Man-made nuclear reactors are devices used to control a sustained nuclear reaction. Most commonly, they’re used to generate electricity and for the propulsion of ships. Nuclear fission reactors can occur naturally on the Earth. They are uranium deposits where self sustaining nuclear reactions have occurred. The existence of such a reactor was discovered in 1972 at Oklo in Gabon, Africa.
French physicist Francis Perrin was the first to discover this site. Oklo is the only known location of a naturally occurring nuclear fission reactor. It consists of 16 sites at which self-sustaining nuclear fission reactors took place about 1.7 billion years ago. These reactors ran for around 100,000 years and averaged 100 kW of power throughout this time.