The heatwave that spread across western and central Europe at the end of June and into early July 2025 forced nuclear operators in Switzerland and France to cut output or shut reactors because river water used for cooling became too warm.
In Switzerland, Axpo first reduced output at the Beznau nuclear plant, on the Aare river in the canton of Aargau, to 50% and then shut both of its units. The company said the Aare had exceeded 25°C, the threshold above which it must limit operations to protect the river ecosystem, in line with requirements set by Swiss authorities. Axpo announced on 8 July that Beznau was back at full operation after the river cooled.
In France, EDF shut the Golfech plant on the Garonne river in the south-west as water temperatures approached 28°C, according to Euronews and other reports. Output was also reduced at the Blayais plant on the Gironde estuary as temperatures there rose.
Why river temperature matters
Most nuclear plants, and many coal and gas plants, use large volumes of water to condense steam after it has passed through the turbines. Plants on rivers either draw water once through the condenser and return it warmer, or use cooling towers that evaporate part of the water. In either case, the discharge of heated water raises the temperature of the river downstream.
Environmental regulations in Switzerland, France and other countries set limits on discharge temperatures and on the temperature rise a plant can cause, to protect fish and other aquatic life. When a river is already warm, the margin for heated discharge shrinks, and plants must reduce output or shut down. The limits are site-specific and depend on the river's flow, ecology and the plant's cooling design.
Beznau uses once-through cooling from the Aare, which makes it particularly sensitive to river temperature. Plants with cooling towers can usually operate at higher river temperatures but still face limits when river flow is low.
How much output was lost
The curtailments in early July were short. Beznau is Switzerland's oldest nuclear plant, and Golfech's two reactors are among the larger units in the French fleet. EDF regularly issues warnings during summer heatwaves that output at specific plants may be reduced.
The broader effect on the European power system was modest. Solar generation was high during the heatwave, which helped meet daytime demand. Prices rose in the evening hours when solar output fell and demand for cooling remained high.
A recurring summer pattern
River-related curtailments have become a regular feature of European summers. In 2022, a severe drought combined with high temperatures led to extensive output reductions across the French nuclear fleet, at a time when many reactors were also offline for stress corrosion inspections. French nuclear output in 2022 fell to its lowest level in decades, and France became a net importer of electricity for the year, which tightened the whole continental market during the gas crisis. In 2003, a severe heatwave also forced large curtailments.
Summer 2025 is different from 2022 in that the French nuclear fleet is in much better condition, and the curtailments so far have affected only a few plants for short periods. Even so, each episode shows a structural vulnerability of thermal power plants on rivers in a warming climate.
Implications for planning
Several European countries are planning new nuclear reactors or extending the life of existing ones. France has announced a programme of new EPR2 reactors, Switzerland is debating whether to allow new nuclear construction, and other countries including Poland, the Czech Republic, the Netherlands and Sweden have new build plans. Siting and cooling design are central to how these plants will perform during future heatwaves.
Coastal sites, which use seawater for cooling, are generally less affected by heat than river sites, although sea temperatures also rise during marine heatwaves. Plants on large rivers with high flow are less exposed than those on smaller rivers. Dry or hybrid cooling systems reduce water use but lower efficiency and increase cost.
The same issue affects other thermal plants. Coal and gas plants on rivers face similar limits, though they are typically smaller. In Germany, low river levels on the Rhine in drought years have also disrupted coal deliveries by barge, a separate climate-related constraint on thermal generation.
The data centre connection
Cooling water constraints are not limited to power plants. Data centres that use evaporative cooling draw large volumes of water, and their cooling systems work harder on hot days. In several European markets, including Ireland, the Netherlands and parts of Spain, local authorities have examined data centre water and power use as part of planning decisions. Heatwaves increase both the water demand of evaporative cooling and the electricity demand of mechanical cooling, at the same time as river-cooled power plants may be curtailed.
What it means for power markets
For traders, river temperature has become a summer variable to monitor alongside solar and wind forecasts. EDF publishes warnings of potential production reductions at specific plants several days in advance when river temperatures are forecast to rise. These warnings move forward prices in France and neighbouring markets.
The direct impact of a few days of curtailment is limited. The risk is concentrated in prolonged heatwaves combined with low river flows, as in 2003 and 2022, when curtailments can affect several plants for weeks and coincide with high demand and low hydro output.
How operators prepare
Operators have several tools to limit the impact. Maintenance outages can be scheduled for summer months, when demand in France is lower than in winter, so that units likely to face cooling limits are already offline for planned work. Grid operators coordinate cross-border flows so that imports can cover shortfalls. In Switzerland, the hydropower fleet, with large reservoirs in the Alps, provides flexibility that can compensate for short nuclear outages, provided reservoir levels are adequate.
Regulators also have discretion. In past heatwaves, French authorities have granted temporary exemptions allowing certain plants to exceed discharge limits when grid security required it, subject to environmental monitoring. Such exemptions are controversial and are used sparingly.
What to watch
The rest of the summer will determine whether early July was an isolated episode or the start of a longer period of constraints. Indicators to watch include river temperatures and flows on the Rhône, Garonne, Loire and Rhine, EDF's production warnings, and hydro reservoir levels in the Alps. Low snowpack or a dry summer would add hydro constraints to the nuclear cooling issue.