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Uranium-235 (235 U 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. Uranium-235 has a half-life of 703.8 million years.
Natural uranium is made weapons-grade through isotopic enrichment. Initially only about 0.7% of it is fissile U-235, with the rest being almost entirely uranium-238 (U-238). They are separated by their differing masses. Highly enriched uranium is considered weapons-grade when it has been enriched to about 90% U-235. [citation needed]
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235 U) has been increased through the process of isotope separation.Naturally occurring uranium is composed of three major isotopes: uranium-238 (238 U with 99.2732–99.2752% natural abundance), uranium-235 (235 U, 0.7198–0.7210%), and uranium-234 (234 U, 0.0049–0.0059%).
The "special nuclear materials" are also plutonium-239, uranium-233, and enriched uranium (U-235). Note that the 1980 Convention on the Physical Protection of Nuclear Material definition of nuclear material does not include thorium. [4] The NRC has a regulatory process for nuclear materials with five main components. [5]
When a uranium nucleus fissions into two daughter nuclei fragments, about 0.1 percent of the mass of the uranium nucleus [15] appears as the fission energy of ~200 MeV. For uranium-235 (total mean fission energy 202.79 MeV [16]), typically ~169 MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about 3% of the speed of ...
Uranium-238 will fission when struck by a neutron with 1.6 megaelectronvolts (0.26 pJ), and about half the neutrons produced by the fission of uranium-235 will exceed this threshold. However, a fast neutron striking a uranium-238 nucleus is eight times as likely to be inelastically scattered as to produce a fission, and when it does so, it is ...
The light-water reactor uses uranium 235 as a fuel, enriched to approximately 3 percent. Although this is its major fuel, the uranium 238 atoms also contribute to the fission process by converting to plutonium 239; about one-half of which is consumed in the reactor. Light-water reactors are generally refueled every 12 to 18 months, at which ...
Because of this, a light-water reactor will require that the 235 U isotope be concentrated in its uranium fuel, as enriched uranium, generally between 3% and 5% 235 U by weight (the by-product from this process enrichment process is known as depleted uranium, and so consisting mainly of 238 U, chemically pure).