Uranium-236 · Lablypedia

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 85.5% of the time, it will fission; about 14.5% 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 14.5%, and the yield of fission products is about 85.5%. 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 transmuted by neutron capture.

The second-most used fissile isotope plutonium-239 can similarly fission or not on absorbing a thermal neutron, the latter giving plutonium-240, a major component of reactor-grade plutonium (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR).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 fission (possibly after more than one neutron capture) before it decays, but ²⁴⁰Pu discarded as nuclear waste will decay over thousands of years. As ²⁴⁰Pu has a shorter half-life than ²³⁹Pu, 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 ²⁴⁰Pu produces uranium-236, while ²³⁹Pu decays to uranium-235.

While the largest part of uranium-236 has been produced by neutron capture in nuclear power reactors, that part is nearly all stored in nuclear reactors and waste repositories and has not been released to the environment. The most significant environmental contribution is the ²³⁸U(n,3n)²³⁶U reaction by fast neutrons in thermonuclear weapons. The nuclear testing of the 1940s, 1950s, and 1960s (atmospheric testing ended 1963) has raised the environmental abundance levels significantly above the expected natural levels.

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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 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 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 23.42 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 81.3 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 has about 190 times as much specific activity. Thus, 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 a sample of ²³⁸U in equilibrium with its decay products (as in natural uranium ore) has eight times the alpha activity of ²³⁸U alone. Even purified natural uranium where the post-uranium decay products have been removed, contains 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, much longer than its own; so its decay chain effectively stops after one step even at long timescales; and the fact that it is an alpha decay means the external exposure hazard is negligible compared to the natural isotopes.

Depleted uranium

Depleted uranium used in kinetic energy penetrators is supposed to be made from uranium enrichment tailings that have never been irradiated in a nuclear reactor, not reprocessed uranium. It should then contain no detectable amount of uranium-236. However, there have been claims of it being found in some depleted uranium.