The 730 MW Krško nuclear power plant has lowered generation because high river temperatures and low flows on the Sava limit available cooling. The reduction extends the effect of summer hydrological stress into the Slovenian and Croatian electricity systems.
Initially, the operator reduced reactor output toward approximately 80% of capacity. The operator also warned that further reductions could be needed if river conditions deteriorate.
Krško’s cross-border role in Slovenia and Croatia
Krško’s operational impact extends beyond Slovenia due to its shared ownership and cross-border function. The plant supplies about 20% of Slovenian electricity demand and around 16% of Croatian demand.
Because of that shared role, a sustained generation reduction affects two national electricity balances at the same time. At full output, a move toward 80% implies roughly one fifth of the plant’s potential generation is temporarily unavailable.
The amount of energy not produced depends on how long the derating lasts and how the plant operates during the restriction. No estimate of lost revenue or the duration of the current limitation was provided in the reviewed information.
Sava temperature limits drive operational constraints
Environmental restrictions are central to the operating decision for Krško. The daily average temperature of the Sava after cooling-water mixing must stay below 28°C, with additional limits on the permitted increase in river temperature caused by plant operations.
When ambient temperatures are high and river flow is low, the river’s ability to absorb heat declines. That can force generation cuts even when the reactor remains technically available.
The constraint reflects how cooling requirements can become more restrictive during heat and drought conditions under thermal discharge regulations. Plants relying on rivers for cooling can therefore face output limits during severe summer weather.
Regional comparison with Cernavoda and market implications
The situation at Krško parallels, in a different technical form, developments at Romania’s Cernavoda nuclear station. Both 680 MW Cernavoda units have been unavailable due to exceptionally low Danube flows reducing cooling-water availability.
Romania has responded by restarting nearly 300 MW of coal-fired capacity at Rovinari. Krško has not faced a complete shutdown, but operating at about 80% removes a meaningful block of low-variable-cost generation from the Slovenian-Croatian market.
The timing matters during hot weather when air-conditioning demand rises while cooling capability for thermal and nuclear plants can fall. Solar output can offset part of daytime demand increases, while evening conditions remain more challenging as photovoltaic generation drops and day-ahead prices show sharp evening ramps in Slovenia and Croatia.
A reduced Krško contribution can increase reliance on imports, hydroelectric production, and other thermal generation. Financial outcomes depend on market prices and how long derating continues, with nuclear plants having comparatively low marginal costs but high fixed costs.
Cooling limits as a longer-term system issue
The wider issue is linked to hydrological conditions that can simultaneously lower river flows and raise water temperatures. Such conditions challenge operating assumptions based on historical patterns .
This does not mean nuclear generation is unreliable, but it increases emphasis on cooling-system design, environmental limits, and contingency planning for long-term adequacy. Krško remains one of the most important shared electricity assets between two southeastern European countries, with its 730 MW capacity anchoring both systems.
Across the Balkans, drought conditions are affecting hydro production, nuclear cooling, thermal dispatch, electricity imports, and petroleum transport . Krško’s reduced output adds to evidence that regional energy security depends not only on installed capacity but also on physical conditions enabling plants to operate.
