A small amount of radioactive gas was discharged into the air yesterday after a series of malfunctions triggered a shutdown of Virginia Electric and Power Co.'s controversial North Anna I nuclear generating plant.

The emissions of radioactive xenon, krypton and rubidium were described by federal nuclear officials as posing "no health hazard" for the public. The plant is located on the North Anna River in Louisa County, about 70 miles south of Washington and 40 miles northwest of Richmond.

At a special briefing in Washington yesterday evening, Victor Stello, director of inspection and enforcement for the Nuclear Regulatory Commission, told commission members that a "very, very small amount" of radioactive gas was vented through smokestacks at the plant -- far too little, he said, to endanger public health.

Stello said he had not yet determined, however, whether any Vepco employes had been exposed to significant amounts of radioactivity.

Vepco Senior Vice President W. L. Proffitt said last night that "less than 10" Vepco workers were exposed to radioactive gas. A statement issued earlier by Vepco said, "Brief, normal precautions were taken to protect station personnel. No one was overexposed during the incident.

A federal investigation was under way last night in an attempt to determine what led to the discharge of radioactive gases from an auxiliary building at the plant. Radioactive emissions also occurred at an auxiliary building during the near-melt-down at the Three Mile Island nuclear plant near Harrisburg, Pa., last March but yesterday's incident was described as far less serious and, in major ways, different from the Three Mile Island crisis.

"The health and safety of the public were not affected," the statement, issued by Vepco Vice President C. M. Stallings, said.

The radioactive emissions were described by Vepco officials as the first to have occurred at the North Anna plant. The incident led to the shutdown of the facility -- Vepco's only operating nuclear power station at the time. North Anna I has previously been the center of several controversies, including the belated discovery that it was located over a geological fault, soil "settlement" that caused part of the plant to sink, and other problems.

Federal regulatory officials traced the radioactive emissions chiefly to three, still-unexplained malfunctions, caused either by mechanical breakdowns or human error.

Yesterday's incident was triggered at 6:13 a.m., federal officials said, when an excessive amount of water appeared in a secondary system designed to create steam to power the plant's turbine. This led to an automatic shutdown of the reactor, they said.

Then a steam valve malfunctioned, causing a decrease in pressure in the reactor area, officials said. The pressure drop, they said, prompted an emergency cooling system to start up automatically, building pressure up again. Then, officials said, a valve opened -- as it was designed to do -- and released radioactive water into tanks inside the building housing the reactor.

Next, a third breakdown or operating error occurred, officials said. The nature of this malfunction has not yet been established. Officials said this still-unexplained development resulted in the discharge of radioactive gasses into an auxiliary building linked to the structure that contains the reactor.

Some of this radioactive gas then flowed through vents and out smokestacks into the air, they said. At its peak, the buildup of radioactivity in the auxiliary building was 155 times the amount permissible on a year-round basis, officials said. The amount measured at 7:30 a.m. at about the maximum permissible level, but it quickly dissipated and lost its radioactivity.