In 1972, analysts at the Pierrelatte uranium enrichment plant in France detected a puzzling anomaly: ore samples from the Oklo mine in Gabon contained barely 0.717% of uranium-235 instead of the universal, constant proportion of 0.720%. In some underground zones, the concentration dropped as low as 0.44%. Physicists and geochemists initially feared a clandestine diversion of fissile material, but the investigation revealed a far more astonishing phenomenon: nature had anticipated Enrico Fermi by nearly two billion years.
During the Proterozoic, the natural abundance of the unstable isotope uranium-235 was approximately 3%, identical to what commercial light-water reactors require today. At Oklo, three extraordinary factors coincided:
- Exceptional concentration: Uranium deposits of high purity and critical thickness in sandstone veins.
- Groundwater as a moderator: Seeping water slowed down the fast neutrons released during decay, making it possible to sustain a self-sustaining chain reaction.
- Thermal self-regulation: When the reactor's heat boiled the water into steam, moderation stopped and the reaction shut down until the stratum cooled.
This colossal geological complex hosted at least 16 active fission zones. It operated in intermittent thermal cycles of about three hours for nearly one million years, generating a continuous average power of 100 kilowatts. Remarkably, the most hazardous radioactive waste remained confined within just a few meters of rock without contaminating the surrounding aquifers.
Source: International Atomic Energy Agency (IAEA) / Scientific American
