"The nuclear power plant Krsko was shut down today, at 05:30 (0330 GMT), and has been placed under safe state of so-called hot-standby," the company said.
Expert teams have begun work to locate the site of the leak, the company said, adding it would be able to estimate the time for repairs and a restart of the plant after the source of the leak had been found.
On Thursday, NEK flagged it would close the plant after finding about the leak in the primary system within the containment building, even though the leak was below technical specification limits.
NEK, in eastern Slovenia close to the border with Croatia, produces more than six billion kilowatt hours of electricity annually which covers about 20% of the electrical energy demand in Slovenia and 16% in Croatia.
NEK was built in cooperation with U.S. company Westinghouse Electric Corporation and started operating in 1982. In 2016 Slovenia and Croatia agreed to prolong its lifespan by 20 years until 2043.
