05000458/FIN-2013007-04
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Finding | |
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Title | Unresolved Item Associated with the Isolation of the Alternative Shutdown System |
Description | The team identified an unresolved item associated with the isolation of post-fire safe shutdown circuitry for control room fire scenarios. Specifically, the team identified that the licensee may not adequately isolate circuitry for the safety relief valves and the main steam isolation valves from the effects of a control room fire. In the event of a fire in the control room, the licensee must ensure control circuitry for equipment credited for post-fire safe shutdown is electrically isolated from the control room so that fire damage could not prevent the ability to achieve and maintain safe shutdown conditions. For valves that are required to close or remain closed for post-fire safe shutdown, the licensee must ensure that control room fires do not prevent the closure of the valves and do not spuriously open the valves once the control room has been isolated and control transferred from the control room to the remote shutdown panel. Example 1: Spurious Opening of the Safety Relief Valves The alternative shutdown procedure provided steps for operators to mitigate the effects of any single spurious actuation or signal resulting from a control room fire that occurred prior to transferring control from the control room to the remote shutdown panel. For the safety relief valves, the procedure directed operators to de-energize two 125 Vdc panels (ENB-PNL02A and ENB-PNL02B) in order to ensure that the 13 non-credited safety relief valves were closed. The three credited safety relief valves were isolated from the control room via the use of transfer switches. The team identified a concern that hot shorts in the control room could cause a spurious actuation that threatened the ability to achieve and maintain safe shutdown conditions. The team noted that the control room cabinets containing the safety relief valves also contained other 125 Vdc circuits that remained energized during an alternative shutdown. The team was concerned that hot shorts from one of these circuits could prevent the closure of a safety relief valve (if spuriously open) or could spuriously open the safety relief valve once the control room was isolated and control transferred from the control room to the remote shutdown panel. The team was also concerned that the safe shutdown analysis did not analyze for one or more safety relief valves remaining open during the plant shutdown. This concern applied to the 13 safety relief valves that did not have control transferred to the remote shutdown panel. In addition, the team noted that circuit failures could spuriously open multiple safety relief valves through the spurious actuation of the automatic depressurization system. The team was concerned that the spurious actuation of the automatic depressurization system could be considered a single spurious actuation or signal that fell within the bounds of the safe shutdown analysis. A similar concern was first identified during the 1997 fire protection functional inspection and documented in Inspection Reports97-201 and 98-16. Example 2: Spurious Opening of the Main Steam Isolation Valves As noted in the previous example, the alternative shutdown procedure provided steps for operators to mitigate the effects of any single spurious actuation or signal resulting from a control room fire that occurred prior to transferring control from the control room to the remote shutdown panel. For the main steam isolation valves, the procedure directed operators to attempt to close the main steam isolation valves inside the control room and then de-energize the reactor protection system motor generator sets outside the control room. The reactor protection system provides power to the circuitry for the main steam isolation valve solenoids. When the solenoids are de-energized, the main steam isolation valves fail closed. The team identified a concern that hot shorts in the control room could cause spurious actuations that threatened the ability to achieve and maintain safe shutdown conditions. Specifically, the team identified that a portion of the trip logic circuitry was connected in the control room to the portion of the circuitry that energizes the solenoid valve for each main steam isolation valve. The trip logic circuitry was located downstream of where the reactor protection system bus was de-energized, and it did not contain a protective circuit device such as fusing or open contacts that would isolate the trip logic portion of the circuitry from the solenoid valve. The control room cabinet containing the trip logic circuitry also contained other 125 Vdc circuits that remained energized during an alternative shutdown. The team was concerned that hot shorts from these circuits could prevent the closure of the main steam isolation valves or could spuriously open the main steam isolation valves after the reactor protection system motor generator sets were de-energized. The team noted that one main steam isolation valve, either inboard or outboard, must close and remain closed in order to maintain inventory. The licensee entered these issues into the corrective action program as Condition Report CR-RBS-2013-03473. The team determined that additional inspection is required to determine if a performance deficiency exists. This issue of concern is being treated as an Unresolved Item URI 05000458/2013007-04, Unresolved Item Associated with the Isolation of the Alternative Shutdown System. |
Site: | River Bend |
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Report | IR 05000458/2013007 Section 1R05 |
Date counted | Dec 31, 2013 (2013Q4) |
Type: | URI: |
cornerstone | Mitigating Systems |
Identified by: | NRC identified |
Inspection Procedure: | IP 71111.05 |
Inspectors (proximate) | E Uribe G Pick G Werner S Alferinka Barrettg Miller J Mateychick S Achen S Alferink |
INPO aspect | |
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Finding - River Bend - IR 05000458/2013007 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (River Bend) @ 2013Q4
Self-Identified List (River Bend)
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