05000361/FIN-2012007-03
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Finding | |
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Title | Evaluation of Retainer Bars Vibration During the Original Design of the Replacement Steam Generators |
Description | The team identified an unresolved item associated with the design of the retainer bars in Unit 2 and Unit 3 replacement steam generators. In February 2012, the licensee identified wear indications in Unit 2 replacement steam generators at the tube locations in contact with the retainer bars. Some of the indications showed excessive wear with a maximum degradation of 90 percent through wall. The team identified that the design of the replacement steam generators did not expect any potential vibration concerns in the area of the tube bundle where the retainer bars were located. The basis for Mitsubishis design philosophy relied on the following factors: Based on the calculated natural frequency of the retainer bar, Mitsubishi considered that there would not be a resonant vibration condition relative to the flow conditions in the location of retainer bars. The vibration analysis of the tube bundle only considered out-of-plane vibration because in-plane vibration was not expected to be an operational concern for the retainer bars. The outermost tubes were considered the least susceptible to flow-elastic instability; therefore retainer bar locations were not included in the vibration analysis. Fluid-elastic instability was found not applicable to the retainer bar because this mechanism did not apply to a single tube in cross flow. Vortex-induced vibration was found not applicable to the retainer bar because it was considered a vibration mode applicable to a single cylinder in uniform cross flow in a large area and the flow condition around the retainer bars was considered slug-froth two phase flow. However, upon identification of retainer bar-to-tube wear in Unit 2 replacement steam generators, Mitsubishi performed an evaluation to identify the cause of excessive wear. The analysis considered three vibration mechanisms: fluid-elastic instability, vortex-induced vibration, and turbulence-induced vibration (random vibration). The analysis for turbulence-induced vibration determined that random vibration was the possible cause of the retainer vibration, based on the peculiar flow around the retainer bar, combined with the rather low natural frequency of the retainer bar. The analysis used the two phase flow conditions around the retainer bars and identified various modes of vibration at those flow conditions that could lead to retainer bar vibration and consequently to tube wear. Additional review by the NRC is required following completion of the Mitsubishis cause evaluation to determine whether this issue represents a performance deficiency or constitutes a violation of NRC requirements. |
Site: | San Onofre |
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Report | IR 05000361/2012007 Section 4OA5 |
Date counted | Jun 30, 2012 (2012Q2) |
Type: | URI: |
cornerstone | Mitigating Systems |
Identified by: | NRC identified |
Inspection Procedure: | IP 93800 |
Inspectors (proximate) | C Thurston E Collins E Murphy J Ortega Luciano J Reynoso J Rivera Ortiz A Johnsonb Hagarg Werner J Reynoso J Rivera-Ortiz |
INPO aspect | |
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Finding - San Onofre - IR 05000361/2012007 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (San Onofre) @ 2012Q2
Self-Identified List (San Onofre)
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