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In a fission nuclear reactor, uranium-238 can be used to generate plutonium-239, which itself can be used in a nuclear weapon or as a nuclear-reactor fuel supply. In a typical nuclear reactor, up to one-third of the generated power comes from the fission of 239 Pu, which is not supplied as a fuel to the reactor, but rather, produced from 238 U. [5] A certain amount of production of 239
Pu-239 is produced artificially in nuclear reactors when a neutron is absorbed by U-238, forming U-239, which then decays in a rapid two-step process into Pu-239. [22] It can then be separated from the uranium in a nuclear reprocessing plant. [23] Weapons-grade plutonium is defined as being predominantly Pu-239, typically about 93% Pu-239. [24]
A single neutron capture in 238 U is sufficient to produce transuranic elements, whereas five captures are generally necessary to do so from 232 Th. 98–99% of thorium-cycle fuel nuclei would fission at either 233 U or 235 U, so fewer long-lived transuranics are produced.
The three long-lived nuclides are uranium-238 (half-life 4.5 billion years), uranium-235 (half-life 700 million years) and thorium-232 (half-life 14 billion years). The fourth chain has no such long-lasting bottleneck nuclide near the top, so almost all of the nuclides in that chain have long since decayed down to just before the end: bismuth-209.
Plutonium-238 (238 Pu or Pu-238) is a radioactive isotope of plutonium that has a half-life of 87.7 years.. Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 isotope suitable for usage in radioisotope thermoelectric generators (RTGs) and radioisotope heater units.
Uranium-238 is the most stable isotope of uranium, with a half-life of about 4.463 × 10 9 years, [7] roughly the age of the Earth. Uranium-238 is predominantly an alpha emitter, decaying to thorium-234. It ultimately decays through the uranium series, which has 18 members, into lead-206. [17]
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While exposed to the neutron flux during normal operation in the core environment, a small percentage of the 238 U in the fuel absorbs excess neutrons and is transmuted into 239 U. 239 U rapidly decays into 239 Np which in turn rapidly decays into 239 Pu. The small percentage of 239 Pu has a higher neutron cross section than 235 U.