View source for Boosted fission weapon

{{Short description|Type of nuclear weapon}}
{{redirect|Fission-fusion-fission|the term as applied to multistage H-bombs|Thermonuclear weapon}}
{{More citations needed|date=August 2018}}
[[File:Greenhouse Item 001.jpg|thumb|The United States' [[Greenhouse Item]] nuclear test, on May 25, 1951, of the world's first boosted fission weapon.]]
A '''boosted fission weapon''' usually refers to a type of [[nuclear bomb]] that uses a small amount of [[Nuclear fusion|fusion]] fuel to increase the rate, and thus yield, of a [[Nuclear fission|fission]] reaction. The [[Fusion neutron|fast fusion neutrons]] released by the [[fusion reaction]]s add to the [[fast neutrons]] released due to fission, allowing for more neutron-induced fission reactions to take place. The rate of fission is thereby greatly increased such that much more of the fissile material undergoes fission before the [[plutonium core|core]] explosively disassembles. The fusion process itself adds only a small amount of energy to the process, perhaps 1%.<ref>[http://www.isanw.org/facts/weapons.html#node8 "Facts about Nuclear Weapons: Boosted Fission Weapons"], Indian Scientists Against Nuclear Weapons {{webarchive |url=https://web.archive.org/web/20080708193625/http://www.isanw.org/facts/weapons.html#node8 |date=July 8, 2008 }}</ref> The fuel is commonly a 50-50 [[deuterium-tritium]] gas mixture, although [[Lithium deuteride|lithium-6-deuteride]] has also been tested.{{Citation needed|date=May 2025}}

The alternative meaning is an obsolete type of single-stage nuclear bomb that uses thermonuclear fusion on a large scale to create fast neutrons that can cause fission in [[depleted uranium]], but which is not a [[Nuclear weapon design#Two-stage thermonuclear weapons|two-stage]] [[hydrogen bomb]]. This type of bomb was referred to by [[Edward Teller]] as "Alarm Clock", and by [[Andrei Sakharov]] as "Sloika" or "Layer Cake" (Teller and Sakharov developed the idea independently, as far as is known).<ref name="Rhodes 1995">{{Cite Q | Q105755363 | last1 = Rhodes | first1 = Richard | author-link1 = Richard Rhodes | df = dmy-all | via = [[Internet Archive]] }}</ref>
{{Nuclear weapons}}

== Terminology ==
The terms "thermonuclear", "fusion" and "hydrogen" bombs or weapons, primarily refer to multi-stage weapons of the [[Teller-Ulam design]]. This is despite most of multi-stage weapon yield deriving from fission, and despite boosted fission weapon usage of thermonuclear reactions between hydrogen isotopes. The term "fusion boosting" and "tritium boosting" are also used, although an equal amount of deuterium is always required.{{Citation needed|date=May 2025}}  

== Development ==
[[File:U.S. Swan Device.svg|thumb|Diagram of the [[Swan (nuclear primary)|Swan]], a 1956 boosted fission weapon design.]]
The idea of boosting was originally developed between late 1947 and late 1949 at [[Los Alamos National Laboratory|Los Alamos]].<ref name="bethe">{{cite web|url=https://fas.org/nuke/guide/usa/nuclear/bethe-52.htm|title=Memorandum on the History Of Thermonuclear Program|publisher=[[Federation of American Scientists]] |last=Bethe |first=Hans A.|editor=Chuck Hansen|editor-link=Chuck Hansen|access-date=19 May 2010 |date=28 May 1952}}</ref> The primary benefit of boosting is further miniaturization of nuclear weapons as it reduces the minimum inertial confinement time required for a supercritical nuclear explosion by providing a sudden influx of fast neutrons before the [[critical mass]] would blow itself apart. This would eliminate the need for an aluminum pusher and uranium tamper and the explosives needed to push them and the fissile material into a supercritical state. While the bulky [[Fat Man]] had a diameter of {{convert|5|feet|m}} and required 3 tons of high explosives for implosion, a boosted fission primary can be fitted on a small nuclear warhead (such as the [[W88]]) to ignite the thermonuclear secondary.

{{First nuclear tests}}

==Gas boosting in modern nuclear weapons==
{{technical|section|date=August 2023}}
In a fission bomb, the [[fissile]] fuel is "assembled" quickly by a uniform spherical implosion [[Explosive lens|created with conventional explosives]], producing a [[Critical mass (nuclear)|supercritical mass]]. In this state, many of the [[neutron]]s released by the fissioning of a nucleus will induce fission of other nuclei in the fuel mass, also releasing additional neutrons, leading to a [[chain reaction]]. This reaction consumes at most 20% of the fuel before the bomb blows itself apart, or possibly much less if conditions are not ideal: the [[Little Boy]] (gun type mechanism) and [[Fat Man]] (implosion type mechanism) bombs had efficiencies of 1.38% and 13%, respectively.

Fusion boosting is achieved by introducing [[tritium]] and [[deuterium]] gas.  Solid [[lithium deuteride]]-tritide has also been used in some cases, but gas allows more flexibility (and can be stored externally) and can be injected into a hollow cavity at the center of the sphere of fission fuel, or into a gap between an outer layer and a "levitated" inner core, sometime before implosion. By the time about 1% of the fission fuel has fissioned, the temperature rises high enough to cause [[thermonuclear fusion]], which produces relatively large numbers of high-energy neutrons. This influx of neutrons speeds up the late stages of the chain reaction, causing approximately twice as much of the fissile material to fission before the explosion disassembles the critical mass.

Deuterium-tritium fusion neutrons are extremely energetic, seven times more energetic than an average fission neutron,<ref name="nwa">{{cite web|url=http://nuclearweaponarchive.org/Nwfaq/Nfaq4-3.html|title=Nuclear Weapon Archive: 4.3 Fission-Fusion Hybrid Weapons}}</ref> which makes them much more likely to be captured in the fissile material and lead to fission. This is due to several reasons:

# When these energetic neutrons strike a fissile nucleus, the fission releases a much larger number of secondary neutrons (e.g. 4.6 vs 2.9 for Pu-239).
# The likelihood of these neutrons interacting with a fissile nucleus is higher than for lower-energy neutrons typical of a fission reaction; the area of the plutonium or uranium nucleus where an 'impact' will lead to fission is much larger. More formally, the fission [[Nuclear cross section|cross section]] is larger for higher-energy neutrons, both in absolute terms and in proportion to the [[scattering]] and [[neutron capture|capture]] cross sections.

Consequently, the time for the neutron population in the core to double is reduced by a factor of about 8.<ref name="nwa"></ref>
A sense of the potential contribution of fusion boosting can be gained by observing that the complete fusion of one [[mole (unit)|mole]] of tritium (3 grams) and one mole of deuterium (2 grams) would produce one mole of neutrons (1 gram), which, neglecting escape losses and scattering, could fission one mole (239 grams) of plutonium directly, producing 4.6 moles of secondary neutrons, which can in turn fission another 4.6 moles of plutonium (1,099 g). The fission of this 1,338 g of plutonium in the first two generations would release 23<ref>{{cite web|url=http://nuclearweaponarchive.org/Nwfaq/Nfaq12.html|title=Nuclear Weapon Archive: 12.0 Useful Tables}}</ref> [[kiloton]]s of TNT equivalent (97 [[terajoule|TJ]]) of energy, and would by itself result in a 29.7% efficiency for a bomb containing 4.5&nbsp;kg of plutonium (a typical small fission trigger). The energy released by the fusion of the 5 g of fusion fuel itself is only 1.73% of the energy released by the fission of 1,338&nbsp;g of plutonium. Larger total yields and higher efficiency are possible, since the chain reaction can continue beyond the second generation after fusion boosting.<ref name="nwa">{{cite web|url=http://nuclearweaponarchive.org/Nwfaq/Nfaq4-3.html|title=Nuclear Weapon Archive: 4.3 Fission-Fusion Hybrid Weapons}}</ref>

Fusion-boosted fission bombs can also be made immune to [[neutron radiation]] from nearby nuclear explosions, which can cause other designs to predetonate, blowing themselves apart without achieving a high yield.
The combination of reduced weight in relation to yield and immunity to radiation has ensured that most modern nuclear weapons are fusion-boosted.

The fusion reaction rate typically becomes significant at 20 to 30 [[megakelvin]]s. This temperature is reached at very low efficiencies, when less than 1% of the fissile material has fissioned (corresponding to a yield in the range of hundreds of tons of TNT). Since implosion weapons can be designed that will achieve yields in this range even if neutrons are present at the moment of criticality, fusion boosting allows the manufacture of efficient weapons that are immune to [[predetonation]]. Elimination of this hazard is a very important advantage in using boosting. It appears that every weapon now in the U.S. arsenal is a boosted design.<ref name="nwa" />

According to one weapons designer, boosting is mainly responsible for the remarkable 100-fold increase in the efficiency of fission weapons since 1945.<ref>{{cite book|url=https://books.google.com/books?id=AXM6WofXkNwC&pg=PA177|title=The Governance of Large Technical Systems|page=177|author=Olivier Coutard|date=2002|publisher=Taylor & Francis|isbn=9780203016893}}</ref>

==Some early non-staged thermonuclear weapon designs==
Early [[thermonuclear weapon]] designs such as the [[Joe-4]], the Soviet "Layer Cake" ("Sloika", {{langx|ru|Слойка}}), used large amounts of fusion to induce fission in the [[uranium-238]] atoms that make up [[depleted uranium]]. These weapons had a fissile core surrounded by a layer of [[lithium deuteride|lithium-6 deuteride]], in turn surrounded by a layer of depleted uranium. Some designs (including the layer cake) had several alternate layers of these materials. The Soviet ''Layer Cake'' was similar to the American ''[[Alarm Clock (nuclear device)|Alarm Clock]]'' design, which was never built, and the British ''[[Green Bamboo]]'' design, which was built but never tested.

When this type of bomb explodes, the fission of the [[highly enriched uranium]] or [[plutonium core]] creates [[neutron]]s, some of which escape and strike atoms of [[lithium-6]], creating [[tritium]]. At the temperature created by fission in the core, tritium and deuterium can undergo thermonuclear fusion without a high level of compression. The fusion of tritium and deuterium produces a neutron with an energy of 14 [[MeV]]—a much higher energy than the 1 MeV of the neutron that began the reaction. This creation of high-energy neutrons, rather than energy yield, is the main purpose of fusion in this kind of weapon. This 14 MeV neutron then strikes an atom of uranium-238, causing fission: without this fusion stage, the original 1 MeV neutron hitting an atom of uranium-238 would probably have just been absorbed. This fission then releases energy and also neutrons, which then create more tritium from the remaining lithium-6, and so on, in a continuous cycle. Energy from fission of uranium-238 is useful in weapons: both because depleted uranium is much cheaper than [[highly enriched uranium]] and because it cannot go [[Critical mass (nuclear)|critical]] and is therefore less likely to be involved in a catastrophic accident.

This kind of thermonuclear weapon can produce up to 20% of its yield from fusion, with the rest coming from fission, and is limited in yield by practical concerns of mass and diameter to less than one [[TNT equivalent|megaton]] of TNT (4 [[petajoule|PJ]]) equivalent. Joe-4 yielded 400 kilotons of TNT (1.7 PJ). In comparison, a "true" hydrogen bomb can produce up to [[Tsar Bomba|97% of its yield from fusion]], and its explosive yield is limited only by device size.

==Maintenance of gas-boosted nuclear weapons==
Tritium is a radioactive isotope with a half-life of 12.355 years. Its main decay product is [[helium-3]], which is among the nuclides with the largest cross-section for neutron capture. Therefore, periodically the weapon must have its helium waste flushed out and its tritium supply recharged. This is because any helium-3 in the weapon's tritium supply would act as a [[neutron poison|poison]] during the weapon's detonation, absorbing neutrons meant to collide with the nuclei of its fission fuel.<ref>{{cite web|title=Section 6.3.1.2 Nuclear Materials Tritium |url=http://nuclearweaponarchive.org/Nwfaq/Nfaq6.html#nfaq6.4|website=High Energy Weapons Archive FAQ|publisher=Carey Sublette|access-date=June 7, 2016}}</ref>

Tritium is relatively expensive to produce because each [[triton (physics)|triton]] - the tritium nucleus - requires production of at least one free neutron, which is used to bombard a feedstock material (lithium-6, deuterium, or helium-3). Furthermore, because of losses and inefficiencies, the number of free neutrons needed is closer to two for each triton, as tritium begins decaying immediately, so there are losses during collection, storage, and transport from the production facility to the weapons in the field. The production of free neutrons demands the operation of either a breeder reactor or a particle accelerator (with a [[spallation]] target) dedicated to the tritium production facility.<ref>{{cite web|title=Section 6.3.1.2 Nuclear Materials Tritium|url=http://nuclearweaponarchive.org/Nwfaq/Nfaq6.html#nfaq6.3|website=High Energy Weapons Archive FAQ|publisher=Carey Sublette|access-date=June 7, 2016}}</ref><ref>{{cite web|title=Section 4.3.1 Fusion Boosted Fission Weapons|url=http://nuclearweaponarchive.org/Nwfaq/Nfaq4-3.html|website=High Energy Weapons Archive FAQ|publisher=Carey Sublette|access-date=June 7, 2016}}</ref>

==See also==
* [[History of the Teller–Ulam design]]
* [[Nuclear weapon design]]
* [[Thermonuclear weapon]]

==References==
{{Reflist}}

[[Category:Nuclear weapon design]]
[[Category:Nuclear fission]]
[[Category:Los Alamos National Laboratory]]
[[Category:American inventions]]
[[Category:1947 in science]]
[[Category:1947 in the United States]]
[[Category:Edward Teller]]