EDF said the EDF Gravelines jellyfish swarm on August 10 sent several dozen tons of jellyfish into the plant’s seawater intakes, forcing three reactors offline and cutting a fourth to half power. The shutdowns hit Gravelines, the largest nuclear plant in Western Europe, and left 3.2 gigawatts unavailable.
The clogging overwhelmed the filter drums that keep debris out of the cooling water. Three of the site’s six 900-megawatt reactors shut themselves down in about two and a half hours, while Unit 1 began winding down to 430 megawatts from 9pm and Unit 2 followed at 10:30pm with a full shutdown.
August 10 at Gravelines
Units 3 and 4 tripped offline just before 8pm local time on Monday, August 10. By Tuesday afternoon, EDF had throttled Unit 6 down to 500 megawatts, leaving roughly two-thirds of the 5,400-megawatt site sidelined at the low point.
EDF later described the jellyfish swarm as “several dozen tons” and said the episode had “no impact on the safety of the facilities, the safety of personnel, or the environment.” The plant’s cooling system draws water from a canal fed by the North Sea.
EDF after 2025 swarm
The same plant lost reactors to a jellyfish swarm on the same August night last year. After that, EDF spent the months in between building a defense system with cameras, thermal imaging and 3 fishing vessels on constant patrol off the plant’s coastline to intercept jellyfish before they reach the intakes.
EDF also spent hundreds of thousands of euros on cameras and monitoring, and in late June formalized a three-year, 1.5 million euro jellyfish-surveillance partnership with a sustainable fishing association and support from France’s volunteer sea rescue service. EDF said the patrols held off a major influx over the preceding weekend, even as Meduseo showed density at the beach next to the plant jumping from 0 to 75 on Monday and local fishermen hauled more than 26 t.
Unit 5 and EDF defenses
Unit 5 was partway through its scheduled 10-year inspection and was not due back until December. That means the plant had less spare room when the swarm hit, and the remaining reactors had to absorb the loss as the filters clogged.
Whether the new surveillance and patrol system will be changed again before the next jellyfish season is not answered by the facts available. For now, the clearest reading is simple: EDF built a barrier, the jellyfish got through it, and 3.2 gigawatts still vanished from Gravelines in one night.
