Natural nuclear fission reactor

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Part of a series on
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] .In August 1945, Paul Kazuo Kuroda, a professor at Tokyo Imperial University, was sent to examine the ruins of Hiroshima. In April 1956, after immigrating to the United States in 1949, Paul Kazuo Kuroda went to the annual spring meeting of the American Geophysical Union (AGU) to suggest that natural, self-sustaining nuclear fission reactors operated spontaneously on Earth around two billion years ago (2 Ga). The hypothesis was confirmed sixteen years later by a group of scientists working for the French Atomic Energy Commission (1972).

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.

Gabon was a French colony when the first analyses of the subsoil were carried out by the CEA from the MABA base in Franceville, more precisely by its industrial direction which later became COGEMA, leading in 1956 to the discovery of uranium deposits in the region.^{[5][citation needed][clarification needed]}

France almost immediately opened mines, managed by the “Compagnie des Mines d'Uranium de Franceville” (COMUF), to exploit the resources, near the village of Mounana. After independence in 1960, the state of Gabon received a small share of the company's profits.

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%. As the quantities of fissile isotopes are precisely catalogued, this discrepancy had to be explained, so an investigation was launched by CEA on samples from all the mines operated by CEA in France, Gabon and Niger, and at all stages of ore processing and uranium purification.

For uranium and ²³⁵U analyses, CEA's Production Division relies on the Analytical Laboratory at the Pierrelatte plant and on CEA's Central Analysis and Control Laboratory at Cadarache, headed by Michele Neuilly, where Jean François Dozol is in charge of mass spectrometry analyses.