Hungary Lets Paks Run Hotter

The Danube measured 30.22°C on Monday morning, 500 metres downstream from Hungary's Paks nuclear plant (HVG). That was above the 30°C limit meant to protect fish and oxygen levels in the river. Under the normal permit, Paks should have cut 640 megawatts of generation.

Hungary's energy minister did not take that route. Kapitány István granted a two-day exemption and raised the permitted discharge temperature to 31.5°C. The cut fell to just 40 MW (Portfolio, Telex). The reason given was grid security, following a recommendation from MAVIR, Hungary's transmission system operator. One minister used one permit to keep one plant running. But the same problem, hot rivers meeting electricity demand, is now turning up across Europe.

Why Hot Rivers Mean Expensive Electricity

Nuclear and large thermal plants work by boiling water into steam. Once that steam has driven the turbines, the plant still has to get rid of enormous leftover heat. Many plants do this by taking river water through their cooling systems and returning it warmer.

That is why environmental permits matter. Warm water holds less dissolved oxygen, which can kill fish and damage river life. The EU Water Framework Directive sets the broad duty to protect water bodies; national permits then turn that duty into plant-by-plant temperature limits (eur-lex).

When a heatwave pushes rivers close to those limits, plants have to reduce output. That happens at the worst possible moment: demand is rising because homes, offices and shops are using more cooling. Malta does not have nuclear plants or major cooling rivers, but it buys electricity through the same European market logic. When supply tightens elsewhere, the price signal does not stay politely on the other side of the continent.

In Europe's wholesale electricity market, the clearing price is set by the last and most expensive plant needed to meet demand. If cheap, steady nuclear output drops out, gas plants often set the price instead. The result is higher prices for everyone connected to that market chain.

France showed how quickly this spreads. EDF cut nuclear output by 4.1 GW last week, about 7% of midday demand, as river temperatures hit permit limits at reactors including Golfech and Nogent-sur-Seine (Reuters, Le Figaro). French electricity exports fell from roughly 10–12 GW to around 3 GW (Le Monde). Neighbours relying on those imports were squeezed. Quarter-hourly wholesale prices in Belgium rose above €1,000/MWh during evening hours, after solar output fell but cooling demand remained high (Euronews).

Heat Hits Factories Too

The constraint is not limited to reactors. Power plants near Mainz-Wiesbaden draw about 80,000 cubic metres of Rhine water an hour for cooling and face partial shutdowns if the river nears 28°C. The Budenheim chemical plant could also have to reduce energy-intensive operations at similar temperatures (Tagesschau).

Rheinland-Pfalz has already issued its first river-heat warning. Higher warning levels were reached in 2018, 2019, 2022 and 2025 (Rhein-Ahr-Anzeiger). This is no longer an odd summer incident. It is becoming part of the operating conditions for European industry.

Who Pays for the Choice

Hungary's exemption kept about 600 MW of cheap nuclear power on the grid and limited the price shock consumers might otherwise have faced. The Danube took the risk instead.

The government says continuous monitoring showed no danger to aquatic life (HVG, Telex). But no independent dissolved-oxygen data or ecological assessment has been published. Days earlier, the same government described Paks's initial output cuts as routine and manageable. By Monday, further cuts were treated as unacceptable (444.hu). The shift was not explained.

France has the same legal escape route. During the 2022 heatwave, four French plants operated under environmental exemptions for a combined 24 days (Le Monde). In 2026, EDF cut output and had not requested a new exemption at the time of reporting. The law is not the difference. The choice is. France absorbed the supply hit; Hungary shifted the risk onto the river.

As heatwaves become more frequent, river-temperature limits will keep colliding with electricity demand created by the heat itself. Every exemption granted without transparent ecological evidence makes the next one easier to sign.