The 1,000-megawatt unit at the Kozloduy plant was reconnected to the power grid late Wednesday, the operator said in a statement.

No radiation was leaked when the emergency protection system shut the unit early Wednesday after a malfunction in the excitation system of the generator increased voltage.

The operator said that it replaced the defective excitation system element and carried out the necessary checks, and that power output is now being "gradually increased."

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

Kozloduy's second 1,000-megawatt unit was working at full capacity.

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

Four smaller units at the plant were shut down in 2002 and 2006 over safety concerns to meet the European Union's conditions for membership.