The Krško nuclear power station is now stable.
The Administration said it was monitoring the situation and would be informing the public regularly.
Its head, Andrej Stritar, told the STA that the exact cause of the halt was not known yet but allegedly there was a glitch in the regulation of the water level in the vaporiser.
Maintenance works and refuelling of Slovenia's only nuclear power plant were last conducted in October 2016, with the plant being brought back on line in early November.
The EUR 20m operation involved more than a thousand contracted workers and more than 60 domestic and international companies. Apart from refuelling and maintenance, it also involved 27 upgrades designed to boost the safety and reliability of the plant's operation.
NEK, which provides around a quarter of all electricity produced in Slovenia, began commercial operation in 1983. It is jointly owned by Slovenia and Croatia.
While it was originally designed to be shut down by 2023, its life cycle has been extended from 40 to 60 years, meaning until 2043.
In 2014, the plant underwent an extensive review performed every ten years as part of certification or continued operation.
The tests, conducted by experts at NEK and its subcontractors and verified by a group of international experts, checked all of its vital safety systems.