Scientists are polarised about possible environmental damage caused by French nuclear tests such as Tuesday's at Mururoa atoll in the Pacific.

Most scientists think that the explosion will create an inner zone of glass around the point of detonation, surrounded by a shatter zone of cracks and fractures. The disagreement is over whether the glass will hold in the radioactivity.

The conventional view is that basalt glass formed naturally has remained intact for millions of years, which suggests that the basalt glass at Mururoa may trap radiation for long periods.

But work by John Clemens of Kingston University has shown that synthetic glass used to encase nuclear waste is unstable at temperatures below 300 degrees centigrade in the presence of water. He argues that the greatest threat may come from later explosions puncturing the glass tomb.

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Rumours that Fungataufa atoll, 16 kilometres south of Mururoa, will be used for larger explosions points to French concern about the Mururoa.

The French claim that the environmental effects of the tests will be minimal but refuse to release data that could support their claims.

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Critics of the tests agree that the environmental effects are less significant than the political ones. The tests provide justification for Chinese testing and for nuclear weapons developments by countries which do not have the bomb.

Suzanna Van Moyland of VERTIC, the Verfication Technology Information Centre, said: "Even if there is no immediate danger of large-scale contamination, as some scientists think, the French still risk continued global condemnation for resuming nuclear testing."