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Gabon: Přírodní jaderný reaktor Oklo

Gabon: Přírodní jaderný reaktor Oklo
The "Oklo phenomenon» was discovered in 1972 at Pierrelattes (France) when routine analyses conducted by the Commissariat ΰ l?Energie Atomique (CEA) pointed out an anomalic depletion in U-235, the fissile isotope used in nuclear power plants, in uranium ore from the Oklo mine (Eastern Gabon). After inquiry, the CEA announced the discovery of the first natural nuclear reactor : the anomaly results from fission chain reactions which took place two billion years ago in the Oklo deposit.

The nuclear phenomenon at Oklo is the only known case of fission products (FP) and transuranians (TU) being stored in a geological, natural environment for a long period of time. Therefore, the Oklo uranium deposit is a valuable site for the study of radionuclide mass transfer processes, and the natural analogy with deep underground radioactive waste disposals. Presently, many countries are concerned about the need for a safe method to dispose of high-level nuclear wastes and prevent potential returns of radioactive wastes from underground repositories to the biosphere. The various national regulatory agencies involved generally require quantitative predictions for at least the first 104 years after disposal, and qualitative predictions of repository safety performance for up to 106 years. The prediction of various scenarii and the sequence of events that are expected to affect the repository site over long periods of time can be achieved by verification and validation using laboratory and field experiments. However, by necessity, these experiments are carried out over relatively short periods of time and therefore are only of limited value. Predicting the long-term behaviour of buried radioactive wastes can be facilitated by comparison with the long-term evolution of natural geochemical systems, or natural analogues, such as Cigar Lake (Canada) and Poηos de Caldas (Brazil). Oklo, by representing a fossil natural reactor which achieved criticality two billion years ago, provides a unique scientific and public awareness opportunity to show whether or not the FP and TU radionuclides have remained confined in the near-vicinity of the reactors subsequent to nuclear reactions.

Of particular interest is the reactor found in the Bangombι uranium deposit, which is located 30 km far from the main Oklo deposit. Due to its shallow location (depth: 10-12 m) within the saturation zone, the Bangombι reactor has undergone extensive weathering phenomena and continues to be affected by the penetration of meteoric waters. This shallow system allows to evaluate the influence of various rock matrices (shales, sandstones) and mineral phases on the mobilization/retardation of various actinides and fission products.


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