Jellyfish alarm in Gravelines

Late on Sunday evening, 10 August 2025, a massive, sudden swarm of jellyfish in the filter drums of the pumping units at the Gravelines nuclear power plant triggered an automatic shutdown. Three reactors (units 2, 3 and 4) were shut down immediately, followed by the fourth in the early hours of 11 August. The other two reactors had already been shut down for scheduled maintenance work. This meant that the entire plant was temporarily out of operation - around 10 % of French electricity production was at a standstill.

Experts attribute such jellyfish occurrences to climate change, warming seawater, overfishing and the increased occurrence of invasive species.

List: Heatwave impacts further power plants

In parallel to the jellyfish crisis, France's nuclear power fleet is struggling with a persistent heatwave:

• Bugey 2 (910 MW) will remain shut down until Thursday (14 August) - an extension of 24 hours - having already been off the grid since Saturday for environmental reasons.

• Bugey 3 (910 MW) will be reduced to 410 MW between 14:30 CET and midnight today.

• Tricastin 4 (915 MW) is to be reduced by 70 % to 296 MW tomorrow between 09:30 and 17:30 CET (originally planned for today).

• Saint-Alban 1 (1.3 GW) will probably also have to reduce its output from Thursday - one day later than initially announced.

The measures are being taken because EDF is legally obliged to reduce output if river water temperatures become too high for cooling the reactors in order to avoid ecological damage.

Heatwave in France

The heatwave began on Friday 8 August in the south-west of France with temperatures of 42°C. In the south-east, 40 °C was measured on Sunday. According to Météo France, temperatures in parts of the country are expected to remain at 36-42 °C until at least 20 August. There is no cooling in sight.

Double shock

EDF wants to clean the filter systems in Gravelines and reconnect the reactors to the grid by Thursday. At the same time, the heatwave will put further pressure on grid stability, as additional power plant curtailments are likely.

The double shock of biological disruptions (jellyfish) and climate-related restrictions (heat) illustrates the extent to which critical infrastructure is now dependent on environmental factors. In addition to technical innovation, ecological early warning systems and climate-proof operating strategies will be crucial to ensure security of supply in the future.