Slovenia has instructed the Krško nuclear power plant to operate 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 has chosen to preserve minimum generation as long as nuclear safety parameters remain satisfied.
Voltage concerns on Slovenia’s 400 kV grid
The operational impact extends beyond replacing lost electricity. Transmission operator ELES 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 Krško provides both active power and essential network support through voltage control. Imported megawatt-hours can replace its output, but they cannot automatically reproduce the same local voltage-control service.
Tightening supply-demand balance and cross-border flows
Slovenia’s system balance was already tightening as the reduction took effect. 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. At the same time, Slovenia continued to export electricity towards Croatia.
Krško is jointly owned by Slovenia and Croatia. Croatian utility HEP said it can cover any further reduction through domestic production and wholesale purchases.
HEP noted that Croatia’s replacement exposure could become expensive during a regional heatwave. The cost risk is tied to Italian demand pulling electricity westward and Serbia competing for imports during evening hours.
Cooling constraints and grid planning implications for nuclear plants on rivers
The situation also points to broader implications for European nuclear assets located on rivers. Cooling constraints driven by climate-related conditions are increasingly becoming an operational and financial risk rather than a distant scenario.
Low river flows can limit output while high temperatures increase electricity demand. Those conditions can also reduce thermal efficiency and increase wholesale prices simultaneously.
For grid planners, the Krško case highlights 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 would therefore need to cover 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 that is largely invisible during normal reactor operation but becomes critical when the unit is unavailable .
