Uranium-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.
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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 of fissile decay products per 100 fissions is about 82%. 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plutonium-240 makes up a large proportion of reactor-grade plutonium (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR).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 A-bomb testing of the 1940s, 1950s, and 1960s has raised the environmental abundance levels significantly above the expected natural levels.
Destruction and decay
236U, on absorption of a thermal neutron, does not undergo fission, but becomes 237U, which quickly beta decays to 237Np. However, the neutron capture cross section of 236U is low, and this process does not happen quickly in a thermal reactor. Spent nuclear fuel typically contains about 0.4% U-236. With a much greater cross-section, 237Np may eventually absorb another neutron, becoming 238Np, which quickly beta decays to plutonium-238, or fissioning(Np237 is fissile).
236U and most other actinides are fissionable by fast neutrons in a nuclear bomb or a fast neutron reactor. A small number of fast reactors have been in research use for decades, but widespread use for power production is still in the future.
Uranium-236 alpha decays with a half-life of 23.420 million years to thorium-232. It is longer-lived than any other artificial actinides or fission products produced in the nuclear fuel cycle. (Plutonium-244 which has a half-life of 80 million years is not produced in significant quantity by the nuclear fuel cycle, and the longer-lived U-235, U-238, and thorium-232 occur in nature.)
Difficulty of separation
Unlike plutonium, minor actinides, fission products, or activation products, chemical processes cannot separate U-236 from U-238, U-235, U-232 or other uranium isotopes. It is even difficult to remove with isotopic separation, as low enrichment will concentrate not only the desirable U-235 and U-233 but the undesirable U-236, U-234 and U-232. On the other hand, U-236 in the environment cannot separate from U-238 and concentrate separately, which limits its radiation hazard in any one place.
Contribution to radioactivity of reprocessed uranium
U-238's halflife is about 190 times as long as U-236; therefore U-236 should have about 190 times as much specific activity. That is, in reprocessed uranium with 0.5% U-236, the U-236 and U-238 will produce about the same level of radioactivity. (U-235 contributes only a few percent.)
The ratio is less than 190 when the decay products of each are included. U-238's decay chain to uranium-234 and eventually lead-206 involves emission of eight alpha particles in a time (hundreds of thousands of years) short compared to the halflife of U-238, so that a sample of U-238 in equilibrium with its decay products (as in natural uranium ore) will have eight times the alpha activity of U-238 alone. Even purified natural uranium where the post-uranium decay products have been removed will contain an equilibrium quantity of U-234 and therefore about twice the alpha activity of pure U-238. Enrichment to increase U-235 content will increase U-234 to an even greater degree, and roughly half of this U-234 will survive in the spent fuel. On the other hand, U-236 decays to thorium-232 which has a halflife of 14 billion years, equivalent to a decay rate only 31.4% as great as that of U-238.
Depleted uranium
Depleted uranium used in kinetic energy penetrators, etc. is supposed to be made from uranium enrichment tailings that have never been irradiated in a nuclear reactor, not reprocessed uranium. However, there have been claims that some depleted uranium has contained small amounts of U-236.