News outlet Ouest France reported that the incident happened at 10am.
Guernsey authorities were informed at 11am and said the small explosion resulted in a fire.
It happened in a machine room outside of the reactor zone.
States Emergency Planning Officer Kevin Murphy said there were no injuries and no further risks associated with the incident.
Medics and fireighters attended the scene.
The plant, on the west coast of Normandy, is about 30 miles from Guernsey.
Flamanville houses two pressurised water reactors that came into service in 1986 and 1987.
Construction on a new reactor began in December 2007 but has been beset by delays and escalating costs.
It is due to start up in the fourth quarter of 2018.
At the start of the year it was reported that France's atomic watchdog, Autorité de Sûreté Nucléaire, was finalising its review into poor quality parts and a reported falsifying of safety documents.