How Groundwater Turned a Gabon Uranium Deposit Into a Self-Running Nuclear Reactor

Groundwater, not human engineering, was the missing ingredient that turned an ordinary uranium deposit into a working nuclear reactor. At Oklo, in what is now Gabon, water seeping into a uranium-rich rock formation around two billion years ago helped create the only known self-sustaining natural nuclear reactor on Earth, according to a study titled ‘The Workings of an Ancient Nuclear Reactor’.

Uranium had concentrated in sandstone deposits at the site, while groundwater moving through the ore acted as a neutron moderator, slowing neutrons enough to sustain a chain reaction, according to research compiled by Nuclear Power and historical studies of Oklo. This was only possible because uranium-235 made up around 3% of all uranium two billion years ago, a concentration high enough to trigger fission without any artificial enrichment.

The same water that made fission possible also kept it in check. Findings published in Nature, under the title ‘Ancient nuclear power controlled by water’, describe how heat from the reaction boiled away the groundwater, removing the moderator and halting fission, until the rock cooled, water returned, and the cycle began again.

A reconstruction in that study estimates the pattern at roughly 30 minutes of active fission followed by about 2.5 hours of inactivity, repeating over an estimated 150,000 years.

French researchers discovered the phenomenon in 1972 after noticing that uranium-235 in Oklo ore was slightly lower than expected, a gap natural radioactive decay could not explain. The U.S. Geological Survey later confirmed that sustained nuclear fission had taken place at the site billions of years earlier, and subsequent surveys found more than a dozen separate reactor zones across Oklo and the nearby Okelobondo deposit, each generating around 100 kilowatts of thermal power.

With radioactive by-products remaining trapped in the surrounding rock for close to two billion years, scientists now treat Oklo as a natural laboratory for studying 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 stayed unchanged over billions of years.

Wikimedia Commons/by MesserWoland