U-235
Uranium-235 (235U or U-235) 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 nuclear chain reaction. It is the only fissile isotope that exists in nature as a primordial nuclide and was discovered in 1935 by Arthur Jeffrey Dempster.
The release of energy from the fission of Uranium-235 powers most nuclear reactors and nuclear weapons. Uranium enrichment is the process of taking natural uranium and increasing the proportion of uranium-235. Some reactor types can use natural uranium directly, while nuclear weapons and some others reactor types require uranium enriched in U-235.
Uranium-235 has a half-life of 704 million years. Its fission cross section for slow thermal neutrons is about 584.3±1 barns. For fast neutrons it is on the order of 1 barn.
Most neutron absorptions induce fission, though a minority (about 15%) result in the formation of uranium-236.
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