Jellyfish have severely impacted operations at France's largest nuclear power plant at Gravelines at the northern end of the English Channel, operator Électricité de France said.
Three reactors are completely shut down, while another is operating only partially. As one of the six reactors is undergoing maintenance, only one reactor is currently operating normally.
EDF issued an assurance that the situation posed no safety risk to the plant, its staff or the environment. It did not say exactly where the jellyfish washed ashore on Monday or how they disrupted the plant.
The plant battled jellyfish last year, when large numbers ended up in the filtration systems of the pumping stations. The reactors resumed operations after a shutdown lasting several days.
Gravelines is one of nearly 20 nuclear power plants in France. It is located on the northern French coast, roughly halfway between Calais and Dunkirk.