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The Oklo chain reaction presumably began shortly after the uranium deposit was formed, when the proportion of U-235 in the ore was at its peak. It ceased for good when a sufficient amount of the fissionable U-235 had burned away. Because they possess no timetable for the depletion of U-235 during spontaneous reactions, the French scientists can only guess how long that process might have taken. Their estimates: from several hundred million to more than a billion years.
Several U.S. nuclear physicists agree that their French colleagues may be right. "It is fantastically and incredibly possible," a top Government scientist said. "I haven't been able to think of any better explanation," admitted Nobel Laureate Glenn T. Seaborg, former head of the U.S. Atomic Energy Commission. "There are plenty of explanations I could give you," said Caltech Geochemist Donald Burnett, "but none are less exotic." The physicists also agree on one further point: if the Oklo reaction did indeed occur, it may not have been an isolated phenomenon. The search for other deposits with the same telltale clues has already begun.