View source for Physics:Uranium-236

{{short description|Isotope of uranium}}
{{Infobox isotope
 | image = 
 | alternate_names =
 | mass_number =236
 | symbol =U
 | num_neutrons =144
 | num_protons =92
 | abundance ={{val|p=<|10|e=−11}}
 | halflife = {{val|2.348|e=7|u=years}}
 | decay_product = Thorium-232
 | decay_mass = 232
 | decay_symbol =Th
 | parent = Protactinium-236
 | parent_mass = 236
 | parent_symbol =Pa
 | parent_decay = 
 | parent2 = Neptunium-236
 | parent2_mass = 236
 | parent2_symbol = Np
 | parent2_decay =
 | parent3 = Plutonium-240
 | parent3_mass = 240
 | parent3_symbol = Pu
 | parent3_decay = 
 | mass = 236.045568(2)
 | spin = 0+
 | excess_energy =
 | binding_energy = {{val|1790415.042|1.974}}
 | decay_mode1 = Alpha
 | decay_energy1 = 4.572
 | decay_mode2 =
 | decay_energy2 =
 | decay_mode3 =
 | decay_energy3 =
 | decay_mode4 =
 | decay_energy4 =
 }} 
'''Uranium-236''' (<sup>236</sup>U) is an [[Physics:Isotopes of uranium|isotope of uranium]] that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived [[Physics:Radioactive waste|radioactive waste]]. It is found in [[Physics:Spent nuclear fuel|spent nuclear fuel]] and in the [[Physics:Reprocessed uranium|reprocessed uranium]] made from spent nuclear fuel.

==Creation and yield==
The fissile isotope [[Physics:Uranium-235|uranium-235]] fuels most nuclear reactors. When <sup>235</sup>U absorbs a thermal neutron, one of two processes can occur. About 82% of the time, it will [[Physics:Nuclear fission|fission]]; about 18%{{citation needed|date=February 2023}} of the time, it will not fission, instead emitting gamma radiation and yielding <sup>236</sup>U. Thus, the yield of <sup>236</sup>U  per <sup>235</sup>U+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 [[Physics:Caesium-137|caesium-137]], [[Physics:Strontium-90|strontium-90]], and [[Physics:Technetium-99|technetium-99]] are between 6% and 7%, and the combined yield of medium-lived (10 years and up) and [[Physics:Long-lived fission product|long-lived fission product]]s is about 32%, or a few percent less as some are [[Physics:Nuclear transmutation|transmutated]] by [[Physics:Neutron capture|neutron capture]]. [[Physics:Caesium-135|Caesium-135]] is the most notable "absent fission product", as it is found far more in [[Physics:Nuclear fallout|nuclear fallout]] than in [[Physics:Spent nuclear fuel|spent nuclear fuel]] since its parent nuclide [[Physics:Xenon-135|xenon-135]] is the strongest known [[Chemistry:Neutron poison|neutron poison]].

The second-most used fissile isotope [[Physics:Plutonium-239|plutonium-239]] can also fission or not fission on absorbing a thermal neutron. The product [[Physics:Plutonium-240|plutonium-240]] makes up a large proportion of [[Physics:Reactor-grade plutonium|reactor-grade plutonium]] (plutonium recycled from spent fuel that was originally made with enriched natural uranium and then used once in an LWR). <sup>240</sup>Pu decays with a half-life of 6561 years into <sup>236</sup>U. In a closed [[Physics:Nuclear fuel cycle|nuclear fuel cycle]], most <sup>240</sup>Pu will be fissioned (possibly after more than one neutron capture) before it decays, but <sup>240</sup>Pu discarded as [[Physics:Nuclear waste|nuclear waste]] will decay over thousands of years. As {{chem|240|Pu}} has a shorter half life than {{chem|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. [[Physics:Alpha decay|Alpha decay]] of {{chem|240|Pu}} produces uranium-236, while {{chem|239|Pu}} decays to uranium-235.

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{{Actinidesvsfissionproducts}}
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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 <sup>238</sup>U(n,3n)<sup>236</sup>U reaction by fast neutrons in [[Engineering:Thermonuclear weapon|thermonuclear weapon]]s. The A-bomb testing of the 1940s, 1950s, and 1960s has raised the environmental abundance levels significantly above the expected natural levels.<ref>{{cite journal|last=Winkler|first=Stephan|author2=Peter Steier |author3=Jessica Carilli |title=Bomb fall-out 236U as a global oceanic tracer using an annually resolved coral core|journal=Earth and Planetary Science Letters|year=2012|volume=359-360|issue=1|pages=124–130|doi=10.1016/j.epsl.2012.10.004|pmid=23564966|pmc=3617727|bibcode=2012E&PSL.359..124W}}</ref>

==Destruction and decay==
<sup>236</sup>U, on absorption of a thermal [[Physics:Neutron|neutron]], does not undergo fission, but becomes <sup>237</sup>U, which quickly undergoes [[Physics:Beta decay|beta decay]] to [[Physics:Neptunium-237|<sup>237</sup>Np]]. However, the [[Physics:Neutron capture|neutron capture]] cross section of <sup>236</sup>U is low, and this process does not happen quickly in a thermal reactor. Spent nuclear fuel typically contains about 0.4% <sup>236</sup>U. With a much greater [[Physics:Cross section|cross-section]], <sup>237</sup>Np may eventually absorb another [[Physics:Neutron|neutron]] and become [[Physics:Neptunium-238|<sup>238</sup>Np]], which quickly [[Physics:Beta decay|beta decay]]s to [[Physics:Plutonium-238|plutonium-238]] (another non-fissile isotope).

<sup>236</sup>U and most other [[Chemistry:Actinide|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 [[Physics:Alpha decay|alpha decay]]s with a [[Physics:Half-life|half-life]] of 23.420 million years to [[Physics:Thorium-232|thorium-232]]. It is longer-lived than any other artificial actinides or fission products produced in the [[Physics:Nuclear fuel cycle|nuclear fuel cycle]]. ([[Physics:Plutonium-244|Plutonium-244]], which has a half-life of 80 million years, is not produced in significant quantity by the [[Physics:Nuclear fuel cycle|nuclear fuel cycle]], and the longer-lived [[Physics:Uranium-235|uranium-235]], [[Physics:Uranium-238|uranium-238]], and [[Physics:Thorium-232|thorium-232]] occur in nature.)

==Difficulty of separation==
Unlike [[Chemistry:Plutonium|plutonium]], minor actinides, fission products, or [[Physics:Neutron activation|activation products]], chemical processes cannot separate <sup>236</sup>U from [[Physics:Uranium-238|<sup>238</sup>U]], <sup>235</sup>U, [[Physics:Uranium-232|<sup>232</sup>U]] or other uranium isotopes. It is even difficult to remove with isotopic separation, as low enrichment will concentrate not only the desirable <sup>235</sup>U and [[Physics:Uranium-233|<sup>233</sup>U]] but the undesirable <sup>236</sup>U, [[Physics:Uranium-234|<sup>234</sup>U]] and <sup>232</sup>U. On the other hand, <sup>236</sup>U in the environment cannot separate from <sup>238</sup>U and concentrate separately, which limits its radiation hazard in any one place.

==Contribution to radioactivity of reprocessed uranium==

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

The ratio is less than 190 when the decay products of each are included. The [[Physics:Uranium-238#Radium series|decay chain of uranium-238]] to [[Physics:Uranium-234|uranium-234]] and eventually [[Physics:Lead-206|lead-206]] involves emission of eight alpha particles in a time (hundreds of thousands of years) short compared to the half-life of <sup>238</sup>U, so that a sample of <sup>238</sup>U in equilibrium with its decay products (as in natural [[Earth:Uranium ore|uranium ore]]) will have eight times the alpha activity of <sup>238</sup>U alone. Even purified [[Physics:Natural uranium|natural uranium]] where the post-uranium decay products have been removed will contain an equilibrium quantity of <sup>234</sup>U and therefore about twice the alpha activity of pure <sup>238</sup>U. Enrichment to increase <sup>235</sup>U content will increase <sup>234</sup>U to an even greater degree, and roughly half of this <sup>234</sup>U will survive in the spent fuel. On the other hand, <sup>236</sup>U decays to [[Physics:Thorium-232|thorium-232]] which has a half-life of 14 billion years, equivalent to a decay rate only 31.4% as great as that of <sup>238</sup>U.

==Depleted uranium==

[[Chemistry:Depleted uranium|Depleted uranium]] used in [[Physics:Kinetic energy penetrator|kinetic energy penetrator]]s, etc. is supposed to be made from uranium enrichment tailings that have never been irradiated in a [[Physics:Nuclear reactor|nuclear reactor]], not [[Physics:Reprocessed uranium|reprocessed uranium]]. However, there have been claims that some depleted uranium has contained small amounts of <sup>236</sup>U.<ref>{{cite web | url = https://www.un.org/News/Press/docs/2001/unep81.doc.htm | url-status = dead | access-date = 10 February 2021 | title = UN ENVIRONMENT PROGRAMME CONFIRMS URANIUM 236 FOUND IN DEPLETED URANIUM PENETRATORS | date = 16 January 2001 | website = United Nations | author = [[Organization:United Nations Environment Programme|UNEP]] | archive-url = https://web.archive.org/web/20010717004828/https://www.un.org/News/Press/docs/2001/unep81.doc.htm | archive-date = 17 July 2001}}</ref>

{{Isotope sequence
|element=uranium
|lighter=[[Physics:Uranium-235|uranium-235]]
|heavier=uranium-237
|before=[[Physics:Protactinium-236|protactinium-236]]<br />[[Physics:Neptunium-236|neptunium-236]]<br />[[Physics:Plutonium-240|plutonium-240]]
|after=[[Physics:Thorium-232|thorium-232]]
}}

== See also ==
* [[Chemistry:Depleted uranium|Depleted uranium]]
* [[Chemistry:Uranium market|Uranium market]]
* [[Physics:Nuclear reprocessing|Nuclear reprocessing]]
* United States Enrichment Corporation
* [[Physics:Nuclear fuel cycle|Nuclear fuel cycle]]
* [[Physics:Nuclear power|Nuclear power]]

== References ==
{{reflist}}

==External links==
* [https://web.archive.org/web/20060427035426/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]
{{Isotopes of uranium}}

[[Category:Actinides]]
[[Category:Isotopes of uranium]]
[[Category:Nuclear materials]]

{{Sourceattribution|Uranium-236}}