"French nuclear transports are among the safest in the world," Cogema spokesman Charles Hufnagel said.

The charges come at a sensitive time for the conservative government, which is wary of criticism before regional elections that will test its popularity. It has also been forced to tighten security on its railway system because of bomb threats.

France is the world's second largest producer of nuclear power.

A group of nuclear experts sounded the alarm bells in a report commissioned by France's Nuclear Safety Authority (ASN) and released on Tuesday.

"France has still not adopted a genuine strategy to cope with major contamination of an area resulting from a nuclear accident or a criminal act that leads to lasting exposure of the population," the report said.

"The experts expressed surprise about the absence of any official program mapping out a strategy to tackle economic and welfare problems in contaminated zones, whether urban or rural."

CONVOYS SAFE?

Environmental group Greenpeace released a report on Wednesday saying France would face a disaster if there was an accident or attack on trucks carrying plutonium for processing. The trucks pass near France's two biggest cities, Paris and Lyon.

"Depending on the gravity of the accidents, the release of plutonium could contaminate up to hundreds of square kilometers and millions of people," Greenpeace said.

The study, by independent nuclear engineering consultants Large & Associates, said two trucks, each with nine plutonium flasks in trailers, leave a reprocessing plant in La Hague in northwestern France every seven to 10 days, escorted by around seven police officers. It said they take radioactive material within 15 km (nine miles) of central Paris on the way to mixed oxide fuel fabrication plants in southeastern France.

"That is about 300 kilograms (660 pounds) of plutonium dioxide, the equivalent of 40 to 60 nuclear bombs traveling from the north to the south of France, accompanied by a minibus and two cars," Greenpeace spokesman Shaun Burnie said.

Cogema, which organizes the convoys, rejected the criticism as non-scientific. It said its transports conformed with standards set by the International Atomic Energy Agency (IAEA) and that it had not had any accident in 15 years of using them.

Hufnagel said Cogema's trucks had escorts and were built to withstand extreme heat, crashes or a fall of nine meters (29 feet). "We comply with the IAEA's standards, not those set by Greenpeace," he said. (Additional reporting by Kerstin Gehmlich)