Plant spokesman Kory Raftery said that a control rod dropped into the reactor unit while performing tests and the shut down occurred.

Raftery said, "the controlled shutdown occurred without any complications and there were no impacts on the public or plant staff."

The U.S. Nuclear Regulatory Commission spokesman Neil Sheehan said, "Plant personnel will now have to troubleshoot what caused the problem.

"Our inspectors will continue to follow those troubleshooting and repair activities."

Raftery said that CENG has manually closed the reactor to perform the required maintenance and followed their training and procedures.

The Calvert Cliffs nuclear plant is located along the Chesapeake Bay, around 70 miles south of Baltimore.