Uranium and thorium are found in many mineral species some of which contain appreciable amounts of these elements. Thorium isotope (Th 232) must be converted into fissionable isotope U 233 to be utilized as a source of power. It is useful in explosive devices and in the generation of power. Uranium is the only fissionable material occurring in nature. Sarkar, in Essentials of Mineral Exploration and Evaluation, 2016 2.9.3.4 Uranium and Geothermal Sources The actual critical masses depend on the precise designs employed and composition of the plutonium and uranium, but these approximations provide a good benchmark to compare with the flows of materials in the civilian nuclear fuel cycle. The significant quantities specified by the International Atomic Energy Agency (IAEA) are 8 kg for plutonium and 25 kg for 90% U-235. The use of a reflector-several inches of a material that will reflect neutrons back into the weapon assembly-could cut the critical mass by as much as one-half, for example, to 4–8 kg for plutonium and 25 kg for uranium. Generally speaking, the critical masses required are 10 kg for plutonium metal and 50 kg for uranium metal (90% U-235). Thus, the critical barrier to proliferation is acquisition of fissile material, and a central question for civilian nuclear power is whether it provides a ready route to such acquisition. The gun-type design is particularly simple (it was used in Hiroshima and was never tested prior to that), and a subnational group may be able to use this design to produce a device, once it obtains the necessary fissile material.
The rudiments of both designs are widely known, and it must be assumed that many countries would be able to put together a team of scientists and engineers to manufacture a weapon once the fissile material was obtained. For reasons discussed later, a gun-type device cannot use plutonium. In an implosion weapon, a subcritical sphere or shell of fissile material is imploded, increasing the density of the material to a supercritical state. In a gun device, two subcritical masses of a fissile material are explosively brought together, as along the barrel of a gun. Fission weapons, such as were used in Hiroshima and Nagasaki, could also be used to ignite thermonuclear reactions in so-called two-stage devices, in which the fission component is termed a fission primary or “pit.” Thus, a potential proliferant, even if ultimately seeking a thermonuclear weapon, will first have to develop a fission device. Once it gathered the fissile material, a country or group could then assemble a fission weapon employing one of two fundamental designs: gun type and implosion. However, U-233, which is produced in a reactor containing thorium, could play a role in the future of civilian nuclear energy and is mentioned later. Other isotopes could in principle be used in a weapon-notably U-233, americium-241, and neptunium-239-but so far these are not of practical significance. Although HEU includes all uranium above 20% U-235, here it generally refers to uranium at or above 90% U-235. Although uranium with concentrations between 20 and 90% U-235 could sustain an explosive chain reaction, the critical masses needed, which go inversely as the square of the U-235 density, would be correspondingly very high. Practically, uranium enriched to less than 20% U-235 cannot be used for weapons, and so-called weapon-grade uranium is generally taken to be more than 90% U-235. Natural uranium contains approximately 0.7% U-235 and 99.3% U-238. The critical fissile materials that can sustain an explosive chain reaction are plutonium (of almost any isotopic composition) and HEU (i.e., uranium enriched in the isotope U-235). Feiveson, in Encyclopedia of Energy, 2004 2 Nuclear Weapons
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