An anomaly in the ore
In 1972, workers at a French nuclear fuel processing plant noticed something strange. Uranium ore arriving from a mine in Oklo, Gabon, showed a small but significant discrepancy. The natural abundance of the fissile isotope Uranium-235 (U-235) is consistently 0.7202% everywhere on Earth, in moon rocks, and in meteorites. Yet the Oklo samples contained as little as 0.7171%, with some ore showing concentrations as low as 0.44%. This deficit meant only one thing: the U-235 had already undergone fission. French physicist Francis Perrin announced the conclusion that the Oklo deposit had been the site of a self-sustaining nuclear chain reaction nearly two billion years before humans built their first reactor.
At least 16 distinct reactor zones have since been identified at Oklo. The evidence was in the missing U-235, but also in the presence of specific fission byproducts. Isotopes of the rare earth element neodymium were found in abundances that matched the signature of uranium fission, confirming the initial hypothesis. The conditions 1.7 billion years ago were perfect for this phenomenon. Due to its faster decay rate, the natural concentration of U-235 was about 3%, similar to the enrichment level used in modern power reactors. This higher concentration, which would not be possible today, was a requirement for the reactor to form.
A self-regulating system
For the chain reaction to begin and sustain itself, several conditions predicted by physicist Paul Kuroda in 1956 had to be met. First, the uranium ore deposits had to be sufficiently rich, with concentrations of at least 10%, and exist in thick seams of at least half a meter. Second, a neutron moderator was needed. At Oklo, this was groundwater, which seeped into the porous sandstone and slowed the neutrons produced by decaying uranium, allowing them to be captured by other U-235 atoms and sustain the chain reaction. There also had to be a lack of neutron-absorbing elements, or "poisons," like boron and lithium, which could halt the process.
The reactors operated in a cyclical, self-regulating manner. As a chain reaction started, the heat it generated, estimated to average about 100 kilowatts, boiled the surrounding groundwater away. Without the water moderator, the reaction would slow and stop. After the rock cooled, groundwater would seep back in, and the process would restart. This on-off cycle is thought to have repeated roughly every three hours, and the reactors operated intermittently for several hundred thousand years before the U-235 concentration dropped too low to continue.