Nature's Reactor

The world's first nuclear chain reaction, according to all the history books, was man-made and occurred in a closely guarded makeshift laboratory under Stagg Field at the University of Chicago on Dec. 2, 1942. Last week a former high commissioner of the French Atomic Energy Commission took issue with that belief. Addressing a meeting of the French Academy of Sciences in Paris, Physicist Francis Perrin argued that nature, not man, produced the first chain reaction, and that it occurred in an African uranium deposit perhaps 1.7 billion years ago.

Evidence of a prehistoric, spontaneous nuclear reaction was discovered by researchers at the French uranium-enrichment center at Pierrelatte. Analyzing uranium from the Oklo mine in Gabon, Africa, they found that it contained an abnormally low proportion of uranium 235, the radioactive isotope that powered the first atomic bomb with its awesome energy. In all other known uranium deposits—including a sample brought back from the moon by Apollo astronauts—U-235 invariably makes up .72% of the uranium ore; but the samples from the Oklo mine, which was opened in 1969, contained as little as .44% of U-235. Until the French discovery, levels that low had been found only in depleted uranium fuel taken from atomic reactors in which the U-235 had been "burned."

Coupled with this unexpected find was the announcement—made by scientists at the French atomic center at Ca-darache—that the Oklo ore also contained four rare elements in forms similar to those found in depleted uranium. To Perrin, those clues suggested that deep under the African soil, random natural conditions had prompted an energetic but nonexplosive chain reaction—a phenomenon that man can produce only by scientific techniques.

How—and when—could this extraordinary event have occurred? Using both hard information and informed guesswork, Perrin and his associates evolved an imaginative but logical hypothesis. The scientists knew that the concentration of U-235 found in contemporary uranium deposits is too low to sustain a chain reaction. But they were also aware that radioactive U-235 decays at a known rate, and that 1.7 billion years ago—the approximate age of the Oklo deposit —U-235 made up 3% of raw uranium deposits. This is roughly the same concentration that is created in artificially enriched uranium fuels and thus is enough to support sustained fission.

Perrin suspects that water, filtering down through the Oklo deposit, became an accessory to the chain reaction. Acting as a "moderator," subsurface water slowed down the neutrons emerging from splitting U-235 atoms enough to allow them to hit and split the nuclei of other U-235 atoms. (Without a moderator, the neutrons escape from the uranium fuel too fast to sustain the reaction.) When the heat from this process became too intense, scientists believe, the water turned to steam, the neutrons speeded up, and the chain reaction halted until the uranium cooled sufficiently for the steam to condense back to water. Thus, Perrin believes, "the fossil pile at Oklo must have functioned intermittently, pulsating as it were."