The component that was shut down was part of its conventional, non-nuclear unit, reported Associated Press.
Officials from the plant were quoted by Associated Press as saying, "there were no changes in the radioactivity level at the plant."
The plant, which is located 200km north of the capital Sofia, has two 1000MW nuclear units built in Russia.
Two older 440MW units at the nuclear plant were permanently decommissioned in 2006 as a result of European Union safety concerns.
The officials were not able to confirm what repairs are necessary at the plant or when the unit might be back in operation.
The plant has two 1000-megawatt Russia-built nuclear units. Two older 440-megawatt units at the nuclear plant were permanently decommissioned in 2006 because of European Union safety concerns.
