Natural nuclear fission reactor

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The Francevillian basin
Preservation Biota History
Geology and localities Francevillian B Formation Francevillian A Formation Oklo Mine
Fossils Francevillian biota (Also see Akouemma)
Evolutionary concepts Great Oxidation Event Shunga-Francevillian event Huronian glaciation Lomagundi event Origin of Life
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A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions occur. The conditions under which a natural nuclear reactor could exist were predicted in 1956 by Paul Kuroda.^[1] 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 ratios of uranium and of the fission products (and the stable daughter nuclides 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 layers. 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 kW of thermal power during that time.^[2][3][4] During that era, life on Earth consisted of little more than aquatic single-cell organisms.

The "Oklo phenomenon" was discovered in June 1972 by the laboratory at the uranium enrichment plant in Pierrelatte, France. Routine analysis of a sample of natural uranium revealed a slight but abnormal deficit of uranium 235 (²³⁵U)⁶. The normal proportion of ²³⁵U is 0.7202%, whereas this sample showed only 0.7171%.