Slovenia’s Krško nuclear power plant will reduce reactor output to 80% of capacity as prolonged drought and high temperatures restrict the plant’s ability to discharge heat into the Sava River.
The reduction is a precautionary environmental measure rather than a response to a reactor fault. Further cuts remain possible if river flow and temperature conditions continue to deteriorate.
Krško’s environmental permit requires the average daily temperature of the Sava downstream of the Brežice hydropower plant to remain below 28°C. The nuclear plant must also ensure that its operation does not increase the river’s average daily temperature by more than 3°C.
The plant uses the Sava in its tertiary cooling system, which removes residual heat from the condensers before water is returned to the environment. Cooling towers and other systems can reduce thermal discharge, but their effectiveness becomes more limited when ambient temperatures are high and river flows are low.
Krško had operated continuously for 280 days following completion of its scheduled overhaul in November 2025. The reactor maintained full output during the earlier part of the heatwave by maximising the use of its cooling towers.
Continuing drought has now reduced the margin available under the environmental permit. The operator will gradually lower generation and monitor the effect on downstream river temperatures before determining whether a deeper reduction is required.
The last extended output restriction caused by river temperature and flow conditions occurred in 2003, when Krško operated below full capacity for more than three months.
The current reduction will tighten the regional electricity balance because nuclear generation normally provides stable, low-marginal-cost baseload output. Slovenia may need additional imports or greater thermal generation during the restriction, particularly after sunset when solar production falls.
Full output will be restored once hydrological and temperature conditions provide sufficient environmental headroom. Until then, the Sava’s capacity to absorb waste heat—not the technical availability of the reactor—will determine how much electricity Krško can produce.
