Editing Natural nuclear fission reactor (section)

== Mechanism ==
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 decays extremely slowly via [[double beta decay]] and thus scientists were able to determine the neutronics of this reactor by calculations based on those isotope ratios almost two billion years after it stopped fissioning uranium.
[[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; the content was 3.65% 2 billion years ago.]]
A key factor that made the reaction possible was that, at the time the reactor went [[critical mass|critical]] 1.7&nbsp;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 {{SimpleNuclide|uranium|238}} and roughly 55 ppm {{chem|234|U}}, neither of which is fissile by [[Neutron temperature|slow]] or moderated neutrons.) 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>

The Oklo uranium ore deposits are the only known sites in which natural nuclear reactors existed. Other rich uranium ore bodies would also have had sufficient uranium to support nuclear reactions at that time, but the combination of uranium, water, and physical conditions needed to support the chain reaction was unique, as far as is currently known, to the Oklo ore bodies. It is also possible that other natural nuclear fission reactors were once operating but have since been geologically disturbed so much as to be unrecognizable, possibly even "diluting" the uranium so far that the isotope ratio would no longer serve as a "fingerprint". Only a small part of the continental crust and no part of the oceanic crust reaches the age of the deposits at Oklo or an age during which isotope ratios of natural uranium would have allowed a self sustaining chain reaction with water as a moderator.

Another factor which probably contributed to the start of the Oklo natural nuclear reactor at 2&nbsp;billion years, rather than earlier, was the [[Great Oxygenation Event|increasing oxygen content in the Earth's atmosphere]].<ref name="Gauthier-Lafaye1996" /> Uranium is naturally present in the rocks of the earth, and the abundance of fissile {{SimpleNuclide|uranium|235}} was at least 3% or higher at all times prior to reactor startup. Uranium is soluble in water only in the presence of [[oxygen]].{{citation needed|date=June 2022}} Therefore, increasing oxygen levels during the aging of the Earth may have allowed uranium to be dissolved and transported with groundwater to places where a high enough concentration could accumulate to form rich uranium ore bodies. Without the new aerobic environment available on Earth at the time, these concentrations probably could not have taken place.

It is estimated that nuclear reactions in the uranium in centimeter- to meter-sized veins consumed about five tons of {{SimpleNuclide|uranium|235}} and elevated temperatures to a few hundred degrees Celsius.<ref name="Gauthier-Lafaye1996" /><ref>{{cite journal |last=De Laeter |first=J. R. |author2=Rosman, K. J. R. |author3=Smith, C. L.  |year=1980 |title=The Oklo Natural Reactor: Cumulative Fission Yields and Retentivity of the Symmetric Mass Region Fission Products |journal=Earth and Planetary Science Letters |volume=50 |issue= 1|pages=238–246 |doi=10.1016/0012-821X(80)90135-1 |bibcode=1980E&PSL..50..238D}}</ref> Most of the non-volatile fission products and actinides have only moved centimeters in the veins during the last 2 billion years.<ref name="Gauthier-Lafaye1996" /> Studies have suggested this as a useful natural analogue for nuclear waste disposal.<ref>{{cite journal |last=Gauthier-Lafaye |first=F. |year=2002 |title=2 billion year old natural analogs for nuclear waste disposal: the natural nuclear fission reactors in Gabon (Africa) |journal=Comptes Rendus Physique |volume=3 |issue=7–8 |pages=839–849 |doi=10.1016/S1631-0705(02)01351-8 |bibcode = 2002CRPhy...3..839G |url=https://comptes-rendus.academie-sciences.fr/physique/articles/10.1016/S1631-0705(02)01351-8/ }}</ref> The overall [[mass defect]] from the fission of five tons of {{chem|235|U}} is about {{convert|4.6|kg}}. Over its lifetime the reactor produced roughly {{convert|100|MtTNT}} in thermal energy, including [[neutrino]]s. If one ignores fission of plutonium (which makes up roughly a third of fission events over the course of normal burnup in modern human-made [[light water reactor]]s), then [[fission product yield]]s amount to roughly {{convert|129|kg}} of technetium-99 (since decayed to ruthenium-99), {{convert|108|kg}} of [[zirconium-93]] (since decayed to [[niobium]]-93), {{convert|198|kg}} of caesium-135 (since decayed to barium-135, but the real value is probably lower as its parent nuclide, xenon-135, is a strong neutron poison and will have absorbed neutrons before decaying to {{SimpleNuclide|caesium|135}} in some cases), {{convert|28|kg}} of [[palladium-107]] (since decayed to silver), {{convert|86|kg}} of [[strontium-90]] (long since decayed to zirconium), and {{convert|185|kg}} of [[caesium-137]] (long since decayed to barium).