Nearly two billion years before humans first achieved sustained nuclear fission-that rather sordid affair set up on a raquets court at the University of Chicago, in 1942-a naturally-occuring vein of uranium ore in the west African nation of Gabon went critical, all on its own.
Since the uranium 235 isotope decays faster than the more common 238, the fissionable isotope was much more abundant two billion years ago. Three per cent of all uranium atoms were the desirable fissionable kind back then, as opposed to just 0.72 per cent today. Abundant enough, in fact, for fission to occur spontaneously-if enough of the stuff was put together within a certain volume.
The key to a sustainable nuclear fission reaction is having a sufficient supply of so-called “slow neutrons,” to split the uranium 235 atoms. Unfortunately, most neutrons are the fast kind, so a so-called moderator is used to slow them down. In most nuclear plants, heavy water or carbon rods are used as such.
In the natural nuclear reactor at Oklo, though-the first such site discovered, in 1975-groundwater served as moderator. And the rate of reaction never once got so fast as to become explosive, during Oklo’s 100,000 years of operation-output averaged 100 kilowatts, enough to light about 1,700 ordinary light bulbs. That’s because the groundwater played an important role in regulating Oklo’s output.
As the fission reaction gathered pace, the uranium ore deposit heated up, boiling away the moderator. This in turn, interrupts the fission process. And as the water boiled away, some of the nasty fission by-products of uranium 235 became sequestered in deposits of aluminum phosphate minerals, a discovery that has led scientists to think about new ways to store nuclear waste for the long haul.
-Mike Ghenu
Source: Scientific American
