Slovenia's sole nuclear power plant Krsko resumed energy production on Sunday, six days after being shut down due to mechanical problems, its operator said.

"The cause for the shut-down on February 25 was a mechanical problem on the isolation valve on the plant's secondary steam system," a statement said, adding the vital parts of the valve had been replaced.

The plant -- jointly built with Croatia in 1984 when they were both part of the former Yugoslavia and situated some 90 kilometres (55 miles) east of Ljubljana -- accounts for about 40 percent of the EU country's electricity production.