A natural nuclear fission reactor is a uranium deposit where analysis of isotope ratios has shown that self-sustaining nuclear chain reactions have occurred. The existence of this phenomenon was discovered in 1972 by French physicist Francis Perrin. The conditions under which a natural nuclear reactor could exist were predicted in 1956 by P. Kuroda[1]. The conditions found at Oklo were very similar to what were predicted.

http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor
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