In 1972, a routine uranium measurement in France came back slightly wrong: 0.717% uranium-235 where scientists expected about 0.720%. Tracing that tiny discrepancy back to a mine in Gabon revealed something extraordinary — two billion years ago, parts of the Earth had sustained nuclear chain reactions on their own. The natural reactors at Oklo operated intermittently for hundreds of thousands of years, regulated by groundwater, and left behind an isotopic record that scientists are still using to study nuclear physics and the movement of radioactive material through rock. △ ───────── △ IN THIS EPISODE — How a 0.003% discrepancy revealed ancient nuclear fission — Why natural uranium could sustain a chain reaction two billion years ago — How groundwater switched the reactors on and off — What Oklo still tells us about fundamental physics and nuclear wasteTHE NUMBER 0.717% — the uranium-235 concentration that triggered the investigation in 1972SOURCES Physical Review Letters — Meshik, Hohenberg & Pravdivtseva, 2004 Nature — Alexander Shlyakhter, 1976 Proceedings of the National Academy of Sciences, 2018 △ ───────── △ Narration in this episode is AI-assisted. Research, writing and editorial judgment are human. Reality Is Strange Real phenomena, documented by science — where the explanation can be stranger than the mystery. Subscribe or follow Reality Is Strange wherever you listen so you don't miss the next episode.

The Nuclear Reactors That Formed Naturally

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