This Uranium Deposit Secretly Ran Itself Like a Reactor for 150,000 Years

For an estimated 150,000 years, a patch of uranium ore beneath what is now Gabon ran itself through cycles of nuclear fission with no human involvement at all, according to a study titled ‘The Workings of an Ancient Nuclear Reactor’.

The deposit, at a site called Oklo, is the only known example of a natural nuclear reactor on Earth. Findings published in Nature under the title ‘Ancient nuclear power controlled by water’ describe how the reactor switched on and off repeatedly as groundwater flowed in and out of the rock, with heat from fission periodically boiling the water away.

A reconstruction in the same Nature study puts the cycle at roughly 30 minutes of active fission followed by about 2.5 hours of dormancy, repeated over that 150,000-year span. The reactor was able to run this way because uranium-235 made up around 3% of all uranium two billion years ago, far higher than today’s levels and high enough to sustain a chain reaction without any artificial enrichment, according to research compiled by Nuclear Power and historical studies of the site.

Groundwater played the key role in keeping the reaction stable: it seeped into the uranium-bearing sandstone and acted as a neutron moderator, slowing neutrons enough to sustain fission, then boiled away once the heat built up, shutting the reaction down until the rock cooled and water returned.

French researchers first identified the phenomenon in 1972 after noticing that uranium-235 in Oklo ore was slightly lower than expected — a discrepancy natural decay alone could not account for. The U.S. Geological Survey later confirmed sustained nuclear fission had occurred at the site billions of years ago, and subsequent surveys found more than a dozen separate reactor zones across Oklo and the nearby Okelobondo deposit, each producing around 100 kilowatts of thermal power.

The site’s ability to keep radioactive by-products contained in rock for nearly two billion years has made it a reference point for studying how deep geological repositories might safely hold spent nuclear fuel, according to research published in the Journal of Contaminant Hydrology and Nature. It has also helped researchers test whether physical constants have remained unchanged across billions of years.

Wikimedia Commons/by MesserWoland