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.

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

Four of its reactors were shut down on Sunday and Monday after a swarm of jellyfish clogged cooling pumps.

"Reactor No. 6 restarted at 7:30 am this morning," an EDF spokeswoman told AFP, adding that work was still going on to bring three other reactors back online "in the coming days".

The plant's two other units are offline for maintenance.

The incident had not affected the safety of the facilities, personnel or the environment, the operator said.

The Gravelines plant was also disrupted by jellyfish in the 1990s, and the creatures have caused plants to close in the past in the United States, Sweden and Japan.

Experts say overfishing, plastic pollution and climate change have created conditions allowing jellyfish to thrive and reproduce.

Nuclear power accounts for around three-fifths of French electricity output and the country boasts one of the globe's largest nuclear power programmes.

France has 18 nuclear power plants with a total of 57 reactors.

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