Local media said firefighters were alerted to an explosion and a fire at EDF's Flamanville plant at around 10am local time. Ouest France newspaper reported there was a possibility of some injuries but no nuclear risk.
EDF said a fire resulting in a minor explosion broke out in the turbine hall on the non-nuclear part of unit 1 at the Flamanville nuclear power plant at 9.40AM.
"The fire was immediately brought under control by the plant’s response team," the firm said.
"As per normal procedure, the fire brigade went to the affected location and confirmed that the fire had been extinguished. Unit 1 was disconnected from the grid.
"There were no casualties. There were no consequences for safety at the plant or for environmental safety."
Flamanville is located on the French Channel coast, around 30km from Cherbourg.
French utility giant EDF is the firm partnering with the British government to build the new £18bn Hinkley Point C nuclear power station in Somerset - and Flamanville is viewed as the sister project to Hinkley.
EDF has been contacted for comment.