A natural nuclear reactor spontaneously activated in Africa about 1.7 billion years ago.

The Oklo fossil reactors were discovered by the French Alternative Energies and Atomic Energy Commission (CEA) after they noticed a deficit in the isotope uranium-235. While standard uranium ore contains 0.72% of U-235, samples from the Oklo mine in Gabon contained only 0.717%. This tiny discrepancy led researchers to conclude that a nuclear fission reaction had consumed the missing material approximately 1.7 billion years ago.During that era, the natural concentration of U-235 was about 3%, which is similar to the enriched uranium used in modern nuclear power plants. The geological conditions at Oklo allowed groundwater to seep into the uranium-rich veins. This water acted as a neutron moderator, slowing down neutrons so they could strike other nuclei and sustain a chain reaction. As the reaction generated heat, the water boiled away, which stopped the reaction and prevented a meltdown.Once the rocks cooled, water returned and the cycle began again, continuing for roughly 100,000 to 500,000 years. Scientists identified at least 16 separate reactor zones in the region. Most impressively, studies by the Los Alamos National Laboratory have shown that the toxic byproducts of these reactions, such as plutonium and cesium, migrated less than 10 meters over two billion years. This discovery provides critical data for modern scientists studying how to safely store nuclear waste in deep geological repositories.