The vibrant colors of the Grand Prismatic Spring in Yellowstone Park are due to the microbial carpet it is covered with. A similar carpet of microbes likely covered the shallow lakes that about two billion years ago, in the territory of today’s Gabon, saw the ignition of a series of natural nuclear reactors, the only ones currently known

Oklo’s Natural Nuclear Reactors

About two billion years ago, in a region of Central Africa located in the current Gabonese Republic, an incredible and unrepeatable series of coincidences caused the ignition of at least seventeen natural nuclear reactors, which remained in operation for hundreds of thousands of years

A surprising discovery

In 1972, samples of uranium-containing minerals from various mines were collected in the uranium enrichment plant in Pierrelatte, France. Analyzing a sample of uranium hexafluoride from the Oklo mine in Gabon with a mass spectrometer, physicist Francis Perrin noticed something strange. The ratio between uranium-235 (²³⁵U) and uranium-238 (²³⁸U) present in the sample was 0.007171, a value slightly lower than the typical value of 0.007252. It was necessary to understand the origin of this difference. Numerous other samples from the same mine were analyzed. It was found that they contained a lower than average amount of ²³⁵U, in some cases much below, up to a minimum ratio of 0.00440 compared to the ²³⁸U isotope. They were exactly the values one would have expected to find in a nuclear fuel used inside a reactor.