However, Energy and Environment Minister Jean-Louis Borloo moved to reassure the public over the latest incident.

"We mustn't over-exaggerate," he told reporters, saying there were 115 such "little anomalies" in France's nuclear industry each year. "This is something which poses no environmental or health risk," he said.

The nuclear watchdog of Europe's biggest atomic energy nation said it had dispatched a team of experts to check the FBFC site, a uranium factory at Romans-sur-Isere.

The leak came from a buried pipe transporting liquid uranium and dated back several years, the ASN said, adding that Areva believed the crack occurred in 2006 at a time when the company carried out works at the site.

Jean-Christophe Niel, director general of the ASN, told a news conference that experts had found around 800 grammes of uranium enriched at 50 percent in a concrete gutter under the leaking pipe. This did not constitute a threat to the environment or to the staff's health, he said.

The ASN said it had classified the event at level one of the International Nuclear Event Scale (INES) which ranges from zero to seven.

"We decided to communicate to the press on this event in particular because of the public reaction triggered by the previous leak at the Tricastin site," Niel said.

The ASN criticised Areva for the way it had handled the previous leak, which was also classified as a level one event, saying it delayed communication of the problem and had unsatisfactory security measures in place.

Areva has since appointed a new manager for the Tricastin plant who has been told to review procedures at the site.

Areva, a state-controlled company which makes nuclear reactors and processes uranium, said the new leak was confined to its FBFC factory and did not pose a danger to the environment.

"The defective pipe was shut by our teams," Areva said in a statement, adding that the pipe linked a workshop making fuel to a uranium treatment station.

The ASN said Areva would start cleaning the affected area on Friday, adding that the pipes did not appear to meet safety standards.

The latest incident is an embarrassment for Areva which is at the forefront of French President Nicolas Sarkozy's effort to sell home-grown nuclear energy technology to the world.

The problems could also cast new doubts on France's recent decision to build a second new-generation European Pressurised Reactor (EPR) by 2017, bringing to 60 the number of nuclear reactors in France. (Additional reporting by Benjamin Mallet; editing by Elizabeth Piper)