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Seventeen natural nuclear reactors have been discovered; the first was the found in the Gambian Oklo Uranium mine in 1972. All told six natural reactors have been found in the Oklo mine and its vicinity. Nine of the seventeen natural reactors that have been discovered have been completely mined out. In the Oklo mine the remains of what has come to be called Oklo reactor 15 are still visible.

Paul Kuroda, a Japanese physicist, was the first to explain the possibility of a naturally occurring nuclear reaction on Earth.

There are two naturally occurring isotopes of Uranium, whose mass numbers are 235 and 238. The half-life of Uranium 235 is 700 million years, while that of Uranium 238 is 4.5 billion years. Uranium 235 decays faster and thus is more radioactive. Current nuclear reactors must enrich the amount of Uranium 235 in their fuel to 3% or more before it is able to sustain a nuclear reaction. The same is true of a natural reactor. However Uranium currently has a percentage of Uranium 235 that is 0.72% or less. Not enough for a natural reaction to occur, this is a good thing. However the percentage of Uranium 235 was not always this low. In fact the further back in time you go the larger the percentage of Uranium 235 becomes. About 2 billion years ago the percentage of Uranium 235 was 3%, and enough to sustain a nuclear reaction.

Sources:
http://www.npp.hu/tortenelem/foldreaktor-e.htm

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