A partial power loss shut down both reactors at the Calvert Cliffs nuclear plant in Maryland at 9:25 p.m. Tuesday.
The Combustion Engineering pressurized water reactors are stable, and the event did not pose a threat to public safety. According to a report filed with the Nuclear Regulatory Commission, safety systems shut the units down automatically after the loss of a 13 KV bus. Emergency diesel generators started. Reactor coolant pumps at both units continued to run. At unit 2, the auxiliary feed water actuation system initiated. According to the report, "The normal unit 2 heat sink was unavailable due to the loss of the operating circulating water pumps resulting in a loss of condenser vacuum. The unit 2 AFW actuation included one of two steam-driven pumps and the motor-driven pump."
A release from Calvert Cliffs owner CENG, majority owned by a subsidiary of Exelon, indicated the power problem originated in the non-nuclear side of the plant. Crews were investigating the shutdown Wednesday and preparing to return both units to service.