Natural Nuclear Fission Reactor - History

History

In May 1972 at the Pierrelatte uranium enrichment facility in France, routine mass spectrometry comparing UF₆ samples from the Oklo Mine, located in Gabon, Central Africa, showed a discrepancy in the amount of the 235U isotope. Normally the concentration is 0.720%; these samples had only 0.717% – a significant difference. This discrepancy required explanation, 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 (CEA) began an investigation. A series of measurements of the relative abundances of the two most significant isotopes of the 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 235U concentration as low as 0.440%. Subsequent examination of other isotopes showed similar anomalies, such as neodymium and ruthenium as described in more detail below.

This loss in 235U is exactly what happens in a nuclear reactor. A possible explanation therefore was that the uranium ore had operated as a natural fission reactor. Other observations led to the same conclusion, and on September 25, 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.