The 730 MW Krško nuclear power plant has reduced generation as high river temperatures and low flows on the Sava constrain its cooling capability, extending the impact of southeastern Europe’s summer hydrological stress into the Slovenian and Croatian electricity systems.
The plant initially reduced reactor output toward approximately 80% of capacity, with the operator warning that additional reductions could be required if river conditions deteriorate further.
Krško’s importance reaches well beyond Slovenia. The plant supplies approximately 20% of Slovenian electricity demand and around 16% of Croatian demand, reflecting the shared ownership and long-standing cross-border role of the nuclear station.
Any sustained reduction therefore affects two national electricity balances simultaneously.
Environmental restrictions are central to the operating decision. The daily average temperature of the Sava after cooling-water mixing is required to remain below 28°C, while separate limits also apply to the permitted increase in river temperature caused by the plant.
During periods of high ambient temperature and low river flow, the amount of heat the river can absorb declines, forcing the plant to reduce generation even when the reactor itself remains technically available.
The constraint is increasingly important because it demonstrates that nuclear generation, although largely insulated from fuel-price and renewable intermittency risk, is not completely independent of weather.
Plants using rivers for cooling can face output restrictions during severe heat and drought conditions, particularly when environmental regulations tightly limit thermal discharge.
The situation at Krško mirrors, in a different technical form, developments at Romania’s Cernavoda nuclear station. Both 680 MW Cernavoda units have been unavailable because exceptionally low Danube flows reduced cooling-water availability.
Romania has responded by restarting nearly 300 MW of coal-fired capacity at Rovinari.
Krško has not faced the same complete shutdown, but its reduction toward 80% removes a meaningful block of low-variable-cost generation from the Slovenian-Croatian market.
At full 730 MW capacity, an output reduction toward 80% implies that roughly one fifth of the plant’s potential generation is temporarily unavailable. The exact lost energy depends on the duration and operating profile of the restriction.
The impact is particularly relevant during hot weather because high temperatures can simultaneously increase air-conditioning demand while reducing cooling capability at thermal and nuclear plants.
This creates a difficult power-market combination: consumption strengthens at precisely the point when some large conventional generators become less able to operate at full capacity.
Solar generation can offset part of the daytime demand increase, but the evening remains more challenging once photovoltaic output falls. Slovenia and Croatia have recently exhibited day-ahead price profiles with sharp evening ramps, underlining the value of dispatchable generation during those hours.
A reduced Krško contribution can therefore increase reliance on imports, hydroelectric production and other thermal generation.
The financial implications depend on market prices and the duration of the derating. Nuclear plants typically have high fixed costs but comparatively low marginal generation costs, meaning electricity not produced during high-price periods can represent economically valuable lost output.
No estimate of lost revenue or the duration of the current restriction has been provided in the information reviewed.
The wider system issue is more structural. Climate conditions capable of simultaneously lowering river flows and increasing water temperatures challenge operating assumptions developed around historical hydrological patterns.
That does not make nuclear generation unreliable, but it increases the importance of cooling-system design, environmental limits and contingency planning in long-term generation adequacy.
Krško remains one of the most important electricity assets shared between two southeastern European countries. Its 730 MW capacity anchors both the Slovenian and Croatian systems, while stable nuclear generation normally helps reduce exposure to volatile wholesale imports.
The current reduction shows that even this low-carbon baseload supply can become weather-sensitive during extreme summer conditions.
Across the Balkans, the same drought episode is now influencing hydro production, nuclear cooling, thermal dispatch, electricity imports and petroleum transport. Krško’s reduced output is another reminder that regional energy security increasingly depends not only on installed capacity but on the physical conditions that allow that capacity to operate.
