Federal regulators have reprimanded the Virginia Electric and Power Co. for allowing employes at its North Anna nuclear plant to improperly jam an emergency switch with a pencil and paper clip and to disconnect a system designed to vent radioactive gas.

"The use of a pencil and paper clip or similar mechanisms . . . is not acceptable nor will it be tolerated by the Nuclear Regulatory Commission," wrote Victor Stello, director of NRC's office of inspection and enforcement, in a letter released yesterday.

NRC investigators said the switch used to control a water tank that is part of the reactor's emergency cooling system, was jammed for about a month and that as many as 20 control room supervisors and workers were probably aware of its blockage.

"They did it to correct a leakage problem but it's a ridiculous risk to take to do that kind of makeshift stuff when you've got a multimillion-dollar power plant," said one NRC official who asked not to be identified.

But the NRC declined to fine Vepco for the two violations, which were uncovered during an investigation into the release Sept. 25 of a small amount of radioactive xenon gas from the plant, located 70 miles south of Washington. In the past, the NRC has fined Vepco a total of $122,400 for errors in its nuclear operations.

The NRC stressed that neither of the latest violations contributed to the radioactive release. NRC investigator Ted Webster said computer records showed the water tank switch was not jammed the morning of the gas release.

Had the vent system been properly connected, Webster said, the xenon gas would have escaped to the atmosphere through it. Because the system was disconnected the gas instead was channeled to the plant's auxiliary building from which it was released.

As described by Webster and other NRC officials, the incident began with a mechanical breakdown in the plant's main turbine, which led to an automatic shutdown of both the turbine and its nuclear reactor. Then, a series of small mechanical problems and errors by plant operators led to a momentary "burping" of a pressure relief valve allowing the radioactive gas to escape.

NRC officials and Vepco have stressed that the amount of radiation released was barely measurable and posed no threat to public health.

Vepco spokesman Doug Cochran said yesterday the company had corrected both violations and had taken steps to avoid repetition of the mistakes.

"Our procedures have changed and this will not happen again," Cochran said.

Cochran said the vent was probably disconnected sometime in 1977 or 1978 and that the company had not been able to determine who did it or why. He said the problem had not been discovered until recently because it was in a remote and highly radioactive section of the plant.

NRC investigators also were unsuccessful in determining which workers had jammed the control room switch.