A leak of coolant prompted Slovenia to completely shut down the reactor at its only nuclear power plant, according to BBC.
The European Commission said parts were still cooling after the shutdown, but the situation was under control.
It said there appears to have been no discharge into the environment at the Krsko plant, which supplies energy to Slovenia and Croatia.
The Commission alerted all 27 EU member states under its Ecurie early warning system for nuclear emergencies.
Slovenia`s nuclear safety chief Andrej Stritar 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.
