Swarm of jellyfish shuts French nuclear plant

Four reactors at France's Gravelines nuclear power plant were shut down late Sunday due to a swarm of jellyfish in the cooling systems, operator EDF said on Monday, likely due to rising water temperatures because of global warming.

The plant in northern France is one of the largest in the country and cooled from a canal connected to the North Sea. Its six units produce 900 megawatts of power each, or 5.4 gigawatts in total.

The entire plant has now temporarily halted production as the other two units are offline for planned maintenance, EDF data showed.

There is no damage to the filter pumps; they just need to be cleaned, said a source with knowledge of the matter, suggesting the reactors will be able to restart quickly.

The beaches around Gravelines, between the major cities of Dunkirk and Calais, have seen an increase in jellyfish in recent years due to warming waters and the introduction of invasive species.

"Jellyfish breed faster when water is warmer, and because areas like the North Sea are becoming warmer, the reproductive window is getting wider and wider," said Derek Wright, marine biology consultant with the U.S. National Oceanic and Atmospheric Administration.

"Jellyfish can also hitch rides on tanker ships, entering the ships' ballast tank in one port and often getting pumped out into waters halfway across the globe," he said.

An invasive species known as the Asian Moon jellyfish, native to the Pacific Northwest, was first sighted in the North Sea in 2020.

The species, which prefers still water with high levels of animal plankton like ports and canals, has caused similar problems before in ports and at nuclear plants in China, Japan and India.

"Everyone talks about nuclear being clean, but we don't think about the unintended consequences of heat pollution," Derek said.

EDF said it did not know the species of jellyfish involved in the shutdown, and that teams were working to restart the reactor safely.

Reactors 2, 3, and 4 at Gravelines stopped automatically just before midnight when the filter drums of the pumping stations became packed with a "massive and unpredictable" swarm of jellyfish, and reactor 6 went offline several hours later, the EDF notice said.

The event did not affect the safety of the facilities, staff or the environment, it said.

The nuclear plant is also near beaches that have become hotspots for migrants attempting to cross into Britain. The invasive jellyfish are not considered a threat, as they do not have a poisonous sting.