Effects of heatwaves on nuclear plants

For the second summer in a row, some nuclear power plants in Europe have been forced to temporarily shut down or reduce power output due to extreme heat waves. Some have suggested that this curtailment demonstrates that nuclear energy is ill-adapted to a warming climate.

Why do some nuclear power plants have to stop producing electricity during heat waves, and what are the implications of this issue? Are there ways to fix the situation? In this new report, by acclaimed author and energy engineer, Dr Staffan Qvist, we explore the impact of the European heat waves of 2003, 2006, 2018 and 2019 on nuclear power output.


  • Nuclear power plants already operate at the highest capacity factor and availability factor of any electricity generating technology. This is also true during heat waves, when wind and hydroelectric output can be substantially depressed.
Annual utilization of installed capacity (capacity factor), United States 2018


  • Curtailment of nuclear power during heat waves is not a result of safety considerations. Curtailment primarily occurs at river sited plants and are due to limitations in the allowed temperature at which cooling water can be returned to a river.
  • Curtailment of nuclear power due to periods of extreme heat has had the effect of reducing European nuclear power output by approx. 0.1% since the year 2000. For France specifically, which with its many inland river-sited plants has been the worst hit country, the corresponding figure is approximately 0.15%
Actual nuclear power plant output and approximate lost output due to heat-wave curtailment in Europe (2000-2019) [preliminary figures for 2019
  • At peak times, less than 5% of European nuclear power capacity has been unavailable specifically due to curtailment during recent heat waves. Even at times of peak curtailment, theavailabilityof nuclear power exceeded that of any other low-carbon electricity generating technology in Europe, including solar PV, wind and hydroelectricity.
  • To address this problem in the near-mid-term, technical fixes are not necessarily required. For example, the French nuclear fleet runs at 60% capacity factor during the summer, heat wave or not, since it is typically deployed to vary with load, and load is lowest in summer, so scheduled outages are common. Electricity prices have remained modest during curtailment periods (€40-60/MWh) and France covers its own demand during the summer. To compensate for lost output at its most vulnerable river-sited plants during heat-wave curtailments, France could increase output at the remainder of its fleet if economic incentives were provided.
  • Over the longer term, there are also technical options readily available for alltypes of power plants, including nuclear, to increase both rates of utilization as well as availability. These will also be driven by economics, not engineering limits. The cost increases associated with these options are relatively modest.

Read the report here: Qvist Nuclear and Heat Waves August 2019 FINAL_