Boosted fission weapon

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"Fission-fusion-fission" redirects here. For the term as applied to multistage H-bombs, see Thermonuclear weapon.
Greenhouse Item nuclear test of the first deuterium-tritium boosted weapon

A boosted fission weapon usually refers to a type of

core explosively disassembles. The fusion process itself adds only a small amount of energy to the process, perhaps 1%.[1]

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

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).[2]

Nuclear weapons
Photograph of a mock-up of the Little Boy nuclear weapon dropped on Hiroshima, Japan, in August 1945.
Background
Nuclear-armed states
NPT recognized
United States
Russia
United Kingdom
France
China
Others
India
Israel (undeclared)
Pakistan
North Korea
Former
South Africa
Belarus
Kazakhstan
Ukraine

Development

The idea of boosting was originally developed between late 1947 and late 1949 at Los Alamos.[3] 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 5 feet (1.5 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.

Gas boosting in modern nuclear weapons

In a fission bomb, 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

thermonuclear fusion, which produces relatively large numbers of neutrons, speeding up the late stages of the chain reaction and approximately doubling its efficiency[clarification needed
].

Deuterium-tritium fusion neutrons are extremely energetic, seven times more energetic than an average fission neutron,[4] which makes them much more likely to be captured in the fissile material and lead to fission. This is due to several reasons:

  1. 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).
  2. The fission cross section is larger both in absolute terms, and in proportion to the scattering and 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 (2.5×109 vs 3×108).

A sense of the potential contribution of fusion boosting can be gained by observing that the complete fusion of one

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 5 g of fusion fuel itself is only 1.73% of the energy released by the fission of 1,338 g of plutonium. Larger total yields and higher efficiency are possible, since the chain reaction can continue beyond the second generation after fusion boosting.[4]

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

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.[4]

According to one weapons designer, boosting is mainly responsible for the remarkable 100-fold increase in the efficiency of fission weapons since 1945.[6]

Some early non-staged thermonuclear weapon designs

Early

Green Bamboo
design, which was built but never tested.

When this type of bomb explodes, the fission of the

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

PJ) equivalent. Joe-4 yielded 400 kilotons of TNT (1.7 PJ). In comparison, a "true" hydrogen bomb can produce up to 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 poison during the weapon's detonation, absorbing neutrons meant to collide with the nuclei of its fission fuel.[7]

Tritium is relatively expensive to produce because each

triton - the tritium nucleus - produced requires production of at least one free neutron which is used to bombard a feedstock material (lithium-6, deuterium, or helium-3). Actually, because of losses and inefficiencies, the number of free neutrons needed is closer to two for each triton produced (and 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.[8][9]

See also

References

  1. ^ "Facts about Nuclear Weapons: Boosted Fission Weapons", Indian Scientists Against Nuclear Weapons Archived July 8, 2008, at the Wayback Machine
  2. Wikidata Q105755363 – via Internet Archive
    .
  3. ^ Bethe, Hans A. (28 May 1952). Chuck Hansen (ed.). "Memorandum on the History Of Thermonuclear Program". Federation of American Scientists. Retrieved 19 May 2010.
  4. ^ a b c "Nuclear Weapon Archive: 4.3 Fission-Fusion Hybrid Weapons".
  5. ^ "Nuclear Weapon Archive: 12.0 Useful Tables".
  6. ISBN 9780203016893
    .
  7. ^ "Section 6.3.1.2 Nuclear Materials Tritium". High Energy Weapons Archive FAQ. Carey Sublette. Retrieved June 7, 2016.
  8. ^ "Section 6.3.1.2 Nuclear Materials Tritium". High Energy Weapons Archive FAQ. Carey Sublette. Retrieved June 7, 2016.
  9. ^ "Section 4.3.1 Fusion Boosted Fission Weapons". High Energy Weapons Archive FAQ. Carey Sublette. Retrieved June 7, 2016.
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