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Uranium: Natural Uranium
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Uranium-235 is of particular interest because it is the only fissile material that occurs in nature in significant quantity, and it can be used to construct a nuclear explosive device if a sufficient quantity can be acquired. In a typical sample of natural uranium, only 0.72 percent of the atoms are 235 U atoms, and it can be assumed that all of the remaining atoms are 238 U atoms. 8 Natural uranium typically has a composition of 0.0055 atom % 234 U, 0.7205 atom % 235 U, and 99.274 atom % 238 U. For most purposes, the tiny fraction of 234 U can be neglected. Higher concentrations of 235 U are required for many applications, and the use of uranium isotope separation processes to increase the assay of 235 U above its natural value of 0.72 percent is called uranium enrichment.
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Uranium has some special chemical and biological characteristics: since natural uranium has a low specific activity, chemical damage to the kidneys is likely to be more limiting than radiation damage. If U-235 is concentrated for fuel or weapons, it is called enriched. Normally radiation damage to the lung and kidney are considered if exposure is to enriched uranium.
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Uranium is naturally radioactive, which means that atoms of uranium are unstable and decay by emitting particles and energy. Uranium decays very slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.5 billion years, which means it is not very radioactive. In fact, its very long half-life (and ... low radioactivity) is the reason uranium still exists on the Earth. Three additional isotopes of uranium are not naturally present but can be produced by nuclear transformations. These are uranium-232, uranium-233, and uranium-236.
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Uranium gained importance with the development of practical uses of nuclear energy. Uranium-235 is the only naturally occurring nuclear fission fuel, but this isotope is only about 1 part in 140 of natural uranium; the balance is mostly uranium-238. Because the supply of uranium-235 is limited, the use of fast breeder reactors that convert nonfissionable uranium-238 to fissionable plutonium-239 is becoming increasingly important (see nuclear reactor). Uranium-235 can be separated from uranium-238 by a diffusion process using the gaseous hexafluoride, UF6; the compound of the lighter isotope diffuses faster.
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Physical Half-life: 7.038 x 108 years Sources: Naturally occurring primordial nuclide, 0.720% of the atoms in natural Uranium in the Earth's crust. Enriched with respect to 238U in fissionable material for reactors and weapons. Specific Activity for naturally occuring uranium: 15.8 nCi of U-235/g of Uranium (582 kBq/kg) or ~64 kg per mCi (37 MBq) Principle Modes and Energies of Decay (MeV): alpha -> 4.22 (5.7%), 4.32 (4.4%), 4.40 (55%), 4.56 (4.2%), 4.37 (17%); gamma 0.196 (61%)
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Natural Uranium