Natural nuclear fission reactor

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 idea of a nuclear reactor existing in situ within an ore body moderated by groundwater was briefly explored by Paul Kuroda in 1956.^[1] The existence of an extinct or fossil nuclear fission reactor, where self-sustaining nuclear reactions have occurred in the past, are established by analysis of isotope ratios of uranium and of the fission products (and the stable daughter nuclides of those fission products). The first such fossil reactor was first discovered in 1972 in Oklo, Gabon, by researchers from the French Alternative Energies and Atomic Energy Commission (CEA) when chemists performing quality control for the French nuclear industry noticed sharp depletions of fissionable ²³⁵
U
in gaseous uranium made from Gabonese ore.

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. Fission in the ore at Oklo continued off and on for a few hundred thousand years and probably never exceeded 100 kW of thermal power.^[2][3][4] Life on Earth at this time consisted largely of sea-bound algae and the first eukaryotes, living under a 2% oxygen atmosphere. However even this meager oxygen was likely essential to the concentration of uranium into fissionable ore bodies, as uranium dissolves in water only in the presence of oxygen. Before the planetary-scale production of oxygen by the early photosynthesizers groundwater-moderated natural nuclear reactors are not thought to have been possible.^[4]