05000321/FIN-2003006-01
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Finding | |
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| Title | Concerns Associated with Potential Opening of SRVs |
| Description | The team identified a potential concern in that the licensee used manual actions to isolate two 4 to 20 ma instrumentation loop control circuits associated with eleven SRVs in lieu of providing physical protection. This did not appear to be consistent with the plants licensing basis nor 10 CFR 50, Appendix R. Spurious action of these SRVs could impact the licensees fire mitigation strategy. In addition, the licensee provided no objective evidence that post-fire safe shutdown equipment could mitigate this event. The SSAR stated that a fire in Fire Area 2104 could cause all eleven SRVs to spuriously actuate as a result of fire damage to two cables located in close proximity in this area. The specific circuits that could cause this event were identified by the licensee as circuits ABE019C08 and ABE019C09. Each circuit separately provides a 4 to 20 ma instrumentation signal from an SRV high-pressure actuation transmitter 2B21-N127B or 2B21-N127D to its respective master trip unit (2B21-N697B or 2B21-N697D). The purpose of this circuitry was to provide an electrical backup to the mechanical trip capability of the individual SRVs. In the event of high reactor pressure, the circuits would provide a signal to the master trip units which would cause all eleven SRVs to actuate (open). The pressure signal from each transmitter would be conveyed to its respective master trip unit through a two-conductor, instrument cable that was routed through this fire area (two separate cables). Each cable consisted of a single twisted pair of insulated conductors, an uninsulated drain wire that was wound around the twisted pair of conductors, and a foil shield. In Fire Area 2104, the two cables were located in close proximity in the same cable tray. Actuation of the SRV electrical backup is completely blind to the operators. That is, unlike ADS, it does not provide any preactuation indication (e.g., actuation of the ADS timer) or an inhibit capability (e.g., ADS inhibit switch). Because the operators typically would not initiate a manual scram until fire damage significantly interfered with control of the plant, it is possible that all eleven SRVs could open at 100% power, prior to scramming the reactor. This event could place the plant in an unanalyzed condition. Unlike a typical control circuit, a direct short or hot short between conductors of a 4 to 20 ma instrument circuit may not be necessary to initiate an undesired (false high) signal. For cables that transmit low-level instrument signals, degradation of the insulation of the individual twisted conductors due to fire damage may be sufficient to cause leakage current to be generated between the two conductors. Such leakage current would appear as a false high pressure signal to the master trip units. If both cables were damaged as a result of fire, false signals generated as a result of leakage current in each cable, could actuate the SRV electrical backup scheme which would cause all eleven SRVs to open. The conductor insulation and jacket material of each cable was cross-linked polyethylene (XLPE). Because both cables were in the same tray and exposed to the same heating rate, there would be a reasonable likelihood that both instrumentation cables could suffer insulation damage at the same time and both circuits could fail high simultaneously. The licensees SSAR recognized the potential safety significance of this event and described methods that have been developed to prevent its occurrence and/or to mitigate its impact on the plants post-fire SSD capability (should it occur). To prevent this event, the licensee developed procedural guidance which directs operators to open link BB-10 in panel 2H11-P927 and link BB-10 in panel 2H11-P928. These panels are located in the main control room. Opening of these links would prevent actuation of the SRV trip units by removing the 4 to 20 ma signal fed by the pressure transmitters (PT) to the master trip units. In the event the SRVs were to open prior to the operators completing this action, the SSAR credits core spray loop A to mitigate the event. The inspection team had several concerns regarding the licensees approach to this potential spurious actuation of the SRVs. Specific concerns identified by the team include: 1. The links may not be opened in time to preclude inadvertent actuation of the SRVs. 2. The use of links to avoid inadvertent actuation of the SRVs did not appear to be consistent with the current licensing basis. 3. No objective evidence existed to demonstrate that the post-fire SSD equipment could adequately mitigate a fire in Fire Area 2104, if the SRVs were to open. 4. The operations staff would be unable to manually control the Group A SRVs, which are credited for mitigating a fire in Fire Area 2104, should they spuriously actuate as a result of fire-induced damage. With regard to the timing of operator actions to prevent fire damage from causing all SRVs to open, the licensee performed an evaluation during the inspection which estimated that approximately thirty minutes would pass from the time of fire detection to the time an operator would implement procedural actions to open the links. The inspectors independently arrived at a similar time estimate based on their review of the procedure. In response to inspectors concerns that this interval may be too lengthy to preclude fire damage to the cables of interest and subsequent actuation of the SRVs, the licensee agreed to enhance its existing procedures so that the action would be taken immediately following confirmation of fire in areas where the spurious actuation could occur. This issue is discussed in Section 1R05.04/.05.b.1 of this report. The team also determined that the opening of terminal board links was not in compliance with the plants licensing basis. Current licensing basis documents, specifically Georgia Power request for exemption dated May 16, 1986, and a subsequent NRC Safety Evaluation Report (SER) dated January 2, 1987, characterized the opening of links as a repair activity that is not permitted as a means of complying with 10 CFR 50, Appendix R, Section III.G. The inspectors concluded that, the opening of links was considered a repair by both the licensee and the NRC staff in 1987. The licensee could not provide any evidence to justify why these actions should not be characterized as a repair activity in its current SSAR. Additionally, because there is a potential for all SRVs to spuriously actuate as a result of fire in Fire Area 2104 at a time when RHR is not available, the SSAR credits the use of core spray loop A to accomplish the reactor coolant makeup function. During the inspection, the licensee performed a simulator exercise of an event which caused all 11 SRVs to open. During this exercise, simulator RPV level instruments indicated that core spray would be capable of maintaining level above the top of active fuel. However, the licensee did not provide any objective evidence (e.g., specific calculation or analysis) which demonstrated that, assuming worst-case fire damage in Fire Area 2104, the limited set of equipment available would be capable of mitigating the event in a manner that satisfied the shutdown performance goals specified in 10 CFR 50, Appendix R, Section III.L.1.e. Finally, the logic that was installed by design change request (DCR)91-134 for the SRVs was a two-out-of-two coincidence taken twice logic in addition to a one-out-of two coincidence taken twice logic. The team determined that the two-out-of-two coincidence logic input from trip unit master relays K310D and K335D represented a common cause failure for Group A SRVs for a fire in Fire Area 2104. Specifically, cable ABE019C08 associated with PT 2B21-N127B current loop, and cable ABE019C09 associated with PT 2B21-N127D current loop, were routed in close proximity to each other in the same cable tray in Fire Area 2104. Both shielded twisted pair instrument cables were unprotected from the effects of a fire in this fire area. Fire-induced insulation damage to both cables could result in leakage currents and cause the instrument loops to fail high. This failure mode would simulate a high nuclear boiler pressure condition and would initiate SRV backup actuation of all the Group A SRVs. Whenever a SRV lifted, it would remain open until pressure reduced to about 85% of its overpressure lift setpoint However, the instrument loops, having failed high, would ensure that the trip unit master relays and the trip unit slave relays continued to energize the pilot valve of the individual SRV and keep the SRV open. This issue is discussed in more detail in Section 1R21.01. Ultimately, this failure mode would prevent the operators from manually controlling the Group A SRVs as required per the SSAR. In response, the licensee initiated CR 2003800152, dated July 24, 2003, to evaluate actions to open links to determine if they are necessary to achieve hot shutdown, and if an exemption from Appendix R is required. Pending additional review by the NRC, this issue is identified as Unresolved Item (URI) 50-366/03-06-01, Concerns Associated with Potential Opening of SRVs. |
| Site: | Hatch  |
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| Report | IR 05000321/2003006 Section 1R05 |
| Date counted | Sep 30, 2003 (2003Q3) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.05 |
| Inspectors (proximate) | C Smith K Sullivan R Schin G Wiseman C Oglef Ehrhardtm Thomas P Fillion G Wiseman R Nease P Braxton J Dymek |
| INPO aspect | |
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Finding - Hatch - IR 05000321/2003006 | |||||||||||||
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Finding List (Hatch) @ 2003Q3
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