All Six Gravelines Reactors Lose Normal Output

Tonnes of jellyfish clogged the seawater filters. Pumps lost flow. Cooling failed. One by one, Gravelines' six reactors dropped off the grid (EDF, France 24). By 27 August, two units were running at reduced power, two had stopped because of jellyfish, one was down for a technical fault, and one was already in maintenance. Not a single unit at the plant near Dunkirk was operating normally.

Gravelines holds six 900 MW reactors, roughly 5.4 GW of capacity, enough to power a mid-sized European country on a calm day (EDF, El Debate). This was the third jellyfish episode in twelve months. No radioactive system was compromised. EDF said there was no consequence for safety or the environment (Le Figaro). But several gigawatts of cheap, reliable electricity vanished from a grid that feeds into Belgium, the UK and beyond.

A filter problem with a market price tag

Gravelines draws seawater to cool the turbine side of the plant, where steam turns into electricity. Jellyfish piled up on the pre-screens and rotating drum filters at the pumping station, choking the water flow needed to reject heat (France 3 Hauts-de-France). When flow drops, operators cut output or shut down entirely. The reactor core stays safe. The electricity disappears.

After a jellyfish episode in August 2025 shut Gravelines for nearly 48 hours, EDF installed cameras on the intake canals and partnered with fishing boats to spot swarms offshore (EDF, France 3 Hauts-de-France). The swarms beat the defences anyway.

So who pays? In Europe's coupled electricity market, generators and buyers trade hourly blocks for next-day delivery in day-ahead auctions. When cheap nuclear power drops out, more expensive gas or imported power fills the gap, and the clearing price rises across countries linked by undersea and overland cables. The shock travels as far as spare cable capacity allows; it weakens where interconnectors fill up.

This particular episode left no smoking-gun spot-price spike. France's day-ahead average on 27 August was €157/MWh, Belgium's €162/MWh, barely above the previous day (Selectra). But Bloomberg reported that the disruption helped push French month-ahead contracts (power bought now for delivery next month) to their highest level since January 2025 (Bloomberg). Traders are pricing in the pattern, not just the incident.

The broader August picture is worse

On 12 August, 20.4% of EDF's nuclear fleet was offline for environmental reasons: heat, low rivers, jellyfish. Thirteen of France's 57 reactors were affected (France 24). That wider crunch did move markets: day-ahead power jumped 21.8% in France and 22.8% in Germany as heatwave-curbed nuclear met low German wind (Euronext/Reuters). The European Commission said the electricity system remained stable (European Commission). Stable and cheap are not the same thing.

The losses land in layers. EDF loses production revenue first. Suppliers who promised fixed-price contracts must buy replacement power at whatever the market charges. Industrial users on variable contracts see bills rise immediately. Governments absorb part of the shock if regulated tariffs or subsidies cap retail prices. On the other side, gas generators and flexible producers gain: they sell at the higher clearing price that nuclear's absence created.

The same class of problem appeared on the Danube this summer, where drought forced Hungary's Paks plant to reduce output. The Hungarian government cited replacement-power costs (buying electricity from more expensive sources to compensate) of at least 50 billion forints per month if Paks lost cooling water (Hungarian Government, MVM Paks). Different trigger, same result: reliable round-the-clock power vanishes, and the bill lands on consumers and treasuries that share an interconnected grid.

Gravelines shows that Europe's nuclear risk is no longer only whether reactors are safe, but whether their cooling systems can keep cheap power flowing during hotter, more biologically volatile summers. EDF, grid operators and energy ministries have not published the thresholds or contingency rules that would let markets and neighbours plan for the next episode. This one did not produce a clear price spike. The pattern, though, is already in the forward curve.