A Natural Nuclear Reactor

The natural atomic reactor that emerged in Gabon in Africa some two billion years ago truly represents one of the most amazing--though little known--scientific discoveries of all time.

In 1972 a French lab technician was running some routine tests on uranium ore samples. These tests were meant to monitor the quality and international movement of ore from mines around the world. But the tests turned up a curious anomaly: some of the ore from the Oklo mine in Gabon, Central Africa, showed a peculiar ratio of uranium isotopes.

Over the next few years as nuclear scientists puzzled over these findings and performed some further studies on radioactive breakdown products in the ore, an amazing picture emerged. Some two billion years ago, because of a curious coincidence of ore type, movement of the Earth's mantle, oxygen concentration in the atmosphere, and groundwater flow, the uranium deposits in this mine went critical--they started a nuclear chain reaction. The radioactive uranium suddenly overheated by several hundred degrees, explosively boiling off the ground water. This reduced the critical concentration of uranium and shut the reaction down. Over the next two and a half hours the rocks cooled enough to let ground water, rich with uranium, seep back into the reactor site; the critical reaction was re-triggered.

This explosive cycle continued every three hours, over and over, for tens of thousands of years. Some five to six tons of uranium was consumed in this process; approximately 15,000,000 kilowatts of thermal energy was produced every year.

Subsequently 16 sites have been found that may have gone critical in this way. These are all in and near the Oklo mine. There are also several other uranium deposits around the world with similar geologic configuration.

Because of this unique thermonuclear experiment, scientists have learned some strange and wonderful things about nuclear physics. These include some unusual insights into the formation and development of our planet, but also challenges to fundamental ideas about the formation of the cosmos, even to the stability of fundamental "constants" including the hallowed constancy of the speed of light.