05000395/FIN-2011006-07
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Finding | |
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| Title | Failure to Use the Most Limiting Design Inputs in an Electrical Calculation |
| Description | The team identified a non-cited violation (NCV) of 10 CFR Part 50, Appendix B, Criterion III, Design Control for the licensees failure to use the most limiting design inputs in an electrical calculation associated with determining settings for the degraded voltage relays to ensure that adequate voltages would be available to safety-related emergency core cooling system equipment during a design basis loss of coolant accident with offsite power available. Specifically, the team identified nine deficiencies in Calculation DC08200-001, ESF Undervoltage Logic and Settings. The licensee entered this issue into the corrective action program as condition report CR-11-01045. The team determined that the use of non-conservative inputs and methodologies in electrical calculation DC08200-001 was a performance deficiency. Specifically, the nine examples of deficiencies in the calculation resulted in a reasonable doubt that a spurious separation from offsite power would occur and that adequate voltages would be available to safety-related emergency core cooling system equipment during a design basis loss of coolant accident with offsite power available. This finding was more than minor because it was associated with the design control attribute of the Mitigating Systems Cornerstone and affected the cornerstone objective of ensuring the availability, reliability, and capability of systems that respond to initiating events to prevent undesirable consequences. The finding affected the availability of the offsite power feed from Transformer XTF-4 and/or XTF-5, and with XTF-6 voltage regulator not in service under specific minimum grid voltage conditions. These transformers are the normal offsite power supply to Safety Bus 1DA and the alternate supply to Safety Bus 1DB. Using Manual Chapter Attachment 0609.04, Phase 1 Initial Screening and Characterization of Findings, the inspectors determined that the finding represented a potential loss of core decay heat removal safety function under the mitigating systems cornerstone conditional upon specific offsite power grid system voltage circumstances. The VC Summer SDP phase 2 pre-solved Worksheet did not have a suitable surrogate for this finding therefore a phase 3 SDP analysis was performed by a regional SRA. The phase 3 SDP analysis utilized the latest VC Summer NRC SPAR risk model and an exposure period of 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> per year. The dominant sequence was a loss of DC Bus B with a failure of emergency feedwater and a failure to implement feed and bleed leading to core damage. The risk was mitigated by equipment available from Safety Bus 1DB and the short exposure period. The SDP phase 3 risk analysis result was an increase in core damage frequency of <1E-6 per year. The finding was characterized as GREEN, a finding of very low safety significance. A cross-cutting aspect was not identified because the finding does not represent current performance. |
| Site: | Summer  |
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| Report | IR 05000395/2011006 Section 1R21 |
| Date counted | Jun 30, 2011 (2011Q2) |
| Type: | NCV: Green |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | D Jones D Mas-Penaranda M Yeminy S Sandal B Desai J Eargle |
| INPO aspect | |
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Finding - Summer - IR 05000395/2011006 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Summer) @ 2011Q2
Self-Identified List (Summer)
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