The exceptional heatwave in late June 2026 reduced France’s daily electricity exports to neighboring countries by nearly fourfold. In just a few days, the volume of available capacity that Europe’s largest electricity exporter could supply to external markets fell from 12 GW to less than 3 GW. The cause was neither accidents nor fuel shortages, but the overheating of rivers used for cooling nuclear power plants. This is the conclusion reached by specialists from British engineering consultancy WSP, who analyzed the performance of the French power system during the June heatwave.
At the end of the month, a powerful high-pressure system settled over Western Europe pushing air temperatures in parts of France up to 42.5°C. Along with the air, the country’s largest rivers – the Rhône, Garonne, and Seine – also warmed, as their water is used to cool eight nuclear power plants.
After passing through the cooling systems, the water is returned to the rivers. Environmental regulations set strict limits on the permissible temperature increase of river water. Consequently, when the Rhône, Garonne, and Seine approached the 28°C threshold, several French nuclear plants began to reduce reactor output. As the rivers warmed gradually, the restrictions were imposed only after nearly a week of extreme heat.
France did not, however, face an electricity shortfall. Throughout June, the country fully met domestic demand and remained Europe’s largest net electricity exporter. But it was precisely external supplies that proved most sensitive to the extreme weather. The output restrictions at some reactors coincided with rising electricity consumption due to air conditioning, causing available capacity for neighboring markets to drop sharply.
While at the start of June France was exporting about 12 GW of daytime capacity abroad, by June 24, that was registered as the hottest day of the month, this figure had fallen down to 2.9 GW. Flows to northern Italy remained largely unchanged, thanks to consistently high prices on the Italian market, while exports to Belgium, Switzerland, and the United Kingdom declined markedly. The most unusual situation occurred at the Spanish border: the country that typically buys French electricity actually began supplying it to France on the hottest days, thanks to high output from its solar plants.
On a monthly scale, however, the heatwave’s impact was much less pronounced. France’s net exports for the month totaled about 7.6 TWh, roughly the same as in June of the two previous years. Moreover, average available capacity from French nuclear plants stood at about 39 GW, compared with about 36 GW a year and two years earlier. In other words, the heat did not so much reduce total export volumes as alter their temporal distribution, sharply curtailing exports precisely during the hottest hours of the day.
Nevertheless, in WSP’s view, this episode clearly illustrates how rising river temperatures are gradually becoming a new constraint on nuclear power generation. As the climate warms, reactors will increasingly be forced to reduce output precisely during peak demand hours. For the European power system, this signals the emergence of a new stress scenario: whereas winter periods with high demand and low wind were previously considered the main threat, extended summer heatwaves can now cause comparable shortages of available capacity.
