Danube drought hits nuclear power

The Danube gauge at Ruse, Bulgaria, is now at -87 centimetres (BTA). The flow entering Romania has fallen to roughly 1,590 cubic metres per second, about a third of its long-term July average (Digi24). That loss of water has taken nuclear capacity offline in Romania and cut it sharply in Hungary. Two countries that usually sell electricity into the region are now buying it during the most expensive hours of the day.

How a river shuts a reactor

Nuclear plants are meant to provide baseload power: the steady generation that keeps a grid anchored day and night. The catch is heat. Most of the energy released by fission does not become electricity; it becomes waste heat, and in river-cooled plants the river is part of the machinery that removes it.

When the river falls too far, intake pumps start meeting physical limits. When the river is already too warm, the plant can also hit legal limits on how much hotter it is allowed to make the water downstream. The reactor may be nuclear, but the weak point is hydraulic.

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

The emergency measures show how tight the margin has become. Authorities plan to blast granite and sink barges loaded with stone to push more water towards the intake, steps that may buy another three to five days (HotNews, Antena 3).

Hungary is already deeper into the same problem. MVM, the operator of the Paks nuclear plant, reported staged cuts during the week: Unit 3 stopped, Unit 2 reduced by half. 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 low-cost nuclear drops out, the gap is filled by gas plants running at peak prices or by imported electricity that is scarce for everyone else as well. Day-ahead prices, the price paid for electricity delivered the following day, rose to €132.87/MWh in Romania and €122.08/MWh in Hungary after the shutdowns, according to Serbia Energy.

Romania's evening power balance shows the pressure clearly: demand of about 7,300 MW, domestic generation of only 4,000 to 4,300 MW, and imports covering 1,700 MW of the shortfall (Profit.ro).

Market coupling, the EU system that allocates cross-border electricity through coordinated auctions, keeps the lights on by moving power to where it is most needed. It also spreads scarcity. A Romanian nuclear outage does not stay Romanian for long; it pushes up prices in neighbouring markets too.

The first to feel the cost are suppliers and industrial buyers exposed to market rates. Then come state budgets, if governments decide to absorb the difference. Households on fixed retail tariffs are protected for now (Romania Insider). Nuclearelectrica has already issued a force majeure notice, a legal warning that it may be unable to honour contracts because the conditions are outside its control, to its customers (ZF).

For Malta, this is not a distant Balkan energy story. A small, import-exposed economy knows what happens when reserve margins narrow and wholesale prices move faster than retail politics. Interconnectors, market coupling and fuel costs are the mechanisms through which weather elsewhere becomes a domestic bill.

Not just a Danube problem

France has already shown that this is not a southeastern European 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 electricity exporter into a net importer at -16.7 TWh. When nuclear generation 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 Danube itself, Bulgaria's Kozloduy plant is still generating, but it is already monitoring 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 already 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). The unresolved question for governments is how much reserve capacity they are prepared to pay for before the next dry summer, and who will carry that cost when the bill arrives.