A unit at Bulgaria's Kozlodui nuclear power station on the Danube River underwent an emergency shutdown Saturday, facility operators said.
A statement from the power plant said the incident did not cause any radioactive leak.
Experts were investigating what triggered the automated safety system shutdown at the 1000-megawatt unit, it said. It was not immediately clear what repairs would be necessary or when the unit might be back in operation.
The unit is of the Russian VVER type.