One of two reactors at the Calvert Cliffs nuclear power plant in Southern Maryland shut down suddenly Wednesday night after an as-yet-unexplained malfunction, a Constellation spokesman said.

Unit 2 went offline at 9:47 p.m. after a shutdown of the turbine that generates electricity from the nuclear reactor, according to Kory Raftery, spokesman for the Constellation Energy Nuclear Group. Some valves feeding steam to the turbine closed unexpectedly, triggering its shutdown, Raftery explained, and that led to the reactor itself shutting down to prevent a buildup of steam pressure in its cooling system.

The reactor's automatic safety system functioned as designed, the company spokesman said, averting any danger to plant workers or the public. The unit is in "stable condition," he said, while plant personnel and federal nuclear safety regulators investigate what caused the shutdown.

Raftery said plant operators don't know what caused the turbine valves to close.

"We're looking at every possible option right now and at this point in time wouldn't want to speculate," he said. He wouldn't predict when the reactor would resume operations, saying plant operators first needed to identify the cause of the shutdown, fix it and test to be sure.

The Nuclear Regulatory Commission said in a statement that its two resident inspectors at Calvert Cliffs will track Constellation's troubleshooting and review any repair plans.

This is the first unplanned "scram," or shutdown, of Unit 2 since 2010, according to Raftery. Unit 1, which is still operating at full power, shut down unexpectedly last year after a control rod suddenly dropped into the reactor core.

tim.wheeler@baltsun.com

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