The blast took place in the engine room at the Flamanville plant, 25 kilometres west of the port of Cherbourg and just across from the Channel Islands.
"It is not a nuclear accident," senior local official Jacques Witkowski told the AFP news agency.
Not a nuclear accident
He said a ventilator had exploded outside the nuclear zone at the plant, which has been in operation since the 1980s.
Five people suffered smoke inhalation but there were no serious injuries, Witkowski said.
One of the two pressurised water reactors at the plant was shut down after the explosion and the incident was declared over at 11.00am GMT, the authorities said.
A new third-generation reactor known as EPR is also being built at Flamanville, which will be the world's largest when it goes into operation in late 2018.
Construction of the new plant at the site in Normandy began in 2007 and was initially due for completion in 2012 but has been delayed several times.