Oklo natural nuclear reactors

In 1972, measurements of uranium ore from the Oklo mine in Gabon revealed a slightly lower proportion of uranium-235 than expected in natural uranium. Further analyses found evidence of fission products in the ore. Researchers concluded that nuclear fission had occurred naturally there in the distant past. Oklo is the only known site where natural, self-sustaining fission reactors have been identified.

A chain reaction is possible when neutrons released by fission trigger further fissions. The IAEA describes several conditions that coincided at Oklo: uranium deposits had sufficient concentration and size, and groundwater acted as a neutron moderator, slowing neutrons so they could sustain fission. The reactors formed under geological and atmospheric conditions that differed from those of today; the site demonstrates that nuclear processes can be shaped by ordinary natural materials and local geology.

The uranium isotope ratios and fission products preserved in the deposits provide evidence of ancient nuclear reactions. The site has therefore attracted interest from nuclear science, geology and the history of science. Oklo is not a human-built power station: it was a naturally occurring geological phenomenon, identified through laboratory analysis of mined ore. Claims about how its reactors operated in detail rely on interpreting evidence preserved in the deposits.