In 1972, during a routine analysis at the Pierrelatte nuclear plant in France, technicians detected a baffling anomaly: a shipment of uranium extracted from the Oklo mine, in Gabon, contained only 0.717% of the fissile isotope uranium-235 (U-235), compared to the invariant 0.720% recorded in any other rock on Earth, meteorites, and even the Moon. That tiny discrepancy hid an astonishing geological secret: part of the nuclear fuel had already been consumed in the past.
Geological investigations revealed that the Earth had created its own nuclear reactors approximately 1,700 million years ago, anticipating humans by eons. In that remote era, U-235 had not decayed as much as today and its natural abundance hovered around 3%, a proportion identical to the one used by modern nuclear plants. For the miracle to occur, three key factors intervened:
- Hyperconcentrated veins: sandstone strata with uraninite concentrations more than one meter thick.
- Groundwater as a moderator: groundwater slowed down the fast neutrons emitted by uranium, allowing a continuous chain reaction to be sustained.
- Absence of absorbing elements: scarcity of boron or cadmium, minerals that would have shut down fission.
The complex also had a sophisticated cyclical thermal self-regulation mechanism: when fission heated the rock to hundreds of degrees, groundwater boiled and evaporated, shutting down the reactor due to a lack of moderator. When the zone cooled down after two and a half hours, liquid water seeped back in and restarted fission. This cycle repeated for hundreds of thousands of years across 16 active zones, releasing constant thermal energy without ever triggering a nuclear disaster.
Source: International Atomic Energy Agency (IAEA)
