Record-low Danube water levels are transforming drought from a predominantly hydropower issue into a broader electricity-system risk across Southeast Europe, after exceptionally low river flows forced nuclear generation reductions in both Romania and Bulgaria.
Romania’s state-controlled Nuclearelectrica shut down Cernavodă Unit 1 on July 28 because of critically low Danube levels and began the controlled shutdown of Unit 2 on August 13 as hydrological conditions deteriorated further. Each reactor has approximately 700 MW of capacity, with the two units normally accounting for around one-fifth of Romania’s electricity production.
By mid-August, Danube flows entering Romania had fallen to approximately 1,350-1,400 cubic metres per second, compared with a multi-year August average of around 3,900 cubic metres per second. The scale of the decline highlighted the severity of the drought and its growing impact on the region’s electricity infrastructure.
Bulgaria subsequently reduced output from Kozloduy Unit 5 by around 120 MW on August 21 because of what the plant described as an unprecedented and critical decline in Danube water levels. It marked the first hydrology-related reduction in reactor output during Kozloduy’s 52-year operating history.
The developments are significantly broadening the market consequences of drought. Low rainfall has traditionally affected Southeast European power markets primarily through reduced hydropower generation, particularly across the Balkans, where Albania, Montenegro, Bosnia and Herzegovina, Serbia, Croatia and Romania all have substantial exposure to hydroelectric production.
The summer of 2026 has demonstrated a second and potentially more significant channel. Large nuclear and thermal power plants located along major rivers also require sufficient water for cooling and other essential plant processes. An extreme drought can therefore reduce output from facilities that are normally considered firm baseload capacity.
This makes hydrology a regional electricity-market risk rather than an issue confined to hydropower producers.
The exposure is particularly important along the Danube, where major electricity systems stretching from Hungary through Serbia, Romania and Bulgaria are closely interconnected both physically and commercially. A reduction in Romanian nuclear generation does not remain isolated within Romania. It can increase the country’s need for imports, tighten supply balances in neighbouring markets and alter electricity flows across Serbia and the wider Balkan region.
The impact became visible at the beginning of September. Romania required approximately 1.03 GW of net imports during the September 1 trading session while nuclear generation remained at zero. Romanian day-ahead electricity prices settled at €177.28/MWh, close to levels in Hungary, Bulgaria and Greece, while Serbian SEEPEX prices increased by around €34/MWh in a single session.
The market impact becomes even greater when low river levels coincide with weak wind generation, elevated cooling demand or reduced availability from thermal plants. Under those conditions, system operators may have to rely more heavily on electricity imports, reservoir hydropower, lignite or gas generation precisely when neighbouring markets may be facing similar weather-related constraints.
The situation also complicates the traditional definition of firm low-carbon generation. Nuclear power remains substantially less dependent on weather conditions than wind or solar over normal operating periods. However, this summer has demonstrated that older river-cooled nuclear facilities can still face significant exposure to extreme hydrological conditions.
For Southeast Europe, drought risk therefore increasingly needs to be assessed across the entire electricity generation stack rather than through hydropower reservoirs alone.
A severely depleted Danube can simultaneously reduce hydropower output, constrain river transport and fuel logistics, and limit the availability of nuclear or thermal generation. The result is a potentially powerful combination of supply pressures that can spread well beyond the areas directly affected by the drought.
Water availability is therefore becoming an increasingly important electricity-market variable, with consequences extending from Romania and Bulgaria to Serbia, Hungary and the wider Western Balkans.
