The Virginia Electric and Power Company's Surry I and II nuclear power plants have been given permission to continue operating at full power with up to 25 per cent of their steam generator tubes plugged.

That ruling by the Nuclear Regulatory Commission means, according to critics of nuclear power plant safety procedures, that regulations thought to be firm have again given way when the utilities pushed. Vepco and the commission say it only shows that safety regulations are realistic but widely misunderstood by the public.

The two viewpoints are another example of the complex issues and communications problems involved in the controversy over nuclear power.

The tubes being plugged are part of a system in which water is circulated around the nuclear reactor itself and heat is carried away for eventual conversion into electric power. A corrosion problem called "denting" has gotten progressively worse at the Surry plants, causing an increasing number of these tubes to spring leaks. The leakers then have to be plugged.

The amount of water that has leaked out has been small and has posed no danger to public health or safety, officials have said. However, only so many tubes can be closed before there won't be enough of them left to carry off the heat efficiently, with a healthy margin to allow for the possibility of an accident that might cause of the reactor's temperature to rise sharply.

When the Surry plants on the James River near Newport News, Va., opened in 1972 and 1973, the regulatory commission said they could only operate at full power if fewer than 12 per cent of their tubes were plugged. The figure was later changed to 19 per cent for one plant and 20 for the other, and now it is 25 per cent for both.

"This is typical," said Robert Pollard of the Union of Concerned Scientists. "Whenever a problem arises, it's solved with a pencil. The safety margins keep getting encroached on."

June Allen, president of the North Anna Environmental Coalition, which is locked in a battle with Vepco over another plant, agreed with Pollard. "The permissible amount keeps climbing. The allowances seem always to depend on need of the utility as opposed to any absolute safety finding . . . the company gets permission that a year ago would have been unthinkable."

Not so, says Vepco and the regulatory commission.

"You start by building in a margin to allow for the plugs you know you're going to need," said Vic Stello, director of the NRC's division of operating reactors. It is axiomatic in the utility industry that all tubes eventually leak. Water transferring heat in oil, coal, gas or nuclear plants alike will corrode the pipes (although at differing rates) and there will be leaks.

"You decide how many tubes will wind up plugged and then set your technical specifications for other aspects of the reactor operation based on that," Stello said. "If you want to operate with more tubes plugged, you just operate the rest of the plant more stringently. You recalculate other limits and operate at those limits.

"It does reduce your flexibility," he went on. "A plant can operate at 25 per cent (tube plugs) but it's getting very iffy."

The limit, in other words, is relative to other rules and can be loosened as they are tightened. An absolute safety limit could be and has been calculated, he said, but it is less expensive for a utility to refigure its dial settings for a series of specific situations as needed than it is to work out a set of readings allowable under every circumstance in advance.

"It's unfortunate that this is regarded as an absolute limit that got changed." Stello said. "It's just not true."

Vepco hopes the 25 per cent plugged-tube limit will be enough to allow the Surry plants to continue operating at full steam until this summer. Then they will be shut down for at least three months each to allow the entire steam generator and tube assembly to be replaced. The job will cost at least $60 million.