Editing Natural nuclear fission reactor (section)

== Discovery of the Oklo fossil reactors ==
In May 1972, at the [[Tricastin Nuclear Power Plant|Tricastin uranium enrichment site]] at Pierrelatte, France, routine [[mass spectrometry]] comparing [[Uranium hexafluoride|UF<sub>6</sub>]] samples from the Oklo mine showed a discrepancy in the amount of the {{SimpleNuclide|uranium|235|link=yes}} isotope. Where the usual concentrations of {{SimpleNuclide|uranium|235|link=yes}} were 0.72% the Oklo samples showed only 0.60%. This was a significant difference—the samples bore 17% less {{SimpleNuclide|uranium|235|link=yes}} than expected.<ref name="DavisGould2014">{{cite journal |last1=Davis |first1=E. D. |last2=Gould |first2=C. R. |last3=Sharapov|first3=E. I. |title=Oklo reactors and implications for nuclear science |journal=International Journal of Modern Physics E |volume=23 |issue=4 |year=2014 |pages=1430007–236 |issn=0218-3013 |doi=10.1142/S0218301314300070 |arxiv = 1404.4948 |bibcode = 2014IJMPE..2330007D |s2cid=118394767}}</ref> This discrepancy required explanation, as all civilian uranium handling facilities must meticulously account for all fissionable isotopes to ensure that none are diverted into the construction of unsanctioned [[nuclear weapon]]s. Further, as [[fissile material]] is the reason for mining uranium in the first place, the missing 17% was also of direct economic concern. [[File:Gabon Geology Oklo.svg|right|thumb|Geological situation in Gabon leading to natural nuclear fission reactors {{ordered list |Nuclear reactor zones |Sandstone |Uranium ore layer |Granite}}]]

Thus, the [[French Alternative Energies and Atomic Energy Commission]] (CEA) began an investigation. A series of measurements of the relative abundances of the two most significant [[isotopes of uranium]] mined at Oklo showed anomalous results compared to those obtained for uranium from other mines. Further investigations into this uranium deposit discovered uranium ore with a {{SimpleNuclide|uranium|235}} concentration as low as 0.44% (almost 40% below the normal value). Subsequent examination of isotopes of fission products such as [[neodymium]] and [[ruthenium]] also showed anomalies, as described in more detail below. However, the [[trace radioisotope]] {{chem|234|U}} did not deviate significantly in its concentration from other natural samples. Both [[depleted uranium]] and [[reprocessed uranium]] will usually have {{chem|234|U}} concentrations significantly different from the [[secular equilibrium]] of 55 [[parts per million|ppm]] {{chem|234|U}} relative to {{chem|238|U}}. This is due to {{chem|234|U}} being enriched together with {{chem|235|U}} and due to it being both consumed by [[neutron capture]] and produced from {{chem|235|U}} by [[fast neutron]] induced (n,2n) reactions in nuclear reactors. In Oklo, any possible deviation of {{chem|234|U}} concentration present at the time the reactor was active would have long since decayed away. {{Chem|236|U}} must have also been present in higher than usual ratios during the time the reactor was operating, but due to its half life of {{val|2.348e7}} years being almost two orders of magnitude shorter than the time elapsed since the reactor operated, it has decayed to roughly {{val|1.4e-22}} its original value and below any abilities of current equipment to detect.

This loss in {{SimpleNuclide|uranium|235}} is exactly what happens in a nuclear reactor. A possible explanation was that the uranium ore had operated as a natural fission reactor in the distant geological past. Other observations led to the same conclusion, and on 25 September 1972, the CEA announced their finding that self-sustaining nuclear chain reactions had occurred on Earth about 2 billion years ago. Later, other natural nuclear fission reactors were discovered in the region.<ref name="Gauthier-Lafaye1996"/>

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