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It is famous as the locale of a number of sites (so far 16 zones discovered) at three ore deposits, where self-sustaining Nuclear Fission reactions took place approximately 1.5 billion years ago. This fact was discovered in 1972 , by French physicist Francis Perrin . In May 1972 at the Pierrelatte Uranium Enrichment facility in France , routine Mass Spectrometry of a UF6 sample from the Oklo Mine , Franceville , Haut-Ogooué , Gabon , Central Africa showed a discrepancy in the amount of the 235U isotope. Normally the concentration is 0.7202%; these samples had only 0.7171%, a significant difference. The discrepancy, tiny as it was, needed to be explained as all uranium handling facilities must meticulously account for all fissionable isotopes to assure that none are diverted for weapons purposes. Thus the French Commissariat à L'Énergie Atomique began an investigation. A series of measurements of the relative abundances of the two most significant Isotope s of the Uranium mined there showed anomalous results compared to those obtained for uranium from other mines. Further investigations into this uranium deposit discovered uranium ore with a 235U to 238U ratio as low as 0.440%. Also, Neodymium and other elements were found with isotopic compositions different from natural. For example, natural neodymium contains 27% 142Nd; the Nd at Oklo contained less than 6% but contained more 143Nd. Subtracting the natural isotopic Nd abundance from the Oklo-Nd, the isotopic composition matched that produced by the fissioning of 235U. Similar investigations into the isotopic ratios of Ruthenium at Oklo found a much higher 99Ru concentration than expected (27-30% vs. 12.7%). This anomaly could be explained by the decay of 99Tc to 99Ru. Other observations led to the same conclusion and on September 25, 1972, the CEA announced their conclusion that self-sustaining nuclear chain reactions had occurred on Earth about 2000 million years ago. Later, other natural nuclear fission reactors were discovered in the region. The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with Groundwater that acted as a Neutron Moderator , and a strong chain reaction took place. The water moderator would boil away as the reaction increased, slowing it back down again and preventing a Meltdown . The fission reaction was sustained for hundreds of thousands of years. It is estimated that secondary enrichment of the uranium in cm to meter sized veins consumed about six tons of 235U and elevated temperatures to a few hundred degrees Celsius. Remarkably the non-volatile fission products have only moved a few cm in the veins during the last 1.5 billion years. This offers a case study of how radioactive isotopes migrate through the earth's crust; a significant area of controversy as opponents of geologic Nuclear Waste disposal fear that releases from stored waste could end up in water supplies or be carried into the environment. A key factor that made the reaction possible was that at the time the reactor went Critical , the Fissionable isotope 235U made up about 3% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 97% was non-fissionable 238U ) Because 235U has a shorter Half Life than 238U, and thus decays more rapidly, the current abundance of 235U in natural uranium is about 0.7%. A natural nuclear reactor is therefore no longer possible on Earth. The natural reactor of Oklo can also be used to check if the Fine-structure Constant might have changed over time, as new evidence suggests is possible. Alex Shlyakhter proposed in 1976 to measure the abundance of 149Sm to estimate the Cross Section for Neutron capture of this isotope at that time and check it against the present value. RELATION TO YUCCA MOUNTAIN AND OTHER GEOLOGIC REPOSITORIES The Deep Geological Repository concept involves the encapsulation of used fuel in long-lived engineered containers which are then placed and sealed within excavated rooms in a naturally occurring geological formation at a design depth of 500 to 1000 metres below ground surface. The ability of natural geologic barriers to isolate radioactive waste is demonstrated by the Oklo reactors. During their long reaction period about 5.4 tonnes of fission products as well as 1.5 tonnes of Plutonium together with other transuranic elements were generated in the orebody. This plutonium and the other transuranics remained immobile until the present day. This is quite remarkable in view of the fact that Ground Water had ready access to the deposits and they were not in a chemically inert form, such as glass. Thus the only known example of underground nuclear waste disposal was successful over a long period in spite of the characteristics of the site. Such a water-logged, Sandstone / Shale structure would not be considered for disposal of modern toxic wastes, nuclear or otherwise, although the Clay s and Bitumen present played an important part in containing the material. The US government assessment of the security of Yucca Mountain for spent nuclear fuel storage, drew comparisons with Oklo. "And when these deep underground natural nuclear chain reactions were over, nature showed that it could effectively contain the radioactive wastes created by the reactions. No nuclear chain reactions will ever happen in a repository for high-level nuclear wastes. But if a repository were to be built at Yucca Mountain, scientists would count on the geology of the area to contain radionuclides generated by these wastes with similar effectiveness." EXTERNAL LINKS
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