The overhaul comes after 18 months of safe and stable operation without shutdowns, SNSA said in a statement on Monday.
The overhaul includes replacing 56 of the 121 fuel elements, as well as performing extensive maintenance works and several technological upgrades, aimed at improving the plant's nuclear and radiation safety, the statement said.
NEK generates some 40% of Slovenia's electricity output. It operates a Westinghouse pressurised light water reactor of 2000 MW thermal power capacity, which generates over five billion kWh of electricity per year.
The plant is jointly managed by its two owners - Slovenia's GEN Energija and Croatian power utility Hrvatska Elektroprivreda (HEP). The two neighbouring countries share the power plant's output.
Krsko was built in Slovenia near the border with Croatia in the 1970s and launched operations in 1982, when the two countries were still part of the former Yugoslavia.