An exterior of one of the two primary containment structures at the Calvert Cliffs nuclear power plant in Lusby, Md., on March 22, 2011. (Marvin Joseph/WASHINGTON POST)

The operators of the Calvert Cliffs nuclear power plant shut down one of the plant’s two nuclear reactors on Sunday after a control rod accidentally dropped entirely into the core where fission occurs, a spokesman for the U.S. Nuclear Regulatory Commission said Monday.

Control rods -- which absorb neutrons that are created by nuclear fission and sustain a chain reaction -- are gradually lowered or withdrawn from the core of the nuclear reactor to regulate the reaction’s pace, NRC spokesman Neil Sheehan said Monday.

When an unplanned drop occurs, however, the fission process can become imbalanced and “pose challenges” for the plant operator, Sheehan said. Such a malfunction also requires operators to shut the reactor down within six hours, he said.

Calvert Cliff’s operators are still trying to determine why a control rod dropped entirely into the core of the plant’s No. 1 reactor at 12:34 p.m. Sunday, Sheehan said. The company filed a formal notification with the NRC at 2:37 p.m. and advised the NRC that it had begun shutting down Unit 1 right away.

“It’s a prudent move in this case,” Sheehan said Monday afternoon. “It sounds more ominous than it is.”

Although any such incident poses challenges to plant workers because of the radiation and heat in the reactor’s core, the event has not endangered the public or the operators, and there has been no release of radiation, Sheehan said. The event was classified as a “non emergency” on the formal notification form submitted to the NRC by Calvert Cliffs, according to a copy of the form .

Plant operators are working to find out what happened and eventually bring the plant’s unit back online, Sheehan said. Although there have been other instances in the industry when a control rod dropped unexpectedly into a reactor, such malfunctions are “infrequent,” Sheehan said.

The plant, located in Lusby, Md., is owned and operated by Constellation Energy Nuclear Group, which is a joint venture between Exelon Corp. and EDF. The company’s Web site says the plant’s two reactors produce 1,700 megawatts, or more enough electricity to power more than 1 million homes. Company officials did not respond immediately Monday to requests for comment made by email and telephone.