One of two reactors at Romania's sole nuclear power plant stopped unexpectedly after its automatic shutdown system was mysteriously triggered, management said Friday.

The state-owned Nuclearelectrica group, which holds a majority stake in the Cernavoda plant, said the reactor would remain out of service until the cause of the "unplanned" shutdown on Wednesday was understood.

"The incident did not endanger staff, residents or the environment," the group added in a statement.

Located in the country's southeast, Cernavoda has been in operation since 1996 and supplies around 18 percent of Romania's electricity.