Creation and Yield
The fissile isotope uranium-235 fuels most nuclear reactors. U-235 that absorbs a thermal neutron may go one of two ways. About 82% of the time, it will fission. About 18% of the time it will not fission, instead emitting gamma radiation and yielding U-236. Thus, the yield of U-236 per 100 U-235+n reactions is about 18%, and the yield per 100 fissions is about 22%. In comparison, the yields of the most abundant individual fission products like Cs-137, Sr-90, Tc-99 are between 6% and 7% per 100 fissions, 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 destroyed by neutron capture.
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plutonium-240 makes up a large proportion of reactor-grade plutonium (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR). Pu-240 decays with a half-life of 6561 years into U-236. In a closed nuclear fuel cycle, most Pu-240 will be fissioned (possibly after more than one neutron capture) before it decays, but Pu-240 discarded as nuclear waste will decay over thousands of years.
While the largest part of uranium-236 has been produced by neutron capture in nuclear power reactors, it is for the most part stored in nuclear reactors and waste repositories. The most significant contribution to uranium-236 abundance in the environment is the 238U(n,3n)236U reaction by fast neutrons in thermonuclear weapons. The bomb-testing of 1950s and 1960s has raised the environmental abundance levels significantly above the expected natural levels.
Read more about this topic: Uranium-236
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