Faulty computer modeling caused the equipment problems that are expected to keep the San Onofre nuclear plant dark through the summer, federal regulators said Monday.
Officials from the U.S. Nuclear Regulatory Commission gave their first public account of the initial findings of their investigation into the plant’s problems at a meeting in San Juan Capistrano.
What they did not give was any indication of how long the plant is likely to remain out of service, saying there are still questions plant operator Southern California Edison needs to answer and more inspections the NRC must do.
The plant has been out of service since Jan. 31, when operators discovered a small leak in one of the thousands of steam generator tubes that carry hot, radioactive water used to create steam to turn turbines that generate electricity.
That led to the discovery that other tubes were rubbing against support structures and adjacent tubes, and wearing out more quickly than expected. Eight tubes failed pressure testing, which NRC officials said Monday is the first time in the nuclear industry that more than one tube at a plant has failed.
The wear is a safety concern because tube ruptures can release radiation. The plant’s operator, Southern California Edison; the NRC; and Mitsubishi Heavy Industries, the manufacturer of the steam generators, have been studying the cause and extent of the wear. The NRC has ordered Edison to keep the plant shuttered until it has determined the cause and how to fix it.
“This is a significant, serious safety issue,” said NRC regional administrator Elmo Collins. “This is a very difficult technical issue, and to be honest, it’s not one we’ve seen before.”
NRC officials said it appears that simulations by Mitsubishi underpredicted the velocity of steam and water flowing among the tubes by a factor of three or four. The high rate of flow caused the tubes to vibrate and knock against each other, leading to the wear.
It was not clear why the computer modeling was so far off. Mitsubishi had no representatives at the meeting and could not be immediately reached for comment. Collins, however, said that ultimate responsibility lies with Edison.
There were also issues with support structures intended to prevent vibration in Unit 3, but apparently not in Unit 2, officials said. In Unit 3, 326 tubes showed the tube-to-tube wear, while only two in Unit 2 did. Many more tubes showed a more common type of wear caused by rubbing against support structures. In total, 9% of tubes in Unit 2 and 12% in Unit 3 showed wear of 10% or more, said Pete Dietrich, Edison’s senior vice president and chief nuclear officer.
Edison officials said earlier this month that they expect the plant to remain out of service through the summer. With contingency plans in place -- including conservation measures, transmission upgrades and temporarily returning to service two retired generating units at a natural gas plant in Huntington Beach -- energy leaders said they expect California can make it through the summer without rolling blackouts under all but the most extreme circumstances.
Residents of the communities surrounding the darkened plant packed the San Juan Capistrano community center to hear the NRC’s report and to ply regulators with questions, while activists took the opportunity to call for the plant to be decommissioned.
Earlier Monday, the environmental group Friends of the Earth filed a legal petition with the NRC seeking to force the agency to put Edison through a license amendment process, including a trial-like public hearing, before the plant can fire up again.
The group contends that when Edison replaced the plant’s steam generators, it failed to report a series of design changes that should have been reviewed by the NRC and sidestepped public scrutiny of the changes -- a charge that Edison denies.