Constellation Energy Nuclear Group said it has shut down one of two nuclear reactors at its Calvert Cliffs power plant after a malfunction during testing.
The reactor was shut down Thursday after a control rod dropped into the reactor during tests at the Southern Maryland plant, said plant spokesman Kory Raftery.
Raferty said electrical maintenance will be performed, but it's not clear when unit 2 will return to service.
Nuclear Regulatory Commission spokesman Neil Sheehan said the plant must shut down within six hours if a rod can't be withdrawn. He said inspectors will follow the maintenance.
Raftery said there was no public safety threat. He said an investigation found no link to a shutdown in May.
The plant is located along the Chesapeake Bay, about 70 miles south of Baltimore.