And just natural? Bashny.Net

On the territory of the Oklo uranium deposit in Gabon discovered ore bodies, in which nearly 2 billion years ago, there is a spontaneous chain reaction of nuclear fission of uranium. In other words, there was a natural nuclear reactor, and it worked a few hundred thousand years. This discovery was made in 1972, when the French enrichment plant in Gabon carried out mass-spectrometric analysis of rocks and found less in comparison with the ordinary meaning to the concentration of uranium isotope 235U, which indicated the presence of spent nuclear fuel.