05000361/FIN-2012007-08
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Finding | |
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| Title | NON-CONSERVATIVE THERMAL-HYDRAULIC Model Results |
| Description | The team identified an unresolved item associated with the adequacy of Mitsubishis FIT-III thermal-hydraulic code. The FIT-III code predicted non-conservative low velocity and low void fraction results which were used as inputs to the vibration code FIVATS. These non-conservative thermal-hydraulic results lead Mitsubishi to conclude that margins to instability were significantly larger than they actually were. Replacement steam generators were designed and manufactured in accordance with SONGS Design Specification SO23-617-1and ASME Section III, Rules for Construction of Nuclear Facility Components. The replacement steam generators had enhanced materials and maintenance. The tube bundle, comprised of 9727 u-tubes, is supported by a set of seven tube support plates which are maintained and spaced by a network of tie-rods. The ends of the u-tubes were welded onto the tube sheet lower face cladding and were full depth expanded in the tube sheet holes. The u-bends are supported by a set of 6 anti-vibration bars, having a maximum of 12 contact points, in the center of the bundle. For shorter tubes near the periphery, a fewer number of anti-vibration bars are present. One of the major enhancements of the replacement steam generators was the use of Alloy-690 tubing versus Alloy-600 for corrosion resistance. Alloy-690 has lower heat conductivity so, to achieve the same power, the heat transfer surface area must be increased by at least 10 percent. This required more tubes to be used in the replacement steam generators. The increased number of tubes resulted in a more tightly compacted tube bundle and elimination of the stay cylinder. The increase in the number of tubes could lead to increases in primary reactor coolant flow through the steam generators. Orifices were machined as part of the steam generator inlet nozzles to ensure maximum allowed primary system flow-rates were not exceeded. The tube layout indexing or incrementation used in these generators was smaller than other replacement steam generator designs. The tighter indexing results in smaller pitch/diameter ratio in critical regions of the tube bundle u-bends. In addition, it was noted that the anti-vibration bar support structure is not connected to the wrapper for lateral or vertical support; instead the anti-vibration bar system structure is only supported vertically by resting on the tubes. Other operational and physical comparisons of the replacement steam generators and original steam generators were reviewed by the team and no significant differences were noted. Additional review by the NRC is required to fully assess the adequacy of Mitsubishis FIT-III thermal-hydraulic code and then 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-Ortiza Howellc Thurston E Murphy G Warrier G Werner J Billoue J Reynoso M Bloodgood R Lantz V Dhir |
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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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