Two-billion-year-old nuclear reactors: Nature goes fission

Once it was thought that the isotopic composition of natural uranium was invariant. It was thus surprising in 1972 when French scientists observed small but significant deficiencies of the minor isotope 235U in uranium ore. Subsequent investigations traced the isotopically anomalous material to the Oklo mine in the African Republic of Gabon. In the mine, cubic-dekametre-sized pods of rock were found to contain extraordinary concentrations of uranium, as much as 65%, with as little as half the normal isotopic abundance of 235U. In these rocks, neodymium was found to be deficient in the premordial isotope 142Nd and enriched in the fission-produced isotopes 143-150Nd. The presence of fission products was unambiguous evidence that the 235U deficiencies were the result of sustained nuclear fission. Within the heart of the natural reactors, the fission densities were on the order of 1020 fissions/cm3, producing hundreds of megajoules of energy and tens of microwatts of power per gram of rock. Nature had forestalled man's great discovery of energy production by nuclear fission