U-235
Uranium metal highly enriched in uranium-235
Uranium-235 (235U) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a fission chain reaction. It is the only fissile isotope that is a primordial nuclide or found in significant quantity in nature.
Uranium-235 has a half-life of 703.8 million years. It was discovered in 1935 by Arthur Jeffrey Dempster. Its (fission) nuclear cross section for slow thermal neutrons is about 584.994 barns. For fast neutrons it is on the order of 1 barn. Most but not all neutron absorptions result in fission; a minority result in neutron capture forming uranium-236.
The fission of one atom of U-235 generates 202.5 MeV = 3.24 × 10−11 J, which translates to 19.54 TJ/mol, or 83.14 TJ/kg. This is around 2.5 million times more than the energy released from burning coal. When 235
92U nuclides are bombarded with neutrons, one of the many fission reactions that it can undergo is the following (shown visually in the adjacent image):
Heavy water reactors, and some graphite moderated reactors can use unenriched uranium, but light water reactors must use low enriched uranium because of light water's neutron absorption. Uranium enrichment removes some of the uranium-238 and increases the proportion of uranium-235. Highly enriched uranium (HEU), which contains an even greater proportion of U-235, is sometimes used in nuclear weapon design.
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