Scientists Found a Nuclear Reactor Nobody Built — It’s 2 Billion Years Old

In 1972, French researchers analysing uranium ore from Oklo in Gabon noticed something odd: the uranium-235 concentration in the ore was slightly lower than it should have been, and natural radioactive decay alone could not explain the gap.

Further investigation showed the uranium had already undergone sustained nuclear fission billions of years earlier. According to the U.S. Geological Survey, these isotopic anomalies confirmed that a natural nuclear chain reaction had once taken place at the site — something no human had triggered.

The reactor is described in a study titled ‘The Workings of an Ancient Nuclear Reactor’, which traces its origin to around two billion years ago, when a uranium-rich deposit at Oklo became the world’s only known self-sustaining natural nuclear reactor. Subsequent surveys identified more than a dozen separate reactor zones across Oklo and the nearby Okelobondo deposit, each generating around 100 kilowatts of thermal power.

The reaction was possible because uranium-235 made up around 3% of all uranium two billion years ago, a concentration high enough to sustain a chain reaction without artificial enrichment, according to research compiled by Nuclear Power and historical studies of the site. Uranium had concentrated in sandstone deposits, while groundwater seeping into the ore acted as a neutron moderator that slowed neutrons enough to keep fission going.

Rather than running continuously, the reactor regulated itself: heat from fission boiled away the surrounding groundwater, removing the moderator and halting the reaction, and once the rock cooled, water returned and the cycle began again. A reconstruction published in Nature, titled ‘Ancient nuclear power controlled by water’, estimates roughly 30 minutes of active fission followed by about 2.5 hours of inactivity, repeating over an estimated 150,000 years.

Scientists now regard Oklo as a natural laboratory for nuclear waste storage, since radioactive by-products have stayed trapped in the surrounding rock for close to two billion years, 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