Authorities say there is no risk of contamination.
The explosion occurred on Thursday at 10am in the engine room of the Flamanville nuclear power plant on the Normandy coast.
Three people were left injured, according to initial reports by Ouest-France newspaper.
"It is a significant technical event but it is not a nuclear accident," senior local official Olivier Marmion said, The Local news website reported.
An emergency intervention plan, designed to protect local residents in case of a leak, has not been triggered because authorities say there is no risk of any contamination.
Firefighters said an explosion and a fire were reported in the heart of the plant, outside the nuclear zone.
Flamanville is France's flagship nuclear plant but it has been beset with problems in recent years leading to it becoming a costly calamity.