A pencil and a paper clip, improperly used to wedge an important water control switch into a closed position, was apparently a major factor in last week's accidental discharge of radioactive gases from Virginia's North Anna nuclear power plant.

Virginia Electric and Power Co. officials informed the Nuclear Regulatory Commission yesterday that unidentified workers in the plant's emergency control room had blocked the switch and that the action "probably contributed to the later release of xenon gas."

NRC spokesman Ken Clark in Atlanta said blockage of the switch was "absolutely not" part of acceptable reactor operating procedure and that two NRC inspectors were to arrive at the plant today to investigate the incident.

Clark said it was "too early to speculate" on whether the NRC would fine Vepco for the incident. The company already has been fined a total of $127,400 for violations of NRC rules, making it the most fined utility in the history of U.S. nuclear power plants.

As described by Vepco and NRC officials yesterday, the switch tampering is the first indication that a deliberate action by employes contributed in a major way to the incident, which officials previously suspected resulted from a combination of mechanical failure and possible human error.

Officials said the switch governs a valve on an automatic control system that regulates the amount of water kept in a tank and used to cool the plant's number one reactor. When functioning properly, the valve serves to make sure that excess water from the tank is diverted to holding tanks in an auxiliary building.

After last Tuesday's emergency shutdown of the reactor, the tank began filling with radioactive water, officials said. But because the switch had been wedged into a nonautomatic position, water was not diverted to other holding tanks but instead overfilled the main tank.

A special pressure relief valve then tripped into action, releasing radioactive gases to yet another tank. A further malfunction in the tank -- caused by the as-yet-unexplained absence of a vent pipe -- led to the release of some of the gases into the atmosphere.

"Had the switch not been blocked, you would not have had the tank overfill or the lifting of the relief valve," said Clark. "It certainly contributed to the problem."

Vepco spokesman August Wallmeyer said yesterday that company workers apparently had wedged the switch to nonautomatic because it was allowing some water to be improperly diverted during regular operation of the reactor.

Wallmeyer said the company had not yet determined how long the switch had been wedged or the identity of the worker or workers who had tampered with it. He and other Vepco officials stressed that the amount of radiation discharged was small and represented no health hazard to either the public or plant personnel.

Vepco officials also said the switch was not part of the plant's emergency safety system designed to keep the reactor's core from overheating.

But Clark of the NRC said, "Even though it's not part of the safety system, the switch is important because you're handling radioactive liquids."

The reactor, located on the North Anna River in Louisa County about 70 miles south of Washington, has been closed since the incident. NRC officials say they expect it will be at least 12 weeks before their investigation is completed and the reactor is allowed to reopen.