There is a risk of a radioactive leak after an explosion on Monday at the Marcoule nuclear site near the city of Nimes in the south of France, emergency services said.
Reports claim one person has been killed in the blast.
The explosion hit the Centraco nuclear waste treatment centre belonging to the Socodei subsidiary of national electricity provider EDF, said a spokesman for the Atomic Energy Commissariat (CEA).
"For the time being nothing has made it outside," the spokesman said, asking not to be named.
A security perimeter has been set up around the installation, firefighters said, without being able to provide further details.
The plant's reactors have been shut down.