San Onofre probe: Edison followed procedure
The U.S. Nuclear Regulatory Commission issued its written report from a special inspection team's probe of the issues that have shuttered the San Onofre nuclear plant for nearly six months.
Apart from discussing the cause of unusual wear on tubes in the plant's newly replaced steam generators, which had been operating for less than two years when one tube sprung a leak and released a small amount of radioactive steam, the NRC addressed the issue of design changes in the new steam generators.
Critics have said Edison did not properly report and the NRC did not properly review some of the changes.
The NRC report found that the design changes were properly reported and reviewed and that the changes between the original and new steam generators were "commonly used in replacement steam generators today," although it also characterized some of them as "major design changes."
Edison claimed vindication, while critics continued to express skepticism.
Dan Hirsch, a lecturer in nuclear policy at the University of California, Santa Cruz, called the report's conclusion "completely unconvincing" and "self-serving."
NRC spokesman Victor Dricks said the agency is looking at whether it needs to update its review processes for design changes in general, but that Edison and the NRC properly followed the processes in place at the time of the replacement.
The report also reiterated some information about the cause of the wear given in a briefing by NRC and Southern California Edison respresentatives last month. The NRC team concluded that errors in computer modeling by Mitsubishi Heavy Industries, the manufacturer of replacement steam generators, led to the tube wear issues.
The models inaccurately predicted the flow of water and steam in the generators, leading the tubes to vibrate and knock against each other and against support structures.
"Unless changes are made to the operation or configuration of the steam generators, high fluid velocities and high void fractions in localized regions in the u-bend will continue to cause excessive tube wear and accelerated wear that could result in tube leakage and/or tube rupture," the report concluded.
To date, Edison has taken 1,317 of the plant's nearly 39,000 steam generator tubes out of service by plugging them -- 387 because they showed wear greater than 35%, the industry standard for plugging, and the rest as a precautionary measure.
Officials have said they believe running the plant at lower power could solve the problem of tube vibration, but Edison has not yet submitted a restart plan. Company officials have said they expect the plant to remain shut down at least through the end of summer.
U.S. Sen. Dianne Feinstein released a statement on the report saying, “I recently met with NRC inspectors and believe the team is carefully and thoroughly investigating the cause of tube degradation at San Onofre. Today’s report indicates there is more work to be done before we can fully understand the problem and determine a possible solution."
The report raised some other issues that the NRC said require follow-up, issues with the shipping of the equipment, design of support structures, the adequacy of alarms meant to alert plant operators to tube vibration and loose parts. Dricks said there were issues with the placement of the alarms, which were placed on structural supports on the bottom rather than the top of the steam generators.
-- Abby Sewell
Photo: The San Onofre nuclear facility has been dark for nearly six months. Credit: Allen J. Schaben / Los Angeles Times