The Nuclear Regulatory Commission imposed a $50,000 fine yesterday on Virginia Power's Surry nuclear plant for failing to detect microorganisms clogging a water spray system designed to cool the reactor core in an emergency.

The problem stemmed mainly from the growth of small marine animals, called hydroids, in the pipes of a spray system. That growth would slow the flow of water that cools and depressurizes the reactor core in an accident, worsening a radiation leak, the NRC said.

Virginia Power reported the clogging problem to the NRC in October, after it was discovered during a test for another purpose at the Surry plant, which is on the James River about 200 miles southeast of Washington and supplies more than 20 percent of Northern Virginia's electricity.

The utility had shut down one of Surry's two 781-megawatt nuclear reactors before the test. It switched off the other one when the problem appeared.

The NRC praised the utility for voluntarily notifying it of the problem, but criticized Virginia Power for not taking more aggressive action sooner.

The commission had warned Virginia Power about potential clogging by microorganisms after a 1988 Surry safety inspection, and it had sent all utilities a letter ordering them to respond to general safety issues, including spray system clogging, in 1989.

The message sent by the fine, said NRC projects branch chief Marvin Sinkule in Atlanta, is: "If you guys would have dug deep enough, we feel you would have found this problem sooner."

NRC officials said that the problem would not cause an accident but would worsen one, and that the fine was the normal amount levied for such an offense.

Virginia Power said it would not contest the fine.

"We're not protesting because we did in fact have a problem," utility spokesman Carl Baab said.

The utility and the NRC said the Surry pipes were unusually clogged last fall because mild weather had encouraged growth of the marine organisms in the James River, source of the plant's cooling water.

The problem has not occurred in lake water drawn to the utility's other nuclear plant, North Anna, about 90 miles south of Washington, Baab said.

Baab said Virginia Power has begun treating Surry's pipes and water to hold back growth of the microorganisms.

The fine was the first at Surry since 1989, when a spate of management and safety problems forced a lengthy shutdown of the nuclear plant.

The plant was included on the NRC's "watch list" of troubled nuclear facilities from June 1989 to June of last year.