Nature's nuclear reactor
A uranium seam in Gabon went critical — and ran for 150,000 years
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Uranium-235 decays faster than uranium-238, so the fissile fraction was around 3 per cent 1.7 billion years ago — roughly that of modern reactor fuel. Where a rich uranium vein sat in porous sandstone, groundwater acted as a moderator and started the chain reaction. The reactor was self-regulating: when the heat boiled the water away it shut down, restarting when the water returned, in cycles of a few hours. Fifteen such zones have been identified at Oklo. They are also the best natural experiment we have in long-term storage of radioactive waste: the fission products have mostly stayed put for two billion years.
For children
In 1972 French scientists noticed that uranium from a mine in Gabon was missing something. The explanation was extraordinary: 1.7 billion years ago, groundwater had made the uranium in the ground start a nuclear reaction all by itself. It cycled on and off for 150,000 years, and nobody built it.
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Part of Terra Saga — the story of Earth on a timeline. terrasaga.org