Slovenia has instructed the Krško nuclear power plant to remain at the lowest output compatible with safe operation until at least 12 August. The directive is linked to high temperatures and exceptionally low water levels in the Sava River, which constrain cooling conditions.
The plant’s reduction is its first weather-related reactor power cut in 23 years. If river conditions deteriorate further, a complete shutdown could be required while Slovenia’s government seeks to preserve minimum generation as long as nuclear safety parameters remain satisfied.
Voltage risks on the 400 kV network if Krško shuts down
The situation extends beyond 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.
ELES said the risk is partly related to reactive-power flows arriving from the Balkans. In response, Krško is operating in a compensation mode intended to reduce voltage at its own substation and elsewhere in the system.
This operating approach means the plant provides both generation and network support. Imported megawatt-hours can replace Krško’s active-power output, but they cannot automatically replicate the same local voltage-control service .
Tightening Slovenian supply-demand balance during August heat
Slovenia’s system balance was already tightening ahead of further constraints. On 7 August, forecast generation fell to approximately 1,004 MW against consumption of 1,327 MW, leaving net imports of 323 MW.
Inflows from Austria reached more than 1,150 MW. Over the same period, Slovenia continued to export electricity towards Croatia.
Shared ownership with Croatia and replacement costs during regional demand spikes
Krško is jointly owned by Slovenia and Croatia. Croatian utility HEP 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. The risk is heightened when Italian demand pulls electricity westward and Serbia competes for imports in evening hours.
Cooling limits and grid services for European river-adjacent nuclear plants
The episode also points to broader implications for European nuclear assets located on rivers. Cooling constraints tied to climate conditions are increasingly becoming an operational and financial risk rather than a distant scenario.
Low river flows can restrict output while high temperatures raise electricity demand, reduce thermal efficiency, and increase wholesale prices. For grid planners, the Krško case indicates that generation adequacy and network security cannot be assessed separately .
Removing a large synchronous generator affects voltage, inertia, fault levels, and reactive-power management across the transmission network. Future investment in Slovenia and neighbouring systems will need synchronous condensers, static compensators, advanced inverter-based resources, and stronger regional voltage-control coordination .
Krško’s reduced output has highlighted a network service that is largely invisible during normal reactor operation but becomes critical when the unit is unavailable .
