Slovenia has instructed the Krško nuclear power plant to remain at the lowest output compatible with safe operation until at least 12 August, as high temperatures and exceptionally low water levels in the Sava River constrain cooling conditions.
The plant’s reduction was its first weather-related reactor power cut in 23 years. Further deterioration in river conditions could require a complete shutdown, but Slovenia’s government has chosen to preserve minimum generation while nuclear safety parameters remain satisfied.
The immediate issue is not limited to replacing lost electricity. Transmission operator ELES has warned that a full Krško shutdown could create excessive voltage in parts of Slovenia’s 400 kV network, partly because of reactive-power flows arriving from the Balkans.
Krško is consequently operating in a compensation mode that helps reduce voltage at its own substation and elsewhere in the system. This makes the plant both an energy producer and a provider of essential network support. Imported megawatt-hours can replace its active-power output, but they cannot automatically reproduce the same local voltage-control service.
Slovenia’s balance was already tightening. Forecast generation fell to approximately 1,004 MW against consumption of 1,327 MW on 7 August, leaving net imports of 323 MW. Inflows from Austria reached more than 1,150 MW, while Slovenia continued to export electricity towards Croatia.
The country jointly owns Krško with Croatia, and Croatian utility HEP has said it can cover any further reduction through domestic production and wholesale purchases. Croatia’s replacement exposure could still become expensive during a regional heatwave, particularly when Italian demand pulls electricity westward and Serbia competes for imports in evening hours.
The episode has broader implications for European nuclear assets located on rivers. Climate-related cooling constraints are increasingly becoming an operational and financial risk rather than a remote environmental scenario. Low river flows can restrict output at the same time that high temperatures raise electricity demand, reduce thermal efficiency and increase wholesale prices.
For grid planners, the Krško case also shows that generation adequacy and network security cannot be evaluated separately. Removing a large synchronous generator changes voltage, inertia, fault levels and reactive-power management across the transmission network.
Future investment must therefore include more than replacement energy. Slovenia and neighbouring systems will require synchronous condensers, static compensators, advanced inverter-based resources and stronger regional voltage-control coordination. Krško’s reduced output has exposed a network service whose value is largely invisible during normal plant operation but becomes critical as soon as the reactor is unavailable.
