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All nuclear weapons that are not pure fission weapons use fusion reactions to enhance their destructive effects. All weapons that use fusion require a fission bomb to provide the energy to initiate the fusion reactions. This does not necessarily mean that fusion generates a significant amount of the explosive energy, or that explosive force is even the desired effect.
Boosted Fission Weapons
The earliest application of fusion to useful weapons was the development of boosted fission weapons. In these weapons a few grams of a deuterium/tritium gas mixture are included in the center of the fissile core. When the bomb core undergoes enough fission, it becomes hot enough to ignite the D-T fusion reaction which proceeds swiftly. This reaction produces an intense burst of high-energy neutrons that causes a correspondingly intense burst of fissions in the core. This greatly accelerates the fission rate in the core, thus allowing a much higher percentage of the material in the core to fission before it blows apart. Typically no more than about 20% of the material in an average size pure fission bomb will split before the reaction ends (it can be much lower, the Hiroshima bomb was 1.4% efficient). By accelerating the fission process a boosted fission bomb increase the yield 100% (an unboosted 20 kt bomb can thus become a 40 kt bomb). The actual amount of energy released by the fusion reaction is negligible, about 1% of the bomb's yield, making boosted bomb tests difficult to distinguish from pure fission tests (detecting traces of tritium is about the only way).
The first boosted weapon test was Greenhouse Item (45.5 kt, 24 May 1951), an oralloy design exploded on island Janet at Enewetak. This experimental device used cryogenic liquid deuterium-tritium instead of gas. The boosting approximately doubled the yield over the expected unboosted value. Variants on the basic boosting approach that have been tested including the use of deuterium gas only, and the use of lithium deuteride/tritide, but it isn't known whether any of these approaches have been used in operational weapons.
Due to the marked increase in yield (as well as other reasons - such as reducing the weight of the fission system, and eliminating the risk of predetonation) today most fission bombs are boosted, including those used as primaries in true fission-fusion weapons. Although boosting can multiply the yield of fission bombs, it still has the same fundamental fission bomb design problems for high yield designs. The boosting technique is most valuable in small light-weight bombs that would otherwise have low efficiency. Tritium is a very expensive material to make, and it decays at a rate of 5.5% per year, but the small amounts required for boosting (a few grams) make its use economical.
Staged Radiation Implosion Weapons
This class of weapons is also called "Teller-Ulam" weapons, or (depending on type) fission-fusion or fission-fusion-fission weapons. These weapons use fusion reactions involving isotopes of light elements (e.g. hydrogen and lithium) to remove the yield limits of fission and boosted fission designs, to reduce weapon cost by reducing the amount of costly enriched uranium or plutonium required for a given yield, and to reduce the weight of the bomb. The fusion reactions occur in a package of fusion fuel ("the secondary") that is physically separate from the fission trigger ("the primary"), thus creating a two-stage bomb (the fission primary counting as the first stage). X-rays from the primary are used to compress the secondary through a process known as radiation implosion. The secondary is then ignited by a fission "spark plug" in its center. The energy produced by the fusion second stage can be used to ignite an even larger fusion third stage. Multiple staging allows in principle the creation of bombs of virtually unlimited size.
The fusion reactions are used to boost the yield in two ways:
Bombs that release a significant amount of energy directly by fusion, but do not use fusion neutrons to fission the fusion stage jacket, are called Fission-Fusion weapons. If they employ the additional step of jacket fissioning using fusion neutrons they are called Fission-Fusion-Fission weapons.
The fast fission of the secondary jacket in a fission-fusion-fission bomb is sometimes thought of, or referred to, as a "third stage" in the bomb, and it is in a sense. But care must be taken not to confuse this with the true three-stage thermonuclear design in which there is another complete tertiary fusion stage.
Bombs that are billed as "clean" bombs (a relative term) obtain a large majority of their total yield from fusion. The last and largest stage of these bombs is always a pure fusion stage (not counting the spark plug), substituting a non-fissionable material for the jacket. The fusion-fraction of these designs as demonstrated in tests has been as high as 97% (this was the Tsar Bomba, see below).
Fission-fusion-fission bomb are dirty, but they have superior "bang for the buck" and "pow per pound". They generate a large amount of fission fallout since fission accounts for most of their yield. The 5 Mt Redwing Tewa test (20 July 1956 GMT, Bikini Atoll) had a fission fraction of 85%. If the emphasis is on cheapness depleted or natural uranium is usually used for the jacket. If the emphasis is on yield per weight (like nearly all modern strategic weapons) enriched uranium is used.
The staging concept allows the use as fuel pure deuterium, or varying mixtures of lithium 6 and 7 in the form of a compound with deuterium (lithium 6/7 deuteride). These natural stable isotopes are vastly cheaper than the artificially made and radioactive tritium.
The staged radiation implosion concept was originally conceived by Stanislaw Ulam and then developed further in a collaboration between Ulam and Edward Teller in early 1950. The first test of a staged thermonuclear device was Ivy Mike on 31 October 1952 (GMT) on Elugelab/Flora island at Enewetak Atoll. This experimental device, called Sausage, used pure deuterium fuel (probably the only time this was ever done) and a natural uranium jacket. It was designed by the Panda Committee led by J. Carson Mark at Los Alamos. Mike a yield of 10.4 Mt, 77% of which was fission.
The Teller-Ulam concept was later rediscovered by the other four nuclear weapon states, all of which have tested and deployed these weapons. No other nation is known to have deployed these designs, although the undeclared nuclear powers of Israel and India almost certainly have done development work on them.
Three stage designs have been tested and deployed to produce very high yield weapons. The first three stage U.S. test, and probably the first three stage weapon test ever, was the Bassoon device detonated in the Redwing Zuni test (27 May 1956 GMT, Bikini Atoll, 3.5 Mt). The largest nuclear explosion ever set off (50 Mt) was the Tsar Bomba (King of Bombs), a Soviet three stage fission-fusion-fission design. It was exploded on 30 October 1961 over Novaya Zemlya at an altitude of 4000 m.
By jacketing the third stage with non-fissionable material, three stage devices can produce high yield clean weapons. Both Zuni and Tsar Bomba were in fact very clean devices - Zuni was 85% fusion and Tsar Bomba was 97% fusion. Both designs permitted replacing the lead or tungsten third stage jacket with U-238 however. A version of Bassoon called Bassoon Prime was tested in the dirty Tewa test mentioned above. A dirty device derived from the Bassoon was weaponized to create the highest yield weapon the U.S. ever fielded, the 25 megaton Mk-41. The Tsar Bomba design was for a fission-fusion-fission bomb with a staggering yield of at least 100 megatons!
A possible variation on the staged radiation implosion design is one in which a second fission stage is imploded instead of a thermonuclear one. This was actually the initial concept developed by Stanislaw Ulam before he realized its possible application to thermnuclear weapons. The advantage of this approach is that radiation implosion speeds are hundreds of times higher, and maximum densities tens of times greater, than those achievable through high explosives. This allows achieving higher yields than is practical with high explosive driven fission weapons, and the use of lower grades of fissile material. If some fusion fuel is included in this second fission stage to boost yield, a sort of hybrid two-stage boosted weapon design results that blurs the distinction between two-stage fission and classic Teller-Ulam thermonuclear weapons. The TX-15 "Zombie" developed by the U.S. was originally planned to be a two stage pure fission device, but later evolved into this sort of hybrid boosted system. The Zombie was tested in the Castle Nectar shot (13 May 1954 GMT; Bikini Atoll; 1.69 Mt), and was fielded as the Mk-15.
The Alarm Clock/Sloika (Layer Cake) Design
This idea predates the invention of staged radiation implosion designs, and was apparently invented independently at least three times. It was first devised by Edward Teller in the United States, who named the design "Alarm Clock". Later Andrei Sakharov and Vitalii Ginzburg in the Soviet Union hit upon it and dubbed it the "sloika" design. A sloika is a layered Russian pastry, rather like a napoleon, and has thus been translated as "Layer Cake". Finally it was developed by the British (inventor unknown). Each of these weapons research programs hit upon this idea before ultimately arriving at the more difficult, but more powerful and efficient, staged thermonuclear approach.
This system was dubbed "Layer Cake" by the Soviets because it uses a spherical assembly of concentric shells. In the center is a fission primary made of U-235/Pu-239, surrounding it is an (optional) layer of U-238 for the fission tamper, then a layer of lithium-6 deuteride/tritide, a U-238 fusion tamper, and finally the high explosive implosion system. The process begins like an ordinary implosion bomb. After the primary in the center completes its reaction, the energy it releases compresses and heats the fusion layer to thermonuclear temperatures. The burst of fission neutrons then initiates a coupled fission-fusion-fission chain reaction. Slower fission neutrons generate tritium from the lithium, which then fuses with deuterium to produce very fast neutrons, that in turn cause fast fission in the fusion tamper, which breed more tritium. In effect the fusion fuel acted as a neutron accelerator allowing a fission chain reaction to occur with a large normally non-fissionable U-238 mass. While spiking the fusion layer with an initial dose of tritium is not strictly necessary for this approach, it helps boost the yield.
The achievable fusion fraction is fairly small, 15-20%, and cannot be increased beyond this point. Its use of fusion fuel is also quite inefficient. This design is also limited to the same yield and yield-to-weight range as high yield pure fission and boosted fission weapons. This was developed into a deliverable weapon by the Soviet Union and the British prior to their development of the staged designs described above. The U.S. did not bother to pursue it, partly because Teller did not feel it was sufficiently destructive to be worthwhile.
The first test of this concept was a device designated RDS-6s, (known as Joe 4 to the U.S.) on 12 August 1953. By using tritium doping it achieved a 10-fold boost over the size of the trigger, for a total yield of 400 kt. The UK Orange Herald Small device tested in Grapple 2 (31 May 1957) was similar but used a much larger fission trigger (300 kt range) apparently without tritium for a total yield of 720 kt, a boost in the order of 2.5-fold. This is probably the largest test of this design.
Although apparently not used in any weapons now in service in the five declared weapons states, it remains a viable design that could be attractive to other states that do not have the resources to develop the technically more demanding radiation implosion design. Information supplied by Mordechai Vanunu indicates that Israel may have developed a weapon of this type.
This design should probably be considered distinct from other classes of nuclear weapons. This design is something of a hybrid and could be considered either a type of boosted fission device, or a one-stage type of fission-fusion-fission bomb.
Neutron bombs, more formally referred to as "enhanced radiation (ER) warheads", are small thermonuclear weapons in which the burst of neutrons generated by the fusion reaction is intentionally not absorbed inside the weapon, but allowed to escape. This intense burst of high-energy neutrons is the principle destructive mechanism. Neutrons are more penetrating than other types of radiation so many shielding materials that work well against gamma rays do not work nearly as well. The term "enhanced radiation" refers only to the burst of ionizing radiation released at the moment of detonation, not to any enhancement of residual radiation in fallout.
The U.S. has developed neutron bombs for use as strategic anti-missile weapons, and as tactical weapons intended for use against armored forces. As an anti-missile weapon ER weapons were developed to protect U.S. ICBM silos from incoming Soviet warheads by damaging the nuclear components of the incoming warhead with the intense neutron flux. Tactical neutron bombs are primarily intended to kill soldiers who are protected by armor. Armored vehicles are extremely resistant to blast and heat produced by nuclear weapons, so the effective range of a nuclear weapon against tanks is determined by the lethal range of the radiation, although this is also reduced by the armor. By emitting large amounts of lethal radiation of the most penetrating kind, ER warheads maximize the lethal range of a given yield of nuclear warhead against armored targets.
One problem with using radiation as a tactical anti-personnel weapon is that to bring about rapid incapacitation of the target, a radiation dose that is many times the lethal level must be administered. A radiation dose of 600 rads is normally considered lethal (it will kill at least half of those who are exposed to it), but no effect is noticeable for several hours. Neutron bombs were intended to deliver a dose of 8000 rads to produce immediate and permanent incapacitation. A 1 kt ER warhead can do this to a T-72 tank crew at a range of 690 m, compared to 360 m for a pure fission bomb. For a "mere" 600 rad dose the distances are 1100 m and 700 m respectively, and for unprotected soldiers 600 rad exposures occur at 1350 m and 900 m. The lethal range for tactical neutron bombs exceeds the lethal range for blast and heat even for unprotected troops.
The neutron flux can induce significant amounts of short lived secondary radioactivity in the environment in the high flux region near the burst point. The alloy steels used in armor can develop radioactivity that is dangerous for 24-48 hours. If a tank exposed to a 1 kt neutron bomb at 690 m (the effective range for immediate crew incapacitation) is immediately occupied by a new crew, they will receive a lethal dose of radiation within 24 hours.
Newer armor designs afford more protection than the Soviet T-72 against with ER warheads were initially targeted. Special neutron absorbing armor techniques have also been developed and deployed, such as armors containing boronated plastics and the use of vehicle fuel as a shield. Some newer types of armor, like that of the M-1 tank, employ depleted uranium which can offset these improvements since it undergoes fast fission, generating additional neutrons and becoming radioactive.
Due to the rapid attenuation of neutron energy by the atmosphere (it drops by a factor of 10 every 500 m in addition to the effects of spreading) ER weapons are only effective at short ranges, and thus are found in relatively low yields. ER warheads are also designed to minimize the amount of fission energy and blast effect produced relative to the neutron yield. The principal reason for this was to allow their use close to friendly forces. The common perception of the neutron bomb as a "landlord bomb" that would kill people but leave buildings undamaged is greatly overstated. At the intended effective combat range (690 m) the blast from a 1 kt neutron bomb will destroy or damage to the point of unusability almost any civilian building. Thus the use of neutron bombs to stop an enemy attack, which requires exploding large numbers of them to blanket the enemy forces, would also destroy all buildings in the area.
Neutron bombs (the tactical versions at least) differ from other thermonuclear weapons in that a deuterium-tritium gas mixture is the only fusion fuel. The reasons are two-fold: the D-T thermonuclear reaction releases 80% of its energy as neutron kinetic energy, and it is also the easiest of all fusion reactions to ignite. This means that only 20% of the fusion energy is available for blast and thermal radiaiton production, that the neutron flux produced consists of extremely penetrating 14.7 Mev neutrons, and that a very small fission explosion (250-400 tons) can be used for igniting the reaction. The more typical lithium deuteride fuel would produce much more blast and flash for each unit of neutron flux, and would require a much larger fission explosion to set it off. The disadvantage of using D-T fuel is that tritium is very expensive, and decays at a rate of 5.5% a year. Combined with its increased complexity this makes ER warheads more expensive to build and maintain than other tactical nuclear weapons. To produce a 1 kt fusion yield 12.5 g of tritium and 5 g of deuterium are required.
The U.S. developed and produced three neutron warheads, a fourth was cancelled prior to production. All have been retired and dismantled.
The Soviet Union, China, and France are all known to have developed neutron bomb designs and may have them in service. A number of reports have claimed that Israel has developed neutron bombs which, though they could be valuable on an armor battleground like the Golan Heights, are difficult to develop and require sigificant testing. This makes it unlikely that Israel has in fact acquired them.