A historical account is presented of the sequence of events which led to the 1956 prediction by the author that nuclear reactors should have existed in nature approximately 2 billion years ago. Following a brief review of the natural reactor theory, some of the results from recent studies of the Oklo Phenomenon are discussed. The report consists of the following chapters: I. Introduction, II. Natural Reactor Theory, III. The Oklo Phenomenon-Models of Natural Reactors, IV. Possibility of the Graphite-type Natural Reactor, V. The Sudbury Phenomenon, and VI. Conclusion. The Chap. IV deals with the studies on the occurrence of fissiogenic xenon isotopes in the carbon-rich mineral thucholite from the Besner Mine, Parry Sound, Ontario, Canada. The thucholite contained fissiogenic xenon isotopes from
238U spontaneous fission, but not from
235U neutron-induced fission. The Chap. V deals with the studies of the abundance pattern of Ne, Ar, Kr and Xe in several granite samples, including the Red Rock granite from the Sudbury structure, Ontario, Canada, which, according to Dietz (1964), was formed by the impact of an asteroid about 1.7 billion years ago. No unusual concentration of fissiogenic xenon isotopes was detected in the Red Rock granite, but the observed rare gas abundance pattern resembled that in meteorites, rather than the terrestrial rare gas abundances.
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