Low river water levels force nuclear power plants to reduce production. If you look at the whole year, the losses are small - but in these critical days it is reflected in electricity networks and prices across Europe, writes DW.
At the moment, on the Danube, but also on the Sava, you can see how many nuclear power plants affected by high temperatures. In Hungary's Paks, which normally covers about 40 percent of the country's electricity needs, only one out of four turbines is currently working. Because of the record low water level Danube the only nuclear power plant in Hungary was temporarily even threatened with complete shutdown.
Due to the low water level and high temperatures of the Sava River, the nuclear power plant Krško in Slovenia started gradually reducing the power of its reactor on the night from Wednesday to Thursday. It is planned to reduce production to 80 percent of capacity, and if necessary, production could be further limited.
However, France is much more important for the European electricity market. No other European country relies as much on nuclear energy as France. If production at its nuclear plants declines, the country may export less electricity to its neighbors or even become an importer of electricity.
Reduced electricity production
The balance of production reduction in French nuclear power plants seems paradoxical at first glance. If you look at the whole year, the losses caused by high temperatures are generally small. But during some weeks with extreme heat, restrictions can be very significant.
According to a 2023 report by France's National Audit Institution on Nuclear Energy, annual production losses between 2000 and 2022, caused by excessively high water temperatures and low river levels, amounted to less than one percent of total nuclear power production almost every year. Even during the extremely hot summer of 2003, the loss was only about 1,5 percent.
Temporary risks
However, annual data mask short-term fluctuations. During the summer of 2003, French nuclear power plants had to temporarily reduce production by more than six gigawatts. The most recent heat waves in late June and mid-July 2026 reached similar proportions. During those periods, the maximum available power was temporarily reduced by eight to nine gigawatts. This sometimes represents as much as a fifth of the nuclear power that France normally has at its disposal during the summer.
However, eight gigawatts less available power does not automatically mean that that much electricity was really missing for many days. Due to lower summer consumption, large production from solar power plants or network congestion, some reactors would not operate at full capacity even without low water levels. According to data from the grid operator, France had sufficient reserves during the heat waves in 2026 and remained a net exporter of electricity.
Other countries are also feeling the consequences
For power grid operators and power traders, such restrictions are nevertheless very important. For them, it is not crucial how much electricity is produced throughout the year, but whether the production capacity is available when it is needed.
Experience shows that heat waves increase electricity consumption due to more intensive use of air conditioning. Since the heat has affected large parts of Europe this year, the available reserves from abroad are also decreasing.
The rivers are warming.
Nuclear power plants especially suffer from low water levels in rivers, because they have to remove large amounts of waste heat per kilowatt-hour produced. Only about a third of the heat generated in the reactor is converted into electricity, while the rest is released into the environment through cooling systems, often into rivers or the sea. When the water level is low, the same amount of heat is distributed to a smaller amount of water, so the temperature of the river rises faster.
Because warmer water contains less oxygen, legal temperature limits protect fish and other aquatic organisms. When operators reduce reactor power in the summer, it is generally not a measure for the safety of the reactor itself, but rather a measure to protect the environment.
Limit values and cooling towers
In situations where there is a danger to the supply of electricity, the French authorities temporarily allow deviations from the prescribed temperature limits. Then the condition of watercourses is additionally monitored. This allows the state company EDF to produce more electricity in the short term.
Power plants with cooling towers draw significantly less water from rivers and return much less heat to the water stream than power plants with an open cooling system. However, even cooling towers do not make a nuclear power plant completely independent of weather conditions. An example of this is the Golfèche power plant on the Garonne River, which often has very warm and low water levels in the summer. This is why, in addition to the cooling towers, it is considered one of the most sensitive locations in France. In 2020, the reactors at the Shu power plant on the Meze River had to slow down their operation so that the supply of drinking water in Belgium would not be threatened.
Breakdowns and overhauls also reduce production
Not every decline in nuclear power generation is due to weather conditions. Four years ago, French reactors produced almost 82 terawatt-hours less electricity than in 2021. Stress corrosion problems and delayed overhauls, not climate conditions, were responsible for the crisis, which made headlines.
Experts of the State Audit Institution estimate that the unavailability of reactors caused by climate change could triple or even quadruple by 2050. Mass production outages are still not expected, but certain locations could become problematic.
Additional cooling towers, deeper catchment systems and better maintenance planning could help France partially adapt to the new conditions. However, increasing the heat resistance of power plants requires large investments and additional space.
When modernizing the nuclear sector, France plans to take more into account the consequences of climate change. Four of the first six new EPR2 type reactors are to be built onshore, at Penley and Gravelin. Only a third pair of reactors is planned along the Rhône River, near Bouzet, where cooling towers are supposed to limit river heating and water consumption.
What about the export of electricity?
The consequences of reduced production in French reactors can be seen in the European electricity market. On June 24, due to high river temperatures, about 4,1 gigawatts of French nuclear capacity was limited. As a result, France's electricity exports fell to around three gigawatts, after only seven days earlier it had been between ten and twelve gigawatts. Together with increased electricity consumption, weaker wind power production and greater use of gas power plants, this contributed to electricity prices in France and Germany reaching their highest level since January 2025 in the short term.
During subsequent heat waves in July, the restrictions were even greater, but the consequences for other countries were significantly less. On July 13, eight reactors with a total power of 6,3 gigawatts were affected, while two reactors were completely disconnected from the grid. Despite this, France exported more than ten gigawatts of electricity and only occasionally imported cheap solar and wind power from Spain.
For now, France appears to be managing the heat-induced reductions in nuclear power generation. Whether it will develop into a serious European bottleneck in the future will depend primarily on the development of electricity demand in other countries, as well as on the availability of wind and solar energy.
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