Unit 2 at the Calvert Cliffs nuclear plant remained offline Friday after a turbine trip shut down the reactor automatically earlier in the week.

Just before 10 p.m., Wednesday, the reactor tripped automatically from full power, according to a report filed with the Nuclear Regulatory Commission. Safety systems performed as designed, and unit 1 remained at full power.

A Constellation Energy spokesman told the Baltimore Sun that steam valves leading to the turbine closed for unknown reasons, shutting down the turbine. The unit 2 reactor had not experienced an automatic shutdown since 2010. According to the NRC, operators shut down unit 1 manually last August following the unexpected insertion of a control rod.

Both units at the plant near Annapolis, Md., are Combustion Engineering pressurized water reactors first licensed in the mid-1970s.