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Uranium-236 (
U
or U-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.

Creation and yield

The fissile isotope uranium-235 fuels most nuclear reactors. When U 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 U. Thus, the yield of U per U+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 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plutonium-240 makes up a large proportion of reactor-grade plutonium (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR). Pu decays with a half-life of 6561 years into U. In a closed nuclear fuel cycle, most Pu will be fissioned (possibly after more than one neutron capture) before it decays, but Pu discarded as nuclear waste will decay over thousands of years. As
Pu has a shorter half life than
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
Pu produces uranium-236, while
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 U(n,3n)U 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

U, on absorption of a thermal neutron, does not undergo fission, but becomes U, which quickly undergoes beta decay to Np. However, the neutron capture cross section of U is low, and this process does not happen quickly in a thermal reactor. Spent nuclear fuel typically contains about 0.4% U. With a much greater cross-section, Np may eventually absorb another neutron and become Np, which quickly beta decays to plutonium-238 (another non-fissile isotope).

U 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 U from U, U, U or other uranium isotopes. It is even difficult to remove with isotopic separation, as low enrichment will concentrate not only the desirable U and U but the undesirable U, U and U. On the other hand, U in the environment cannot separate from U and concentrate separately, which limits its radiation hazard in any one place.

Contribution to radioactivity of reprocessed uranium

The half-life of U is about 190 times as long as that of U; therefore, U should have about 190 times as much specific activity. That is, in reprocessed uranium with 0.5% U, the U and U will produce about the same level of radioactivity. (U 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 U, so that a sample of U in equilibrium with its decay products (as in natural uranium ore) will have eight times the alpha activity of U alone. Even purified natural uranium where the post-uranium decay products have been removed will contain an equilibrium quantity of U and therefore about twice the alpha activity of pure U. Enrichment to increase U content will increase U to an even greater degree, and roughly half of this U will survive in the spent fuel. On the other hand, U 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 U.

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.

See also

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

References

1. "Capture-to-fission Ratio". nuclear-power.com. Retrieved June 26, 2024.
2. Cabell, M. J.; Slee, L. J. (1962). "The ratio of neutron capture to fission for uranium-235". Journal of Inorganic and Nuclear Chemistry. 24 (12): 1493–1500. doi:10.1016/0022-1902(62)80002-5.
3. Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
4. Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
5. Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
   "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk with a half-life greater than 9 . No growth of Cf was detected, and a lower limit for the β half-life can be set at about 10 . No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 ."
6. This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
7. Excluding those "classically stable" nuclides with half-lives significantly in excess of Th; e.g., while Cd has a half-life of only fourteen years, that of Cd is eight quadrillion years.
8. Winkler, Stephan; Peter Steier; Jessica Carilli (2012). "Bomb fall-out 236U as a global oceanic tracer using an annually resolved coral core". Earth and Planetary Science Letters. 359–360 (1): 124–130. Bibcode:2012E&PSL.359..124W. doi:10.1016/j.epsl.2012.10.004. PMC 3617727. PMID 23564966.
9. UNEP (16 January 2001). "UN ENVIRONMENT PROGRAMME CONFIRMS URANIUM 236 FOUND IN DEPLETED URANIUM PENETRATORS". United Nations. Archived from the original on 17 July 2001. Retrieved 10 February 2021.

External links

• Uranium | Radiation Protection Program | US EPA
• NLM Hazardous Substances Databank - Uranium, Radioactive

• Actinides
• Isotopes of uranium
• Nuclear materials