Jellyfish influx forces reactor shutdowns at France's Gravelines nuclear plant

Disruption at the cooling filters

A swarm of jellyfish entered the seawater filtration system at the Gravelines nuclear power plant in northern France on 26 August, forcing operator Électricité de France to take reactor units offline. The incident occurred around 16:00 on Wednesday, when marine life accumulated in the seawater pumping station that supplies cooling water to the facility. Unit 3 was disconnected from the grid just eight days after returning to service following an earlier jellyfish blockage on 10 August.

Teams at Gravelines proceeded, as a preventive measure, to shut down production unit no. 3 due to a new massive arrival of jellyfish at the filtration systems of the seawater pumping station.

The operational status across the station varied across its six reactors following the incident. Unit 4 remained offline after being shut down during the initial jellyfish surge on 10 August, while Unit 5 was already out of service for scheduled maintenance. Production at Unit 2 was adjusted to lower capacity as a precaution. Discrepancies appeared between operational reports on Thursday, with EDF stating that Unit 3, Unit 4, and Unit 6 were halted while Units 1 and 2 ran at reduced capacity, whereas regional French outlets reported Units 1 and 6 were operating normally.

Cooling mechanics and marine behavior

The Gravelines station relies on water drawn through an intake canal connected to the North Sea to cool its steam condensers and reactor systems. Cooling systems depend on large intake volumes, making their filtration screens vulnerable to floating debris and dense concentrations of marine organisms. When large swarms of jellyfish enter these intake points, they clog the protective drum screens, restricting the flow of seawater required for cooling.

Warm summer temperatures drive jellyfish populations to peak activity along the northern coastline of France. Warmer waters accelerate the growth of surface plankton, which serves as the primary food supply for several native jellyfish species. As plankton concentrates near the surface during warm weather, jellyfish migrate upward and drift with coastal currents directly into industrial intake channels.

Plant capacity and mitigation measures

Located in the Hauts-de-France region between Dunkirk and Calais, Gravelines is one of the largest nuclear power facilities in Western Europe. The complex consists of six pressurized water reactors, each capable of generating 900 megawatts of electricity. The facility provides baseload electricity to the French national grid when operating at full capability.

To manage the ecological risk, plant operators have deployed dedicated marine countermeasures. Électricité de France established an operational partnership with local commercial fishermen to intercept swarms before they reach the plant. Crews capture jellyfish in nets near the canal entrance and transport them offshore for release. Despite these measures, dense concentrations of marine life during peak summer periods continue to reach intake barriers.

Precedents in European nuclear operations

Disruptions caused by marine wildlife have affected Gravelines and other northern European coastal facilities in prior years. The Gravelines station experienced a similar closure in August 2025 when seasonal jellyfish blocked intake screens. The recurrence in August 2026 marks another summer cooling disruption at the Hauts-de-France complex.

Similar incidents have occurred across Europe over the past two decades. The Torness nuclear power station in Scotland was forced to shut down its reactors in 2011 after a jellyfish swarm clogged its seawater intake filters. In 2013, the Oskarshamn nuclear plant in Sweden halted generation at its facility under similar circumstances.