Danube Drought Halts Nuclear Reactors

By late July, the Danube had stopped behaving like the river Romania's power system was built around. Near the Cernavoda nuclear plant, it was 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 gauge had fallen to -118 cm, a local record (Telex).

That matters because nuclear power is only as steady as its cooling system. A reactor converts part of its heat into electricity. The rest has to be taken away safely, usually by river water. If the river is too low or too warm, the plant cannot dispose of that heat properly, so operators have to pull the reactor back or shut it down.

That is what happened this week at both ends of the Danube basin. Romania shut both Cernavoda reactors, taking about 1,300 MW off the grid. President Ilie Bolojan said that was close to a fifth of the country's electricity consumption (HotNews). In Hungary, state-owned MVM began shutting Paks Unit 3 on 29 July; the plant normally supplies close to half of Hungary's power (TVP World, HVG).

Neither shutdown was an accident. Both were defensive. Romania's Energy Ministry warned that damage to a single cooling pump could leave the plant facing repairs lasting up to a year (Mediafax). A fortnight without output is painful. A year without a critical pump would be far worse.

Cheap power drops out, expensive power fills in

Nuclear power is expensive to build but cheap to run. Once the plant is there, each extra unit of electricity costs little to produce. When that supply disappears, the gap is filled by gas plants, imports, or other flexible generators that can move quickly.

Those sources cost more. In Europe's wholesale electricity markets, the price is usually set by the most expensive plant needed to meet demand. So when cheap nuclear drops out and gas or imports set the marginal price, everyone selling into the market benefits from the higher clearing price.

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

Romania also moved from being a net electricity exporter to a net importer within hours. The pinch comes between 20:00 and 23:00, when household demand is high and solar generation has faded. During those evening hours, Romania was importing about 1,700 MW, mainly from Bulgaria (Digi24, Profit.ro).

That fallback is not as secure as it looks. Bulgarian Danube gauges at Ruse are already showing -85 cm, below previous historic lows (ClubZ). Romania's emergency supply is coming from a neighbour facing the same river problem.

Who pays, who profits

Romanian households are partly insulated for now. Officials said domestic contracts have fixed prices through the year (Romania Insider). But fixed prices do not make costs disappear. They push them on to suppliers buying dearer replacement power, into next year's price resets, or on to the state budget.

Industrial users feel the wholesale spike more directly. Bolojan appealed publicly to citizens and businesses to reduce evening consumption. Both Romania and Hungary prepared contingency plans to cut large consumers' load while exempting hospitals (444.hu, Portfolio).

The winners are gas plant operators and power exporters able to sell into shortage hours. The losers are nuclear operators, who lose output and revenue, and large consumers exposed to the price jump.

Not a freak event

This is not a freak event. France's Cour des comptes, the national audit office, has documented climate-related 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. For Ireland, watching from outside the nuclear club, the lesson is still relevant. A low-carbon power system depends not only on the fuel burned, or not burned, but on the physical systems around it: rivers, grids, interconnectors and weather.

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 wider problem will last beyond this heatwave. Europe is building a cleaner power system, but some of its most reliable low-carbon plants still need cold, plentiful river water. Climate change is making that a less dependable assumption.