Calvert Cliffs nuclear power plant in Lusby powered down one of its two generating units Thursday night after a leak was discovered in the coolant system keeping the pressurized water reactor from overheating, the Nuclear Regulatory Commission said Friday.

The plant 70 miles south of Baltimore began a gradual shutdown of Unit 1 shortly after 11 p.m., as required when a loss of coolant cannot be quickly halted. Workers were able to enter the reactor containment building to deal with a leaking water line once power had been reduced to around 12 percent, according to Kory Raftery, spokesman for Exelon Corp., which owns the plant.

The leak was fixed, and plant operators have since brought the reactor back up to 80 percent power, Raftery said. Unit 1 will remain at that level for the next few days while some scheduled maintenance is performed, then restored to full power, the spokesman said.

An NRC inspector at Calvert Cliffs monitoring the shutdown and leak repair saw "no immediate safety concerns," the agency said.

Tim.wheeler@baltsun.com