EDF has temporarily shut down nuclear reactors at the Nogent-sur-Seine plant on the Seine and the Bugey plant on the Rhône as river temperatures rose during a record heatwave, Euronews reported on 25 June 2026. A reactor at the Golfech plant on the Garonne in south-western France had been taken offline earlier in the week, and output was reduced at several other sites across EDF's 57-reactor fleet.
Météo-France reported that Wednesday 24 June was the hottest day recorded in France since national measurements began in 1947. More than half of France's 96 departments were placed under a red alert, the highest level of heat warning.
Why reactors are curtailed
French nuclear plants located on rivers draw water to cool their reactors and return it to the river slightly warmer. Environmental rules set limits on the temperature of discharged water and on the temperature increase between water withdrawn and water returned, to protect fish and other aquatic life. When rivers warm naturally during heatwaves, operators must reduce output or shut reactors to stay within those limits.
EDF said earlier in the week that it had reduced output on one of the Nogent reactors "to limit the temperature increase between the water withdrawn from the Seine and the water discharged back into it, thereby protecting aquatic plant and animal life." Reuters reported on 18 June that EDF had warned high river temperatures were expected to limit output in the following week, naming Golfech, Blayais and Bugey among the plants at risk.
According to a report citing EDF data, reductions in nuclear output reached about 4.1 GW on 24 June, roughly 7% of midday demand.
System adequacy
French grid operator RTE said on 24 June that "France has sufficient generation capacity to meet electricity demand, including in the event of outages at certain production facilities." Nuclear power accounted for close to 70% of France's electricity generation in 2025, according to Euronews, and France has been a large net exporter of electricity since its nuclear fleet recovered from the corrosion problems and outages of 2022.
Summer heatwaves coincide with strong solar output, which helps offset reduced nuclear generation during the day. The tighter periods tend to be in the evening, when solar output falls but temperatures remain high and air conditioning demand continues. Imports from neighbouring countries can also help, although heat affects plants across the region at the same time.
Prices
Heatwaves tend to raise power prices in Europe. Euronews, citing analysis of the previous summer, said that during the peak days of the June and July 2025 heatwave daily power demand rose by up to 14%, driving a two- to three-fold increase in average daily power prices. Reduced nuclear output in France affects prices across interconnected markets, including Germany, Belgium, Switzerland, Italy and Spain.
A recurring pattern
Heat-related curtailments at French nuclear plants have become more frequent. In the summer of 2022, during a drought and heatwave, the French regulator granted temporary waivers allowing some plants to exceed discharge temperature limits to maintain supply, at a time when many reactors were already offline for corrosion checks. In July 2025, EDF shut a reactor at Golfech and Swiss operator Axpo shut down both units at the Beznau plant on the Aare river because of high water temperatures.
Plants on the Rhône and Garonne are particularly exposed, because these rivers are warmer and their summer flows are more variable. Coastal plants, such as Blayais on the Gironde estuary, Gravelines on the North Sea and Flamanville on the Channel, use seawater and are less affected, though Blayais sits on an estuary where water temperatures can also rise.
Adaptation measures
EDF has been studying measures to make its fleet more resilient to heat and low water, including additional cooling towers, changes to operating procedures and better forecasting of river temperatures. The planned new EPR2 reactors are being sited mostly at coastal locations or on large rivers, and their design takes climate projections into account. Operators also coordinate with water management authorities, because reservoirs upstream can sometimes release cooler water to support river flows.
The French nuclear safety authority has said that the operating limits are environmental rather than safety limits. Reactors can continue to operate safely in heat, but discharge limits protect ecosystems downstream. The distinction matters for public communication, because curtailments are sometimes misread as safety incidents.
The wider European heatwave
The heat was not limited to France. Germany, Spain, Portugal and Switzerland also faced very high temperatures, and Euronews reported it was Europe's third heatwave of the year, with forecasters warning temperatures could reach 43°C around the Mediterranean. Schools closed early in some places, and rail services in Paris and Brussels ran reduced schedules to lower the risk of breakdowns. At least 18 heat-related deaths had been confirmed in France as of 22 June.
Scientists have linked the growing frequency and intensity of European heatwaves to climate change. Euronews cited a recent study finding that parts of Europe now experience up to 40 additional days of extreme heat stress compared with the 1970s.
Hydropower and water levels
Heat and dry conditions also affect hydropower, France's second-largest source of electricity after nuclear. Alpine reservoirs filled by snowmelt typically support output into the summer, but low rainfall reduces run-of-river generation on major rivers. The same river flows matter for nuclear cooling, because lower flows warm faster and dilute heated discharge less. The combination of reduced hydro and curtailed nuclear output during prolonged heat can tighten the French system more than either effect alone.
Data centres and cooling
The same physical constraints affect other water-cooled infrastructure, including data centres, many of which use evaporative cooling or water-based systems. In heatwaves, data centres use more energy for cooling and, depending on design, more water. The EU now requires data centres above a size threshold to report energy and water use, and operators in southern Europe have increasingly turned to air-cooled and closed-loop designs to reduce water dependence.
What to watch
Key items include how long the heat persists, whether EDF extends curtailments to other plants, whether regulators grant temporary waivers if supply tightens, and how river levels evolve through July and August, which affect both nuclear cooling and hydropower output.