Around 2 billion years ago, Oklo became a natural nuclear reactor beneath Africa, and its radioactive remains are still there
A routine uranium test uncovered a 2-billion-year-old mystery How Earth accidentally built and controlled its own nuclear reactor PC: IAEA How oxygen helped create Earth's
A routine uranium test uncovered a 2-billion-year-old mystery How Earth accidentally built and controlled its own nuclear reactor PC: IAEA How oxygen helped create Earth's first natural nuclear reactor How a 2-billion-year-old reactor is testing the laws of physics What the buried waste revealed The only known place where nature built its own nuclear reactor Long before forests spread across the continents, before the first animals walked on land and even before the age of dinosaurs, a remarkable process was unfolding beneath Earth's surface. Deep underground, a series of natural nuclear reactions began inside unusually rich uranium deposits, releasing energy in cycles over an immense stretch of time. Nothing had been designed or built. There were no engineers, no machinery and no human intervention of any kind. Yet the conditions were just right for nature to produce something that closely resembles a modern nuclear reactor. The traces of that ancient phenomenon still exist today in central Africa, offering an unexpected glimpse into Earth's distant geological past and raising questions that continue to interest physicists and geologists alike.As reported by the International Atomic Energy Agency, the story only came to light in the early 1970s during routine inspections of uranium ore from a mine near Oklo in present-day Gabon. Material from the site appeared almost identical to uranium found elsewhere, apart from one tiny inconsistency.
A small amount of the uranium-235 isotope seemed to be missing.The difference was so slight that it could easily have been dismissed as a measurement error.Instead, scientists looked closer. The missing uranium-235 pointed towards something extraordinary: part of the radioactive material had already undergone nuclear fission long before the ore was mined. Since the deposits were roughly 2 billion years old, the reaction could not have been caused by people. It had happened naturally, deep underground, in Earth's distant past.Natural nuclear reactors are exceptionally difficult to create because several uncommon conditions have to occur together.At Oklo, uranium had accumulated in unusually high concentrations within ancient rock formations. Groundwater slowly filtered through these deposits, acting in much the same way as the moderator used inside certain modern nuclear power stations. The water slowed down neutrons enough for a sustained chain reaction to begin.The process did not continue without interruption. As heat built up, the surrounding groundwater eventually boiled away, removing the conditions needed for fission. The reaction then stopped. Once the rocks cooled and water returned, the cycle began again. These natural on-and-off phases repeated over long periods, with some operating cycles lasting from hours to months.Scientists believe around 16 separate reactor zones developed across the area. They did not all operate simultaneously. Instead, activity shifted between different sections over roughly 200,000 years.The existence of the Oklo reactors depended on more than just uranium and water.Their formation appears to have been tied to a major turning point in Earth's history known as the Great Oxidation Event, when microscopic organisms dramatically increased the amount of oxygen in the atmosphere.
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