One person was killed and four others injured after a blast at the Marcoule nuclear waste treatment site in southern France.
A furnace exploded at the Centraco site, owned by French power utility EDF and adjacent to the Carcoule nuclear research centre. The site does not house any nuclear reactors.
France's ASN nuclear safety watchdog said there was no leak of radioactive material outside the furnace.
The furnace that exploded is used to melt waste with levels of radioactivity ranging from low to very high, it added.
It did not immediately give a reason for the blast but said it was completely contained within the furnace.
Police also said there was no contamination outside the site, which is about 30 km from the city of Avignon and about 80 km from the Mediterranean coast.
France, the world's most nuclear-reliant country, is carrying out stress tests on its 58 nuclear reactors in the aftermath of the Fukushima disaster in Japan.
The U.N. atomic agency said its incident and emergency centre had been activated and it is seeking information from France's nuclear authorities about the explosion
The International Atomic Energy Agency (IAEA) board is expected to approve an IAEA action plan to strengthen global nuclear safety in the wake of Japan's Fukushima accident in March.
IAEA director general Yukiya Amano suggested the news from France showed the need to act on nuclear safety issues.
The IAEA came under fire in the first few days after the Fukushima emergency six months ago, with critics saying it initially provided scant and out-of-date information about the world's worst nuclear disaster in a quarter of a century.
The agency said it could only give the information it received from Japan.
EDF's share price has dropped over six per cent on the news of the blast.