MARSEILLE, France, October 15, 2009 (AFP) - The French government on Thursday demanded to know how a nuclear research facility lost track of its plutonium stocks after several previously undeclared kilos of the material were found.

The Cadarache research centre near Marseille had declared that eight kilogrammes (nearly 18 pounds) of plutonium were stored in one of its facilities slated for decommissioning, but 22 kilograms were found at the location during the operation.

The Atomic Energy Commission (CEA) that runs the Cadarache centre said as many as 39 kilogrammes may be stored at the facility that was being dismantled. A full inventory is to be completed in one month.

"We need to have some transparency in this matter because this is not normal," said junior environment minister Chantal Jouanno.

French nuclear safety authorities classified the incident as a level-2 on the seven-point international nuclear event scale.

Anti-nuclear activists raised the alarm and accused industry officials of having lost control of France's vast network of reactors, treatment plants and research facilities.

"This is one of the most serious and critical situations that we have seen at a nuclear installation for a while," said a Greenpeace spokesman, who claimed that the extra plutonium was the equivalent of five nuclear bombs.

The incident did not jeopardise safety at the facility, said the ASN nuclear safety agency, but it did prompt authorities to review procedures to ensure no chain reaction of the fissile material could be triggered.

"A nuclear facility must be able to control the quantity of its material," said ASN spokesman Laurent Kueny.

But Henri Maubert, spokesman of the Cadarache facility, argued that the additional plutonium did not pose a danger as it was stored in 450 separate sealed containers.

Adding fuel to the controversy was the fact that the discrepancy was discovered in June but was not reported until now.

"We did not try to hide the situation. We were just waiting to get a better overall view," said Maubert.

Ecology Minister Jean-Louis Borloo said he "profoundly regretted that there was a delay between the discovery of the stocks and the decision to report them."

Decomissioning work at the research centre was halted after the discovery.

The Cadarache facility came under scrutiny as the state electricity giant EDF faced questions over reports that it was sending spent nuclear fuel to Siberia.

An Arte television documentary and Liberation newspaper said that hundreds of tonnes of depleted uranium from French nuclear power plants have been stored in an open-air complex in Siberia.

EDF said the materials sent to Russia were recyclable and not nuclear waste.

France runs 58 nuclear reactors, the second largest network after the United States, and considers itself a world leader in developing atomic technology.