Virginia Power's North Anna nuclear power plant has a long history of problems with defective steam generator tubes like the one that ruptured July 15 and released small amounts of radiation into the atmosphere, Nuclear Regulatory Commission records show.
Chronic problems with the same type of steam generators at Virginia Power's other nuclear power plant at Surry, where operations began in 1973, resulted in replacement of six of those models in 1980 and 1981 at a cost of $137 million, according to company officials.
A utility official said yesterday that the company believes it has arrested the progress of the defects in the tubing at the North Anna facility, however, and that the steam generators will not have to be replaced soon.
"I don't think we have a disease in the steam generator, that we have a large number of tubes about to rupture," said William L. Stewart, Virginia Power vice president for nuclear operations. Because of the recurrent problems, the company will have to continue to do extensive -- and costly -- testing on the tubes, he said.
When the Surry and North Anna facilities were built, the generators were expected to last the entire 30- to 40-year life of the plant. The North Anna plant, 90 miles southwest of Washington, began operation in 1978.
A report filed by Virginia Power officials Nov. 25 recounted that in January 1980, less than two years after the plant started operation, the company found tube deformation in several spots.
The North Anna Unit 1 reactor, where the recent rupture occurred, was shut down for testing in January 1984 after substantial leakage in two of the three steam generators there, the report noted. Four of the more than 3,300 tubes in each steam generator were plugged.
Periodic testing and plugging continued, and by the end of the company's May-to-June refueling this year, 271 tubes in the "C" steam generator, or about 8 percent of them, had been plugged because of suspected defects.
The utility has made periodic reports to the NRC about efforts to determine the reasons for the tube degradation and to correct it.
"During the present refueling outage on Unit 1, eddy current inspections in 'C' steam generator identified greater than 1 percent of the sampled tubes to be defective . . . . In order to help determine the cause of the defects, two of the defective tubes were pulled from the steam generator," a November 1985 report to the NRC said. In an eddy current inspection, an electrical charge is passed through metal to detect aberrations.
In June, just a month before the rupture, two NRC inspection reports addressed the tube defect issue.
"As noted in the last inspection report concerning this subject, the continuing formation of cracks in these tubes in both units indicated that degradation was occurring through mechanical erosion and several corrosion mechanisms," a June 24 report stated. It went on to outline a number of actions taken by the company to reduce corrosion.
Another report referred to a June 3 meeting between the NRC and utility officials in Washington in which the company outlined its tube-plugging criteria so it could get NRC permission to restart the unit. The NRC granted permission.
Both company and NRC officials have said that the tube rupture occurred in a spot where there had not been problems in the past, so none could have been anticipated.
Virginia Power officials have expanded the inspections they are doing to determine the cause of the generator rupture and have said more tubes probably will be plugged as a result of more sensitive tests. Stewart said that at least 12 percent and probably more of the tubes can be plugged without affecting operations.
Critics of the industry have said the rupture is another example of unpredictable problems developing with aging nuclear power plants. In the case of older steam generators, the cost of testing and repairing will grow rapidly, they say.
"Virginia Power is a leader in the discovery of problems with steam generators," said Kennedy Maize, senior energy analyst for the Union of Concerned Scientists. " . . . It isn't going to be as easy as they think. They are going to spend a lot of money just to keep these plants operating."
Maize called steam generators the Achilles' heel of nuclear power plants, because tube ruptures result in a release of radiation into the atmosphere.
Utility officials have said the amount of radiation released at North Anna was miniscule, less than in a chest X-ray, and other experts in the field have not challenged that assessment. But, Maize said, if a number of tubes ruptured at the same time it could have "enormous consequences."