Heat waves and nuclear power: Europe faces its water limits

The heatwaves of summer 2026 forced France to temporarily reduce its nuclear capacity by up to nine gigawatts, nearly a fifth of its usual summer output. Hungary came close to a complete shutdown of its only nuclear power plant, in Paks, due to insufficient water in the Danube.

The Paks nuclear power plant in Hungary, which normally supplies 40% of the country's electricity needs, operated only one of its four turbine units this summer. The exceptionally low water level of the Danube River even briefly threatened a complete shutdown of the site, the country's only nuclear reactor. This situation illustrates the structural dependence of power plants on local conditions, whether it be river flow or water temperature.

However, France faces the greatest challenges for the European electricity market. No other country on the continent relies as heavily on nuclear power, and any drop in French production immediately impacts exports to neighboring countries, or even transforms France into a net importer.

A report by the French Court of Auditors published in 2023 estimated that, between 2000 and 2022, annual losses due to high temperatures and low water levels never exceeded 1% of national nuclear production. Even during the 2003 heatwave, the annual figure reached only 1,5%. But these averages mask much more severe short-term shocks: that summer, power plants had to reduce their output by more than six gigawatts for several weeks.

During the heat waves of late June and mid-July 2026, available capacity was temporarily reduced by eight to nine gigawatts. By June 24, the reduction had reached 4,1 gigawatts due to high river temperatures, causing French exports to plummet from 10 to 12 gigawatts the previous week to around 3 gigawatts. Combined with increased consumption, weak winds, and a greater reliance on gas-fired power plants, this contraction pushed electricity prices in France and Germany to their highest levels since January 2025.

In July, the restrictions were further tightened: on July 13, eight reactors totaling 6,3 gigawatts were affected, two of which were completely shut down. France exported only intermittently more than 10 gigawatts and imported inexpensive solar and wind power from Spain. Grid operators nevertheless indicated that the country maintained sufficient reserves and remained, overall, a net exporter.

The physical mechanism is simple: a nuclear power plant converts only about a third of the heat produced by the reactor into electricity. The remaining two-thirds are released into the environment, most often via a waterway. When the flow rate decreases, the same amount of heat is distributed over a smaller volume of water, and the temperature rises more quickly. Legal thresholds protect aquatic life, and it is generally compliance with these thresholds, not the safety of the reactor itself, that necessitates reducing power output.

Cooling towers mitigate the problem but do not eliminate it. The Golfech nuclear power plant on the Garonne River is a prime example: despite its towers, it remains one of the most vulnerable sites in France because the river there is particularly warm and prone to low water levels. At Chooz, on the Meuse River, the reactors had to be reduced in 2020 to avoid compromising Belgium's drinking water supply, despite the presence of cooling towers.

French authorities sometimes grant temporary exemptions to temperature limits, coupled with increased monitoring of waterways, allowing EDF to produce more in the short term. But this room for maneuver remains limited.

The Court of Auditors anticipates that climate-related shutdowns could triple or quadruple by 2050. Starting from a currently low baseline, this would not necessarily mean widespread outages, but some sites could find themselves in critical situations. Solutions exist: additional cooling towers, deeper water intakes, and improved maintenance planning. These solutions are expensive and require more space.

France is taking this into account in its nuclear modernization program. Of the six planned EPR2 reactors, four will be built on the coast, at Penly and Gravelines. Only the third pair will be located on the Rhône River, near Bugey, with cooling towers to limit the thermal load on the river.

For now, heat-related restrictions remain a manageable problem. Whether this becomes a Europe-wide shortage will largely depend on the evolution of foreign demand and the rise of renewable energy on the continent.