Slovenia's only nuclear power plant reduced output to one third for repair work, the operator said on Saturday.

The plant at Krsko, 90 kilometres (55 miles) east of capital Ljubljana, requires lubricating oil in a reactor pump, said a spokeswoman.

"The reduction was preventive and agreed in advance... there is no risk (of nuclear leaks) at all," spokeswoman Ida Novak Jerele told AFP.

She said the reduction would last some hours "to preventively add lubricating oil into a reactor's pump bearing."

The Krsko power plant -- jointly built with Croatia in 1984 when they were both part of the former Yugoslavia -- generates about 40 percent of the EU country's electricity production.

It was shut down in March 2011 when a loss of coolant in the reactor's cooling system forced an automatic shutdown.

As part of its 18-month production cycle, it is scheduled to interrupt production on April 14 for a regular one-month maintanence.