Slovenia completely shut down the reactor at its only nuclear power plant because of a leak of coolant.
Parts were still cooling after the shutdown, but the situation was under control, said the European Commission.
It added that there appears to have been no discharge into the environment at the Krsko plant, that supplies energy to Slovenia and Croatia.
All 27 EU member states were alerted by the Commissions, under its Ecurie early warning system for nuclear emergencies.
Andrej Stritar, Slovenia’s nuclear safety chief, said there was a water leak from the primary circuit, inside the containment area. He said operators had shut down the plant safely.
“Situation is under control” he said in a statement. “The plant is in stable condition. There is no off-site impact and there is no need for off-site protective measures. “
The Krsko plant has a US-made pressurised water reactor.