Danube Drought Hits Nuclear Supply

The Danube is doing what rivers do in a hot, dry summer: falling. At Ruse in Bulgaria, the gauge now reads -87 centimetres (BTA). By the time the river enters Romania, the flow is down to roughly 1,590 cubic metres per second, about a third of its long-term July average (Digi24).

That has become an energy problem. Romania and Hungary, both normally exporters of electricity, have been pushed into importing power during the most expensive hours of the day after drought knocked nuclear capacity offline in Romania and half-offline in Hungary.

How a river shuts a reactor

Nuclear power is usually treated as the dependable part of the system: baseload generation, running day and night, holding the grid steady. But a reactor is also a heat machine. Most of the energy released by fission becomes waste heat, and in river-cooled plants the river is the way that heat is carried away.

When the river falls too low, the intake pumps run up against physical limits. When the water is too warm, the plant can breach environmental rules on how much hotter it is allowed to make the river downstream. The constraint is not ideological or market-driven. It is hydraulic.

Romania's state nuclear operator, Nuclearelectrica, shut Cernavoda Unit 1 on 28 July after flows fell to about 1,630 m³/s (BVB/Nuclearelectrica). Unit 2 is still running, but close to its limit. Sorin Rindasu, deputy director of Apele Romane, Romania's national water authority, said on 1 August it could hold for another four to five days (Agerpres).

Rain upstream would not solve the problem quickly. Even if it fell in Austria tomorrow, the pulse would take roughly two weeks to reach Cernavoda (Business Magazin).

The emergency work gives the measure of the pressure. Authorities plan to blast granite and sink barges loaded with stone to push more flow towards the intake, measures that might buy another three to five days (HotNews, Antena 3).

Hungary is further into the squeeze. MVM, operator of the Paks nuclear 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 that a full shutdown could follow within 72 hours (MVM, kormany.hu).

Who pays for the missing megawatts

When cheap nuclear drops out, the system does not get to wait for better weather. The gap is filled by gas plants running at peak cost, or by electricity brought across borders from neighbours with power to spare.

Day-ahead prices, the prices paid 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 clearly: demand at about 7,300 MW, domestic generation at just 4,000-4,300 MW, and imports covering 1,700 MW of the gap (Profit.ro).

Market coupling, the EU system that allocates cross-border electricity through coordinated auctions, keeps the lights on. It also spreads scarcity. When Romania or Hungary needs more power, the price pressure does not stop at the border.

The first bill lands with suppliers and industrial buyers exposed to market-rate contracts. After that, it moves towards state budgets where governments absorb the difference. Households on fixed retail tariffs are protected for now (Romania Insider). Nuclearelectrica has already issued a force majeure notice to customers, a legal warning that it may not be able to fulfil contracts because conditions are outside its control (ZF).

Not just a Danube problem

France has already shown that this is not a Balkan oddity. During the 2022 heatwave, the French 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). France's Cour des comptes, the public audit body, found that 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 it is already watching low-water thresholds (Kozloduy, BTA).

The pattern is now visible: shutdown at Cernavoda, deep cuts at Paks, monitoring at Kozloduy, regulatory waivers in France. Drought has turned firm nuclear power into conditional power in parts of Europe. The Danube flow is forecast to fall towards 1,450 m³/s by 7 August (Agerpres).

For governments, the unresolved question is not whether dry summers can take nuclear capacity out of the system. They can. It is how much reserve capacity they are willing to pay for before the next one arrives.