An explosion in a furnace at the Marcoule nuclear waste treatment site in southern France on Monday killed one person, but there was no leak of radioactive material outside the site, officials said.
Four people were injured, one with serious burns, in the blast at the Centraco site, owned by French power utility EDF and adjacent to the Marcoule nuclear research centre. The Centraco site does not house any nuclear reactors.
Fatalities in accidents at active nuclear sites in France are extremely rare, although deaths occasionally occur during plant construction.Monday's blast comes amid heightened safety concerns in the wake of Japan's Fukushima disaster in March.
The United Nations nuclear watchdog, the International Atomic Energy Agency, said that it was seeking information from France about the explosion and had activated its incident and emergency centre.
France's ASN nuclear safety watchdog said it was launching an investigation into the cause of the explosion at 13.06pm (1106 GMT) that hit shares in EDF, France's main power utility.
EDF said the blast was contained within the furnace, which is used to melt waste with levels of radioactivity ranging from low to very low.
"There was an explosion at the site of Marcoule at 13.06pm causing one death and four others were injured," an EDF spokeswoman said.
"We don't know the cause," she added.
Local emergency services said there were no traces of radioactivity on the four people injured.
"The risk of fire is over and there is no radioactive or chemical contamination of either the interior or exterior of the site," a rescue worker said.
The police also said there was no contamination outside the Centraco complex, in operation since 1999.
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 March when the Japanese nuclear site was damaged in an earthquake and tsunami.