Two previous incidents including generator malfunctions at Kozloduy's reactors, located in northwest Bulgaria on the Danube River, have triggered shutdowns in June and October.

The leak at the Kozloduy Nuclear Power Plant was within safety limits when it was detected last week in a steam generator, its operator said on Friday, adding that it had decided to monitor the reactor at the time.

Kozloduy NPP said Friday that it decided to "shut down the reactor before the limits in the technological regulation will be reached".

It added that there was "no change in the radiation situation in the working areas and at Kozloduy's site".

Reactor 6 is due to be switched off on Saturday morning. It was not immediately clear when it would be reconnected to the grid.

Kozloduy's second active reactor, the 1,000-megawatt unit 5, was operating at full capacity.

Kozloduy generates about a third of the country's electricity and is vital for Bulgaria's energy security especially during winter months.