The Oklo natural nuclear reactors
Uranium in Gabon goes critical on its own and burns for hundreds of thousands of years.
- When
- Known to
- ± 50 million years
- Era
- Paleoproterozoic, Proterozoic
Around 1.7 billion years ago, groundwater seeping through unusually rich uranium ore in Gabon moderated neutrons well enough to sustain nuclear fission. Sixteen natural reactors ran intermittently, generating perhaps 100 kilowatts on average. It was only possible then because uranium-235 was about 3% of natural uranium rather than today's 0.7% — the fuel has since decayed below criticality, so this can never happen again. It was discovered in 1972 when French analysts found ore that was measurably depleted.