P. K. Kuroda: Feasibility of Natural Nuclear Reactors in Uranium Mines

Early in 1939, Otto Hahn and Fritz Strassmann published in Die Naturwissenschaften their discovery of nuclear fission, the splitting of the nucleus of uranium atoms. Later in the same year and in the same journal, Siegfried Flügge speculated on the possibility of nuclear fission chain reactions occurring in nature. But as the clouds of World War II darkened over Europe and the rest of the world, interest in nuclear fission focused upon the design and production of nuclear fission devices (atom bombs). After the war, attention focused upon commercial nuclear electric power production and nuclear submarine propulsion. Little attention was directed to nuclear fission in nature, very little attention was paid to the possibility of nuclear fission in nature.

Think about a nuclear reactor and the image that may come to mind is of extreme complexity, the seemingly exotic blue glow surrounding the core of some types of reactors or perhaps the image of a nuclear reactor control room with as many gauges and controls as the cockpit of a 747 jetliner. But, stripped of man-made control apparatus, a nuclear reactor is surprisingly simple, just a mass of uranium with two important properties. The mass must be sufficiently large so that not too many neutrons escape and the fissionable uranium must be sufficiently pure so that not too many neutrons are used for non-fission activities. The readily fissionable component of uranium, U-235, comprises less than one percent of naturally occurring uranium; most is U-238. The low abundance of U-235 in natural uranium makes natural nuclear reactors virtually impossible to begin operating today, which is the main reason that little interest was paid to the possibility of natural nuclear reactors.

But then along came Paul Kazuo Kuroda, who realized that because U-235 has a considerably shorter half-life than U-238, the relative abundance of U-235 would have been greater in times past. In 1956, Paul K. Kuroda, pictured at left, published a short paper in the Journal of Chemical Physics demonstrating the feasibility that thick seams of uranium ore about one meter thick might, 2,000 million years ago, have been able to support nuclear fission chain reactions and function as natural nuclear reactors (click here for pdf). For 16 years the idea languished in obscurity, ignored by geo-scientists, until proof came dramatically and unexpectedly by the discovery of fossil natural nuclear reactors in a uranium mine at Oklo, near Franceville, in the Republic of Gabon. There is much, much more to this story and to the consequences.