A massive influx of jellyfish into the seawater pumping stations at the Gravelines nuclear power plant in northern France forced EDF to shut down three reactors on Monday, August 10. Units 3 and 4 automatically shut down in accordance with safety procedures, while reactor number 2 also had to be stopped.
The power output of reactor number 1 has been reduced by half. Only unit number 6 is currently operating at full capacity, as reactor number 5 is already unavailable due to a scheduled maintenance outage. EDF assures that this event has had no impact on the safety of the facilities, the security of employees, or the environment.
A power plant dependent on pumping seawater
Nuclear reactors require significant amounts of water to cool their facilities. At Gravelines, this water is drawn from the North Sea by pumping stations. The massive influx of jellyfish can clog the filtration systems and reduce the flow rate needed for cooling, thus causing the automatic shutdown of the affected units.
The incident comes at a time when the French nuclear power plant fleet is already facing difficulties. On Sunday, 15,6% of its total capacity was unavailable due to the extreme heat and drought, a record since EDF began publishing data in 2015. Several power plants had to reduce their output to comply with environmental limits related to river temperatures.
A phenomenon already observed in 2025
The Gravelines nuclear power plant, considered the most powerful in Western Europe, had experienced a similar incident in August 2025. Four reactors had then automatically shut down and the complete return to production took about ten days.
The presence of jellyfish on the northern French coast is a seasonal phenomenon, but reports of large swarms are becoming more frequent. Rising ocean temperatures, combined with climate change, can promote their proliferation and increase disruptions around coastal infrastructure.
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