Four people were critically injured and one killed in an industrial accident at the Maracoule nuclear site in the south of France on Monday.
The explosion was caused by a fire near a furnace in the Centraco radioactive waste storage site and workers were not contaminated by radiation, company officials said.
The blast occurred in the foundry of the waste processing plant, which was melting about 4t of used, mildly radioactive metal objects.
Ministers said the blast was an industrial accident and not an explosion in, or near, a nuclear reactor. There had been no radioactive leak and no need to evacuate workers or local people.
Socodei, a subsidiary of EDF, owns the Centraco facility, which is located adjacent to the Marcoule nuclear site.