Record low Danube shuts Cernavoda reactors

The Danube near Romania's Cernavoda nuclear plant is carrying about 1,630 cubic metres of water per second, roughly a third of its normal July flow (Agerpres, Știrile ProTV). Three hundred kilometres upstream, at Hungary's Paks plant, the river gauge has dropped to -118 cm, a local record (Telex).

Nuclear plants need rivers for cooling. A reactor turns only part of its heat into electricity. The rest has to be carried away by water safely. When the river runs too low or too warm, the plant cannot shed that heat, and operators have to shut down.

That is what happened this week at both ends of the Danube basin. Romania shut both Cernavoda reactors, removing about 1,300 MW from the grid. President Ilie Bolojan described that as close to a fifth of the country's electricity consumption (HotNews). Hungary's state-owned MVM began shutting Paks Unit 3 on 29 July; the plant normally produces close to half of Hungary's power (TVP World, HVG). Neither shutdown was an accident. Both were preventive. Romania's Energy Ministry warned that damage to a single cooling pump could mean repairs lasting up to a year (Mediafax). A two-week outage beats that.

Cheap power drops out, expensive power fills in

Nuclear electricity costs very little to produce once the plant is built. When it disappears from the grid, the replacement comes from gas plants, cross-border imports, or whatever flexible source can start up fast. All of those cost more. And because wholesale electricity markets pay every seller the price set by the most expensive producer needed to meet demand, the whole market price rises.

After Cernavoda went offline, Romania's day-ahead wholesale price jumped €22.1 to €132.87 per megawatt-hour. Hungary's equivalent rose €26.50 to €122.08/MWh (Serbia Energy, Serbia Energy).

Romania also flipped from a net electricity exporter to a net importer within hours. The stress is worst between 20:00 and 23:00, when household demand peaks and solar output vanishes. During those evening hours, Romania was importing about 1,700 MW, with Bulgaria as the main supplier (Digi24, Profit.ro). That dependency is itself fragile: Bulgarian Danube gauges at Ruse already show -85 cm, below previous historic lows (ClubZ).

Who pays, who profits

Romanian households are partly shielded for now. Officials said domestic contracts carry fixed prices through the year (Romania Insider). But fixed prices don't erase costs. They shift them to suppliers who must buy expensive replacement power, to next year's price resets, or to the state budget. Industrial users feel the wholesale spike directly. Bolojan publicly appealed to both citizens and industry to cut evening consumption, and both governments prepared contingency plans to reduce large consumers' load while exempting hospitals (444.hu, Portfolio).

The winners are gas plant operators and power exporters selling electricity at the higher price set by shortage hours. The nuclear operators lose output and revenue.

Not a freak event

This is not a one-off. France's Cour des comptes (national audit office) has documented climate-driven nuclear output losses as a recurring problem: French plants lost at least 1 TWh of output in each of 2018, 2019, and 2020 because rivers were too warm or too low (Argus). The Danube basin is now showing the same pattern, on a river that cools reactors in four countries.

Romania's grid operator estimates Cernavoda Unit 1 could restart around 4 August and Unit 2 around 10 August (Profit.ro). Those dates depend on rainfall that forecasters have not promised. The broader problem outlasts any single heatwave. Europe is building a cleaner power system, but its most reliable low-carbon plants still depend on cold, abundant river water, a resource that climate change is steadily reducing.