Uranium-236 (²³⁶U) 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.

Creation and yield

The fissile isotope uranium-235 fuels most nuclear reactors. When ²³⁵U absorbs a thermal neutron, one of two processes can occur. About 82% of the time, it will fission; about 18%^{[citation needed]} of the time, it will not fission, instead emitting gamma radiation and yielding ²³⁶U. Thus, the yield of ²³⁶U per ²³⁵U+n reaction is about 18%, and the yield of fission products is about 82%. 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 transmutated by neutron capture. Caesium-135 is the most notable "absent fission product", as it is found far more in nuclear fallout than in spent nuclear fuel since its parent nuclide xenon-135 is the strongest known neutron poison.

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plutonium-240 makes up a large proportion of reactor-grade plutonium (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR). ²⁴⁰Pu decays with a half-life of 6561 years into ²³⁶U. In a closed nuclear fuel cycle, most ²⁴⁰Pu will be fissioned (possibly after more than one neutron capture) before it decays, but ²⁴⁰Pu discarded as nuclear waste will decay over thousands of years. As ²⁴⁰
Pu has a shorter half life than ²³⁹
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 ²⁴⁰
Pu produces uranium-236, while ²³⁹
Pu decays to uranium-235.

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.^[6]

Destruction and decay

²³⁶U, on absorption of a thermal neutron, does not undergo fission, but becomes ²³⁷U, which quickly undergoes beta decay 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% ²³⁶U. With a much greater cross-section, ²³⁷Np may eventually absorb another neutron and become ²³⁸Np, which quickly beta decays to plutonium-238 (another non-fissile isotope).

²³⁶U and most other actinide isotopes 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 uranium-235, uranium-238, and thorium-232 occur in nature.)

Difficulty of separation

Unlike plutonium, minor actinides, fission products, or activation products, chemical processes cannot separate ²³⁶U from ²³⁸U, ²³⁵U, ²³²U or other uranium isotopes. It is even difficult to remove with isotopic separation, as low enrichment will concentrate not only the desirable ²³⁵U and ²³³U but the undesirable ²³⁶U, ²³⁴U and ²³²U. On the other hand, ²³⁶U in the environment cannot separate from ²³⁸U and concentrate separately, which limits its radiation hazard in any one place.

Contribution to radioactivity of reprocessed uranium

The half-life of ²³⁸U is about 190 times as long as that of ²³⁶U; therefore, ²³⁶U should have about 190 times as much specific activity. That is, in reprocessed uranium with 0.5% ²³⁶U, the ²³⁶U and ²³⁸U will produce about the same level of radioactivity. (²³⁵U contributes only a few percent.)

The ratio is less than 190 when the decay products of each are included. The decay chain of uranium-238 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 half-life of ²³⁸U, so that a sample of ²³⁸U in equilibrium with its decay products (as in natural uranium ore) will have eight times the alpha activity of ²³⁸U alone. Even purified natural uranium where the post-uranium decay products have been removed will contain an equilibrium quantity of ²³⁴U and therefore about twice the alpha activity of pure ²³⁸U. Enrichment to increase ²³⁵U content will increase ²³⁴U to an even greater degree, and roughly half of this ²³⁴U will survive in the spent fuel. On the other hand, ²³⁶U decays to thorium-232 which has a half-life of 14 billion years, equivalent to a decay rate only 31.4% as great as that of ²³⁸U.

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 ²³⁶U.^[7]