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Posted Mar. 8, 2013 / Posted by: Adam Russell
The just-released Mitsubishi Heavy Industries report on the San Onofre steam generators conclusively reveals that, as far back as 2005-2006, the joint Southern California Edison/Mitsubishi anti-vibration bar design team had identified worrisome problems with Edison’s proposed design for the steam generators MHI was contracted to build. The report confirms that Edison’s contract with Mitsubishi specified that the new, radically redesigned replacement steam generators meet the Nuclear Regulatory Commission’s “like for like” standard (10 C.F.R. §50.59) thereby hoping not to trigger a license amendment. The report further reveals that while the design team considered significant design changes that might have addressed the void fraction/steam quality problem, it was decided that the changes not be made. The specific and only reason for not making those changes cited in the report was Edison’s requirement not to trigger a license amendment process by the Nuclear Regulatory Commission.
The MHI report therefore reveals that Edison was aware of the void fraction problem -- a problem which the utility itself has said is the basis for fluid elastic instability, which in turn is what they have told the NRC is the basis for the unusual wear and damage of the steam generators. This wear and damage led to the unprecedented thinning of thousands of tubes and release of radiation from Unit 3 -- the triggering event for the closure of both reactors, now in its 14th month.
Below is Friends of the Earth’s initial analysis of key points from the Mitsubishi report and related documents:
Sen. Boxer and Rep. Markey stated that unreleased Mitsubishi Heavy Industries document reveals that Southern California Edison was aware of problems with design of replacement steam generators prior to their construction and installation at San Onofre. On February 6, 2013, Sen. Barbara Boxer and Rep. Edward Markey sent a letter to NRC Chairwoman Allison Macfarlane, stating that they have seen a Mitsubishi Heavy Industries report that:
“…indicates that Southern California Edison (SCE) and MHI were aware of serious problems with the design of San Onofre nuclear power plant’s replacement steam generators before they were installed. Further, the Report asserts that SCE and MHI rejected enhanced safety modifications and avoided triggering a more rigorous license amendment and safety review analysis.”
The redacted version of the MHI report released today confirms that Edison was aware of the problems and that design changes were rejected, at least in part, because they would trigger a license amendment process. On March 8th 2013, under pressure from federal, state and local officials, a non-proprietary version of the October 12, 2012 Mitsubishi report, “Root Cause Analysis Report for tube wear identified in the Unit 2 and Unit 3 Steam Generators of San Onofre Nuclear Generating Station”, was released by the NRC. This report confirms the charges made in the Boxer/Markey letter -- namely that SCE, as a member of the Edison/Mitsubishi AVB design team, was in fact aware of significant problems in the steam generator design, which they hired Mitsubishi to fabricate. The report shows that the Edison/Mitsubishi design team were aware of problems with the radically redesigned steam generators during 2005 (see chart p.55), and that they rejected design change solutions, at least in part, because the contract specifically stipulated that the replacement steam generator project was not to trigger a license amendment.
“Early in the project, SCE and MHI formed an AVB Design Team with the goal of minimizing U-bend tube vibration and wear.” (p17/64)
“However, the AVB Design Team recognized that the design for the SONGS RSG resulted in higher steam quality (void fraction) than previous designs and had considered making changes to the design to reduce the void fraction (eg., using a larger downcomer, using larger flow slot design for the tub support plates, and even removing a TSP). But each of the considered changes had unacceptable consequences and the AVB Design Team agreed not to implement them. Among the difficulties associated with the potential changes was the possibility that making them could impede the ability to justify the RSG design under the provisions of the 10 C.F.R. §50.59.” (p22/64)
“The Certified Design Specification SO23-617-01, Rev. 3 stated that SCE intended to use the provisions of 10 C.F.R. §50.59 as the justification for the RSG design, which imposed physical and other constraints on the characteristics of the RSG design in order to assure compliance with that regulation. The RSGs were also required to fit within the same space occupied by the OSGs.” (p. 8/64)
Edison stated that “Void fraction is a critical factor in the determination of whether FEI will exist.” In its January 30, 2013, filing before the Atomic Safety and Licensing Board, Edison made a categorical link between void fraction -- the problem identified by the design team -- and fluid elastic instability, the vibration problem that Edison subsequently stated was the primary cause of damage to the steam generators.
“Void fraction is a critical factor in the determination of whether FEI will exist.” (Southern California Edison Company’s Brief on Issues Referred by the Commission, Docket Nos. 50-361-CAL & 50-362-CAL, January 30, 2013, p. 90.)
Edison notifies the NRC that the prime cause of damage in the steam generators was caused by fluid elastic instability. According to the NRC, in its official Augmented Inspection Team report of July 18, 2012, the NRC was notified by Edison that FEI had caused “ ... the excessive wear and loss of structural integrity” at the San Onofre replacement steam generators:
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“SONGS Unit 3 steam generators had experienced excessive vibration of tubes in the U-bend region of the steam generators to the extent that the tubes rubbed against each other (tube-to-tube interactions) causing excessive wear and loss of structural integrity. Your staff determined that the vibration was caused by the steam conditions in the U-bend region of the steam generators by a phenomenon called “fluid elastic instability.” The NRC inspection team concluded that the steam generators’ design and configuration did not provide the necessary margin to prevent this phenomenon.
Although the steam generator tube degradation from this phenomenon observed in Unit 2 steam generators was not as severe, the NRC team concluded that both units’ steam generators were of similar design with similar thermal hydraulic conditions and configurations. Therefore, SONGS Unit 2 steam generators are also susceptible to this phenomenon.” (NRC AIT report Cover Letter from Elmo Collins, Region 4 Administrator, to Peter Dietrich, Senior Vice President and Chief Nuclear Officer, Southern California Edison Company, San Onofre Nuclear Generating Station, July 18, 2012.)
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