New safety measures ordered by the Nuclear Regulatory Commission after the Three Mile Island accident apparently were inadequate to prevent the accident discharge of radioactive gases from Virginia's North Anna power station.

After the Pennsylvania power plant accident earlier this year -- the most serious in the history of the nation's nuclear power industry -- the NRC ordered a series of new actions designed to prevent radioactive materials from escaping into the atmosphere during any future accidents.

But at North Anna, as at Three Mile Island, contaminated gases were once again shunted from the containment building into an auxiliary building and released.

Virginia Electric and Power Co. officials insisted yesterday that the release Tuesday morning of small amounts of contaminated gases from the plant had a "negligible" impact on five exposed utility workers and the public.

But company officials had no ready explanation for what triggered series of mechanical malfunctions that resulted in the gases being pumped from a containment building and into an auxiliary building.

There were other problems linked to the incident at the plant, 70 miles south of Washington, that were disclosed yesterday. Among them:

A rupture in the water system used to heat water before it is pumped into the plant's steam generator.

A malfunction of a steam pump valve that was supposed to close, but did not.

A control room operator's failure to readjust the water replenishment system in the reactor's coolant, causing the water to overflow into another tank located in an auxilliary building.

The absence of a vent pipe that is normally connected to the chimney stack in the auxilliary building and used as a last precuation against the discharge of radioactive gases into the air.

NRC investigators noted that mechanical and human mistakes exacerbated a turbine malfunction at North Anna. The officials seemed particularly perplexed to discover late yesterday that the vent line safeguard was not hooked up.

"It's supposed to bottle up most of the gas in the stack, and I was surprised to find it wasn't attached," said Ed Case, deputy director of NRC'S office of nucelar reactor regulation.

The Vepco reactor, automatically shut down at the first indication of mechanical trouble, will remain shut down for previously scheduled refueling and maintenance work.

This will give NRC and other federal investigators more time to probe mechanical and plant operations that contributed to the discharge -- the second known release of radioactive material into the atmosphere in the nation this year. The first came at Three Mile Island in March.

While NRC inspectors checked the plant, located on the North Anna River in Louisa County, Vepco and antinuclear spokesmen drew opposing conclusions from the incident.

"With the exception of two little things that went wrong, all the equipment functioned normally," said Vepco spokesman Doug Cochran. He said the North Anna problems were "very different" from those at Three Mile Island, primarily because the Virginia plant's turbine and reactor shut down within "milliseconds" of each other.

But Robert Pollard, a physicist with the Union of Concerned Scientists, said emergency procedures drawn up the NRC after the Three Mile Island accident actually contributed to the release of gas at North Anna.

At Three Mile Island, Pollard recalled, operators shut off an emergency cooling system. The NRC directed on Aug. 24 that one of a plant's cooling pumps be kept running for at least 20 minutes after a reactor is shut off, he said.

"But because [a] North Anna [operator] left the pump running so long, it put too much water in the reactor and caused the reactor pressure to go too high," Pollard said.

This, Pollard said, caused a relief valve on a tank to open which, in turn, became the source for the radio-active discharge.

"One of the major lessons of TMI is that power operated relief valves are not that reliable, but NRC'S order to keep the pump running only puts reliance on the same component that caused the trouble," Pollard complained.

At Three Mile Island, Pollard recalled, the relief valve opened and stayed open, releasing greater quantities of radioactivity into the air. At North Anna, the valve opened but quickly closed again.

One Vepco worker, a technical operator, received seven millirems of radioactivity. Four employees who are assigned to monitor any release of gas in the plant received five millirems.

Cochran said that amount of exposure is equal to about what an airplane pilot would experience during two flights between Richmond and California. By comparison, he said, a person who is X-rayed typically is exposed to 50 millirems of radioactivity.

He said the five employes returned to work at the plant within an hour and a half after the accident.

The NRC'S Case differed with Pollard's assessment of the two power station accidents. Case streessed that the problems at Three Mile Island and North Anna "were different" and he said that the gas released in Virginia came from a different source and was emitted for a different reason than in Pennsylvania.

Leaving the one of the two pumps running wouldn't have caused a problem, Case said, if an operator "hadn't forgot he still had one pump working." Even this human error might have been overcome if Vepco officials had connected the vent pipe in the auxiliary building, Case said.

"At least some of the gas wouldn't have escaped, but I can't answer as to how much difference it would have made" in the amount of discharge, Case said.