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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.

Fusion bombs, also called thermonuclear bombs, have higher kiloton yields and greater efficiencies than fission bombs. To design a fusion bomb, some problems have to be solved:

The first boosted fission weapon test was Greenhouse Item (45.5 kt, 24 May 1951), an oralloy design exploded on island Janet at Enewetak. The design of the boosted fission weapon is a bit more complex than the pure fission weapons. Contained in these weapons is a deuterium and tritium gas mixture, isotopes of hydrogen. 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. The deuterium/tritium gas mixture is included in the center of the fissile material. When the bomb core undergoes enough fission, it becomes hot enough to ignite the D-T fusion reaction which proceeds swiftly. The reaction of the D-T fusion produces an intense burst of high-energy neutrons that in turns 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).

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. Soon later, this lead to the Staged Radiation Implosion Weapons which is more commonly know as the Teller-Ulam weapon or (depending on type) fission-fusion or fission-fusion-fission weapons. These class of weapons incorporate light elements such as hydrogen and lithium to overcome the limitation yield of the fission type device and, as well, the boosted fission weapons designs. Other reason for the new design of the Teller-Ulam device is the reduction of weapon cost by reducing the amount of costly enriched uranium-235 or plutonium-239. The new design, Teller-Ulam device, also allows for lighter weight than previous designs. Obvious advantages includes: transportation (smaller aircraft can be used to deploy weapon) and rocket size and fuel to deploy is reduced.

It is inconvenient to carry deuterium and tritium as gases in a thermonuclear weapon, and certainly impractical to carry them as liquefied gases, which requires high pressures and cryogenic temperatures. Instead, one can make a “dry” device in which ⁶Li is combined with deuterium to form the compound ⁶Li D (lithium-6 deuteride). Neutrons from a fission “primary” device bombard the ⁶ Li in the compound, liberating tritium, which quickly fuses with the nearby deuterium. The alpha particles, being electrically charged and at high temperatures, contribute directly to forming the nuclear fireball. The neutrons can bombard additional ⁶Li nuclei or cause the remaining uranium and plutonium in the weapon to undergo fission. This two-stage thermonuclear weapon has explosive yields far greater than can be achieved with one point safe designs of pure fission weapons. Thermonuclear fusion stages can be ignited in sequence to deliver any desired yield. Such bombs, in theory, can be designed with arbitrarily large yields: the Soviet Union once tested a device with a yield of about 59 megatons.

In a relatively crude sense, 6 Li can be thought of as consisting of an alpha particle ( ⁴He) and a deuteron ( ²H) bound together. When bombarded by neutrons, ⁶Li disintegrates into a triton ( ³H) and an alpha: 6 Li + Neutron = ³H + ³He + Energy

The nuclear fusion reaction which ignites most readily is: D + T = ⁴He + n + 17.6 MeV.

In other words, the Teller-Ulam configuration makes use of the fact that at high temperatures of a fission bomb about 80% or more of the energy exists as soft X-rays, not as kinetic energy (heat). The rate at which the energy from radiation expands from the core is about 1000km/sec faster than the heat. The energy that is released from the core as heat is by far slower than the radiation (X-rays). This, of course is taken advantage, in the Teller-Ulam design. These dynamics of the Teller-Ulam design make it possible to compress, and ignite a physically separate mass of fusion fuel (the second stage) through radiation implosion (X-rays) before the expanding trigger disrupts it.

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.

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.

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 radiation 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.

A "salted" nuclear weapon is reminiscent of fission-fusion-fission weapons, but instead of a fissionable jacket around the secondary stage fusion fuel, a non-fissionable blanket of a specially chosen salting isotope is used (cobalt-59 in the case of the cobalt bomb). This blanket captures the escaping fusion neutrons to breed a radioactive isotope that maximizes the fallout hazard from the weapon rather than generating additional explosive force (and dangerous fission fallout) from fast fission of U-238.

Variable fallout effects can be obtained by using different salting isotopes. Gold has been proposed for short-term fallout (days), tantalum and zinc for fallout of intermediate duration (months), and cobalt for long term contamination (years). To be useful for salting, the parent isotopes must be abundant in the natural element, and the neutron-bred radioactive product must be a strong emitter of penetrating gamma rays.

Candidate Salting Agents
Parent Isotope	Natural abundance	Radioactive Product	Half-Life
Cobalt-59	100%	Co-60	5.26 years
Gold-197	100%	Au-198	2.697years
Tantalum-181	99.99%	Ta-182	115days
Zinc-64	48.89%	Zn-65	244days

The idea of the cobalt bomb originated with Leo Szilard who publicized it in Feb. 1950, not as a serious proposal for weapon, but to point out that it would soon be possible in principle to build a weapon that could kill everybody on earth. To design such a theoretical weapon a radioactive isotope is needed that can be dispersed world wide before it decays. Such dispersal takes many months to a few years so the half-life of Co-60 is ideal.

The Co-60 fallout hazard is greater than the fission products from a U-238 blanket because

The half-life of Co-60 on the other hand is long enough to settle out before significant decay has occurred, and to make it impractical to wait out in shelters, yet is short enough that intense radiation is produced.

Initially gamma radiation fission products from an equivalent size fission-fusion-fission bomb are much more intense than Co-60: 15,000 times more intense at 1 hour; 35 times more intense at 1 week; 5 times more intense at 1 month; and about equal at 6 months. Thereafter fission drops off rapidly so that Co-60 fallout is 8 times more intense than fission at 1 year and 150 times more intense at 5 years. The very long lived isotopes produced by fission would overtake the again Co-60 after about 75 years.

Zinc has been proposed as an alternate candidate for the "doomsday role". The advantage of Zn-64 is that its faster decay leads to greater initial intensity. Disadvantages are that since it makes up only half of natural zinc, it must either be isotopically enriched or the yield will be cut in half; that it is a weaker gamma emitter than Co-60, putting out only one-fourth as many gammas for the same molar quantity; and that substantially amounts will decay during the world-wide dispersal process. Assuming pure Zn-64 is used, the radiation intensity of Zn-65 would initially be twice as much as Co-60. This would decline to being equal in 8 months, in 5 years Co-60 would be 110 times as intense.

Militarily useful radiological weapons would use local (as opposed to world-wide) contamination, and high initial intensities for rapid effects. Prolonged contamination is also undesirable. In this light Zn-64 is possibly better suited to military applications than cobalt, but probably inferior to tantalum or gold. As noted above ordinary "dirty" fusion-fission bombs have very high initial radiation intensities and must also be considered radiological weapons.

No cobalt or other salted bomb has ever been atmospherically tested, and as far as is publicly known none have ever been built. In light of the ready availability of fission-fusion-fission bombs, it is unlikely any special-purpose fallout contamination weapon will ever be developed.

These are weapons that only use fission reactions as a source of energy. Fission bombs operate by rapidly assembling a sub critical configuration of fissile material (plutonium or enriched uranium) into one that is highly supercritical. The original atomic bombs tested in 16 July 1945 (device name: Gadget, test name: Trinity) and dropped on Japan in 6 August 1945 (Little Boy, over Hiroshima) and 9 August 1945 (Fat Man, over Nagasaki) were pure fission weapons.

A fission weapon utilizes Plutonium-239 or Uranium-235 to create a high energy explosion. U-235 and Pu-239 are the only two elements that are used in fission type nuclear weapons. U-235 and Pu-239 hold unique properties in which when a neutron collides with the nucleus of the element the U-235 or Po-239 will split into two halves, releasing neutrons. The U-235 releases 2-3 neutrons per fission causing other fissions. However, Pu-239 is preferred over U-235. Pu-239 has higher fission and scattering cross sections than U-235, and a larger number of neutrons produced per fission, and consequently a smaller critical mass is needed. The amount of neutron release is dependent on how the atom splits. The two new atoms that are formed release gamma radiation while settling into their new states.

The condition known as super-criticality is referring to the high probability of a U-235 or Pu-239 capturing a neutron. In a bomb working correctly, more than one neutron ejected from each fission causes another fission to occur. The process of capturing the neutron and splitting happens very quickly, on the order of picoseconds (1*10E-12 seconds). Enormous amounts of energy is released on the occurrence of a fission and can be calculated. Multiples effects occur when the atom splits: new elements are formed, neutrons are released, and gamma radiation is emitted. However, if the masses of the new elements formed are added to the neutrons emitted, it weighs less than the original element (U-235 or Pu-239). The difference in weight is converted into energy with Einstein's formula E=mc^2 (where: E=energy, m=mass (kg), c=speed of light (3.0x10^8m/s) ). To put in perspective a pound of highly enriched uranium is equal to on the order of a million gallons of gasoline. A pound of U-235 is smaller than a baseball, while a million gallons of gasoline would fill a building with dimension 50 x 50 x 50feet.

In order for a sustained nuclear reaction enriched U-235 must be used. The majority of Uranium in the world is U-238 only a small percentage of U -235 exists. Special facilities such as the Isotope separation production facilities at Oak Ridge during WWII can yield enriched uranium. Furthermore, the fuel in a fission bomb must be kept separate (sub-critical) to prevent premature detonation. Only at detonation do the masses come together to ensure critical mass (critical mass - is the minimum mass of fissionable material required to sustain a nuclear fission reaction). This procedure of keeping the masses separate (sub-critical) to prevent detonation bring about problems in design. To sustain a fission reaction the sub-critical masses must be brought together, which will provide adequate supply of neutrons to ensure a fission reaction. Neutrons must be introduced into the critical-mass to let fission begin. Perhaps the most concern is with fizzle. A reaction starts, but not fast enough and the bomb fizzles out. So when detonated, as much of the material as possible must be fissioned before the bomb explodes.

These problems are solved by two processes; the gun-triggered device or the implosion device. Neutrons are introduced to the supercritical mass by a neutron generator. The neutron generator consists of a small pellet of polonium and beryllium. The Neutron generator is separated from the supercritical mass by a foil. When detonation is needed the two sub critical masses are brought together by either the gun-triggered device or the implosion device and the foil is breached. Immediately the polonium begins to emit alpha particles. The alpha particles, in turn, collide with the beryllium-9 to produce beryllium-8 and emit neutrons. The neutrons then collide into the core of the supercritical masses beginning the chain reaction.

The core of the supercritical mass is designated to a tamper during fission. This tamper is made of U-238. As the reaction intensifies the tamper is heated and is expanding exerting pressure back onto the super critical core. Furthermore, the tamper slows down the cores expansion ensuring as much as a complete detonation of material as possible. Another purpose the tamper serves is a neutron reflector. Neutrons from the reaction reflect neutron back to the core providing a more efficient reaction.

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