Boosted fission weapon
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/* Fusion boosting to increase efficiency */
{{Unreferenced|date=November 2007}}
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 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 two-stage [[hydrogen bomb]].
==Fusion boosting to increase efficiency==
In a fission bomb, the [[fissile]] fuel is "assembled" quickly by a uniform spherical implosion created with conventional explosives, producing a [[Critical mass (nuclear)|supercritical mass]]. In this state, each [[neutron]] from a fission reaction will induce enough other atoms in the fuel to undergo fission as well, 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.
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) 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 neutrons speeding up the early stages of the chain reaction and approximately doubling its efficiency.
Deuterium-tritium fusion neutrons are extremely energetic, seven times more energetic than an average fission neutron, which causes them to boost the overall alpha far out of proportion to their numbers. This is due to several reasons:
# Their high velocity creates the opposite of time absorption: time magnification.
# 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 fission [[cross section]] is larger both in absolute terms, and in proportion to the [[scattering]] and [[neutron capture|capture]] cross sections.
Taking these factors into account, the maximum alpha value for D-T fusion neutrons in plutonium (density 19.8 g/cm³) is some 8 times higher than for an average fission neutron (2.5×10<sup>9</sup> vs 3×10<sup>8</sup>).
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 for the moment, 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 (1099 g). The fission of this 1.338 kg 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 kg of plutonium (a typical small fission trigger). The energy released by the fusion of the 5g of fusion fuel itself is only 1.73% of the energy released by the fission of 1.338 kg 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 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 web|url=http://books.google.com/books?id=AXM6WofXkNwC&pg=PA177&lpg=PA177|title=The Governance of Large Technical Systems}}</ref>
==Fusion boosting in single stage thermonuclear weapons==
Early thermonuclear weapon designs such as the [[Joe-4]], the Soviet "Layer Cake", 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-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'', which was never built, and the British ''Green Bamboo,'' which was built but never tested.
When this type of bomb explodes, the fission of the [[highly enriched uranium]] or [[plutonium]] core creates neutrons, 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 very 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 to less than one [[megaton]] of TNT (4 [[petajoule|PJ]]) equivalent. Joe-4 yielded 400 kilotons of TNT (1.7 PJ). In comparison, a true hydrogen bomb produces typically 50% of its yield from fusion, with 97% having been achieved, and there is no upper limit to its explosive yield.
==See also==
*[[Nuclear weapons design]]
==References==
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[[Category:Nuclear weapons]]
[[bg:Усилено ядрено устройство]]