Oklo
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Oklo is a place in the West African state of Gabon.
It is famous as the locale of a number of sites (so far 16 different 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.
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.
A key to the creation was that at the time, the abundance of fissionable U-235 was about 3%, which is comparable to the amount used in today's reactors. 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.
The natural reactor of Oklo can also be used to check, if the fine-structure constant <math>\alpha<math> 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.
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 (http://www.physics.isu.edu/radinf/Files/Okloreactor.pdf)
- Oklo: Ancient African Nuclear Reactors (http://antwrp.gsfc.nasa.gov/apod/ap021016.html)
- yucca Mountain Project: Oklo:Natural Nuclear Reactors (http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml)de:Naturreaktor Oklo
es:Oklo fr:Réacteur nucléaire naturel d'Oklo pl:Oklo sv:Oklo