Oklo

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

Related Topics:
West Africa - Gabon

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It is famous as the locale of a number of sites (so far 16 zones discovered) 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.

Related Topics:
Nuclear fission - Reactions - 1972 - Francis Perrin - Isotope - Uranium

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The natural nuclear reactor formed when a uranium-rich mineral deposit became 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.

Related Topics:
Groundwater - Neutron moderator

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A key factor that made the reaction possible was that fissionable isotope U-235 made up about 3% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 97% was non-fissionable U-238) Due to the fact that U-235 has a shorter half life than U-238, and therefore decays more rapidly, the current abundance of U-235 in natural uranium is about 0.7%. Therefore a natural nuclear reactor is no longer possible on Earth.

Related Topics:
Isotope - U-235 - U-238 - Half life

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The natural reactor of Oklo can also be used to check if the fine-structure constant lpha might have changed over time, since there is no physical reason why it should be exactly constant. Alex Shlyakhter proposed in 1976 to measure the abundance of Sm-149 to estimate the cross section for neutron capture of this isotope at that time and check it against the present value.

Related Topics:
Fine-structure constant - Alex Shlyakhter - Cross section - Neutron - Isotope

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