Why Europe's Heatwave Is Shutting Down Nuclear Reactors

Europe is in the middle of its hottest summer on record, and the heat is starting to hit an unexpected part of the energy system: nuclear power.

According to the EU's Copernicus Climate Change Service, June and July 2026 were the hottest on record in Western Europe, with average temperatures almost 2.8°C above the long-term norm. This is already the fifth heatwave to hit the continent since May. Slovakia recorded a national record of 42.2°C in early August, and the UK saw its hottest day of the year, 38.1°C in west London, in the second week of the month. Sea temperatures in the western Mediterranean may also have hit an all-time high.

For most people, that means fans, air conditioning and warnings to stay hydrated. For nuclear power plants, it means something more specific: a growing struggle to keep cool.

Why heat is a problem for nuclear power

Nuclear reactors generate enormous amounts of heat, and most rely on nearby rivers to cool their systems, drawing in cool water and releasing warmer water back into the river afterwards. Environmental rules limit how hot that returned water can be, to protect fish and other river life. When a river is already low and warm from weeks of drought and heat, plants often have no choice but to reduce output, or shut down entirely, to stay within those limits.

This isn't a rare or theoretical risk. It's happening across several countries at once this summer.

France: repeated cuts to Europe's largest nuclear fleet

France generates around 70% of its electricity from nuclear power, more than any other country in Europe, which makes it especially exposed. This is already the fifth heatwave to hit France since June, and EDF, the state utility, has been cutting reactor output repeatedly as a result.

On Friday 14 August, EDF expected to lose around 15% of its nuclear fleet's capacity, roughly 9.4 gigawatts across nine reactors, with six reactors fully offline. Combined with scheduled maintenance, that pushed nuclear availability down to its lowest level since August 2023. Reactors at Chooz and Cattenom have also been shut or reduced specifically because of low water levels on the Meuse and Moselle rivers.

The knock-on effects have been felt well beyond France. French day-ahead power prices have jumped by more than 20% on some days this month, and price contracts in Germany and Italy have hit their highest levels in three and a half years, a reminder of how closely connected Europe's power markets are.

Hungary: a nuclear plant pushed to the edge

The more dramatic story has played out in Hungary, where record-low water levels on the Danube pushed the country's only nuclear plant, Paks, to the brink of a full shutdown for the first time in its 44-year history.

Paks normally supplies close to half of Hungary's electricity. In late July, one reactor was shut down entirely, and at one point the whole plant was running at only around 10% of its capacity on a single turbine. Prime Minister Péter Magyar said running at that reduced level was costing the Hungarian economy roughly $158 million a month.

Rather than simply wait for rain, the government intervened directly, sinking large barges into the Danube near the plant to raise the water level enough to keep the cooling system running. As river levels have slowly recovered, Paks has been restarting gradually, though officials say it could take more than a month to return to full output.

Not just France and Hungary

The same pattern has shown up elsewhere along the Danube. Romania's Cernavodă plant, which also depends on the river for cooling, shut down entirely last month, prompting the government to declare a nationwide energy emergency. Slovakia's nuclear plant has been running at around 80% of capacity for much of August for the same reason.

A growing collision between climate and cooling

None of this means nuclear power is becoming unsafe. Plants are shutting down or cutting output precisely because operators are following environmental and safety rules designed to protect rivers and equipment, not despite them.

But it does point to a challenge that is likely to recur. Nuclear power has long been valued as a steady, low-carbon source of electricity, largely unaffected by the weather in the way that wind and solar can be. This summer has shown that isn't entirely true. As heatwaves become more frequent and rivers run lower for longer, water availability may increasingly shape how much power nuclear plants can actually deliver, right at the moments demand for electricity is at its highest.