Uranium-236

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Uranium-236 (U or U-236) 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 U absorbs a thermal neutron, one of two processes can occur. About 85.5% of the time, it will fission; about 14.5% of the time, it will not fission, instead emitting gamma radiation and yielding U. Thus, the yield of U per U+n reaction is about 14.5%, and the yield of fission products is about 85.5%. 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 transmuted by neutron capture. The second-most used fissile isotope plutonium-239 can similarly fission or not on absorbing a thermal neutron, the latter giving plutonium-240, a major component of reactor-grade plutonium (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR). Pu decays with a half-life of 6561 years into U. In a closed nuclear fuel cycle, most Pu will fission (possibly after more than one neutron capture) before it decays, but Pu discarded as nuclear waste will decay over thousands of years. As Pu has a shorter half-life than Pu, 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 Pu produces uranium-236, while Pu decays to uranium-235.