A Natural Nuclear Reactor In Gabon

Since uranium-235 undergoes self-sustaining fission in commercial reactors and since uranium lies in the Earth in great quantities, Paul Kuroda predicted that naturally operating reactors are possible under special conditions. Not nowadays, when the ratio of uranium-235 to uranium-238 is only about 0.7%, but in the past, when the ratio was much higher owing to the fact that U-235 has a shorter half life than U-238.

The conditions necessary for self-sustained fission would be as follows: a uranium deposit where U-235 was present at the 3% level (the level at which modern reactors operate); the presence of material (such as water, carbon, and most organic compounds) that could moderate, or slow down, the neutrons issuing from fission reactions; and the absence of material (such as Fe, K, Be, Gd) that would absorb the neutrons outright.

In 1972, such a natural reactor was found at the Oklo mine in Gabon, in West Africa. There a 2-billion-year-old uranium deposit some 5-10 meters thick and 600-900 meters wide was bathed by an ancient river. This “reactor” is reckoned to have released 15 gigawatt-years of energy and operated at an average power of 100 kilowatts.

Now physicists at Washington University in St. Louis have defined a likely mode of operation for this ancient reactor and confirmed one of the proposed mechanisms of its self regulation. According to Alex Meshik (am@wustl.edu), the reactor cycled on (producing heat that boiled the nearby water) typically for 30 minutes and then off (when the now-scarce water failed to moderate the nuclear fission process) typically for 2.5 hours.

This cycling saga is deduced from microscopic mass-spectrometric examination of the rock samples from the area. Meshik says that tiny alumophosphate grains found in the material of ancient reactor preserve a signature of the reactor's operational mode. "It is fascinating that xenon isotopic composition measured today provides us with such pristine timing records for a natural reactor operated 2 billion years ago." (Meshik et al., Physical Review Letters, 29 October 2004.)