Over 200 Tonnes of Jellyfish Disrupt All Six Reactors at France's Gravelines Nuclear Plant

Swarm triggers precautionary shutdowns

A mass influx of more than 200 tonnes of jellyfish clogged the cooling water intake systems at the Gravelines nuclear power plant in northern France overnight into 27 August. Plant operator EDF halted reactor 6 as a precaution after large swarms of the marine animals reached the seawater pumping station. The jellyfish accumulated directly against the mechanical pre-filters and rotating filter drums that clean North Sea water before it enters the facility's cooling circuits. The intake disruption forced the operator to ramp down generation at units 1 and 2 while taking units 3 and 6 completely offline. EDF confirmed that the automated and manual safety interventions prevented physical damage to internal pumping equipment.

There is no impact on the safety of the installations.

Operational status across the six units

Following the overnight actions, none of the six reactor units at the Gravelines site are functioning under normal operating conditions. Units 1 and 2 remain connected to the transmission network at curtailed generation levels to prevent further blockage of their intake channels. Unit 3, which had resumed electricity generation on 18 August after an earlier jellyfish stoppage, was shut down again nine days later. Unit 4 is offline due to an unrelated technical malfunction, while unit 5 remains disconnected for scheduled ten-year maintenance work. The utility stated that the combined outages at the facility, situated on the North Sea coast between Calais and Dunkirk, do not threaten the stability of the national electricity grid.

Offshore containment and previous incidents

To clear the cooling water intakes, EDF deployed seven specialized boats operating 24 hours a day in the coastal waters near the station. Maritime crews work alongside the environmental organization France Pêche Durable to collect the animals in nets and transport them further out to sea for release. The utility had already introduced enhanced detection protocols on 8 August following recurrent jellyfish activity along the coast. Similar swarms caused operating disruptions at Gravelines in August 2025 and earlier in August 2026, when an influx on 10 August triggered automatic shutdowns of units 3 and 4 alongside a manual power reduction at unit 2. Company spokespersons noted that the exact environmental causes of the sudden population surges remain under study.

Climate conditions and nuclear generation

The jellyfish disruption coincides with wider weather-related strains across the French nuclear fleet, which comprises 57 operational reactors across nearly 20 sites. Nuclear power typically provides approximately 70% of electricity generation in France. In mid-August, high summer temperatures, drought conditions, and marine infestations temporarily curtailed roughly 20% of the country's installed nuclear capacity, leaving up to 13 reactors unavailable or operating at reduced capacity. While inland power stations must lower output during hot spells to avoid heating river ecosystems beyond regulatory limits, coastal plants like Gravelines rely directly on sea water where warming marine trends contribute to seasonal jellyfish blooms.