The trouble-plagued North Anna 1 nuclear generating plant was shut down late Thursday night after a water-control valve malfunctioned, the Virginia Electric and Power Co. reported yesterday.

No radiation was discharged during the incident, according to Vepco and Nuclear Regulatory Commission officials.

The shutdown -- the third this year for the North Anna 1 unit -- is expected to last for about two weeks, Vepco officials said. The unit will undergo repairs as well as previously planned control-room modifications. A Vepco spokesman estimated that the shutdown would result in increased fuel costs of about $200,000 a day.

North Anna 1 was out of operation for four days last month after another valve malfunctioned. It was shut down in February, also for four days, when a worker's shirt accidentally snagged an electrical switch.

A small amount of radioactive xenon gas was released from the nuclear unit during an incident last Sept. 25, leading to criticism by NRC officials. The plant is situated on the shore of Lake Anna about 80 miles southwest of Washington.

The shutdown of North Anna 1 left Vepco with one nuclear generating unit -- Surry 1 -- in operation. Surry 1 resumed service May 12 after undergoing prolonged repairs and tests. Surry 2 is scheduled to go back into service next month. The Nuclear Regulatory Commission recently authorized testing of North Anna 2, but the unit is not expected to go into full operation for some time.

The North Anna 1 unit was shut down shortly before midnight Thursday, according to Vepco and NRC officials, when a valve became stuck while it was open. The valve regulates the amount of water flowing into the unit's steam system. As the water level in the system rose, the generating unit automatically shut itself down, officials said.

A second incident occurred about two hours later, according to NRC and Vepco officials, when an emergency cooling systems was activated, apparently because of a loss of current in an electrical connector. An NRC spokesman said officials have not yet determined the causes of the valve malfunction or the cooling system's activation.