Two of Gravelines’ six reactors were already offline for maintenance, meaning the site was completely offline on Monday.
Gravelines is the largest nuclear power plant in Western Europe, with each reactor able to produce 900 megawatts. It provides about 70% of the annual electricity consumption of the Hauts-de-France region of northern France.
The plant is due to open two next-generation reactors, each with a capacity of 1600 megawatts by 2040.
Rising water temperatures at the plant’s surrounding beaches have led to an increase in jellyfish in recent years.
The UK Met Office said waters to the south of Britain are currently experiencing a significant marine heatwave, with waters around the English Channel registering anomalies of 1.5C to 3C above average levels between 1982 and 2012.
It said that the conditions were favourable for species such as jellyfish – as well as bluefin tuna, sea bass and stingrays.
Derek Wright, a marine biology consultant at the US National Oceanic and Atmospheric Administration Fisheries, said the incident was likely to have been a result of climate change.
“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,” he told Reuters.
“They 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.
“Everyone talks about nuclear being clean, but we don’t think about the unintended consequences of heat pollution.”
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.
