The blast occurred at 10:00 local time (09:00 GMT) in an engine room, the Ouest-France newspaper reported earlier.
Several workers are feared to have been injured as a consequence of the explosion. Emergency services arrived at the scene, where huge fire were burning according to the newspaper.
The newspaper quoted the local prefecture, saying that there is ‘no nuclear risk’ for local citizens living nearby the reactor.
The nuclear plant located near Flamanville has two pressurized water reactors that produce 1.3 GWe each. The two reactors were built in 1986 and 1987. A third reactor is planned to be completed by 2018.