A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions occur. The idea of a nuclear reactor existing in situ within an ore body moderated by groundwater was briefly explored by Paul Kuroda in 1956. The existence of an extinct or fossil nuclear fission reactor, where self-sustaining nuclear reactions occurred in the past, was established by analysis of isotope ratios of uranium and of the fission products. The first discovery of such a reactor happened in 1972 in Oklo, Gabon, by researchers from the French Alternative Energies and Atomic Energy Commission (CEA) when chemists performing quality control for the French nuclear industry noticed sharp depletions of fissile 235U in gaseous uranium hexafluoride made from Gabonese ore.