A Uranium Mine in Gabon Ran a Nuclear Reactor 2 Billion Years Ago
The fuel was just rocks and groundwater. It cycled on for thirty minutes at a time and ran for a hundred thousand years.
In May 1972, French physicist Francis Perrin sat at a uranium-processing plant in Pierrelatte and stared at a number that should not have existed. A routine mass-spec check on ore from the Oklo mine in Gabon showed only 0.60% uranium-235, instead of the 0.7202% you find in every other uranium sample, terrestrial or lunar. Some samples were as low as 0.44%. Something had been eating the U-235.
The only known process that selectively burns U-235 is nuclear fission. But the reactor at Pierrelatte hadn't seen this ore yet. Whatever ran the reaction wasn't human.
Perrin's team worked it out by September. Two billion years ago, before any animal existed, U-235 made up about 3% of natural uranium — enough to go critical if you concentrated it and added a moderator. Groundwater at Oklo did both. It seeped into a uranium-rich seam and slowed neutrons until the rock started fissioning. The heat boiled the water off, the chain reaction shut down, and once the rock cooled the water came back. Roughly 30 minutes of criticality, then 2.5 hours of cool-down, on a 3-hour cycle.
Sixteen reactor zones have been mapped at Oklo, plus more nearby at Bangombé. They ran on and off for a few hundred thousand years at under 100 kilowatts — roughly a household's worth of heat, geologically speaking. Then U-235 decayed below the threshold for natural criticality, and the rock went quiet. It cannot happen again. The fuel is too dilute now.
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