As reported by John Lichfield at the Independent:
"[A] recent spate of five co-ordinated “visits” in one evening to nuclear reactors hundreds of miles apart has now placed the French government on high alert...A campaign of harassment by anti-nuclear campaigners is considered the most likely explanation. Surveillance flights by a terrorist group testing the security of France’s 19 nuclear sites have not been ruled out...[a] score of intrusions by...elaborate drones in the restricted airspace over 13 nuclear power stations [have happened] since early October."
Despite the allegation that a "Campaign by anti-nuclear campaigners is considered the most likely explanation," Greenpeace France "has denied any link with the flights." So too has Greenpeace International.
To the contrary, the group "has accused the government of “minimising the risk” and covering up the fact that France’s nuclear power stations are vulnerable to terrorist attack. “A medium-sized drone could carry an explosive charge big enough to damage the pools [of cold water] in which spent nuclear fuels are stocked,” a spokesman said.