The Nuclear Regulatory Commission yesterday expanded its investigation of the tube rupture in the radioactive section of Virginia Power's North Anna nuclear power station after learning that the rupture Wednesday was larger than first believed.

However, NRC and Virginia Power officials said, their original measurements of the small amounts of radioactive gas that were released into the atmosphere remain unchanged, and no safety threat is posed to residents or plant employes.

Kenneth Clark, a spokesman in NRC's regional headquarters in Atlanta, said the commission will augment its seven-person inspection team at the site with two more NRC investigators and a consultant from Washington. The additional inspectors have expertise in metallurgy and in the engineering design of the steam generator where the rupture occurred, Clark said.

Clark said initial calculations suggested that the rupture caused a leak rate of between 50 and 150 gallons of radioactive water per minute. However, subsequent calculations, aided by a computer, show that the leak rate was about 627 gallons per minute, he said.

The discovery of the higher rate "certainly puts a taller hat on NRC concerns," Clark said. He said the inspection team is being beefed up because of "heightened interest in what happened inside the steam generator, and what needs to be done, not only at the North Anna plant, but at other nuclear power plants to minimize the possibility of a reoccurrence."

"Obviously, the higher leak rate means the break in that tube was bigger than first thought," Clark said. "But we don't know whether that's one tube or more than one tube." There are about 3,300 tubes inside the generator.

Tubing inside steam generator C of North Anna's Unit 1 ruptured about 6:30 a.m. Wednesday, releasing small amounts of radiactive gas into the atmosphere and forcing Virginia Power to shut down the reactor. There were no injuries. North Anna Unit 2, the other reactor on the site, continues in operation.

The total amount of radiation released was a fraction of the level of one chest X-ray and less than 1 percent of the amount that NRC allows to be released from the plant, officials said.

Clark and utility officials explained that the amount of released radioactive gas has not changed, because the radioactive material was actually measured at the time, whereas the leak rate was calculated based on preliminary information.

The tubes in the steam generator are used to exchange heat between the primary water system, which flows from the nuclear reactor, and the secondary cooling system.

The heat boils water into steam, which drives the plant's turbines.

When the tubing ruptured, some of the radioactive water spilled from the reactor into the secondary cooling system, which does not normally contain radioactive material.

Virginia Power and NRC officials plan to probe the affected steam generator this weekend.

William N. Curry, a Virginia Power spokesman, said the higher leak rate is still consistent with a one-tube rupture and that the leak rate in just one tube could go as high as 800 gallons per minute.

"We have had no surprises that give us reason to believe that events are worse than we initially thought them to be," Curry said.

The incident at the North Anna power plant, which was built in 1978, has renewed concern about potential risks at aging nuclear plants.

But Robert Pollard, a nuclear safety engineer and a member of the Union of Concerned Scientists, said the higher leak rate is not surprising and that "it simply confirms they probably ruptured a whole tube . . . .

"It doesn't mean the public was in any greater danger," he added.