The first nuclear reactor was in Africa, 2 billion years ago. Two billion years ago, there was enough uranium 235 in a naturally occurring deposit in Africa to fuel a nuclear fission reaction. In 16 separate locations, spontaneously occurring fission reactions went on for some hundreds of thousands of years, cycling multiple times per day. A picture of Fossil Reactor 15. The American Nuclear Society info site.
The site of the world's first nuclear reactor? Gabon. About 1.7 billion years ago several deposits of uranium in Oklo, Gabon spontaneously began to undergo nuclear chain reactions fed by small drips of water. These natural breeder reactors ran for almost a million years, producing both intense heat and plutonium byproducts. Aside from the strangeness of naturally occurring reactors, Oklo provides the only existing case of how highly radioactive waste behaves over a period of tens of millions of years -- exactly the problem faced by the DOE's Yucca Mountain nuclear waste site.