This has been reported by the interior minister of France.
As per reports, unidentified drones flew over seven nuclear plants across the country between October 5 and October 20 – this has not have had any impact on the plants' safety or functioning. However, a judicial investigation has been launched and necessary measures are being taken to know what these drones are and neutralize them.
Interior Minister Bernard Cazeneuve has reported about these but has not indicated any specific measures. Incidentally, France is the world's most nuclear-reliant country with 58 reactors on 19 sites operated by EDF and the drone sightings may renew concerns about the safety of nuclear plants in France.
The reason are the activists from environmental campaigning group Greenpeace who had forced their way into the Fessenheim plant on the German border earlier this year and they have a history of breaking into nuclear plants in France. Greenpeace has, of course, denied any involvement in the pilotless flight activity.