Long before humans split the atom, a self-sustaining nuclear reactor quietly ran beneath the ground in what is now Gabon.
A Puzzling Shortfall
In May 1972, technicians at the Tricastin enrichment plant in Pierrelatte, France, ran routine checks on uranium hexafluoride made from ore mined in Oklo, Gabon. They noticed something odd. The proportion of uranium-235 in the samples was only 0.60%, not the usual 0.72% found in natural uranium.
That 17% shortfall raised immediate concerns. Every gram of fissile uranium must be carefully accounted for, both for security reasons and for the economics of the nuclear industry. French scientists began investigating.
Discovery in the Ore
The French Atomic Energy Commission measured isotope ratios in Oklo uranium and found uranium-235 levels as low as 0.44%. Similar anomalies were seen in isotopes of neodymium and ruthenium.
These matched patterns typically found in the spent fuel of man-made reactors, including elevated concentrations of certain fission products. The evidence pointed to one explanation: the ore itself had operated as a natural nuclear fission reactor about 1.7 billion years ago.
How a Natural Reactor Worked
Back then, uranium-235 made up about 3.1% of natural uranium, comparable to levels used in some modern reactors. At Oklo, uranium-rich veins lay within rocks that allowed groundwater to flow through. The water acted as a neutron moderator, enabling a chain reaction.
As fission proceeded, the heat boiled the water away, stopping the reaction. Once the rocks cooled, the water returned, restarting the process. These cycles occurred every three hours—around 30 minutes of criticality followed by 2.5 hours of cooling—and continued intermittently for hundreds of thousands of years, with an estimated output of up to 100 kW of thermal power.
Lasting Traces
Over its lifetime, the Oklo reactors consumed about five tons of uranium-235, releasing roughly 100 megatonnes of TNT worth of energy. The fission gases, including xenon isotopes, became trapped in mineral structures, allowing scientists billions of years later to reconstruct the operation cycle.
Studies of isotope ratios in samarium and other elements have even been used to test whether the fine-structure constant, a fundamental physical parameter, has changed over time. Measurements showed that nuclear reactions then were essentially the same as today.
A Singular Phenomenon
Sixteen natural reactor zones have been identified at Oklo. No other location is currently known to have hosted such reactors, though others may have existed and been erased by geological processes.
The discovery provided unique evidence of natural chain reactions, operating long before humans designed reactors, and remains a key site for both geology and nuclear science.
In 1972, French technicians examining uranium from a mine in Gabon noticed something strange.
The proportion of uranium-235 was lower than expected, as if part of it had already been used.
What they uncovered would reveal a reactor that had run long before humans existed…🧵 pic.twitter.com/WuypqFsgQa
— Fascinating True Stories (@FascinatingTrue) October 7, 2025