Four reactors at the Gravelines nuclear power plant in northern France were taken offline this week following an unexpected and overwhelming influx of jellyfish that clogged the facility’s cooling water intake system, French energy firm Électricité de France (EDF) confirmed.
“These shutdowns are the result of the massive and unforeseeable presence of jellyfish in the filter drums of the pumping stations, located in the non-nuclear part of the installations,” an EDF spokesperson said in a statement.
The incident, which began late Sunday evening and continued into Monday morning, triggered automatic safety mechanisms that shut down three of the six reactors on Sunday night. A fourth reactor followed early Monday. The remaining two units were already offline for scheduled maintenance, resulting in a complete halt of operations at the plant.
Operational disruption amid broader environmental oressures
Gravelines is one of France’s largest nuclear plants, producing a total of 5.4 gigawatts of power across six units, each with a capacity of 900 megawatts. The facility draws its cooling water from a canal connected to the North Sea, an increasingly jellyfish-prone region.
Citizen scientist Aäron Fabrice de Kisangani identified the species involved as English Barrel jellyfish, a native, non-stinging type that can grow up to 90 centimeters in diameter. “The current would have pushed the jellyfish from the harbor and into the canal towards the nuclear plant, and then they would not have been able to escape the suction of the water coolant system, pulling them into the filters,” he explained.
EDF stressed there was no threat to safety. “The automatic shutdowns of four units had no impact on the safety of the facilities, the safety of personnel, or the environment,” the company stated on its website.
However, the timing coincides with a broader regional heatwave that has been straining power infrastructure and complicating nuclear plant operations across France. Although EDF has not explicitly linked the shutdown to heat-related complications, the heatwave could be contributing to conditions favoring jellyfish blooms.
Plant restart plans delayed as Financial impact mounts
EDF data initially showed that all four affected reactors were expected to restart by Tuesday. However, revised schedules released on Tuesday indicated a delayed return to full operations. Unit 6 was projected to restart first, followed by one reactor per day until Friday.
By 12:50 GMT on Tuesday, the first unit had yet to resume operations. EDF did not provide a reason for the delay, though elevated sea temperatures and biofouling risks are known stressors on nuclear cooling systems.
The financial impact is not insignificant. A single day of downtime for each reactor can cost EDF approximately $1.16 million, according to estimates cited by Reuters.
Operational risk
Jellyfish-related shutdowns are not new. Gravelines experienced a similar incident in 1993. In 2011, Scotland’s Torness nuclear plant faced an almost identical disruption.
Scientists are increasingly concerned that such ecological events will become more frequent. Factors contributing to jellyfish proliferation include global warming, habitat degradation for natural predators, overfishing, and shifts in ocean salinity and temperature.
“This year, higher average sea temperatures could cause larger jellyfish blooms,” Kisangani said. He also noted that “jellyfish were staying longer in the North Sea due to mild autumns and winters.”
Such disruptions pose growing operational challenges for the nuclear energy sector, particularly in coastal regions where marine life dynamics are shifting rapidly. While nuclear facilities are designed with safety redundancies, events like these highlight the increasing influence of climate variability and ecological imbalance on critical energy infrastructure.
EDF has reiterated that the presence of jellyfish in the plant’s filtration system remains a non-nuclear issue and does not compromise reactor safety. Diagnostic checks and necessary interventions are ongoing to bring the units back online safely.
As the French nuclear sector grapples with environmental stressors, ranging from marine invasions to heat-related cooling issues, operators are likely to face intensified scrutiny over resilience planning and biodiversity monitoring in plant design and operation.