Meet Oklo, the Earth's two-billion-year-old only known natural nuclear reactor (French Edition)

Physicist Francis Perrin sat at a nuclearfuel-processing plant down in the south of France, thinking to himself: "This cannot be possible." It was 1972. On the one hand, there was a dark piece of radioactive natural uranium ore, extracted from a mine in Africa. On the other, accepted scientific data about the constant ratio of radioactive uranium in ore. Examination of this high-grade ore from a mine in Gabon was found to contain a lower proportion of uranium-235 (U-235) — the fissile sort. Only a tiny bit less, but enough to make the researchers sit back and scratch their heads. The physicists' first, logical response to such an unusual ratio of U-235 was that this was not natural uranium. All natural uranium today contains 0.720% of U-235. If you were to extract it from the Earth's crust, or from rocks from the moon or in meteorites, that's what you would find. But that bit of rock from Oklo contained only 0.717%. What did this mean? At first, all the physicists could think of was that the uranium ore had gone through artificial fission, i.e. that some of the U-235 isotopes had been forced to split in a nuclear chain reaction. This could explain why the ratio was lower than normal. But after complementary analyses, Perrin and his peers confirmed that the uranium ore was completely natural. Even more bedazzling, they discovered a footprint of fission products in the ore. The conclusion: the uranium ore was natural and had gone through fission. There was only one possible explanation — the rock was evidence of natural fission that occurred over two billion years ago.