Steam secret of natural fission
22 Nov 2004
The world's only known natural nuclear reactor, which decommissioned itself over two billion years ago, could provide insights into how modern nuclear plants can operate more safely.
The site, in Gabon, West Africa, ran for 150million years without blowing up, and storing its own waste in a safe manner.
The reactor was a natural deposit of uranium. Today, and for the last two billion years, natural uranium will not undergo nuclear reactions, because it contains too little of the fissionable isotope, uranium-235 (U235).
But in the distant past, U235 was more abundant, comprising 3% of the total amount - the approximate concentration of enriched uranium used in nuclear fuel today. The Gabon deposit also contained, by a quirk of geology, a mixture of minerals which acted as a neutron moderator, slowing the neutron flux enough to allow the fission process to take place.
In a nuclear reactor, it takes large numbers of specialists and serious application of high technology to prevent reactions from running away. 'The big question we addressed was: when the uranium reached criticality, why didn't it blow up?' says Alexander Meschik of Washington University in St Louis, Missouri.
The answer, it appears, is that the site functioned like a geyser.
The energy generated by the nuclear reaction boiled the groundwater around the deposit. Water is a natural neutron moderator, so as it was converted into steam, it stopped absorbing neutrons and shut down the chain reaction. As the rocks cooled down, the steam condensed, and the presence of water once again slowed the neutrons down and restarted the chain reaction. Meschik calculates that the reactor operated for about half an hour at a time, then shut down for two and a half hours.
Meschik deduced this by analysing the other neutron moderator in the deposit, a 'mineral assembly' containing lanthanum, cerium, strontium and calcium and known as alumophosphate. This also acted as a waste storage medium, the researchers found; it absorbed the isotopes of xenon which were formed by the fission of the U235.
Xenon is extremely rare on Earth and is a characteristic marker of a fission process. It occurs in nine isotopes, and it was the analysis of the relative abundances of these which gave the researchers the clue to the way the reactor operated.
The find could provide insight into how to operate industrial reactors more safely. 'This is very impressive, to think that this natural system not only went critical, it also safely stored the waste,' Meschik says. 'Just using the fact that the water boiled at the reactor site might give contemporary nuclear reactor researchers ideas on how to operate more safely and efficiently.'
