French find prehistoric atomic reactor

Press, Volume CXII, Issue 33047, 14 October 1972, Page 23

French find prehistoric atomic reactor

(By SERGE BERG, of Agence France-Presse, through N.Z.P.A.) PARIS. There was a prehistoric atomic reactor in the jungle of Gabon several hundred million years ago, French scientists have discovered.

The discovery of evidence of the natural atomic pile was announced at a meeting of the French Academy of Sciences by Mr Francis Perin, the former Commissioner for Atomic Energy.

The reactor formed naturally in the Oklo uranium deposits, 40 miles north-west of Franceville in the former French colony, and ivorked for undetermined periods.

The self - generating chain reaction of the pile occurred in uranium deposits whose uranium-235 content was 3 per cent about 1700 million years ago. This is the uranium-235 content of the nuclear fuel used in modem, light-water atomic reactors. The now-fossilised reactor naturally stopped working when the uranium content fell below the necessary level to form the critical mass needed for the chain reaction.

DATES NOT DETERMINED Exact dates of when the reaction began and stopped working have not yet been accurately determined. It may have begun working 1700 million years ago—which is the age of the deposits themselves — and stopped more than 100 million years ago, but probably stopped more than 1000 million years ago. Mr Perrin presented two reports on the discovery, at the request of the President of the Gabonese Republic (Mr Albert-Bernard Bongo).

The first was from three members of the French Atomic Energy Commission (Messrs Robert Bodu, Henry Bouzigues and Noel Morin, and Mr Jean-Paul Pfiffelman, of the Franceville Uranium Mining Company, Gabon. The second was from six other members of the commission — Messrs Michele Nuiily, Jean Bussac, Claude Frejacques, Guy Neif, Georges Vendryes and Jacques Yvon. FIRST INDICATION The first indication of the existence of the bumt-out reactor was discovered during control analyses at the French nuclear centre of Pierrelatte. Research workers found that uranium from the Oklo mine was not normal. Up till then, all natural uranium ore known, even samples brought back from the Moon by Apollo missions, showed a

uranium - 235 content (uranium-235 is the only fissionable uranium) of 0.720 per cent. The French scientists were amazed to find that samples from Oklo showed a ura-nium-235 content of between 0.621 and 0.640 per cent in some cases, and in others rose instead to 0.730 per cent.

TWO HYPOTHESES Two hypotheses were put forward to explain these anomalies: either isotopic separation had occurred naturally over the ages, or a very ancient chain reaction had taken place. The second research group, at the Cadarache nuclear centre, adopted the second explanation after more detailed chemical analysis. The group discovered in the samples the presence of rare earths, particularly neddymium, samarium, europium and cerium, whose isotopic composition corresponded perfectly to that found in fission products. This was proof that the sample came from an agesold nuclear chain reaction. Mr Perrin said the discovery was of considerable importance. If it were found that this was not an isolated case it would be necessary to redefine natural uranium, he said. MINING STOPPED Mr Bongo has agreed to halt all mining temporarily at Oklo to give the scientists time to carry out further research. Uranium was discovered in Gabon in 1956 at Mounana and exploitation began in 1961. In 1967, the Boyndzi deposits were found 400 yards further north. In 1969, the Oklo deposits were discovered about a mile southwest of Mounana.

Since mining began in 1961, the tree deposits have produced more than 5000 tons of uranium metal.

Underground mining at Mounana was halted early this year after a large rock fall. It will be continued later by quarrying. Oklo is an open-cut mine.