Uranium-238
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Uranium-238 is the most common isotope of uranium found. When hit by a neutron, it becomes uranium-239, an unstable element which decays into neptunium-239, which then itself decays, with a half-life of 2.355 days, into plutonium-239.
Around 99.284% of naturally occurring uranium is uranium-238, which has a half-life of 1.41 × 1017 seconds (4.46 × 109 years). Depleted uranium consists mainly of the 238 isotope, and enriched uranium has a higher-than-natural quantity of the uranium-235 isotope.
Uranium-238 is relevant to nuclear weapons and nuclear reactors in two ways. In a weapon, it impedes the nuclear fission reaction, and so much care and effort must be expended to make sure the levels of it in weapons grade uranium are extremely low. However, in a nuclear reactor, uranium-238 can be used to breed plutonium, which itself can be used in a nuclear weapon or as a reactor fuel source. In fact, in a typical nuclear reactor, up to a third of the generated power does come from the fission of Plutonium-239 (not supplied as a fuel to the reactor, but transmuted from Uranium-238).