Uranium-236

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.

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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plutonium-240 makes up a large proportion of reactor-grade plutonium (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR).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

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. [1]

See also

• Depleted uranium
• Uranium market
• Nuclear reprocessing
• United States Enrichment Corporation
• Nuclear fuel cycle
• Nuclear power