The incident comes less than a month after the reactor, located in northwest Bulgaria on the Danube River, was switched off for scheduled maintenance and refuelling.

The plant operator said in a statement that unit 5 of the Kozloduy plant was shut down at 6:05 am (0305 GMT) on Wednesday "due to a malfunction in the generator excitation system and the activation of electrical protection in the conventional, non-nuclear part."

Its emergency protection was activated and there was "no change in the radiation situation at the site" after the glitch, the statement said.

Kozloduy's second reactor, the 1,000-megawatt unit 6, was operating at full capacity, the plant said. It is unclear when unit 5 can be re-connected to the grid.

The power plant generates about a third of the country's electricity.

It has only two operational reactors after four smaller units were shut down in 2002 and 2006 to meet the European Union's conditions for membership.