Vibrations that damaged steam generator tubes at the San Onofre Nuclear Generating Station were the result of inaccurate computer modeling by their manufacturer, according to a report released today by a Nuclear Regulatory Commission inspection team.
The nuclear plant along the northern San Diego County coastline has been shut down since Jan. 31, when a tube in one of its two reactor units sprang a leak. The other unit was inoperative at the time for planned maintenance.
The NRC sent what it calls an "augmented inspection team'' to the plant to investigate the cause of the leak and the response by the plant's operator, Southern California Edison.
The inspection team agreed with the utility's findings that vibrations from fluid elastic instability caused excess wear in the tubes that were less than 2 years old. The team further concluded that computer modeling by the manufacturer, Mitsubishi Heavy Industries, did not result in a design that provides a safety margin for vibrations, according to the report.
The two units both have thousands of steam generator tubes. SCE has capped several hundred because of excessive wear.
The report also said SCE employees responded to the leak "in accordance with procedures and in a manner that protected public health and safety,'' and that safety systems worked properly.
"We are committed to continuing to work with the NRC on the steam generator issues and will continue to use conservative decision making as we work on repairs and planning for the future,'' said Pete Dietrich, SCE's senior vice president and chief nuclear officer. "The number one priority is the safety of the public and our employees.''
Fluid elastic instability results from the interaction of steam velocity, moisture content of the steam and effectiveness of the supports in the areas where the vibration occurs, according to SCE.
An anti-nuclear group called "Friends of the Earth'' released a statement saying the report confirms "serious and widespread damage'' to the reactors.
"The NRC is completely clear that these reactors can not be restarted unless the licensee has developed a plan to prevent further steam generator tube degradation and the NRC independently verifies that it can be operated safely,'' said Damon Moglen, the FOE's climate and energy campaign director.
He said the NRC erred in allowing equipment of a new design to be installed at San Onofre without a full regulatory vetting and is now trying to justify its mistake.
Sen. Dianne Feinstein, D-Calif., said she said she was closely monitoring the San Onofre situation.
"I recently met with NRC inspectors and believe the team is carefully and thoroughly investigating the cause of tube degradation at San Onofre,'' Feinstein said. "Today's report indicates there is more work to be done before we can fully understand the problem and determine a possible solution.''
Both units will remain off-line until both the utility and NRC are satisfied it is safe to restart, SCE officials said.
