Hungary Lets Paks Run Hotter

By Monday morning, the Danube had already crossed the line Hungary itself had drawn. Five hundred metres downstream from the Paks nuclear plant, the river was measured at 30.22°C (HVG). The legal ceiling is 30°C, set to protect fish and oxygen levels in the water. Under the ordinary rules, Paks would have had to cut 640 megawatts of electricity output.

That is not what happened. Hungary's energy minister, Kapitány István, granted a two-day exemption and allowed the plant to raise the maximum discharge temperature to 31.5°C. The planned cut fell to just 40 MW (Portfolio, Telex). The reason given was grid security, on the advice of MAVIR, Hungary's transmission system operator. It was a local decision, taken beside one stretch of one river, but it exposed a wider European problem: the power system still depends on cool water, just as heat is making that water harder to find.

Why Hot Rivers Mean Expensive Electricity

Nuclear and large thermal plants work by turning water into steam, using that steam to drive turbines, and then getting rid of the remaining heat. For many plants, rivers do that job. Water is drawn in, passed through cooling systems, and returned warmer than before.

The environmental rule is simple enough. Warmer water holds less dissolved oxygen, and fish suffer first. The EU Water Framework Directive sets the broad obligation to protect rivers and other water bodies; the detail is then written into each plant's permit as a temperature limit (eur-lex).

When a heatwave pushes a river beyond that limit, the plant has to reduce output. The awkward timing is the point. Supply falls at the same moment air-conditioning demand is rising. In Europe's wholesale electricity market, the price is set by the last and most expensive plant needed to meet demand. If cheap nuclear power drops out and gas has to fill the gap, the price rises for everyone on the system.

France has already shown how quickly that travels across borders. 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). Neighbouring countries that rely on those exports were squeezed. In Belgium, quarter-hourly wholesale prices rose above €1,000/MWh during evening hours, after solar output faded but cooling demand remained high (Euronews).

Heat Hits Factories Too

The same constraint reaches beyond nuclear reactors. Power plants near Mainz-Wiesbaden use about 80,000 cubic metres of Rhine water an hour for cooling and face partial shutdowns if the river approaches 28°C. The Budenheim chemical plant could also have to scale back energy-intensive operations at similar temperatures (Tagesschau).

Rheinland-Pfalz has already issued its first river-heat warning. Higher alert levels were reached in 2018, 2019, 2022 and 2025 (Rhein-Ahr-Anzeiger). What used to look like an exceptional summer stress is becoming part of the operating environment for European industry.

Who Pays for the Choice

Hungary's exemption kept roughly 600 MW of cheap nuclear power on the grid. Consumers were spared at least some of the price pressure that would have followed a larger cut. The Danube carried 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 judged unacceptable (444.hu). The change in position was not explained.

France has the same legal route available. During the 2022 heatwave, four French plants operated under environmental exemptions for a combined 24 days (Le Monde). In 2026, EDF curtailed output and had not requested a new exemption at the time of reporting. The difference was not the law. It was the political choice: France took the supply hit; Hungary shifted more of the burden onto the river.

As heatwaves become more frequent, these clashes will come around again. River-temperature limits will meet electricity demand created by the heat itself. Every exemption granted without clear ecological evidence makes the next one easier to sign.