River Heat Shuts Beznau and Golfech: Europe's Early-July Heatwave Tests Nuclear Cooling Limits

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.

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. 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.

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. Beznau uses once-through cooling from the Aare, which makes it particularly sensitive to river temperature. 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.

Several European countries are planning new nuclear reactors or extending the life of existing ones. Coastal sites, which use seawater for cooling, are generally less affected by heat than river sites, although sea temperatures also rise during marine heatwaves. For traders, river temperature has become a summer variable to monitor alongside solar and wind forecasts. The rest of the summer will determine whether early July was an isolated episode or the start of a longer period of constraints.