The French nuclear safety watchdog ASN has suspended work dismantling a plutonium technology plant over worker safety fears, after almost three times as much plutonium was found at the site than expected.

The watchdog said it was only told of the problem on October 6, although the Atomic Energy Commission (CEA), the state body that supervises the plant in Cadarache near the southern port of Marseille, had been aware of the problem since early June.

Around eight kilograms (16 pounds) of plutonium were believed to have been stored at the site when it was in operation, but some 22 kilograms had been discovered to date and the final figure could be closer to 39 kilograms, the nuclear safety watchdog ASN said.

French industrial giant Areva, which produces nuclear reactors, operated the plant.

The underestimation of the quantity of plutonium sharply reduced safety margins designed to prevent fissile material reaching critical mass, which could potentially have serious consequences for the workforce, the ASN statement said.

The incident revealed gaps in the safety culture of those responsible for the plant, which produced fuel for nuclear power stations. ASN said dismantling work could only resume with its agreement.

The watchdog ranked the event as a level two incident on the International Nuclear Event Scale (INES), which runs from zero to a maximum of seven for a major incident.

The ASN said their had been no repercussions from the Cadarache incident, the latest to cloud the reputation of France's well-developed nuclear power industry.

In October, French media reported that waste from French power stations was being deposited in the open air in Russia.

Ecology Minister Jean-Louis Borloo urged full transparency and expressed regret over the delayed declaration by the CEA, while Greenpeace accused the commission and Areva of revealing their inability to manage plutonium stocks.

The incident constituted one the most serious and critical situations that we could have faced in a nuclear installation in a long time, said Greenpeace official Yannick Rousselet.

(Reporting by Tamora Vidaillet; Editing by Anna Willard and Jon Boyle)