Oklo is a place in the West African state of Gabon.

It is famous as the locale of a number of sites at which self-sustaining nuclear fission reactions took place approximately 2 billion years ago. This fact was discovered in 1972, by French physicist Francis Perrin. Measurements of the relative abundances of the two most significant isotopes of the uranium mined there showed an anomalous result compared to those obtained for uranium from other mines. The levels were not merely detectable by statistical analysis: in some samples they were reduced to half what would have been expected. At first nuclear skullduggery was feared,

The natural nuclear reactor formed when a uranium-rich mineral deposit got inundated with groundwater that acted as a neutron moderator, and a strong chain reaction took place. The water moderator would boil away as the reaction increased, slowing it back down again and preventing a meltdown. The fission reaction was sustained for hundreds of thousands of years.

A key to the creation was that at the time, the abundance of fissionable U-235 was about 3%. Due to U-235's shorter half life than U-238, the current abundance of U-235 in natural uranium is about 0.7%. Therefore a natural nuclear reactor is no longer possible on Earth.

Relation To Yucca Mountain

The US government assessment of the security of Yucca mountain for spent nuclear fuel storage, drew a comparison with Oklo.

"And when these deep underground natural nuclear chain reactions were over, nature showed that it could effectively contain the radioactive wastes created by the reactions. No nuclear chain reactions will ever happen in a repository for high-level nuclear wastes. But if a repository were to be built at Yucca Mountain, scientists would count on the geology of the area to contain radionuclides generated by these wastes with similar effectiveness."

External links

• The natural nuclear reactor at Oklo: A comparison with modern nuclear reactors
• Oklo: Ancient African Nuclear Reactors
• yucca Mountain Project: Oklo:Natural Nuclear Reactors