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Concept
Uranium-236
Summary
Uranium-236 (236U) is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.
The fissile isotope uranium-235 fuels most nuclear reactors. When 235U absorbs a thermal neutron, one of two processes can occur. About 82% of the time, it will fission; about 18% of the time, it will not fission, instead emitting gamma radiation and yielding 236U. Thus, the yield of 236U per 235U+n reaction is about 18%, and the yield of fission products is about 82%. In comparison, the yields of the most abundant individual fission products like caesium-137, strontium-90, and technetium-99 are between 6% and 7%, and the combined yield of medium-lived (10 years and up) and long-lived fission products is about 32%, or a few percent less as some are transmutated by neutron capture. Caesium-135 is the most notable "absent fission product", as it is found far more in nuclear fallout than in spent nuclear fuel since its parent nuclide Xenon-135 is the strongest known neutron poison.
The second-most used fissile isotope plutonium-239 can also fission or not fission on absorbing a thermal neutron. The product plutonium-240 makes up a large proportion of reactor-grade plutonium (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR). 240Pu decays with a half-life of 6561 years into 236U. In a closed nuclear fuel cycle, most 240Pu will be fissioned (possibly after more than one neutron capture) before it decays, but 240Pu discarded as nuclear waste will decay over thousands of years. As 240Pu has a shorter half life than 239Pu the grade of any sample of plutonium mostly composed of those two isotopes will slowly increase while the total amount of plutonium in the sample will slowly decrease over centuries and millennia. Alpha decay of 240Pu will yield uranium-236 while 239Pu decays to uranium-235.
Official source
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