Bulgaria said Tuesday that it would close for maintenance one of the two 1,100 megawatt reactors at its sole nuclear power plant at Kozloduy on the Danube.
The plant would start lowering the reactor's capacity from 10:00 pm (2000 GMT) Tuesday, the power station's management said in a statement.
The planned maintenance, expected to be completed in 10 days, would include repairs on a valve regulating steam into the reactor turbine, it said.
The other reactor was working at maximum capacity, the statement said.
Four 440 MW units at Kozloduy were closed in 2002 and 2006 as a condition for joining the European Union in 2007. The old Soviet-built reactors were deemed unsafe.