A prehistoric natural “nuclear reactor”, which was in operation about 2 bn years ago, was discovered in the uraniumNatural radioactive element with the atomic number 92. The n... deposit Oklo/Gabon in 1972. In past years, six further locations in this deposit were found where a self-perpetuating chain reactionSelf-perpetuating reaction. In a fission chain reaction, a f... must have taken place due to the reduced U-235 content in the natural uraniumUranium in the isotope composition occurring in nature, Natu.... For the Oklo II location, it can be calculated from the depletionReduction of the relative frequency of a nuclide or several ... of uranium-235 entailed by fissionSee 'nuclear fission'. that a minimum of 4 t U-235 must have been fissioned, 1 t Pu-239 formed, and a volume of heat of about 100 bn kWh generated. As a comparison: in the reactorSystem used to initiate, maintain and control a fission chai... of a nuclear power plantThermal power plant, primarily used for electricity generati... of the 1,300 MWeMegawatt electric; electric output of a power plant in megaw... category, about 30 bn kWh heat is generated by fissionSee 'nuclear fission'. annually.