Unmanned aerial vehicles (UAVs) were seen flying over seven of France's nuclear plants. For the last three weeks, drones "big enough to carry explosives" breached the no-fly zone, according to The Telegraph.

The state-owned operator of France's nuclear reactors, Electricité de France, filed a legal complaint against the unknown operators, according to The Telegraph. France has 58 reactors over 19 states - France has the second largest nuclear unit after the United States. France's nuclear plants produce 75 percent of the country's electricity - the most of any country in the world.

Fears ignited in France that their nuclear plants are open to attack. "We're not talking about just one type of drone identified, but several," a nuclear expert told Le Parisien, according to The Telegraph.

"Some were only a few dozen centimeters long with a very short range of several hundred meters at most," the expert continued. "So you'd need to be very close to the reactor. But others, and this is much more worryingly, were far bigger - perhaps two meters long so sufficiently big to carry an explosive charge."

Electricité de France tried to squash rumors that the drones could harm the reactors. "We have no fear concerning a drone flying over out installations," a representative said, according to The Telegraph. "These objects are not capable of damaging anything if they fall, nor is any object they might drop."

The power plants are designed to "withstand a direct hit by an airliner," according to The Telegraph.

The environmental group, Greenpeace, is known for calling attention to safety issues and flew over a reactor in 2012 with an ultralight aircraft, according to The Telegraph. They filmed the stunt with a drone. Greenpeace denies involvement with this episode, but said they had proof that the no-fly zone over two other plant sites had been breached.