11 Mar (NucNet): The US Nuclear Regulatory Commission has made public two redacted reports that Mitsubishi Heavy Industries prepared as part of an effort to determine what caused the unusual tube wear found in nearly new replacement steam generators at Southern California Edison’s (SCE) San Onofre nuclear plant in California.
The reports are a root-cause analysis of the steam generator tube wear found in units 2 and 3 at the two-unit plant, and a supplemental technical evaluation. Both units are pressurised water reactors.
The reports offer a detailed technical breakdown of the operational conditions and design elements that caused tubes in the steam generators’ U sections to wear against support bars and each other. Among its conclusions, MHI found that sections of tubes were not held in place firmly enough to control the vibration that caused the wear.
The NRC said the reports are among documents under NRC review as it investigates whether SCE showed sufficient due diligence in its oversight of the redesign of the steam generators; how design changes that were made or rejected may have affected the safety of the steam generators; and the truthfulness and accuracy of all the information Edison has provided to the NRC regarding the redesign and replacement of the steam generators.
San Onofre-2 and -3 have been shut since January 2012 because of tube wear found in replacement steam generators that were installed in 2010 and 2011, respectively. San Onofre-1 was shut down permanently in 1992.
SCE said in a statement on 8 March 2013 that the MHI evaluation cites ineffective tube supports, dry steam and high steam flow velocity as causes of excessive wear in the steam generators tubes.
The reports are on the NRC website:
pbadupws.nrc.gov/docs/ML1306/ML13065A097.pdf
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