Earth’s Only Natural Nuclear Reactor Switched Itself On and Off for Millennia

Deep beneath what is now Gabon, a uranium deposit once switched itself on and off like a machine with a mind of its own — cycling through nuclear fission for an estimated 150,000 years without any human intervention, according to a study titled ‘The Workings of an Ancient Nuclear Reactor’.

The site, known as Oklo, became the world’s only known self-sustaining natural nuclear reactor around two billion years ago. Findings published in Nature, titled ‘Ancient nuclear power controlled by water’, describe how the reactor switched on and off as groundwater moved in and out of the rock, with fission heat boiling the water away before the cycle restarted.

A reconstruction in that Nature study estimates the pattern at roughly 30 minutes of active fission followed by about 2.5 hours of dormancy, repeating over the reactor’s 150,000-year lifespan. The self-regulation happened because groundwater acted as a neutron moderator: it seeped into uranium-bearing sandstone and slowed neutrons enough to sustain the chain reaction, then boiled away once heat built up, shutting the reaction down until the rock cooled and water seeped back in.

This was only possible because Earth’s uranium looked very different two billion years ago — the fissile isotope uranium-235 made up around 3% of all uranium then, a concentration high enough to sustain fission without any artificial enrichment, according to research compiled by Nuclear Power and historical studies of the site.

The phenomenon went unnoticed until 1972, when French researchers analysing Oklo ore found its uranium-235 concentration slightly lower than expected, a gap natural decay could not explain. The U.S. Geological Survey later confirmed sustained fission had occurred there billions of years earlier, and further surveys located more than a dozen separate reactor zones across Oklo and the nearby Okelobondo deposit, each producing around 100 kilowatts of thermal power.

Because radioactive by-products have remained trapped in the surrounding rock for nearly two billion years, scientists now use Oklo as a natural reference point for how deep geological repositories might safely contain spent nuclear fuel, according to research published in the Journal of Contaminant Hydrology and Nature. The site has also been used to test whether fundamental physical constants have changed over billions of years.

Wikimedia Commons/by MesserWoland