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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 the reprocessed uranium made from it.
Additional recommended knowledge
Creation and yield
The fissile isotope uranium-235 which fuels most nuclear reactors will fission after absorbing a thermal neutron about 82% of the time. About 18% of the time, it merely emits gamma radiation and remains 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 long and medium-lived (10 years and up) 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plutonium-240 makes up a large proportion of reactor-grade plutonium (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR).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.
Destruction and decay
236U, on absorption of a thermal neutron, does not 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 .4% U-236.
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 8 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 8 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 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 DU has contained small amounts of U-236. 
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Uranium-236". A list of authors is available in Wikipedia.|