A physics professor at the prestigious College de France has walked out of a scientific committee that has approved turning the fast-breeder reactor Superphenix into a research reactor.

Raymond Sen accused other academics on the committee of being determined to "make this costly instrument profitable . . . not according to scientific reasoning but according to economic and political reasoning".

In its report published last week, the committee refers directly to the huge investment already made in the reactor. It argues that it is "legitimate to want to draw all possible lessons from the considerable financial and intellectual investment which went into building Superphenix".

"The big problem with French scientists is that they do not know when to stop an experiment," Professor Sen told a press conference organised by a scientists' nuclear watchdog association of which he is a member, the Groupement des Scientifiques pour l'Information sur l'Energie Nucleaire.

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Nobel physicist Georges Charpak is on the committee which is chaired by Raymond Castaing, of the Academie des Sciences. Most of Professor Sen's criticism was directed at Mr Castaing.

According to Professor Sen, the total cost of the trouble-plagued reactor tops 50 billion francs (Pounds 6.25 billion). Superphenix began producing electricity in the 1980s as the world's first commercial-scale, fast-breeder reactor.

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It was first shut down in 1987 and again in 1990 after renewed leaks in its liquid sodium cooling circuits. Superphenix, which created more plutonium than it used up, was designed at a time when plutonium was in demand.

The French government decided in 1992 to give the go-ahead to a plan to turn the reactor from a fast-breeder into an experimental "fast-burner", turning long-lived radioactive elements into less hazardous waste by a process known as artificial transmutation.

Superphenix was officially designated as a research reactor in 1994. The committee was set up last September to advise on a research programme into the transmutation/incineration process.

Professor Sen rejects the principle of a research project and sees no future for the reactor. "I do not believe in the industrial rationale. It leads to a machine which is far too complex to attain industrial reliability which remains compatible with safety demands," he said.

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It was this fundamental disagreement which lead Professor Sen to walk out before the report was released. He said it would be a waste of time to "discuss point by point, to try to amend a text with which I cannot associate myself".

There has been no comment on his walk-out from the other members of the committee, which includes seven French scientists and two foreign experts.