Jellyfish swarm shuts down three reactors at France's Gravelines nuclear plant
A massive jellyfish swarm forced EDF to shut down three of six reactors at the Gravelines nuclear plant in northern France on Monday, leaving only one unit at full power at Western Europe's largest nuclear facility.
Reactors taken offline
A massive influx of jellyfish into the seawater pumping stations of the Gravelines nuclear plant in northern France forced operator EDF to shut down three of its six reactors late on Monday 10 August. Reactors 2, 3 and 4 were brought to a complete halt, while reactor 1 was reduced to 50% of its output as a preventative measure. Reactor 5 was already offline for scheduled maintenance, leaving only reactor 6 running at full power. Each of the six units produces 900 megawatts, giving the plant a total capacity of 5.4 gigawatts. Gravelines, situated on the North Sea coast roughly halfway between Calais and Dunkirk, is described as the largest nuclear power station in Western Europe.
EDF stated that the situation has no consequences for the safety of the installations, personnel or the environment. The jellyfish entered the seawater pumps and obstructed the filter drum screens, triggering the shutdown procedure.
- Reactor 1
- 50 %
- Reactor 2
- 0 %
- Reactor 3
- 0 %
- Reactor 4
- 0 %
- Reactor 5
- 0 %
- Reactor 6
- 100 %
Preventative measures and response
After a complete plant outage caused by jellyfish in the summer of 2025, when four reactors shut down automatically and the last was reconnected only after about ten days, EDF implemented new monitoring and trawling techniques. Fixed cameras were installed at the intake canals and on the filter drum screens to detect incoming blockages early. The company also partnered with a French fishing organisation and a sea rescue organisation that monitor for swarms and can raise an alert to launch targeted trawling operations.
Fishing boats were deployed on Saturday after an initial alarm was raised, which limited the impact of the swarm. Jellyfish already on site are being removed with vacuum trucks. An EDF engineer told Reuters that no new jellyfish are arriving, cleanup is finishing, and a quick restart of the units is expected.
- Initial alarm raised; fishing boats deployed to trawl and reduce swarm density before it reaches the plant.
- Three reactors (2, 3, 4) shut down late Monday; reactor 1 reduced to 50% output.
- EDF announces the shutdown on its website; cleanup finishing, no new jellyfish arriving.
A recurring and worsening phenomenon
The presence of jellyfish along the northern French coast is regular and seasonal, but reports of large swarms each summer are becoming more frequent. A similar incident occurred in 1993, when pumps had to be stopped and reactors shut down to avoid invasion and cooling system failure. In summer 2025, a swarm described by an EDF representative as "massif et non prévisible" obstructed the filter drums and triggered automatic shutdown of four reactors; it took three days to restart the first reactor.
Dominique Mallevoy, head of aquariology at the Nausicaá national sea centre in Boulogne-sur-Mer, said that nearly every summer now brings reports of large jellyfish shoals. Scientists cited in the articles point to several factors behind the proliferation: rising ocean temperatures linked to global warming, the arrival of invasive species, habitat loss of predators, and overfishing.
Broader strain on French nuclear fleet
The jellyfish incident comes as the French nuclear fleet is already under pressure from drought and heat. AFP calculations based on EDF data published since 2015 show that 15.6% of nuclear power was missing on Sunday, a record for heat-related unavailability. A fifth heatwave is expected to grow throughout the week and further curtail French nuclear output. One reactor in the north has been offline for most of the summer due to low river levels, while another had limited restarts and remained largely offline for most of July.
