Slovenia’s Krško nuclear power plant will reduce reactor output to 80% of capacity due to prolonged drought and high temperatures affecting heat discharge into the Sava River. The change is intended as a precautionary environmental measure rather than a response to a reactor fault. Additional reductions are possible if river flow and temperature conditions continue to worsen.
Environmental permit limits for Sava river 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 its operation does not raise the river’s average daily temperature by more than 3°C. These limits shape how much heat can be released under current conditions.
The plant draws on 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 is more limited when ambient temperatures are high and river flows are low. As those constraints tighten, available operating headroom under the permit declines.
Cooling system performance and prior heatwave operation
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 by maximizing use of its cooling towers. That approach supported stable generation while environmental conditions allowed sufficient thermal discharge.
With continuing drought, the margin available under the environmental permit has narrowed. The operator plans to lower generation gradually while monitoring downstream river temperatures before deciding whether a deeper reduction is required. The timing and extent of any further cuts depend on ongoing changes in river conditions.
Previous restrictions and market implications during reduced output
The last 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 by the ability to manage waste heat within permitted temperature thresholds.
The reduction is expected to tighten the regional electricity balance because nuclear generation typically provides stable, low-marginal-cost baseload power. Slovenia may need additional imports or higher thermal generation during the restriction, particularly after sunset when solar production falls. Full output is planned to resume once hydrological and temperature conditions restore sufficient environmental headroom.
Until then, the Sava’s capacity to absorb waste heat—not technical availability of the reactor—will determine how much electricity Krško can produce.
