Record-low Danube levels are turning drought from a predominantly hydropower problem into a broader electricity-system risk across Southeast Europe, after falling river levels forced nuclear output reductions in Romania and Bulgaria.
Romania’s state-controlled Nuclearelectrica shut Cernavodă Unit 1 on July 28 because of exceptionally low Danube levels and began the controlled shutdown of Unit 2 on Aug. 13 after conditions deteriorated further.
Each unit has roughly 700 MW of capacity and together the reactors normally account for around a fifth of Romanian electricity production.
The Danube flow entering Romania had fallen to around 1,350-1,400 cubic metres per second in mid-August, compared with a multi-year August average of about 3,900 cubic metres per second.
Bulgaria then reduced output from Kozloduy Unit 5 by around 120 MW on Aug. 21 because of what the plant described as an unprecedented and critical decline in Danube levels.
It was the first hydrology-related reduction of reactor output in Kozloduy’s 52-year operating history.
The developments broaden the market consequences of drought.
Low rainfall has traditionally affected Southeast European electricity markets mainly by reducing hydropower production, particularly in the Balkans where Albania, Montenegro, Bosnia, Serbia, Croatia and Romania all have significant hydro exposure.
The summer of 2026 has demonstrated a second channel.
Large thermal and nuclear plants located along major rivers also depend on sufficient water availability for cooling and associated plant processes. A sufficiently severe drought can therefore reduce output from plants normally regarded as firm baseload capacity.
That makes hydrology a regional rather than purely hydroelectric risk.
The effect is particularly important along the Danube because major electricity systems from Hungary through Serbia, Romania and Bulgaria are connected both physically and commercially.
A reduction in Romanian nuclear generation does not stop at Romania’s border. It can increase Romanian imports, tighten Hungarian and Bulgarian balances and alter power flows across Serbia and the wider Balkan market.
That was visible at the start of September.
Romania required roughly 1.03 GW of net imports during the Sept. 1 trading session while nuclear generation remained at zero. Romanian day-ahead electricity settled at €177.28/MWh, close to Hungary, Bulgaria and Greece, while Serbian SEEPEX jumped €34/MWh in a single session.
The market impact is amplified when low river levels coincide with weak wind, high cooling demand or reduced thermal availability.
That combination forces system operators to draw more heavily on imports, reservoir hydro, lignite or gas generation precisely when neighbouring systems may be experiencing the same weather.
The issue also complicates the concept of firm low-carbon generation.
Nuclear remains considerably less weather-dependent than wind or solar on an annual basis. But this summer has shown that older river-cooled plants can still be exposed to extreme hydrological conditions.
For Southeast Europe, drought risk therefore increasingly has to be analysed across the entire generation stack rather than only hydroelectric reservoirs.
A dry Danube can remove hydro output, constrain navigation and fuel logistics, and reduce nuclear or thermal capacity at the same time.
That makes water availability an electricity-market variable with consequences from Romania and Bulgaria to Serbia, Hungary and the western Balkans.
