Hungary waives Paks heat limit at 30°C

The Danube measured 30.22°C on Monday morning, 500 metres downstream from Hungary's Paks nuclear plant (HVG). That broke through the 30°C ceiling designed to protect the river's fish and oxygen levels. Under normal rules, Paks would have cut 640 megawatts of output.

Hungary's energy minister chose differently. Kapitány István granted a two-day exemption, lifting the maximum discharge temperature to 31.5°C. The planned output cut shrank to just 40 MW (Portfolio, Telex). The justification: grid security, on the recommendation of MAVIR, Hungary's transmission system operator. One plant, one river, one ministerial decision. But the same collision between hot water and electricity supply is playing out across the continent.

Why Hot Rivers Mean Expensive Electricity

Nuclear and large thermal plants generate power by boiling water into steam. After the steam drives the turbines, the plant still needs to dump vast amounts of leftover heat. Most do this by cycling river water through their cooling systems and returning it warmer. Environmental permits cap how much extra heat a river can absorb, because warm water holds less dissolved oxygen and kills fish. The EU Water Framework Directive sets the overarching requirement to protect water bodies; each plant's permit translates that into a specific temperature ceiling (eur-lex).

When a heatwave pushes rivers past those ceilings, plants must cut output. Supply drops exactly when air-conditioning demand is surging. In Europe's wholesale electricity market, the price is set by the last, most expensive plant needed to meet demand. When cheap, steady nuclear power disappears, gas plants often fill that role, and the price jumps for everyone connected to the grid.

France showed how fast the damage 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 that count on those imports got squeezed. Quarter-hourly wholesale prices spiked above €1,000/MWh in Belgium during evening hours, when solar output faded but cooling demand stayed high (Euronews).

Heat Hits Factories Too

The constraint goes beyond reactors. Power plants near Mainz-Wiesbaden draw about 80,000 cubic metres of Rhine water per hour for cooling and face partial shutdowns if the river nears 28°C. The Budenheim chemical plant could also have to scale back energy-intensive operations at similar temperatures (Tagesschau). Rheinland-Pfalz has already triggered its first river-heat warning; the higher levels were reached in 2018, 2019, 2022 and 2025 (Rhein-Ahr-Anzeiger). This is no longer exceptional.

Who Pays for the Choice

Hungary's exemption kept about 600 MW of cheap nuclear power on the grid, limiting the price spike consumers would have faced. The Danube absorbed the cost. The government claims 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 called Paks's initial output cuts routine and manageable. By Monday, further cuts were deemed unacceptable (444.hu). The reversal went unexplained.

France faces the same legal option. During the 2022 heatwave, four French plants ran 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 is not the law but the choice: France absorbed the supply hit; Hungary absorbed the ecological risk.

As heatwaves grow more frequent, river-temperature limits will keep colliding with electricity demand driven by the heat itself. Each exemption granted without transparent ecological evidence makes the next one easier to justify.