When extreme heat threatens Europe's nuclear power

Warm rivers and low water levels are forcing nuclear power plants to reduce output across Europe. Over a year, the losses are minimal, but on critical days the loss of power has consequences for grids and prices.

https://p.dw.com/p/5IWpO Advertisement The state of the Danube is currently showing just how dependent nuclear power plants are on their local conditions . At the power station in Paks, Hungary — which normally produces about 40% of the country's electricity needs — only one of four turbine units is still operating. Due to record-low water levels in the Danube, the country's only nuclear power plant was even briefly at risk of a complete shutdown.

But for the European electricity market, France is of far greater significance. No other country in Europe relies so heavily on nuclear power, and if output in France drops, the country supplies less electricity to its neighbors, and may even become an electricity importer itself.

Over the course of the entire year, heat-related production losses at French power plants are minimal, but during individual weeks of extreme heat , the restrictions can be significant.

A 2023 report by France's official audit office estimated that, between 2000 and 2022, annual losses across all of the country's nuclear reactors due to high water temperatures and low river levels amounted to less than 1% of annual nuclear power production in any given year. Even in the extremely hot summer of 2003, they amounted to only about 1.5%.

But the annual figures obscure short-term fluctuations. In the summer of 2003, French nuclear power plants had to temporarily reduce their output by more than six gigawatts, and the most recent heat waves at the end of June and in mid-July 2026 saw the maximum available capacity was temporarily limited by eight to nine gigawatts. This corresponds to up to one-fifth of the nuclear power capacity normally available to France in the summer.

But eight gigawatts less capacity is not the same as an eight gigawatt overall shortfall. Even without the low water levels, some reactors would not be running at full capacity — either because of lower summer demand, high solar-power production, or bottlenecks in the power grid. According to grid operators, France still had sufficient reserves even during the 2026 heat waves and, on balance, remained an electricity exporter.

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Nevertheless, such restrictions are relevant for grid operators and electricity traders. For them, it doesn't matter how much electricity was ultimately produced by nuclear power plants at the end of the year: What is crucial for the stability of the power grids and short-term price surges is whether the power is actually available when it is needed.

And here, experience shows that heat waves increase electricity demand due to air conditioning. Not only that: As this year's heat waves are affecting large parts of Europe, reserves from abroad are also declining.

Nuclear power plants suffer from low water levels because they must discharge a comparatively large amount of waste heat per kilowatt-hour generated. Only about one-third of the heat generated in a nuclear reactor is converted into electricity; the rest is released into the environment via the cooling system — often into rivers or the sea. But if the river is carrying little water , the waste heat is distributed over a smaller volume of water, and the water temperature rises faster.

Warm water also contains less oxygen, and legal temperature limits are in place to protect fish and other aquatic life. When operators throttle back a reactor in the summer, it is therefore usually not an emergency shutdown for reactor safety, but rather a measure to protect the local river life.

In critical situations, French authorities occasionally grant exemptions from these limits. In such cases, water bodies are monitored more closely, allowing the state-owned power company EdF to produce additional electricity in the short term.

A power plant with cooling towers draws significantly less water from the river and discharges considerably less heat directly into the water than a plant with an open cooling circuit. However, cooling towers do not make a nuclear power plant independent of weather conditions, as the Golfech plant demonstrates.

Located on the Garonne River, which is particularly warm in summer and frequently experiences low water levels, it is one of the most vulnerable sites, despite its cooling towers.

Meanwhile in Chooz on the Meuse River, the reactors had to be throttled back in 2020 despite the cooling towers, to avoid jeopardizing the drinking water supply in Belgium.

Not every decline in nuclear power production can be explained by the weather. Four years ago, France's reactors generated nearly 82 terawatt-hours less electricity than in 2021. This headline-grabbing crisis was caused less by climate change than by stress corrosion cracking and postponed maintenance work.

But looking ahead, the French audit office expects that climate-related reactor downtime could triple or even quadruple by 2050. Given the low baseline so far, even that would not yet constitute a widespread outage, but it could become critical for individual sites.

With additional cooling towers, deeper water intake structures, and better maintenance planning, France can at least partially counteract this trend. However, making the power plants more heat-resistant costs a lot of money and requires more land.

In its upcoming nuclear power plant modernization program, France intends to take the consequences of climate change into account. Four of the six modern EPR2 reactors initially planned are to be built on the coast, in Penly and Gravelines. Only the third pair of reactors will be built on the Rhone River near Bugey. Cooling towers there are intended to limit th...

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