Explosive Lenses Nuclear Weapons
Ilse Marseau
In a nuclear weapon, an array of explosive lenses is used to change the several approximately spherical diverging detonation waves into a single spherical converging one. The converging wave is then used to collapse the various shells (tamper, reflector, pusher, etc.) and finally compresses the core (pit) of fissionable material to a prompt critical state. They are usually machined from a plastic bonded explosive and an inert insert, called a wave-shaper, which is often a dense foam or plastic, though many other materials can be used. Other, mainly older explosive lenses do not include a wave shaper, but employ two explosive types that have significantly different velocities of detonation (VoD), which are in the range from 5 to 9 km/s. The use of the low- and high-speed explosives again results in a spherical converging detonation wave to compress the physics package. The original Gadget device used in the Trinity test and Fat Man dropped on Nagasaki used Baratol as the low-VoD explosive and Composition B as the fast, but other combinations can be used.[2]
A series of experiments was performed in 1944 and 1945 during the Manhattan Project to develop the lenses for a satisfactory implosion. One of the most important tests was the series of RaLa Experiments.
Finally, with the success of the Swan test nuclear explosive device, a two "point" assembly became feasible. Swan used an "air lens" system in addition to shaped charges and became the basis of all U.S. successor designs, nuclear and thermonuclear alike, and featured small size, light weight, and exceptional reliability and safety, as well as using the least amount of strategic material of any design.
The physical basis of a nuclear weapon lies in creating this supercritical state. When a fissile nucleus is struck by a neutron, the nucleus splits and emits additional neutrons and a large amount of energy. These newly freed neutrons can then strike and fission other nuclei, which produces a chain reaction. When the fissile material is arranged in such a manner that the fission of one nucleus leads to the fission of one other nucleus, the chain reaction is self-sustaining and the material is said to have reached its critical mass. Thus, supercriticality is when the fission of one nucleus in the chain reaction leads to the fission of more than one other nucleus.
Each fission event releases a large amount of energy in the form of light, heat, and radiation, so successive generations of fission events in the chain reaction will produce exponentially increasing amounts of energy. The key is to create and sustain a chain reaction long enough to produce the desired explosive energy before the fissile core rips itself apart due to the internal pressure created by the energy release. For example, 99.9% of the energy released in a 100 kiloton (1 kiloton = 1,000 tons of TNT) nuclear explosion is released in the last 7 generations, out of a total of over 50 generations, and occurs in approximately 0.07 microseconds.
2.2 Mechanics
The basic physics of this design is similar to that of the implosion design. Both weapons assemble a supercritical mass of fissile material and use a tamper to hold the core together long enough to produce the desired nuclear explosion. However, the mechanics of a gun design are much simpler, which means that the device is much easier to make.
3.1 Legend for Figure 3
Primary stage: fission implosion device as described in section 1, typically boosted with deuterium-tritium gas.
Secondary stage: a fusion fuel charge composed of lithium deuteride, which contains at its center a cylindrical rod of uranium-235 or plutonium-239, and is surrounded by a casing of uranium metal. The fusion reaction commonly employed is that of deuterium and tritium. The tritium is created when the lithium in the lithium deuteride reacts with a neutron.
Fusion is not limited by the requirement of a critical mass, so these weapons can reach theoretically limitless power. Often they are on the order of a few megatons (1 megaton = 1,000,000 tons of TNT). The largest nuclear weapon ever detonated was an approximately 59 megaton thermonuclear bomb produced by the Soviet Union. Fusion, however, requires higher temperatures and densities than can be achieved by chemical high explosives, so a nuclear fission explosion is used to create the necessary temperature and density. The result is a two-stage reaction in which a fission bomb explodes first and sets off the secondary, fusion part of the weapon. As can be concluded from this discussion, thermonuclear weapons are not a primary proliferation concern because fission weapon technology must first be mastered before a thermonuclear weapon can be developed.
A multi-stage thermonuclear weapon is called a Teller-Ulam configuration. The primary stage has the same basic design as an implosion fission weapon, described in section 1. After the primary stage is detonated, the x-rays it releases cause the pressure and temperature inside the weapon casing to reach the conditions necessary to achieve a thermonuclear reaction in the fusion fuel. The yield of the fusion fuel is increased when the fissile rod in its center reaches a supercritical state and begins itself to fission. As the fusion fuel reacts, it releases high-energy neutrons that also fission the uranium-238 nuclei that are in the uranium metal casing wrapped around the fusion fuel. In a typical configuration, fission and fusion each contribute about half the overall energy yield.
3.3 Enhanced radiation (neutron) weapons
Another class of thermonuclear weapons creates the maximum amount of radiation possible while minimizing the effects caused by blast. These are called enhanced radiation, or neutron bombs. They rely on fusion between deuterium and tritium to produce a lethal radius of neutrons and gamma rays. The goal is to produce a low yield weapon (deliverable by an artillery shell, for example) that inflicts prompt casualties on troops by radiation but leaves intact structures that otherwise would be destroyed by blast effects.
Because fusion releases many times more neutrons than fission for a given weight of fuel, a neutron bomb can create a larger radius inside which there is a lethal dose of nuclear radiation than a small fission bomb can. A one kiloton neutron bomb, for example, creates about the same lethal radius of nuclear radiation as a 10 kiloton fission weapon. This means that by using a neutron bomb, it is possible to achieve a given radius of lethality with only one tenth of the blast damage that would otherwise be required. These are tactical, not strategic weapons because of their small size. When detonated in the air, they have the additional advantage of producing little residual radiation (fallout) so it is plausible to think of them as battlefield weapons.
4.1.4 Chemical extraction of plutonium
Before being used in a bomb, plutonium must be separated from the intensely hot, and highly radioactive fuel rods in which it is created in a reactor. To achieve this separation, a specially shielded chemical plant is needed to chop the fuel rods into pieces, dissolve the radioactive spent fuel in acid, and then extract the plutonium in pure form.
4.2.3 Critical equipment needed to enrich uranium
Most of the enrichment processes also require that the natural uranium be converted into a gaseous form prior to enrichment, typically uranium hexafluoride (UF6). Thus, a separate chemical processing plant must be constructed to convert the uranium into gaseous form.
4.3 Weapon design and production
In addition to the plutonium or highly enriched uranium needed to fuel a weapon, other components are required to achieve a successful detonation. These typically require high-precision manufacturing, which can be accomplished only with specialized equipment or materials. Such components also require specialized testing equipment. Selected components and equipment are listed below.
The energy released by a nuclear explosion comes in several forms: pressure from the blast, thermal radiation, nuclear radiation, and an electromagnetic pulse. The damage inflicted by the various effects depends upon the size and type of the explosion.
5.3 Nuclear radiation
The nuclear radiation resulting from a nuclear explosion can be divided into two categories: initial and residual. The initial radiation consists of neutrons and gamma rays, which can travel great distances, penetrate considerable thicknesses of material, and inflict fatal damage on human tissue. Initial radiation can be intense but has a limited range. For large nuclear weapons, the range of initial radiation is less than the range of lethal blast and thermal effects. For small weapons, direct radiation may be the lethal effect with the greatest range.
While there is some controversy about the effect of nuclear radiation on the human body, it is estimated that an exposure of 600 rem or greater within one week will result in a 90% chance of death within a few weeks. For a one kiloton blast, initial radiation levels of at least 600 rem extend out 0.8 km from the blast. For a one megaton surface blast, the 600 rem exposure radius would be about 2.7 km.
Residual radiation is often termed fallout, and it can affect both the immediate blast area and areas farther away. Fallout is caused by particles that are scooped up when the nuclear fireball touches the earth. If the nuclear burst is high in the air, fallout is minimal. The scooped-up particles can be carried some distance by the wind before falling back to earth, and their concentration in any one location depends on local weather conditions. Fallout can cause severe contamination to soil, vegetation and groundwater. A steady northwest wind, for example, blowing across a one megaton ground burst in Detroit, could carry enough residual radiation to inflict acute radiation sickness to exposed persons in Cleveland. The residual radiation decays over time, by a factor of ten after seven hours, a factor of 100 after 49 hours and a factor of 1,000 after two weeks. Depending on the conditions of the blast, radiation levels can persist above permissible peace time levels for months or years in areas around the explosion.
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