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{{Short description|Naturally occurring uranium self-sustaining nuclear chain reactions}}
{{Franceville basin}}
A '''natural nuclear fission reactor''' is a [[uranium]] [[mineral deposit|deposit]] where self-sustaining [[nuclear chain reaction]]s occur. The conditions under which a natural [[nuclear reactor]] could exist were predicted in 1956 by [[Paul Kuroda]].<ref>{{cite journal |last=Kuroda |first=P. K. |year=1956 |title=On the Nuclear Physical Stability of the Uranium Minerals |journal=Journal of Chemical Physics |volume=25 |issue= 4|pages=781–782; 1295–1296 |doi=10.1063/1.1743058 |bibcode = 1956JChPh..25..781K }}</ref> The remnants of an extinct or '''fossil nuclear fission reactor''', where self-sustaining nuclear reactions have occurred in the past, are verified by analysis of [[isotope]] [[ratio]]s of uranium and of the [[fission product]]s (and the stable [[daughter nuclide]]s of those fission products). This was first discovered in 1972 in [[Oklo]], [[Gabon]] by researchers from French ''[[Commissariat à l'énergie atomique]]'' (CEA) under conditions very similar to Kuroda's predictions.
Oklo is the only location where this phenomenon is known to have occurred, and consists of 16 sites with patches of centimeter-sized [[Ore deposit|ore layer]]s. There, self-sustaining [[nuclear fission]] reactions are thought to have taken place approximately 1.7 [[billion]] years ago, during the [[Statherian]] period of the [[Paleoproterozoic]], and continued for a few hundred thousand years, probably averaging less than 100 [[Watt#Kilowatt|kW]] of thermal power during that time.<ref>{{cite journal |last=Meshik |first=A. P. |date=November 2005 |title=The Workings of an Ancient Nuclear Reactor |url=http://www.sciam.com/article.cfm?id=ancient-nuclear-reactor |journal=Scientific American |volume=
|url=https://blogs.scientificamerican.com/guest-blog/natures-nuclear-reactors-the-2-billion-year-old-natural-fission-reactors-in-gabon-western-africa/
|title= Nature's Nuclear Reactors: The 2-Billion-Year-Old Natural Fission Reactors in Gabon, Western Africa
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The natural nuclear reactor at Oklo formed when a uranium-rich mineral deposit became inundated with [[groundwater]], which could act as a [[neutron moderator|moderator]] for the neutrons produced by nuclear fission. A [[nuclear chain reaction|chain reaction]] took place, producing heat that caused the groundwater to boil away; without a moderator that could slow the neutrons, however, the reaction slowed or stopped. The reactor thus had a negative [[void coefficient]] of reactivity, something employed as a safety mechanism in human-made [[light water reactor]]s. After cooling of the mineral deposit, the water returned, and the reaction restarted, completing a full cycle every 3 hours. The fission reaction cycles continued for hundreds of thousands of years and ended when the ever-decreasing fissile materials, coupled with the build-up of [[neutron poison]]s, no longer could sustain a chain reaction.
Fission of uranium normally produces five known isotopes of the fission-product gas [[xenon]]; all five have been found trapped in the remnants of the natural reactor, in varying concentrations. The concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 30 minutes of criticality followed by 2 hours and 30 minutes of cooling down (exponentially decreasing residual [[decay heat]]) to complete a 3-hour cycle.<ref>{{cite journal |last=Meshik |first=A. P. |year=2004 |title=Record of Cycling Operation of the Natural Nuclear Reactor in the Oklo/Okelobondo Area in Gabon |journal=[[Physical Review Letters]]|volume=93 |issue=18 |pages=182302 |doi=10.1103/PhysRevLett.93.182302 |pmid=15525157 |bibcode=2004PhRvL..93r2302M|display-authors=etal}}</ref> [[Xenon-135]] is the strongest known neutron poison. However, it is not produced directly in appreciable amounts but rather as a decay product of [[Iodine-135]] (or one of its [[parent nuclide]]s). Xenon-135 itself is unstable and decays to [[caesium-135]] if not allowed to absorb neutrons. While caesium-135 is relatively long lived, all caesium-135 produced by the Oklo reactor has since decayed further to stable [[barium-135]]. Meanwhile, Xenon-136, the product of [[neutron capture]] in xenon-135
[[File:Anteil Uran235 im Natururan.png|alt=A graph showing the exponential decay of Uranium-235 over time.|thumb|upright=1.2|Change of content of Uranium-235 in natural uranium
A key factor that made the reaction possible was that, at the time the reactor went [[critical mass|critical]] 1.7 billion years ago, the [[fissile]] isotope {{SimpleNuclide|uranium|235}} made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 96.9% was non-fissile {{SimpleNuclide|uranium|238}} and roughly 55 ppm {{chem|234|U}}.) Because {{SimpleNuclide|uranium|235}} has a shorter [[half-life]] than {{SimpleNuclide|uranium|238}}, and thus decays more rapidly, the current abundance of {{SimpleNuclide|uranium|235}} in natural uranium is only 0.72%. A natural nuclear reactor is therefore no longer possible on Earth without [[heavy water]] or [[graphite]].<ref name="Greenwood1257">{{Greenwood&Earnshaw2nd|page=1257}}</ref>
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