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Aboosted fission weapon usually refers to a type ofnuclear bomb that uses a small amount offusion fuel to increase the rate, and thus yield, of afission reaction. Thefast fusion neutrons released by thefusion reactions add to thefast 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 thecore explosively disassembles. The fusion process itself adds only a small amount of energy to the process, perhaps 1%.[1] The fuel is commonly a 50-50deuterium-tritium gas mixture, althoughlithium-6-deuteride has also been tested.[citation needed]
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 indepleted uranium, but which is not atwo-stagehydrogen bomb. This type of bomb was referred to byEdward Teller as "Alarm Clock", and byAndrei Sakharov as "Sloika" or "Layer Cake" (Teller and Sakharov developed the idea independently, as far as is known).[2]
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The terms "thermonuclear", "fusion" and "hydrogen" bombs or weapons, primarily refer to multi-stage weapons of theTeller-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]
The idea of boosting was originally developed between late 1947 and late 1949 atLos 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 thecritical 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 bulkyFat 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 theW88) to ignite the thermonuclear secondary.
| Country | First tests bynuclear weapon design | |||||||
|---|---|---|---|---|---|---|---|---|
| Fission | Year | Boosted fission | Year | Multi-stage | Year | Multi-stage above one megaton | Year | |
 United States | Trinity | 1945 | GreenhouseItem | 1951 | GreenhouseGeorge | 1951 | IvyMike | 1952 |
 Soviet Union | RDS-1 | 1949 | RDS-6s | 1953 | RDS-37 | 1955 | RDS-37 | 1955 |
 United Kingdom | Operation Hurricane | 1952 | Mosaic G1 | 1956 | Grapple 1 | 1957 | Grapple X | 1957 |
 China | 596 | 1964 | 596L | 1966 | 629 | 1966 | 639 | 1967 |
 France | Gerboise Bleue | 1960 | Rigel | 1966 | Canopus | 1968 | Canopus | 1968 |
 India | Smiling Buddha | 1974 | Shakti I (unconfirmed) | 1998 | Shakti I (unconfirmed) | 1998 | n/a | |
 Pakistan | Chagai I | 1998 | Chagai I | 1998 | n/a | n/a | ||
 North Korea | #1 | 2006 | #4 (unconfirmed) | 2016 | #6 (unconfirmed) | 2017 | n/a | |
 Israel | SeeNuclear weapons and Israel § Nuclear testing | n/a | ||||||
 South Africa | SeeSouth Africa and weapons of mass destruction § Nuclear weapons | n/a | ||||||
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In a fission bomb, thefissile fuel is "assembled" quickly by a uniform spherical implosioncreated with conventional explosives, producing asupercritical mass. In this state, many of theneutrons released by the fissioning of a nucleus will induce fission of other nuclei in the fuel mass, also releasing additional neutrons, leading to achain 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: theLittle Boy (gun type mechanism) andFat Man (implosion type mechanism) bombs had efficiencies of 1.38% and 13%, respectively.
Fusion boosting is achieved by introducingtritium anddeuterium gas. Solidlithium 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 causethermonuclear 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,[4] which makes them much more likely to be captured in the fissile material and lead to fission. This is due to several reasons:
Consequently, the time for the neutron population in the core to double is reduced by a factor of about 8.[4]A sense of the potential contribution of fusion boosting can be gained by observing that the complete fusion of onemole 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[5]kilotons of TNT equivalent (97TJ) 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 toneutron 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 30megakelvins. 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 topredetonation. 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]
Earlythermonuclear weapon designs such as theJoe-4, the Soviet "Layer Cake" ("Sloika",Russian:Слойка), used large amounts of fusion to induce fission in theuranium-238 atoms that make updepleted uranium. These weapons had a fissile core surrounded by a layer oflithium-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 SovietLayer Cake was similar to the AmericanAlarm Clock design, which was never built, and the BritishGreen Bamboo design, which was built but never tested.
When this type of bomb explodes, the fission of thehighly enriched uranium orplutonium core createsneutrons, some of which escape and strike atoms oflithium-6, creatingtritium. 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 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 thanhighly enriched uranium and because it cannot gocritical 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 onemegaton of TNT (4PJ) equivalent. Joe-4 yielded 400 kilotons of TNT (1.7 PJ). In comparison, a "true" hydrogen bomb can produce up to97% of its yield from fusion, and its explosive yield is limited only by device size.
Tritium is a radioactive isotope with a half-life of 12.355 years. Its main decay product ishelium-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 apoison 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 eachtriton - the tritium nucleus - requires production of at least one free neutron, which is used to irradiate a feedstock material (lithium-6,deuterium, orhelium-3). Because of losses and inefficiencies, the number of free neutrons needed is closer to two for each triton. Furthermore, because tritium decays, there are losses during collection, storage, and transport from the production facility to the weapons in the field, and tritium supplies must be replenished periodically. Other ways of producing tritium include the operation of abreeder reactor or aparticle accelerator (with aspallation target).[8][9]