Wetherill of the University of California at Los Angeles and Mark G. The answer came only when someone recalled a prediction published 19 years earlier. Further analyses showed that ore from at least one part of the mine was far short on uranium 235: some 200 kilograms appeared to be missing- enough to make half a dozen or so nuclear bombs.įor weeks, specialists at the French Atomic Energy Commission (CEA) remained perplexed. That tiny discrepancy was enough to alert French scientists that something strange had happened. But in these samples, which came from the Oklo deposit in Gabon (a former French colony in west equatorial Africa), uranium 235 constituted just 0.717 percent. Elsewhere in the earth’s crust, on the moon and even in meteorites, uranium 235 atoms make up 0.720 percent of the total. As is the case with all natural uranium, the material under study contained three isotopes- that is to say, three forms with differing atomic masses: uranium 238, the most abundant variety uranium 234, the rarest and uranium 235, the isotope that is coveted because it can sustain a nuclear chain reaction. He had been conducting a routine analysis of uranium derived from a seemingly ordinary source of ore. In May 1972 a worker at a nuclear fuel–processing plant in France noticed something suspicious.

Editor's Note: This article originally appeared in the October 2005 issue of Scientific American.

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