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No easy answers at SONGS - Update

Dan Yurman's picture
Editor & Publisher NeutronBytes, a blog about nuclear energy

Publisher of NeutronBytes, a blog about nuclear energy online since 2007.  Consultant and project manager for technology innovation processes and new product / program development for commercial...

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Faulty computer analysis, design flaws, and manufacturing changes are among the reasons there is excessive tube wear in the steam generator at San Onofre Nuclear Generating Station Unit 3

California Song Bird

In a presentation at an NRC public meeting held in San Juan Capistrano, CA, on Monday June 18, Southern California Edison VP for Nuclear Engineering Tom Palmisano said there are a combination of factors that have been identified as causes of excessive wear.

Grouping them together under the term “fluid elastic instability,” he said the causes are high velocity dry steam and inadequate support structures. These factors cause the steam tubes to vibrate and rub against each other which a cause of the wear. (see video from the hearing below)

In terms of next steps, Palisano said the utility and other firms in the nuclear industry  supporting it, are developing a corrective action plan. Long terms solutions, he said, will take extensive work. He did not address the issue of whether the utility has decided to replace or repair the steam generators. He did say that the steam equipment for the Unit 2 reactor are in much better shape because the support structure is different in terms of the design and method assembly compared to that in Unit 3.  He didn’t go into specifics on those differences.

NRC calls situation a serious safety issue

Elmo Collins, the regional administrator for the NRC, said the situation at SONGS is a “serious safety issue.”  He said the computer design simulation used by Mitsubishi and Edison failed to predict with sufficient accuracy the velocity of the steam flowing through the tubes. This isn’t a minor mathematical difference. What they found was an error that underestimated these factors by three or four times the original reported numbers.

In an interview with the Associated Press on Sunday June 17 NRC’s Collins said mistakes in fabrication, installation, or both, are possible causes of the excessive wear relative to the age of  the units. Collins told AP that on the issue of “like for like” replacement, the agency is reviewing how that regulatory process was handled at the time.

He said the original steam generators and the replacements “look substantially different.”  He added that while Mitsubishi manufactured the steam generators, SCE is responsible for safety at the reactor site and could be held accountable for any violations of NRC regulations should there be findings along those lines.

Operators shut down Unit 3 on Jan. 31, after a tube leak in one steam generator was identified; Unit 2 had been shut down for a scheduled maintenance outage. Both reactors have remained safely shutdown. On March 27, the NRC issued a Confirmatory Action letter documenting actions that Southern California Edison officials have agreed to take prior to seeking permission to restart the reactors. The NRC has been conducting inspections to determine the extent and cause of the tube degradation. Steam generators do experience some wear during the first year of operation but the level of tube wear at both units is unusual.

Between March 13 and 21, 129 steam generator tubes were pressure tested. Eight tubes failed acceptance criterion – indicating that they could rupture during some operating conditions. The integrity of steam generator tubes is important because the tubes provide an additional barrier inside the containment building to prevent a radioactive steam release.

It is becoming clear that the steam generators as originally designed by Mitsubishi underwent substantial modification. A total of 400 tubes were added and the total weight of the units was increased by 24 tons. Collins said that significant technical issues need to be worked out before the all the causes of wear are known and what to do about them. He noted that “tube-to-tube interaction is substantially proportional to power.”

Similar steam generators manufactured by Mitsubishi were installed at the Ft. Calhoun nuclear power station in Nebraska, but they have not had the excessive wear seen at SONGS. The reasons may be they are smaller and the reactors don’t generate as much power. It is a single unit 484 MW plant. The twin reactors at SONGS come in at just over 1,100 MW each.

SONGS to remain shut down

The steam generators were installed in 2009-2010 at a cost of $617 million. Collins said that ultimately Southern California Edison is responsible for resolving the problem. So far SCE has plugged 510 tubes in Unit 2 and 807 tubes in Unit 3. These numbers will not affect plant power output. Peter Dietrich, SCE VP and Chief Nuclear Officer, said both SONGS units will remain shut down until the utility can satisfy the NRC they can be restarted safely.  

Environmental groups attending the NRC meeting claimed that SCE made changes to the design without getting approval from the agency. Damon Moglen, a spokesman for Friends of the Earth, which has commissioned several speculative reports about the changes, said his organization has submitted a petition to the NRC requiring SCE to get an entirely new license to operate the reactors. He complained that the agency “was asleep at the wheel” when SCE made the design changes.

For its part, the NRC has said it was notified of the changes at the time, but is now conducting a review. Collins promised a written report will be ready in July to address these and other issues.

– SONGS – 
NRC Augmented Inspection Team 
Public Exit Meeting (6/18/2012) 




Image Credit: Esteban De Armas/Shutterstock

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