05000305/FIN-2007006-03
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Finding | |
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| Title | No Analysis for Out of Phase Fast Transfer |
| Description | The inspectors identified an unresolved item regarding the ability of the plant to perform a fast transfer. The electrical distribution system features a voltage restoration scheme that will automatically transfer a safety related 4160V bus to an alternate source of power if an undervoltage condition develops, and voltage is available on an alternate source. The inspectors identified that an out of phase transfer could occur due the delay in the relaying scheme for the transfer that is sufficient to allow motor speed to decay while the bus is disconnected from both power sources during the transfer. An out of phase transfer could also be caused by a transfer between sources that are not synchronized. For very fast transfers between synchronized sources, motor speed does not have an opportunity to decay to unacceptable levels before the bus is connected to the new source. For slow transfers, voltage on the transferred bus will decay to levels where adverse effects will not occur. A review of the transfer relaying showed that the expected transfer time is in the region where adverse effects could occur. The magnitude of the effects would depend on the relay tolerances and the type and number of loads connected to the transferred bus. The electrical distribution system is also susceptible to transfers between unsynchronized sources when the Bank No. 10 transformer is disconnected from the 138 kV switchyard. The potential for adverse effects also depends on the type of undervoltage signal triggering the voltage restoration scheme. For transfers initiated by a loss of voltage signal, the delay in the loss of voltage scheme will enable bus voltage to decay to safe levels. However, for transfers initiated by the16 Attachment degraded voltage scheme, voltage levels could remain at unsafe levels during the transfer. The inspectors determined that there were several alignments allowed by Technical Specifications and operating procedures that could result in damaging transfers. Of particular concern were alignments where both Bus 5 and 6 were aligned to the same offsite source, and the other source remained in service. Although this configuration was infrequently entered, it was allowable under station procedures without time limits. The inspectors was concerned that in this configuration, simultaneous transfer of both buses to the other offsite power source could, as a worst case, result in a complete loss of both redundant safety divisions. The inspectors was also concerned that when the Bank No. 10 transformer was disconnected from the 138 kV switchyard, and the redundant safety buses were aligned to their normal sources (Bus 5 aligned to the TAT and Bus 6 aligned to the RAT), that degraded voltage could occur on Bus 5 causing an out of phase transfer to the RAT. Once again, although this configuration was infrequently entered, it was allowable under station procedures without time limits. The operability discussion in CAP 041804 presented arguments that damaging fast transfers were precluded by compensatory measures previously implemented, that would avoid certain vulnerable configurations, and that the licensing basis excluded a partial loss of offsite power (LOOP). In other words, the LOOP from only one of the two required offsite circuits was not required to be considered since the design basis was a loss of coolant accident (LOCA) concurrent with a LOOP, with a LOOP being defined as a complete loss of offsite power. Consequently, no technical discussion was provided regarding actual effects of a fast transfer. The inspectors noted that the compensatory measures taken in response to NCV 05000305/2007006-01 and NCV 05000305/2007006-02 addressed this concern. This issue has been forwarded to the Office of Nuclear Reactor Regulation to determine the design and licensing basis with respect to a LOOP event. Pending resolution, this item will be tracked as an unresolved item (URI 05000305/2007006-03 (DRS)). |
| Site: | Kewaunee  |
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| Report | IR 05000305/2007006 Section 1R21 |
| Date counted | Jun 30, 2007 (2007Q2) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | J Cassidy T Bilik J Neurauter M Kunowski S Burton S Bakhsh S Atwater V Meghani K Barclay J Pearson M Learn E Love E Sanchez-Santiagog Skinnerm Yeminy R Langstaff S Burgess Z Falevits A Dahbur A Stone C Brown F Trap |
| INPO aspect | |
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Finding - Kewaunee - IR 05000305/2007006 | |||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Kewaunee) @ 2007Q2
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