[Nuclear by the numbers] Extreme heat: nuclear generation losses average just 0.3%
With its “Nuclear in numbers” series, RGN analyses the major trends shaping the sector through key data. During periods of extreme heat, some nuclear power plants may have to temporarily reduce their output in order to comply with environmental limits on river temperatures. However, the impact remains very limited: between 2000 and 2023, these reductions represented just 0.3% of French nuclear generation on average.
During periods of extreme heat, some nuclear power plants may occasionally have to temporarily reduce their output. This is not because the reactors are no longer operating properly, but in order to comply with environmental limits set for the temperature of the rivers used for cooling, as RGN explained in a recently published article. These power reductions regularly make the news, yet their impact on annual generation remains very limited.
Following the second heatwave at the end of June 2026, Étienne Dutheil, Director of EDF’s Nuclear Generation Division, gave a very positive assessment on LinkedIn: “Our equipment performed well, safety was ensured at all times and we experienced no events related to weather conditions.” The energy impact of these outages remains limited for the time being. Between 2000 and 2020, the energy not produced represented “less than 1%, except in 2003, which was marked by a major heatwave,” RTE explains in its Energy Pathways 2050 report. EDF thus estimates that “generation losses due to these phenomena represented 0.3% of annual generation on average”, “but could represent 1.5% by 2050”, according to RTE’s 2023 Electricity Report.
Climate-related unavailability concentrated at a handful of sites
The risk of nuclear power plant unavailability during periods of heatwave or drought remains concentrated at a limited number of sites today. Over the past fourteen years, 90% of generation losses caused by climate-related unavailability have affected only a handful of the eighteen nuclear sites. These notably include Saint-Alban, Tricastin, Bugey, Blayais, Golfech and Chooz.
Conversely, some sites appear to have little or no exposure to these phenomena. The Civaux nuclear power plant, for example, has not been affected by these events thanks to equipment that is unique in France. In addition to its two large cooling towers, the plant has four smaller fan-assisted towers that further reduce the water temperature. This system “makes it possible to discharge water into the Vienne at a temperature lower than when we withdraw it,” explains Laurent Leloup, Director of Environmental Risk Prevention at Civaux.
After 2003, regulations were adapted
In the early 2000s, losses of potential generation due to weather conditions reached significant levels. The 2003 heatwave caused an exceptional loss of around 6 TWh of nuclear generation, equivalent to nearly 1.4% of the nuclear fleet’s annual output. Following this crisis, thermal discharge regulations were amended in 2006, with the introduction of exceptional conditions allowing certain discharge temperature limits to be temporarily raised.
Since this change, losses related to high temperatures and low river flows have represented an average of just 0.3% of annual nuclear generation, or around 1.2 TWh/year.
Table 1: Nuclear generation losses by year – Source: EDF
Generation reductions occurring increasingly early in the year
Table 2: Nuclear unavailability related to extreme heat – Source: Callendar
Since the beginning of the year, extreme heat has already resulted in cumulative generation losses of 2 TWh, equivalent to around 1% of the nuclear generation recorded in 2026. Although this impact remains limited across the fleet, the episode nevertheless marks a turning point: since 2015, French nuclear power has never had to modulate its output this early in the year with such volumes as early as June. Faced with the expected increase in heat episodes, EDF teams are already preparing to adapt operation of the nuclear fleet. As Étienne Duteil points out: “Summer is only just beginning, but teams across the nuclear fleet are already at work to be ready for the forthcoming heat episodes and to make the maximum available capacity available to the power grid again for the winter.”
Even if France marginally reduces its generation, it continues to export large amounts of electricity to support neighbouring countries, as demonstrated by an Aurora Energy Research study: “Beyond the few power reductions observed at certain sites, the June 2026 heatwave above all illustrates the robustness of the French nuclear fleet. Despite a historic heat episode across Europe, France remained a major electricity exporter, with flows reaching nearly 10 GW to neighbouring countries.” ■