During a jellyfish bloom, there is too much biomass at once. The screens clog, water flow drops, and pumps start to strain. Under pressure, jellyfish can also break into a thick gel that slips through meshes and fouls equipment downstream.

Located on the Channel coast near Dunkirk, Gravelines is the largest nuclear power plant in western Europe, with six 900 megawatt reactors.

Four reactors at France's Gravelines nuclear plant will return to operation throughout the week, slightly later than initially planned, EDF data showed on Tuesday, after they were forced offline by a swarm of jellyfish in the cooling systems.