Slovenia’s Krško nuclear power plant will reduce reactor output to 80% of capacity as prolonged drought and high temperatures limit the plant’s ability to discharge heat into the Sava River. The change is being implemented as a precautionary environmental measure rather than a response to a reactor fault. Additional reductions could follow if river flow and temperature conditions continue to worsen.
Environmental permit limits on Sava temperatures
Krško’s environmental permit requires the average daily temperature of the Sava downstream of the Brežice hydropower plant to stay below 28°C. The plant must also ensure that its operation does not raise the river’s average daily temperature by more than 3°C. These requirements are tied to how much heat the river can absorb under current hydrological conditions.
The plant draws on the Sava in its tertiary cooling system, which removes residual heat from condensers before water is returned to the environment. Cooling towers and other systems can reduce thermal discharge, but their effectiveness is more constrained when ambient temperatures are high and river flows are low. As those conditions persist, available headroom under the permit narrows.
Cooling approach during earlier heatwave period
Krško completed its scheduled overhaul in November 2025 and then operated continuously for 280 days. During the earlier part of the heatwave, the reactor maintained full output while maximising use of its cooling towers. That operating approach helped manage thermal discharge while meeting environmental constraints.
With continuing drought now reducing the margin under the permit, the operator plans to lower generation gradually. It will monitor downstream river temperatures after each adjustment before deciding whether a deeper reduction is needed. The sequence of changes is intended to align output with evolving temperature and flow conditions.
Past restrictions and market implications
The most recent extended output restriction linked to river temperature and flow occurred in 2003, when Krško operated below full capacity for more than three months. The current reduction follows a similar constraint-driven mechanism based on thermal discharge limits rather than equipment availability. Full output is expected to return once hydrological and temperature conditions restore sufficient environmental headroom.
Until then, electricity production from Krško will be determined by the Sava’s ability to absorb waste heat rather than by technical availability of the reactor. The restriction is expected to tighten the regional electricity balance because nuclear generation typically provides stable, low-marginal-cost baseload output. Slovenia may need additional imports or higher thermal generation during the period, particularly after sunset when solar production declines.
