Natural nuclear fission reactor
![Geological situation in Gabon leading to natural nuclear fission reactors1. Nuclear reactor zones2. Sandstone3. Uranium ore layer4. Granite](/Images/godic/202501/29/Gabon_Geology_Oklo.svg5833.png")
![Isotope signatures of natural neodymium and fission product neodymium from 235U which had been subjected to thermal neutrons.](/Images/godic/202501/29/Neodymium_isotope_signatures.svg5833.png")
![Isotope signatures of natural ruthenium and fission product ruthenium from 235U which had been subjected to thermal neutrons. The 100Mo (a long-lived double beta emitter) has not had time to decay to 100Ru over the time since the reactors stopped working.](/Images/godic/202501/29/Ruthenium_isotope_signatures.svg5833.png")
A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions have occurred. This can be examined by analysis of isotope ratios. The existence of this phenomenon was discovered in 1972 at Oklo in Gabon, Africa, by French physicist Francis Perrin. The conditions under which a natural nuclear reactor could exist had been predicted in 1956 by Paul Kazuo Kuroda. The conditions found were very similar to what was predicted.