Some of the mined uranium was found to have a lower concentration of uranium-235 than expected, as if it had already been in a nuclear reactor. When geologists investigated they also found products typical of a reactor. They concluded that the deposit had been in a reactor: a natural nuclear fission reactor, around 1.8 to 1.7 billion years BP – in the Paleoproterozoic Era during Precambrian times, during the Statherian period – and continued for a few hundred thousand years, probably averaging less than 100 kW of thermal power during that time. At that time the natural uranium had a concentration of about 3% 235U and could have reached criticality with natural water as neutron moderator allowed by the special geometry of the deposit.[1][2][3][4][5][6][7]
^Cowan, George (1976). "Oklo – A Natural Fission Reactor"(PDF). Federation of American Scientists. Los Alamos Scientific Laboratory. Archived(PDF) from the original on 29 March 2024. Retrieved 15 August 2023.