05000348/FIN-2011010-05
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Finding | |
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| Title | Non-Conservative Assumptions Regarding AFW Net Positive Suction Head |
| Description | The team identified an URI regarding the use of non-conservative assumptions in design bases analyses used to demonstrate adequate available AFW pump net positive suction head (NPSH) and subsequent analysis of the impact of AFW system operation during a loss of instrument air event on available NPSH. The AFW system is a safety-related, seismic category I system that is credited to provide the required cooling water from the CST to each of the three Steam Generators (SG). The AFW system design provides for motive force redundancy by assuring that either a single TDAFW pump, or 2 MDAFW pumps can deliver the required flows. The licensee established acceptability of the design based, in part, on results of Unit 1 calculation 40.02, Verification of AFW Flow Bases, Rev. 4, Unit 2 calculation 38.04, Verification of AFW Flow Bases, Rev. 4, and calculation 11.13, Available NPSH for Auxiliary Feedwater Pumps, Rev. 1. The AFW flow bases calculations identified a main feed water line break (MFLB) accident with the failure of the TDAFW pump as the most limiting case in terms of design flow margin to the other two steam generators. Calculation 40.02 identifies a flow margin of 2.21 gpm per SG for SG A and B, and calculation 38.4 identifies a flow margin of 0.82 gpm per SG for SG A and C vs. the required flow of 150 gpm. Additionally, NPSH calculation 11.13 established that for a bounding case (2 MDAFW pumps operating) the margin between available NPSH (NPSHA) and required NPSH (NPSHR) was less than 1 foot. The team reviewed calculations 40.02, 38.04 and 11.13 and identified examples where non-conservative design inputs, assumptions, or limiting plant conditions were relied upon in design basis analyses used to demonstrate that the MDAFW pumps would have a sufficient capacity and head to perform their safety function. The following specific examples were identified: Calculation 11.13 AFW flowrates were based on the flow rates developed in calculation 40.02 for the MSLB case with all three AFW pumps operating. However, for the NPSHA calculation, a conservative assumption would have been a failure of TDAFW pump, since it would maximize the flow through the remaining MDAFW pumps. NPSHA value was based on a CST temperature of 100F and not 110F as specified in FSAR Section 9.2.6.3 as the maximum CST temperature. In response to this observation, Farley performed an evaluation that established that use of 110 F temperature resulted in decrease of NPSHA by 0.75 feet. Based on the team observations, the licensee entered this issue into the CAP as CR 355025 and CR 352168 and performed an operability determination to address the impact of the non-conservative assumptions on AFW safety function. The licensees evaluation documented in IDO 355898 and DIAJ-FRSNNC2689C-I005 concluded that reasonable assurance existed that AFW safety function would not be adversely impacted by crediting operator actions to manually isolate the faulted SG after 30 minutes of operation (for MFLB) and by crediting the remaining CST level to add an approximately 9 foot increase in the NPSHA value in comparison to the one used in the calculation 11.13. This CST level increase leads to the corresponding NPSHA greater than the NPSHR. The teams review of IDO 355898 and DIAJ-FRSNNC2689C-I005 identified the following concerns and observations: DIAJ-FRSNNC2689C-I005 did not address NPSHA vs. NPSHR conditions at the lower CST levels. The team noted that abnormal operating procedures used to combat a loss of non-safety related instrument air allow cycling of AFW header MOVs in lieu of local-manual throttling of the AFW flow control valves. Since the pump flow rates will remain virtually the same every time the operators will open AFW isolation MOVs, the NPSHA advantage credited in the DIAJ-FRSNNC2689C- I005 will be decreasing until it will become negative prior to the CST becoming empty (since the 9 foot of added margin is less than the height of the 13 feet of protected volume). The licensees analyses did not address the potential for long-term AFW flow restricting orifice erosion that could lead to increased AFW flowrates and decreased NPSHA. Because the actual performance of the AFW flow restricting orifices is not periodically compared to the performance assumed in the design basis analyses, the team did not have sufficient information to conclude that the orifice bores would not erode undetected resulting in degraded performance. |
| Site: | Farley  |
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| Report | IR 05000348/2011010 Section 1R21 |
| Date counted | Dec 31, 2011 (2011Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | E Crowe A Nielsen F Ehrhardt G Kuzo S Sandal B Collins J Sowa C Dykesd Jonesm Shlyamberg S Sandal R Nease J Eargle P Wagner |
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Finding - Farley - IR 05000348/2011010 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Farley) @ 2011Q4
Self-Identified List (Farley)
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