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A 'boosted fission weapon' usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a 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.

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Fusion boosting to increase efficiency

Fusion boosting in single stage thermonuclear weapons

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Fusion boosting to increase efficiency

In a traditional fission design, the fissile fuel is "assembled" quickly by a uniform spherical implosion created with conventional explosives, producing a supercritical mass. In this state the neutrons given out from a fission reaction will induce other atoms in the fuel to undergo fission as well, leading to a chain reaction. This reaction requires a source of a small number of neutrons to initate it and consumes at most one quarter of the fuel before the bomb blows itself apart.
Fusion boosting is achieved by introducing tritium gas or solid lithium deuteride-tritide into the centre of the sphere of fission fuel. The shockwave from the implosion converges to a point in the centre. At that point the temperature rises high enough to cause a small amount of thermonuclear fusion which produces relatively large numbers of neutrons speeding up the early stages of the chain reaction and approximately doubling its efficiency.
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 means that most modern nuclear weapons are fusion boosted.

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 14MeV -a much higher energy than the 1MeV 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 14MeV neutron then strikes an atom of uranium-238 causing fission; without this fusion stage, the original 1MeV 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 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. Joe-4 yielded 400 kilotons. 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