Slovenia started Saturday shutting down its only nuclear power plant for 40 days of regular maintenance after 18 months of production, the Krsko nuclear plant authorities said.

"The plant successfully concluded its 25th fuel cycle... during which production was safe and stable," the plant said on its website, adding that during the last cycle it produced 8.5 terawatt hours (TWh) of electricity.

The plant is to resume production by May 25, the operator said.

In February, the Krsko plant, 90 kilometres (55 miles) east of the capital Ljubljana, briefly reduced output to one third to take preventive measures 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 shutdown.

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.