The European Commission issued an EU-wide alert late Wednesday after Slovenia began shutting down a nuclear power plant following a leak in the cooling system, AFP reported.
"A loss of coolant has occurred in the primary cooling system of the Krsko nuclear power plant," in the southwest, the commission said in a statement.
"The nuclear power plant is proceeding to the safe shut-down procedure," it added.
The EU's executive arm activated its European Community Urgent Radiological Information Exchange (ECURIE) system, immediately transmitting the information to all 27 EU member states.
"At this point in time, no discharge to the environment has been identified," the commission said, adding that the power of the reactor was at 22 percent and "the safe shutdown is still in progress."
The ECURIE system requires early notification and exchange of information "in the event of a radiological or nuclear emergency".
All member states are required to inform the commission "at appropriate intervals" about the measures they are taking and the radioactivity levels they have measured.
Slovenian authorities said in a statement the nuclear power plant at Krsko, about 100 kilometres (60 miles) east of the capital Ljubljana, had been "preventively shut down" after a leak in the cooling system was detected.
"The plant has been stopped preventively for some hours to allow the personnel to determine the cause of the failure and to fix it up," the statement said.
It added "there was no need for an emergency shutdown of the plant and the failure did not and is not expected to affect the environment."
The Krsko nuclear power plant, co-owned by Slovenia and neighbouring Croatia, was constructed in the late 80s and has a net electrical output of 696 MW.