05000338/FIN-2015007-01
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Finding | |
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| Title | Adequacy of Class 1E 120VAC Vital Bus Design |
| Description | Calculation 14258.79-E-4, Short Circuit Currents 120V AC Vital Buses and Miscellaneous Circuits Appendix R Evaluation, Rev.1, Addendum C, stated that the maximum short circuit available to the 120VAC vital buses from the Units 1 and 2 20KVA inverters was 200% of rated full load current, equaling 334A, and 175% of full rated current, equaling 365A, when supplied by the 25KVA voltage regulating transformer. These values were input into Technical Report EE-0118, 10 CFR Part 50 Appendix R Electrical Distribution System Coordination Study, Rev. 2, to verify proper breaker coordination for the 120 VAC Vital instrumentation buses. In 2003, the licensee received concurrence from the vital inverter vendor stating that while the steady state short circuit current limit was indeed approximately 200% of rated full load current for the inverter, the steady state short circuit current was approximately 200% of rated full load current for the regulating transformer, which was different than what was assumed in the technical report. The memo also stated that the 12 cycle instantaneous fault current for the inverter and regulating transformer would be approximately 500% of rated full load current, equaling 833A and 1042A for the inverter and transformer respectively. These values were not evaluated in the technical report. The team noticed that when the 120VAC instrument buses were supplied by the regulating transformer, a condition allowed by TS, breaker coordination could not be verified for the 120VAC buses based on the instantaneous fault values concurred on by the vendor. Specifically, coordination could not be verified for the breakers associated with the 1-I and 2-I 120VAC buses. TS 3.8.7 allows the regulating transformer to supply the vital buses for <=24 hours while the batteries are being equalized and TS 3.8.9 allows the licensee to consider the vital buses operable while they are energized from this transformer. The team was concerned that TS could allow the licensee to be on the regulating transformer when coordination could not be verified for the 120VAC buses. The lack of breaker coordination could result in the loss of additional engineered safety features during a design bases event. The licensee entered this issue into their corrective action program as CR1006865. This issue is a URI pending the determination of whether a violation of NRC requirements exists. |
| Site: | North Anna  |
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| Report | IR 05000338/2015007 Section 1R21 |
| Date counted | Sep 30, 2015 (2015Q3) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | C Baron G Ottenberg J Bartley M Riley O Mazzoni S Herrick T Skaggs-Ryang Croong Eatmon G Ottenberg J Panfel J Rivera M Riley R Kellner S Rose |
| INPO aspect | |
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Finding - North Anna - IR 05000338/2015007 | ||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (North Anna) @ 2015Q3
Self-Identified List (North Anna)
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