Editing Boosted fission weapon (section)

==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 [[neutrons]] 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 critical mass is disassembled by the explosion.

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, a much larger number of secondary neutrons are released by the fission (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>