Nuclear scientists at the Pierrelatte uranium enrichment plant in south-east France noticed a strange deficit in the amount of uranium-235 they were processing in June 1972. That’s a serious problem in a uranium enrichment plant where every gram of fissionable material has to be carefully accounted for. (...)

Physicists soon traced the anomaly to the supply of uranium ore from Gabon in West Africa, which contained far less uranium-235 than the ore from anywhere else on the planet, a problem that caused some consternation among nuclear scientists. (...)

Nuclear scientists believe that the Oklo reactors operated in pulses. As water flowed into the rock, it moderated the neutrons, allowing a chain reaction to occur. But this increased the temperature of the rock, boiling the water into steam which escaped. (...)

Today, nuclear scientists have calculated that the chain reaction probably lasted for 30 minutes and then switched off for about 2.5 hours, a pulsing process that continued for about 300,000 years.

https://www.iafrikan.com/2015/11/02/did-you-hear-about-how-scientists-discovered-a-two-billion-year-old-nuclear-reactor-in-west-africa/