Three reactors at France’s Gravelines nuclear power plant were temporarily shut down on August 10, 2026, after a large swarm of jellyfish reached the facility’s cooling-water systems. Reactors 2, 3 and 4 were taken offline, while the output of reactor 1 was reduced to 50% as a precaution. Reactor 5 was already offline for scheduled maintenance, while reactor 6 remained fully operational. Operator EDF said the incident posed no risk to the safety of the nuclear facility, its personnel or the environment.
The Gravelines nuclear power plant is located in northern France between Calais and Dunkirk, close to the North Sea coast. The facility has six pressurised-water reactors, each with an electrical capacity of approximately 900 megawatts, giving the plant a total capacity of around 5.4 gigawatts. Cooling water is supplied through a canal connected to the North Sea. Jellyfish entering through this system can accumulate around the intake structures and filter drums used by the pumping stations.
EDF received an initial warning about a large concentration of jellyfish approaching the area over the weekend. Fishing vessels were deployed in an attempt to reduce the density of the swarm before it reached the plant. According to the company, the operation limited the scale of the problem, although some jellyfish still reached the cooling infrastructure. The organisms already at the facility are being removed using vacuum trucks. EDF said no new jellyfish were arriving and that the cleanup was nearing completion, which should allow the affected reactors to return to operation after the necessary checks.
A similar incident occurred at Gravelines in August 2025. At that time, a massive jellyfish swarm caused four reactors to shut down automatically, while the remaining two units were already offline for scheduled maintenance. As a result, electricity production at the entire nuclear plant was temporarily halted. Following that incident, EDF introduced additional monitoring and early-warning measures designed to reduce the impact of future jellyfish swarms.
The new measures include fixed cameras installed around the intake canals and filter drum screens, allowing operators to identify potential blockages earlier. EDF has also established cooperation with fishing and sea-rescue organisations that monitor the surrounding waters and can warn the plant when unusually large concentrations of jellyfish are detected. Fishing vessels can then be deployed in targeted operations before the swarm reaches the cooling infrastructure.
The causes behind large jellyfish concentrations are complex. Scientists identify several possible factors, including warmer seawater, the spread of invasive species, changes in marine ecosystems, the loss of predator habitats and overfishing. Warmer waters can also extend the reproductive period of some jellyfish species. However, individual incidents cannot automatically be attributed to a single cause.
The situation at Gravelines comes as high temperatures are also creating difficulties for some other French nuclear plants. Facilities that rely on river water for cooling can be required to reduce electricity production during heatwaves or periods of low river levels in order to comply with environmental limits on water temperatures.
The incident at Gravelines is not a nuclear accident. The reactors were shut down or had their output reduced as part of normal safety and operational procedures, while EDF says cooling and other safety systems continue to function normally. The immediate priority is to complete the removal of the jellyfish, inspect the affected equipment and safely return the reactors to electricity production.
