Danube drought shuts Romanian nuclear reactor

The Danube gauge at Ruse, Bulgaria, reads -87 centimetres (BTA). Flow entering Romania has fallen to roughly 1,590 cubic metres per second, about a third of its long-term July average (Digi24). That drop has knocked nuclear capacity offline in Romania and half-offline in Hungary, turning two countries that normally export electricity into importers during the most expensive hours of the day.

How a river shuts a reactor

Nuclear plants provide baseload, the steady round-the-clock generation that anchors a power grid. But most of the energy from fission becomes waste heat, and that heat has to go somewhere. In river-cooled plants, the river carries it away. When flow drops too low, intake pumps hit physical limits. When the water runs too warm, plants breach legal caps on how much they can heat the river downstream.

Romania's state nuclear operator Nuclearelectrica shut Cernavoda Unit 1 on 28 July after flow fell to about 1,630 m³/s (BVB/Nuclearelectrica). Unit 2 is still running, but near its limit. Sorin Rindasu, deputy director of Apele Romane (Romania's national water authority), said on 1 August it can hold for four to five more days (Agerpres). Even if rain falls in Austria tomorrow, the pulse takes roughly two weeks to reach Cernavoda (Business Magazin).

The improvisation tells the story: authorities plan to blast granite and sink stone-loaded barges to redirect flow toward the intake, measures that might buy three to five extra days (HotNews, Antena 3).

Hungary is further along. MVM, operator of the Paks plant, reported staged cuts through the week: Unit 3 stopped, Unit 2 halved. By 30 July Paks was producing 960 MW instead of roughly 2,000 MW, and MVM warned a complete shutdown could follow within 72 hours (MVM, kormany.hu).

Who pays for the missing megawatts

When cheap nuclear drops out, the replacement is gas plants running at peak cost or scarce cross-border supply. Day-ahead prices (what generators charge for next-day delivery) rose to €132.87/MWh in Romania and €122.08/MWh in Hungary after the shutdowns, according to Serbia Energy. Romania's evening balance shows the strain: demand around 7,300 MW, domestic generation just 4,000–4,300 MW, with imports covering 1,700 MW of the gap (Profit.ro).

Market coupling (the system that allocates cross-border electricity through coordinated auctions) keeps lights on but spreads scarcity into neighbouring prices. The cost lands first on suppliers and industrial buyers on market-rate contracts, then on state budgets absorbing the gap. Households on fixed retail tariffs are shielded for now (Romania Insider). Nuclearelectrica has already issued a force majeure notice (a legal warning that it may not fulfil contracts because conditions are outside its control) to its customers (ZF).

Not just a Danube problem

France proves this is not a regional oddity. During the 2022 heatwave, France's nuclear regulator ASN granted temporary environmental waivers to four river-cooled plants (ASN). Reduced nuclear output helped turn Europe's largest power exporter into a net importer at -16.7 TWh; when nuclear recovered in 2023, exports rebounded to +50.1 TWh (RTE 2022, RTE 2023). The country's Cour des comptes (public audit body) found climate conditions cut at least 1 TWh of nuclear output in each of 2018, 2019 and 2020 (Cour des comptes). On the same Danube, Bulgaria's Kozloduy plant is still generating but already monitoring low-water thresholds (Kozloduy, BTA).

These are points on a gradient: shutdown at Cernavoda, deep cuts at Paks, monitoring at Kozloduy, regulatory waivers in France. Drought has already turned firm nuclear power into conditional power in southeastern Europe. Flow is forecast to drop toward 1,450 m³/s by 7 August (Agerpres). What governments have not settled is how much reserve capacity they are willing to pay for before the next dry summer.