H.pdf |access-date=2014-10-06 |url-status=live |archive-url=https://web.archive.org/web/20160304063659/http://documents.theblackvault.com/documents/foia/FOIA%2014-00108-H.pdf |archive-date=2016-03-04}}</ref><ref>{{cite web |title=Case No. FIC-15-0005 |url=https://www.energy.gov/sites/prod/files/2016/04/f30/FIC-15-0005.pdf |access-date=2016-10-25 |url-status=live |archive-url=https://web.archive.org/web/20161025114419/http://energy.gov/sites/prod/files/2016/04/f30/FIC-15-0005.pdf |archive-date=2016-10-25}}</ref> but such "gigaton" range weapons do not appear to have made it beyond theoretical investigations.

While both the US and Soviet Union investigated (and in the case of the Soviets, tested) "very high yield" (e.g. 50 to 100-megaton) weapons designs in the 1950s and early 1960s,<ref>{{cite web|url=https://thebulletin.org/2021/11/the-untold-story-of-the-worlds-biggest-nuclear-bomb/|title=An Unearthly Spectacle: The Untold Story of the World's Biggest Bomb|last=Wellerstein|first=Alex|publisher=Bulletin of the Atomic Scientists|date=29 October 2021}}</ref> these appear to represent the upper-limit of Cold War weapon yields pursued seriously, and were so physically heavy and massive that they could not be carried entirely within the bomb bays of the largest bombers. Cold War warhead development trends from the mid-1960s onward, and especially after the [[Limited Test Ban Treaty]], instead resulted in highly-compact warheads with yields in the range from hundreds of kilotons to the low megatons that gave greater options for deliverability.

Following the concern caused by the estimated gigaton scale of the 1994 [[Comet Shoemaker-Levy 9]] impacts on the planet [[Jupiter]], in a 1995 meeting at [[Lawrence Livermore National Laboratory]] (LLNL), [[Edward Teller]] proposed to a collective of U.S. and Russian ex-[[Cold War]] weapons designers that they collaborate on designing a 1,000-megaton [[asteroid impact avoidance#Nuclear explosive device|nuclear explosive device for diverting extinction-class asteroids]] (10+ km in diameter), which would be employed in the event that one of these asteroids were on an impact trajectory with Earth.<ref>{{cite web |title=A new use for nuclear weapons: hunting rogue asteroids A persistent campaign by weapons designers to develop a nuclear defense against extraterrestrial rocks slowly wins government support 2013 |website=Center for Public Integrity |date=2013-10-16 |url=https://publicintegrity.org/national-security/a-new-use-for-nuclear-weapons-hunting-rogue-asteroids/ |access-date=7 October 2014 |url-status=live |archive-url=https://web.archive.org/web/20160320055111/http://www.publicintegrity.org/2013/10/16/13547/new-use-nuclear-weapons-hunting-rogue-asteroids |archive-date=2016-03-20}}</ref><ref>{{cite web |title=The mother of all bombs would sit in wait in an orbitary platform |author=Jason Mick |date=October 17, 2013 |url=http://www.dailytech.com/Russia+US+Eye+Teamup+to+Build+Massive+Nuke+to+Save+Planet+from+an+Asteroid/article33569.htm |url-status=dead |archive-url=https://web.archive.org/web/20141009190305/http://www.dailytech.com/Russia+US+Eye+Teamup+to+Build+Massive+Nuke+to+Save+Planet+from+an+Asteroid/article33569.htm#sthash.rQvVzS6m.dpuf |archive-date=October 9, 2014}}</ref><ref>[https://web.archive.org/web/20150909023233/https://e-reports-ext.llnl.gov/pdf/232015.pdf planetary defense workshop LLNL 1995]</ref>

===Neutron bombs===
{{Main|Neutron bomb}}

A neutron bomb, technically referred to as an enhanced radiation weapon (ERW), is a type of tactical nuclear weapon designed specifically to release a large portion of its energy as energetic neutron radiation. This contrasts with standard thermonuclear weapons, which are designed to capture this intense neutron radiation to increase its overall explosive yield. In terms of yield, ERWs typically produce about one-tenth that of a fission-type atomic weapon. Even with their significantly lower explosive power, ERWs are still capable of much greater destruction than any conventional bomb. Meanwhile, relative to other nuclear weapons, damage is more focused on biological material than on material infrastructure (though extreme blast and heat effects are not eliminated).{{Citation needed|date=June 2021}}

ERWs are more accurately described as suppressed yield weapons. When the yield of a nuclear weapon is less than one kiloton, its lethal radius from blast, {{convert|700|m|ft|abbr=on}}, is less than that from its neutron radiation. However, the blast is more than potent enough to destroy most structures, which are less resistant to blast effects than even unprotected human beings. Blast pressures of upwards of {{cvt|20|psi|kPa}} are survivable, whereas most buildings will collapse with a pressure of only {{cvt|5|psi|kPa|sigfig=1}}.{{Citation needed|date=June 2021}}

Commonly misconceived as a weapon designed to kill populations and leave infrastructure intact, these bombs (as mentioned above) are still very capable of leveling buildings over a large radius. The intent of their design was to kill tank crews – tanks giving excellent protection against blast and heat, surviving (relatively) very close to a detonation. Given the Soviets' vast tank forces during the Cold War, this was the perfect weapon to counter them. The neutron radiation could instantly incapacitate a tank crew out to roughly the same distance that the heat and blast would incapacitate an unprotected human (depending on design). The tank chassis would also be rendered highly radioactive, temporarily preventing its re-use by a fresh crew.{{Citation needed|date=June 2021}}

Neutron weapons were also intended for use in other applications, however. For example, they are effective in anti-nuclear defenses – the neutron flux being capable of neutralising an incoming warhead at a greater range than heat or blast. Nuclear warheads are very resistant to physical damage, but are very difficult to harden against extreme neutron flux.{{Citation needed|date=June 2021}}

{| class="wikitable" style="float:right; text-align:center;"
|+ Energy distribution of weapon
|-
! !! Standard !! Enhanced
|-
| Blast || 50% || 40%
|-
| Thermal energy || 35% || 25%
|-
| Instant radiation || 5% || 30%
|-
| Residual radiation || 10% || 5%
|}

ERWs were two-stage thermonuclears with all non-essential uranium removed to minimize fission yield. Fusion provided the neutrons. Developed in the 1950s, they were first deployed in the 1970s, by U.S. forces in Europe. The last ones were retired in the 1990s.{{Citation needed|date=June 2021}}

A neutron bomb is only feasible if the yield is sufficiently high that efficient fusion stage ignition is possible, and if the yield is low enough that the case thickness will not absorb too many neutrons. This means that neutron bombs have a yield range of 1–10 kilotons, with fission proportion varying from 50% at 1&nbsp;kiloton to 25% at 10&nbsp;kilotons (all of which comes from the primary stage). The neutron output per kiloton is then 10 to 15 times greater than for a pure fission implosion weapon or for a strategic warhead like a [[W87]] or [[W88]].<ref name="Neutron bomb: Why 'clean' is deadly"/>

==Weapon design laboratories==

All the nuclear weapon design innovations discussed in this article originated from the following three labs in the manner described. Other nuclear weapon design labs in other countries duplicated those design innovations independently, reverse-engineered them from fallout analysis, or acquired them by espionage.<ref>William J. Broad, "The Hidden Travels of The Bomb: Atomic insiders say the weapon was invented only once, and its secrets were spread around the globe by spies, scientists and the covert acts of nuclear states", ''New York Times'', December 9, 2008, p. D1.</ref>

===Lawrence Berkeley===
{{Main|Lawrence Berkeley National Laboratory}}
The first systematic exploration of nuclear weapon design concepts took place in mid-1942 at the [[University of California, Berkeley]]. Important early discoveries had been made at the adjacent [[Lawrence Berkeley Laboratory]], such as the 1940 cyclotron-made production and isolation of plutonium. A Berkeley professor, [[J. Robert Oppenheimer]], had just been hired to run the nation's secret bomb design effort. His first act was to convene the 1942 summer conference.{{Citation needed|date=June 2021}}

By the time he moved his operation to the new secret town of Los Alamos, New Mexico, in the spring of 1943, the accumulated wisdom on nuclear weapon design consisted of five lectures by Berkeley professor [[Robert Serber]], transcribed and distributed as the (classified but now fully declassified and widely available online as a PDF) [[Los Alamos Primer]].<ref name="Primer">{{cite book |last1=Server |first1=Robert |title=The Los Alamos Primer |publisher=University of California Press |location=Berkeley |isbn=978-0520075764 |edition=1st |date=1992}}</ref> The Primer addressed fission energy, [[neutron]] production and [[neutron capture|capture]], [[nuclear chain reaction]]s, [[critical mass]], tampers, predetonation, and three methods of assembling a bomb: gun assembly, implosion, and "autocatalytic methods", the one approach that turned out to be a dead end.{{Citation needed|date=June 2021}}

===Los Alamos===
{{Main|Los Alamos National Laboratory}}
At Los Alamos, it was found in April 1944 by [[Emilio Segrè]] that the proposed [[Thin Man nuclear bomb|Thin Man]] Gun assembly type bomb would not work for plutonium because of predetonation problems caused by [[Pu-240]] impurities. So Fat Man, the implosion-type bomb, was given high priority as the only option for plutonium. The Berkeley discussions had generated theoretical estimates of critical mass, but nothing precise. The main wartime job at Los Alamos was the experimental determination of critical mass, which had to wait until sufficient amounts of fissile material arrived from the production plants: uranium from [[Oak Ridge, Tennessee]], and plutonium from the [[Hanford Site]] in Washington.{{Citation needed|date=June 2021}}

In 1945, using the results of critical mass experiments, Los Alamos technicians fabricated and assembled components for four bombs: the ''[[Trinity test|Trinity]]'' [[the gadget|Gadget]], Little Boy, Fat Man, and an unused spare Fat Man. After the war, those who could, including Oppenheimer, returned to university teaching positions. Those who remained worked on levitated and hollow pits and conducted weapon effects tests such as [[Operation Crossroads|Crossroads]] Able and Baker at [[Bikini Atoll]] in 1946.{{Citation needed|date=June 2021}}

All of the essential ideas for incorporating fusion into nuclear weapons originated at Los Alamos between 1946 and 1952. After the [[Teller-Ulam design|Teller-Ulam]] radiation implosion breakthrough of 1951, the technical implications and possibilities were fully explored, but ideas not directly relevant to making the largest possible bombs for long-range Air Force bombers were shelved.{{Citation needed|date=June 2021}}

Because of Oppenheimer's initial position in the H-bomb debate, in opposition to large thermonuclear weapons, and the assumption that he still had influence over Los Alamos despite his departure, political allies of [[Edward Teller]] decided he needed his own laboratory in order to pursue H-bombs. By the time it was opened in 1952, in [[Lawrence Livermore National Laboratory|Livermore]], California, Los Alamos had finished the job Livermore was designed to do.{{Citation needed|date=June 2021}}

===Lawrence Livermore===
{{Main|Lawrence Livermore National Laboratory}}
With its original mission no longer available, the Livermore lab tried radical new designs that failed. Its first three nuclear tests were [[fizzle (nuclear test)|fizzles]]: in 1953, two single-stage [[uranium hydride bomb|fission devices with uranium hydride pits]], and in 1954, a two-stage thermonuclear device in which the secondary heated up prematurely, too fast for radiation implosion to work properly.{{Citation needed|date=June 2021}}

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>

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 (nuclear warhead)|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.{{Citation needed|date=June 2021}}

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-235 into the secondary, as it became available with continued [[uranium enrichment]] and the dismantlement of the large high-yield bombs.{{Citation needed|date=June 2021}}

Starting with the [[Nova (laser)#Fusion in Nova|Nova]] facility at Livermore in the mid-1980s, nuclear design activity pertaining to radiation-driven implosion was informed by research with ''indirect drive'' laser fusion. This work was part of the effort to investigate [[Inertial confinement fusion#Nuclear weapons|Inertial Confinement Fusion]]. Similar work continues at the more powerful [[National Ignition Facility]]. The [[Stockpile stewardship#Stockpile Stewardship Management Program|Stockpile Stewardship and Management Program]] also benefited from research performed at [[National Ignition Facility#Stockpile experiments, 2013–2015|NIF]].{{Citation needed|date=June 2021}}

==Explosive testing==
Nuclear weapons are in large part designed by trial and error. The trial often involves test explosion of a prototype.

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, [https://fas.org/irp/ops/ci/goad.html Declaration for the Wen Ho Lee case] {{webarchive |url=https://web.archive.org/web/20160308031512/https://fas.org/irp/ops/ci/goad.html |date=March 8, 2016}}, 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>

It was easy enough to design reliable weapons for the stockpile. If the prototype worked, it could be weaponized and mass-produced.{{Citation needed|date=June 2021}}

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 [[wikt:fudge factor|fudge factors]] 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.{{Citation needed|date=June 2021}}

{{Anchor|Light pipes}}

===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 by instruments located at the far end of a long, very straight pipe.{{Citation needed|date=June 2021}}

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 actually 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-rays 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>
[[File:Castle Bravo Shrimp composite.png|600 px|centre]]

From the shot cab, the pipes turned horizontally and traveled {{convert|7500|ft|km|abbr=on|order=flip}} along a causeway built on the Bikini reef to a remote-controlled data collection bunker on Namu Island.{{Citation needed|date=June 2021}}

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 creates a relatively opaque radiation front in the channel filler, which acts like a slow-moving logjam to retard the passage of [[radiant energy]]. While the secondary is being compressed via radiation-induced ablation, neutrons from the primary catch up with the x-rays, penetrate into the secondary, and start breeding tritium via the third reaction noted in the first section above. This [[lithium-6|<sup>6</sup>Li]] + n reaction is exothermic, producing 5 MeV per event. The spark plug has not yet been compressed and thus remains subcritical, so no significant fission or fusion takes place as a result. If enough neutrons arrive before implosion of the secondary is complete, though, the crucial temperature differential between the outer and inner parts of the secondary can be degraded, potentially causing the secondary to fail to ignite. The first Livermore-designed thermonuclear weapon, the Morgenstern device, failed in this manner when it was tested as [[Castle Koon]] on April 7, 1954. The primary ignited, but the secondary, preheated by the primary's neutron wave, suffered what was termed as an ''inefficient detonation'';<ref name="swordsIV">{{cite book |author-link=Chuck Hansen |first=Chuck |last=Hansen |title=Swords of Armageddon |volume=IV |date=1995 |url=https://www.uscoldwar.com/ |access-date=2016-05-20 |url-status=live |archive-url=https://web.archive.org/web/20161230020259/http://www.uscoldwar.com/ |archive-date=2016-12-30}}</ref>{{rp|165}} thus, a weapon with a predicted one-megaton yield produced only 110 kilotons, of which merely 10 kt were attributed to fusion.<ref name="swordsIII">{{cite book |author-link=Chuck Hansen |first=Chuck |last=Hansen |title=Swords of Armageddon |volume=III |date=1995 |url=https://www.uscoldwar.com/ |access-date=2016-05-20 |url-status=live |archive-url=https://web.archive.org/web/20161230020259/http://www.uscoldwar.com/ |archive-date=2016-12-30}}</ref>{{rp|316}}

These timing effects, and any problems they cause, 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.{{Citation needed|date=June 2021}}

It is not clear from the public record how successful the Shrimp light pipes were. The unmanned data bunker was far enough back to remain outside the mile-wide crater, but the 15-megaton blast, two and a half times as powerful as expected, breached the bunker by blowing its 20-ton door off the hinges and across the inside of the bunker. (The nearest people were {{convert|20|mi|km|order=flip}} 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.</ref>

===Fallout analysis===
{{See also|Nuclear forensics}}
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.{{Citation needed|date=June 2021}}

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>{{cite book |first=Richard |last=Rhodes |title=Dark Sun; the Making of the Hydrogen Bomb |url-access=limited |publisher=Simon and Schuster |year=1995 |page=[https://archive.org/details/darksunmakinghyd00rhod/page/n568 541] |isbn=9780684804002 |url=https://archive.org/details/darksunmakinghyd00rhod}}</ref>

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, ''[[Daigo Fukuryū Maru]]'', steamed home with enough fallout on her 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.{{Citation needed|date=June 2021}}

===Underground testing===
{{Main|Underground nuclear weapons testing}}
[[File: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 literally 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.{{Citation needed|date=June 2021}}

The [[Yucca Flat]] section of the NTS is covered with subsidence craters resulting from the collapse of terrain over radioactive 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 tritium 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.{{Citation needed|date=June 2021|reason=W88 was test full yield before ban}}

==Production facilities==
{{Globalize|section|USA|2name=the United States|date=June 2014}}
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 near Boulder, Colorado, was built in 1952 for pit production and consequently became the plutonium and beryllium fabrication facility.{{Citation needed|date=June 2021}}

The Y-12 plant in [[Oak Ridge, Tennessee|Oak Ridge]], [[Tennessee]], where [[mass spectrometer]]s called [[calutron]]s 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.{{Citation needed|date=June 2021}}

Y-12 made lithium-6 deuteride fusion fuel and U-238 parts, the other two ingredients of secondaries.{{Citation needed|date=June 2021}}

The Hanford Site near Richland WA operated Plutonium production nuclear reactors and separations facilities during World War 2 and the Cold War. Nine Plutonium production reactors were built and operated there. The first being the B-Reactor which began operations in September 1944 and the last being the N-Reactor which ceased operations in January 1987.{{Citation needed|date=June 2021}}

The [[Savannah River Site]] in [[Aiken, South Carolina|Aiken]], [[South Carolina]], also built in 1952, operated [[nuclear reactor]]s 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.{{Citation needed|date=June 2021}}

==Warhead design safety==
Because even low-yield nuclear warheads have astounding destructive power, weapon designers have always recognised the need to incorporate mechanisms and associated procedures intended to prevent accidental detonation.{{Citation needed|date=June 2021}}

[[File:Steel balls.png|thumb|right|300px|A diagram of the ''[[Green Grass (nuclear warhead)|Green Grass]]'' warhead's steel ball 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===
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 propellant in Little Boy (four bags of [[cordite]]) was inserted into the bomb in flight, shortly after takeoff on August 6, 1945. This was the first time a gun-type nuclear weapon had ever been fully assembled.{{Citation needed|date=June 2021}}

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. Similarly, a fire caused by an aircraft crashing could easily ignite the propellant, with catastrophic results. Gun-type weapons have always been inherently unsafe.{{Citation needed|date=June 2021|reason=Safing schemes for the reliable earth penetrator warhead describes safing schemes for gun-type weapons}}

===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.{{Citation needed|date=June 2021|reason=modern weapons were still one-point tested}}

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 use 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.{{Citation needed|date=June 2021|reason=images of IFI systems show a cylinder with HE one end and the pit on the other being inserted}}

In-flight pit insertion will not work with a hollow pit in contact with its tamper.{{Citation needed|date=June 2021|reason=utter nonsense. Whoever wrote that has not even done basic research about IFI}}

===Steel ball safety method===
As shown in the diagram above, one method used to decrease the likelihood of accidental detonation employed [[ball (bearing)|metal balls]]. The balls were emptied into the pit: this prevented detonation by increasing the density of the hollow pit, thereby preventing symmetrical implosion in the event of an accident. 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 (nuclear weapon)|Yellow Sun Mk.1]] bombs.{{Citation needed|date=June 2021}}

[[File:One-Point Safety Test.svg|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 cannot 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 military nuclear accidents|several weapons accidents]], it could not cause a nuclear explosion.{{Citation needed|date=June 2021}}

===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.{{Citation needed|date=June 2021|reason=utter nonsense. Big citation needed for the claim hollow pits are one-point safe. Also boosting requires significant yield to function. A weapon making 0.1kt of yield from a one point detonation is not safe}}

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.{{Citation needed|date=June 2021|reason=}}

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.{{Citation needed|date=June 2021|reason=}}

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.{{Citation needed|date=June 2021|reason=}}

Of particular concern was Livermore's [[W47]], which generated unacceptably high yields in one-point testing. To prevent an accidental detonation, Livermore decided to use mechanical safing on the W47. The wire safety scheme described below was the result.{{Citation needed|date=June 2021|reason=wan device, failed massively, would suggest above claims are very wrong}}

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.{{Citation needed|date=June 2021|reason=}}

===Wire safety method===
In the last test before the 1958 moratorium the W47 warhead for the Polaris SLBM was found to not be one-point safe, producing an unacceptably high nuclear yield of {{convert|200|kg|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. A solution was devised consisting of a [[boron]]-coated wire inserted into the weapon's hollow pit at manufacture. The warhead was armed by withdrawing the wire onto a spool driven by an electric motor. Once withdrawn, the wire could not be re-inserted.<ref>Chuck Hansen, ''The Swords of Armageddon'', Volume VII, pp. 396–397.</ref> The wire had a tendency to become brittle during storage, and break or get stuck during arming, preventing complete removal and rendering the warhead a dud.<ref name="dud"/> It was estimated that 50–75% of warheads would fail. This required a complete rebuild of all W47 primaries.<ref>{{cite journal |last1=Harvey |first1=John R. |last2=Michalowski |first2=Stefan |title=Nuclear Weapons Safety:The Case of Trident |journal=Science & Global Security |volume=4 |issue=3 |pages=261–337 |bibcode=1994S&GS....4..261H |doi=10.1080/08929889408426405 |date=1994 |url=https://scienceandglobalsecurity.org/archive/sgs04harvey.pdf |url-status=live |archive-url=https://web.archive.org/web/20121016101827/http://www.princeton.edu/sgs/publications/sgs/pdf/4_3harvey.pdf |archive-date=2012-10-16}}</ref> The oil used for lubricating the wire also promoted corrosion of the pit.<ref>{{cite book |isbn=978-0521054010 |title=From Polaris to Trident: The Development of the U.S. Fleet Ballistic Missile Technology |url=https://books.google.com/books?id=95eoQSNDp6gC&q=warhead+corrosion&pg=PA214}}.{{dead link|date=November 2016|bot=InternetArchiveBot |fix-attempted=yes}}</ref>

===Strong link/weak link===
{{See also|Strong link/weak link}}
Under the strong link/weak link system, "weak links" are constructed between critical nuclear weapon components (the "hard links"). In the event of an accident the weak links are designed to fail first in a manner that precludes energy transfer between them. Then, if a hard link fails in a manner that transfers or releases energy, energy can't be transferred into other weapon systems, potentially starting a nuclear detonation. Hard links are usually critical weapon components that have been hardened to survive extreme environments, while weak links can be both components deliberately inserted into the system to act as a weak link and critical nuclear components that can fail predictably.{{Citation needed|date=June 2021|reason=}}

An example of a weak link would be an electrical connector that contains electrical wires made from a low melting point alloy. During a fire, those wires would melt, breaking any electrical connection.{{Citation needed|date=June 2021|reason=}}

===Permissive action link===
{{See also|Permissive action link}}
A ''permissive action link'' is an [[access control]] device designed to prevent unauthorised use of nuclear weapons. Early PALs were simple electromechanical switches and have evolved into complex arming systems that include integrated yield control options, lockout devices and anti-tamper devices.{{Citation needed|date=April 2024}}

==References==

===Notes===
{{Reflist|30em|refs=
<ref name="CLR">Howard Morland, [https://fas.org/sgp/eprint/cardozo.pdf "Born Secret"] {{Webarchive|url=https://web.archive.org/web/20171212004751/https://fas.org/sgp/eprint/cardozo.pdf |date=2017-12-12}}, '''Cardozo Law Review''', March 2005, pp. 1401–1408.</ref>
<ref name="dud">Sybil Francis, ''Warhead Politics'', pp. 141, 160.</ref>
<ref name="fusionmath">Glasstone and Dolan, ''Effects'', p. 21.</ref>
<ref name="NWFAQ-6.2">[https://nuclearweaponarchive.org/Nwfaq/Nfaq6.html#nfaq6.2 Fissionable Materials] {{webarchive |url=https://web.archive.org/web/20061003233329/https://nuclearweaponarchive.org/Nwfaq/Nfaq6.html#nfaq6.2 |date=October 3, 2006}} section of the [https://nuclearweaponarchive.org/Nqfaq Nuclear Weapons FAQ],{{dead link|date=September 2018|bot=medic}}{{cbignore|bot=medic}} Carey Sublette, accessed Sept 23, 2006</ref>
<ref name="Neutron bomb: Why 'clean' is deadly">{{cite news |title=Neutron bomb: Why 'clean' is deadly |work=BBC News |date=July 15, 1999 |url=http://news.bbc.co.uk/1/hi/sci/tech/395689.stm |access-date=January 6, 2010 |url-status=live |archive-url=https://web.archive.org/web/20090407070250/http://news.bbc.co.uk/1/hi/sci/tech/395689.stm |archive-date=April 7, 2009}}</ref>
<ref name="Nuclear Weapons FAQ: 1.6">{{cite web |title=Nuclear Weapons FAQ: 1.6 |last=Sublette |first=Carey |url=https://nuclearweaponarchive.org/Nwfaq/Nfaq1.html#nfaq1.6}}</ref>
<ref name="RDD-7">{{cite web |title=Restricted Data Declassification Decisions from 1946 until Present |url=https://fas.org/sgp/othergov/doe/rdd-7.html |access-date=7 October 2014 |url-status=live |archive-url=https://web.archive.org/web/20200404220155/https://fas.org/sgp/othergov/doe/rdd-7.html |archive-date=4 April 2020}}</ref>
}}

===Bibliography===
{{Refbegin|30em}}
* [[Samuel T. Cohen|Cohen, Sam]], ''The Truth About the Neutron Bomb: The Inventor of the Bomb Speaks Out'', William Morrow & Co., 1983
* Coster-Mullen, John, "Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man", Self-Published, 2011
* Glasstone, Samuel and Dolan, Philip J., editors, ''[https://www.deepspace.ucsb.edu/wp-content/uploads/2013/01/Effects-of-Nuclear-Weapons-1977-3rd-edition-complete.pdf The Effects of Nuclear Weapons (third edition)] {{Webarchive|url=https://web.archive.org/web/20160303175040/http://www.deepspace.ucsb.edu/wp-content/uploads/2013/01/Effects-of-Nuclear-Weapons-1977-3rd-edition-complete.pdf |date=2016-03-03}}'' (PDF), U.S. Government Printing Office, 1977.
* Grace, S. Charles, ''Nuclear Weapons: Principles, Effects and Survivability (Land Warfare: Brassey's New Battlefield Weapons Systems and Technology, vol 10)''
* [[Chuck Hansen|Hansen, Chuck]], "[https://www.uscoldwar.com/ Swords of Armageddon: U.S. Nuclear Weapons Development since 1945] {{Webarchive|url=https://web.archive.org/web/20161230020259/http://www.uscoldwar.com/ |date=2016-12-30}}" (CD-ROM & download available). PDF. 2,600 pages, Sunnyvale, California, Chucklea Publications, 1995, 2007. {{ISBN|978-0-9791915-0-3}} (2nd Ed.)
* ''[https://ota.fas.org/reports/7906.pdf The Effects of Nuclear War] {{Webarchive|url=https://web.archive.org/web/20150418011842/http://fas.org/nuke/intro/nuke/7906/index.html |date=2015-04-18}}'', Office of Technology Assessment (May 1979).
* Rhodes, Richard. ''The Making of the Atomic Bomb''. Simon and Schuster, New York, (1986 {{ISBN|978-0-684-81378-3}})
* [[Richard Rhodes|Rhodes, Richard]]. ''Dark Sun: The Making of the Hydrogen Bomb''. Simon and Schuster, New York, (1995 {{ISBN|978-0-684-82414-7}})
* [[Henry DeWolf Smyth|Smyth, Henry DeWolf]], ''[https://www.atomicarchive.com/resources/documents/smyth-report/index.html Atomic Energy for Military Purposes] {{Webarchive|url=https://web.archive.org/web/20170421015824/http://www.atomicarchive.com/Docs/SmythReport/index.shtml |date=2017-04-21}}'', Princeton University Press, 1945. (see: [[Smyth Report]])
{{Refend}}
{{Free-content attribution|howto=no|title=Nuclear Weapons FAQ: 1.6|author=Carey Sublette|documentURL=https://nuclearweaponarchive.org/Nwfaq/Nfaq1.html#nfaq1.6}}

==External links==
{{Commons category|Nuclear weapon design}}
* [https://nuclearweaponarchive.org/ Carey Sublette's Nuclear Weapon Archive] is a reliable source of information and has links to other sources.
** Nuclear Weapons Frequently Asked Questions: [https://nuclearweaponarchive.org/Nwfaq/Nfaq4.html Section 4.0 Engineering and Design of Nuclear Weapons]
* The [https://fas.org/ Federation of American Scientists] provides solid information on weapons of mass destruction, including [https://nuke.fas.org/ nuclear weapons] and their [https://web.archive.org/web/20150329105730/http://fas.org/nuke/intro/nuke/effects.htm effects]
* [https://nuclearweaponarchive.org/Library/Teller.html More information on the design of two-stage fusion bombs]
* [https://fas.org/irp/threat/mctl98-2/mctl98-2.pdf Militarily Critical Technologies List (MCTL), Part II (1998)] (PDF) from the US Department of Defense at the Federation of American Scientists website.
* [https://fas.org/sgp/othergov/doe/rdd-7.html "Restricted Data Declassification Decisions from 1946 until Present"], Department of Energy report series published from 1994 until January 2001 which lists all known declassification actions and their dates. Hosted by Federation of American Scientists.
* [https://fas.org/sgp/eprint/morland.html The Holocaust Bomb: A Question of Time] is an update of the 1979 court case ''USA v. The Progressive'', with links to supporting documents on nuclear weapon design.
* [https://web.archive.org/web/20080226054314/http://alsos.wlu.edu/qsearch.aspx?browse=science%2FNuclear+Weapons+Design Annotated bibliography on nuclear weapons design from the Alsos Digital Library for Nuclear Issues]
* [https://www.wilsoncenter.org/nuclear-history-documents The Woodrow Wilson Center's Nuclear Proliferation International History Project] or NPIHP is a global network of individuals and institutions engaged in the study of international nuclear history through archival documents, oral history interviews and other empirical sources.

{{Nuclear technology}}

[[Category:Nuclear weapon design| ]]
[[Category:Nuclear weapons|Design]]
[[Category:Weapon design]]