Virginia Power officials have found evidence of more flaws in steam generator tubes at the North Anna nuclear power plant and are expanding inspections that began there when a tube section ruptured July 15 and released a small amount of radiation, officials said yesterday.

Testing done since the rupture shows that more tubes may have to be plugged because of defects, they said. The tubing in the three steam generators in North Anna's Unit 1 had just been inspected during May and June, when the reactor was shut down for refueling, and 118 defective tubes had been plugged in the "C" generator, where the rupture occurred the next month.

"While the inspections and analysis of findings are still under way, there are preliminary indications that a few additional tubes may have to be taken out of service." William L. Stewart, Virginia Power's vice president in charge of nuclear operations, said in a statement. "However, we are still reviewing the data we have collected on this. These tentative findings result from the company's use of more sensitive inspection devices than required during the last refueling outage."

Each steam generator has more than 3,300 arch-shaped tubes, and utility officials said yesterday that each one in the "C" generator would be subjected to three distinct tests.

The rupture occurred in an area of tubing that company officials said had not posed a problem in the past, either at North Anna or at any similar nuclear facility.

The amount of radiation released as a result of the July accident totaled significantly less than the amount from a chest X-ray and posed no danger to plant employes or the public, they have said.

Company experts believe the rupture was caused by "fatigue-assisted cracking," resulting from stress on the outside of the tubing, and not from corrosion on the inside, said company spokesman William N. Curry. If that is the case, it is a relatively well-known phenomenon in the industry, though in the past the problem has shown up in areas where utility officials have focused their inspections, he said.

As a result of the expanded investigation, the company revised its estimated restart date for the reactor to late September. Initially, officials had said it might be back in operation as early as mid-August. Curry said the company would be able to meet all customer energy demands from alternative sources.

Virginia Power and Nuclear Regulatory Commission (NRC) officials have said they want to learn more about the rupture to find out what implications it might have for safety and inspection requirements for the industry as a whole.

Industry critics have said the rupture is an indication that nuclear power plants are wearing out faster than expected, and that unexpected problems are inevitable. North Anna was placed in operation in 1978.

Richard Starostecki, NRC associate director for inspections and technical assessment, said that once the cause of the rupture is known the NRC will have a second team of specialists assess what the findings mean for the industry. NRC inspectors expanded their investigation after the North Anna rupture proved larger than first believed.

"This is really only the first round," Starostecki commented. Since the location of the rupture was unusual, "there is a new phenomenon we don't understand," he added.