A nuclear power plant in northern France hit by a jellyfish invasion was getting back online on Wednesday, operator EDF said, with service restored at the first of four closed reactors.

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.

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.