05000220/FIN-2008008-02
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Finding | |
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| Title | Vital Bus Degraded Voltage Relay Time Delay Licensing Bases |
| Description | The team identified an unresolved issue pertaining to the adequacy of the time delay setting for the Unit 1 degraded voltage relays. Specifically, the team questioned what Constellations licensing bases requirement was with respect to a degraded voltage condition concurrent with a postulated loss-of-coolant-accident (LOCA). The team noted that the existing time delay of a nominal 21 seconds, from the detection of a sustained degraded voltage condition until vital bus transfer to the EDGs was longer than the time assumed by the 10 CFR 50.46 LOCA analysis sequential loading time following the receipt of a LOCA signal. NRC letter dated June 2, 1977, sent to all operating plants at that time, stated in position B.1.c.1 that the allowable time delay for the degraded voltage protection scheme, including margin, shall not exceed the maximum time delay that is assumed in the UFSAR accident analysis. By letter dated July 14, 1977 to the NRC, NMP stated that the time delay associated with the degraded voltage protection scheme met this criterion. The NRC accepted the proposed modifications and Technical Specification changes based, in part, on the conclusion that changes satisfied the criteria stated in position B.1.c.1. By letter dated November 9, 1984, the NRC issued Amendment No. 67 to the NMP Unit 1 operating license. This included a revised Technical Specification Table 3.6.2.i, incorporating a degraded voltage relay setpoint of 3600V and a time delay of a nominal 18.5 seconds at 3580V. Subsequently by letter dated April 17, 1994, the NRC issued Amendment No. 148 to the NMP Unit 1 operating license. This included a revised Technical Specification Table 3.6.2.i incorporating a degraded voltage relay (DVR) setpoint of 3705V and a time delay of greater than 3.4 seconds but less than 60 seconds. The 3.4 second minimum time delay was described as the minimum time required to clear the voltage transients due to load sequencing to avoid separation from offsite power. The 60 second maximum time delay was described as the maximum time allowable to preclude load damage of the trip device actuation at voltages below the DVR setpoint of 3705V. These setpoints were still in place at the time of this inspection. In 2005, calculation 4.16KVAC-PB102/103SETPT/27 disposition 002B changed the time dial setting to 5 in order to allow additional time (21 seconds) at the DVR setpoint of 3705V for the automatic load tap changers on the reserve auxiliary transformers to improve voltage and avoid spurious grid separation. Modification N1-02-029 concluded that the 21 second time delay was acceptable because it was less than the 60 second time delay allowed by the TS Table 3.6.2i. The team noted that the current Updated Final Safety Analysis Report (UFSAR) accident analysis assumes that the last core spray topping pump attains full speed within 35 seconds. Based on a 21 second time delay, the time before the last core spray topping pump attains full speed was estimated by Constellation to be approximately 56 seconds. Constellation initiated a condition report to evaluate this issue. They performed an operability review for the condition report associated with this issue (CR-NM-2008-7746). They concluded that the additional time delay would result in a peak cladding temperature of approximately 1950F versus 1850F with the original time delay. This provided a reasonable basis that they would remain bellow the 10 CFR50.46 limit of 2200F assuming a postulated LOCA concurrent with a degraded voltage condition. Constellation identified that the requirement for degraded voltage concurrent with a postulated LOCA was not well defined in their licensing bases and initiated the condition report for additional review. The team concluded that the existing allowable time delay for the NMP Unit 1 degraded voltage scheme was an issue requiring further NRC review to determine if NMP Unit 1 is in compliance with their licensing bases for degraded voltage protection. (URI 05000220/2008008-02, Vital Bus Degraded Voltage Time Delay Licensing Bases |
| Site: | Nine Mile Point  |
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| Report | IR 05000220/2008008 Section 1R21 |
| Date counted | Dec 31, 2008 (2008Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | F Arner G Skinner J Schoppy L Casey L Doerflein M Gotch M Patel M Shlyambergd Kernl Doerflein |
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Finding - Nine Mile Point - IR 05000220/2008008 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Nine Mile Point) @ 2008Q4
Self-Identified List (Nine Mile Point)
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