Slovenia's only nuclear power plant resumed production on Sunday after a regular 42-day maintenance shutdown, authorities at the Krsko nuclear plant said.

"All the maintanence tasks were successfully performed including a check up of the primary equipment, preventive maintenance, replacement of the nuclear fuel and the technological upgrade of the plant," a statement said.

The plant was shut down on April 14 after its 25th fuel cycle during which it produced 8.5 terawatt hours (TWh) of electricity.

Although the plant was expected to resume production on May 25, the maintenance works "extended for some days due to the complexity of the work," the statement said.

In February, the Krsko plant, 90 kilometres (55 miles) east of the capital Ljubljana, briefly reduced output to one third following a small oil leak.

Before that in March 2011 it had temporarily halted operation when a loss of coolant in the reactor's cooling system forced an automatic shut down.

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.