Slovenia's Krško Nuclear Plant Faces Possible Shutdown as Sava River Heats Up

Why Krško Is Closing In on Its Cooling Limits

Slovenia's nuclear power station at Krško may have to cut reactor output, and in the worst case shut down entirely, because of environmental limits tied to the Sava River, the plant's operator has warned. For now, Nuklearna elektrarna Krško (NEK) continues to run at full reactor power, relying on its cooling towers to stay within the temperature thresholds set by its permit.

The margins are thin. NEK must keep the daily average temperature of the Sava at the complete mixing point below the Brežice hydropower plant at or below 28°C, and the average daily temperature increase to no more than 3°C. With little rainfall keeping the river's flow low, the constraints tighten as the heat builds. Around 15:30 local time, the flow at the plant stood at 47.5 cubic metres per second, the river temperature at 24.7°C, and the current temperature increase at 2.6°C — close to the ceiling.

CEO Gorazd Pfeifer stressed the plant itself is in good condition and that the issue is purely a matter of river conditions, which meteorologists say are unlikely to change significantly for at least 14 days. A shutdown, if it came, would be gradual over about a week, while a return to full power would take only a day or two.

The situation has already drawn in Slovenia's grid operator, Eles, whose director Aleksander Mervar warned that a full shutdown would create voltage profile problems in the transmission network. Eles has proposed a temporary exemption from the environmental obligations in an extreme case, a suggestion now being reviewed by the Environment Ministry, which says it will inform the public once a decision is known.

What a Krško Shutdown Would Mean for Slovenia's Grid and Regulators

How close Krško is to its limits

The binding constraint is the river, not the reactor. NEK's discharge heats the Sava, and its permit caps both the absolute daily average temperature (28°C) and the average daily increase (3°C) at the mixing point below the Brežice hydropower station. At 15:30, the increase was already 2.6°C against a flow of 47.5 cubic metres per second, leaving little room before the plant must throttle back.

During June's heat wave, the plant raised the Sava's temperature by an average of 2.4°C and at the peak by exactly 3°C — right at the legal boundary. The June episode shows how quickly the buffers disappear in hot, dry summers.

Cooling towers come at a cost

NEK is already using its cooling towers to add extra cooling in the third cooling loop. That keeps the plant compliant, but not for free: the towers raise electricity consumption and reduce the thermodynamic efficiency of the secondary circuit, cutting both output and the amount of electricity sold. The 657 MWh generated between 14:00 and 15:00 reflects a plant operating at full reactor power but with added internal load.

What a shutdown would mean for the grid

Eles has flagged the main consequence in its opinion to the ministry: losing Krško would create problems with voltage profiles in the transmission network. As Slovenia's only nuclear unit and a large baseload generator, the plant is hard to replace quickly; other domestic plants and imports would need to cover the gap, and network stability would need managing during the transition.

A regulatory dilemma for the ministry

The Environment Ministry now faces a direct trade-off between the permit limits it enforces and the security of electricity supply. Eles has formally proposed a temporary waiver in an extreme case — a request the ministry says its expert services will examine. The precedent exists elsewhere: France and Hungary have both taken reactors offline under heat-related restrictions, with Hungary doing so for the first time in nearly half a century. The Slovenian decision, when it comes, will signal how the state weighs environmental compliance against supply security during a prolonged heat event.

Signals to Track as the Sava's Heat Squeeze Tightens

For grid operators, energy traders and anyone relying on Slovenian supply, the next two weeks carry the main risk window.

• Track NEK and Eles data on the Sava: flow, temperature and the daily increase. A sustained move toward 47.5 m³/s flow, 28°C river temperature, or a 3°C increase raises the probability of an output cut or gradual shutdown.
• Watch the Environment Ministry's response to Eles's proposed temporary exemption — a decision in favour would change the risk outlook quickly.
• Plan for a slow, not sudden, loss of capacity: any shutdown would unfold gradually over about a week, while a restart to full power would take just one to two days.
• Expect pressure on Slovenia's transmission network — Eles has already warned of voltage profile problems if the plant goes offline — and prepare for tighter regional supply margins during the heatwave.

No official timeline for a possible shutdown has been given. The determining factors are the Sava's flow and temperature, with no significant change expected in the next 14 days.