Paris
A GROUP of European scientists is trying to stop a mining company from
excavating the world鈥檚 last untouched natural nuclear fission 鈥渞eactor鈥�. The
reactor was formed 2 billion years ago when pockets of high-grade uranium ore
went critical in what is now Gabon, in western Africa.
The reactor, the only one of 15 in the region to escape mining, is
essentially a prehistoric nuclear waste repository. 鈥淚t contains fission
products very similar to spent fuel from modern reactors,鈥� says Fran莽ois
Gauthier-Lafaye of the Centre for Surface Geochemistry in Strasbourg, who leads
the campaign to preserve the site. 鈥淏ecause the surrounding hydrologic
environment is still intact, it is an opportunity for us to observe how buried
uranium isotopes and fission products behave鈥攚hich ones are contained,
which ones migrate and how far they travel.鈥�
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The first of Gabon鈥檚 natural reactors was discovered in 1972, when mining
geologists noticed that some ore samples contained unusually low amounts of
uranium-235, the radioisotope consumed in nuclear fission. Further studies
revealed the presence of fission products such as isotopes of neodymium.
Researchers deduced that around 2 billion years ago uranium-235 became so
concentrated in some deposits that the fission process was capable of sustaining
itself. The resulting chain reactions lasted hundreds of thousands of years,
first because they were moderated鈥攕lowed down鈥攂y groundwater, and
secondly because the deposits lacked neutron-absorbing elements such as boron
and vanadium that would have damped down the fission process.
The 14 reactors that have been mined for their uranium lie in the Franceville
basin, at Oklo. The one intact reactor that the scientists hope to preserve is
at Bangomb茅, 30 kilometres southeast of Oklo. The ore deposit there lies
10 metres below the surface and contains 100 to 200 tonnes of uranium.
Gauthier-Lafaye, who is in Gabon studying the reactors at Oklo, says that the
French government, acting on behalf of the scientists, has reached an unofficial
agreement with the French-Gabonese mining company COMUF to postpone work at the
Bangomb茅 deposit for a year.
The Bangomb茅 reactor should be of particular importance to the US
government, says Gauthier-Lafaye, because it is very similar in structure to the
proposed nuclear waste repository at Yucca Mountain, Nevada.
