background = #fc6
isotope_name = Uranium-236
abundance =< 10^-10
halflife = 2.348 x10^7 years
decay_product = Thorium-232
decay_mass = 232
parent = Protactinium-236
parent_mass = 236
parent2 = Neptunium-236
parent2_mass = 236
parent2_symbol = Np
parent3 = Plutonium-240
parent3_mass = 240
parent3_symbol = Pu
mass = 236.045568(2) u
spin = 0+
binding_energy = 1783870.285 ± 1.996 keV
decay_mode1 = Alpha 4.572
Uranium-236 is an isotope of Uranium that is neither
fissilewith thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fueland the reprocessed uraniumcontains it.
Creation and yield
fissileisotope uranium-235which fuels most nuclear reactorswill fission after absorbing a thermal neutronabout 82% of the time. About 18% of the time, it merely emits gamma radiationand 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 productslike Cs-137, Sr-90, Tc-99are between 6% and 7% per 100 fissions, 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 destroyed by neutron capture.
The second most used fissile isotope
plutonium-239can also fission or not fission on absorbing a thermal neutron. The product plutonium-240makes up a large proportion of "reactor-grade plutonium" (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR).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 wastewill 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 sectionof 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
actinidesare fissionable by fast neutrons in a nuclear bombor 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.
alpha decays with a half-lifeof 23.420 million years to Thorium-232. It is longer-lived than any other artificial actinidesor fission productsproduced in the nuclear fuel cycle.( Plutonium-244which 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-232occur in nature.)
Difficulty of separation
plutonium, minor actinides, fission products, or activation products, chemical processes cannot separate U-236 from U-238, U-235, U-232or 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-233but 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 productsof each are included. U-238's decay chain to Uranium-234and eventually Lead-206involves emission of 8 alpha particlesin 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 uraniumwhere 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-232which has a halflife of 14 billion years, equivalent to a decay rate only 31.4% as great as that of U-238.
Depleted uraniumused in kinetic energy penetrators, etc. is supposed to be made from uranium enrichmenttailings 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. [http://www.un.org/News/Press/docs/2001/unep81.doc.htm]
Protactinium-236 Neptunium-236 Plutonium-240
United States Enrichment Corporation
Nuclear fuel cycle
* [http://www.epa.gov/radiation/radionuclides/uranium.htm Uranium | Radiation Protection Program | US EPA]
* [http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+uranium,+radioactive NLM Hazardous Substances Databank - Uranium, Radioactive]
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