05000416/FIN-2014007-01
From kanterella
Jump to navigation
Jump to search
Finding | |
|---|---|
| Title | Possible Spurious Actuation of The Safety Relief Valves During Control Room Fire Scenarios |
| Description | The team reviewed the licensees safe shutdown analyses, thermal hydraulic analysis, and the licensing basis for control room fire scenarios. The team identified three issues of concern that require additional information and inspection for resolution. Concern 1: Required Alternative Shutdown Scenarios The first issue of concern was associated with the identification of the control room fire scenarios that were required by the licensing basis to be considered and mitigated. The NRC promulgated guidance for alternative and dedicated shutdown capability in Generic Letter 86-10, Implementation of Fire Protection Requirements. In Question 5.3.10, Design Basis Plant Transients, the staff addressed the plant transients that should be considered in the design of alternative or dedicated shutdown systems. The staff stated that a loss of offsite power shall be assumed for any alternative shutdown area. In addition, the staff stated that the safe shutdown capability should not be adversely affected by a fire in any plant area which results in the loss of all automatic function (signals, logic) from the circuits located in the area in conjunction with one worst case spurious actuation or signal resulting from the fire. On February 24, 2005, the licensee identified a concern with the transient analysis for a control room fire and documented it in Condition Report CR-GGN-2005-00770. The licensee stated that, it appears that it does not assume the loss of offsite power concurrent with the loss of all automatic functions (signals, logic) in conjunction with the worst case spurious actuation or signal resulting from the fire as required by Section III.L of Appendix R and Generic Letter 86-10. On August 24, 2005, the licensee evaluated this concern and determined that the fire induced spurious actuation of the safety relief valves could occur in any of three ways: 1. A single intra-cable short within the control circuit cable could actuate an individual safety relief valve. The safety relief valve control cables were routed together within the control room with each cable containing multiple control conductors and multiple 125 Vdc conductors. 2. Two intra-cable shorts within two separate instrument cables or two intra-cable shorts within a single instrument cable could actuate all eight of the safety relief valves associated with the automatic depressurization system. 3. Two intra-cable shorts within two separate instrument cables to either Division I or Division II circuits could open all 20 safety relief valves. The licensee concluded in their evaluation that it is expected that the worst case spurious actuation or signal resulting from a fire in the control room would involve opening of 20 safety relief valves. On November 2, 2009, the NRC provided the following additional guidance for alternative and dedicated shutdown capability in Regulatory Guide 1.189, Fire Protection for Nuclear Power Plants, Revision 2: The licensee should consider one spurious actuation or signal to occur before control of the plant is achieved through the alternative or dedicated shutdown system for fires in areas that require alternate or dedicated shutdown. After the operators transfer control from the control room to the alternative or dedicated shutdown system, single or multiple spurious actuations that could occur in the fire-affected area should be considered, in accordance with the plants approved fire protection program. The approach outlined in Appendix D to NEI 00-01 provides an acceptable methodology for evaluating alternative and dedicated shutdown, when applied in conjunction with this regulatory guide. In addition, the second paragraph of Appendix G to NEI 00-01 provides information regarding the analysis of multiple spurious actuations for alternative and dedicated shutdown systems. The licensee continued to evaluate this concern through their corrective action program. On August 17, 2012, the licensee developed Engineering Report GGNS-EE-10-00003, Safe Shutdown Evaluation of Control Room Fire Scenarios, Revision 0. In this report, the licensee relied on guidance contained in Appendix G, Generic List of MSOs, to NEI 00-01, Guidance for Post Fire Safe Shutdown Circuit Analysis, Revision 2, when evaluating the effect of a control room fire spuriously actuating all 20 safety relief valves. Specifically, the licensee stated: In accordance with the second paragraph of Appendix G to NEI 00-01, scenarios that involve spurious operation of multiple safe shutdown components concurrent with failure of automatic functions need not be included in the analysis of impacts for the main control room. However, if plant response to such transients could negate the capability to achieve and maintain post-fire safe shutdown, a voluntary review of the postulated scenarios may be warranted to supplement a previously approved alternative shutdown capability that scenarios that involve the spurious operation of multiple safe shutdown components concurrent with the failure of automatic functions need not be included in the analysis of impacts for the control room. The team reviewed the NRC positions provided in Regulatory Guide 1.189, Revision 2, and discussed these positions with personnel in the Office of Nuclear Reactor Regulation. The team determined that the NRC endorsed the approach outlined in Appendix D, Alternative/Dedicated Shutdown Requirements, to NEI 00-01, Revision 2 (when applied in conjunction with Regulatory Guide 1.189, Revision 2). The team also determined that the NRC neither endorsed nor rejected the approach for analyzing multiple spurious actuations for alternative and dedicated shutdown systems outlined in Appendix G to NEI 00-01, Revision 2. The team was concerned that the licensee may not have adequately followed the guidance for identifying the control room fire scenarios that were required to be considered and mitigated. The NRC staff will need to perform additional review to determine if the plants licensing basis required the licensee to analyze and mitigate the spurious actuation of a single safety relief valve (due to a single intra-cable hot short in a single cable), the spurious actuation of the automatic depressurization system (due to two intra-cable hot shorts in a single cable), or the spurious actuation of all twenty safety relief valves (due to a single intra-cable hot short in two separate cables). Concern 2: Time Available for Operators to Depressurize the Reactor The second issue of concern was associated with the amount of time available for operators to depressurize the reactor during control room fire scenarios. For control room fires, the licensees alternative shutdown strategy required operators to take immediate actions to restore electrical power, align a residual heat removal pump in the low pressure coolant injection mode, and depressurize the reactor using six safety relief valves prior to the reactor vessel level reaching -160 . The licensee described the plant response during an alternative shutdown in Engineering Report GGNS-NE-10-00003, GGNS EPU Appendix R Fire Protection, Revision 2. This thermal hydraulic analysis calculated the amount of time for the reactor vessel level to reach -160 (assuming no high pressure injection sources were available) and the resulting maximum peak clad temperature. Using a nominal scenario with no spurious actuations, the licensee determined the reactor vessel level would reach -160 within 14.3 minutes and the reactor would experience a maximum peak clad temperature of 597 F. The team performed a timed walkdown of the alternative shutdown procedure and determined that it took operators approximately 12 minutes to align the equipment required for depressurizing the reactor and injecting with the residual heat removal pump. The licensee also analyzed the plant response using an alternate scenario with the spurious actuation of all 20 safety relief valves. In this alternate scenario, the licensee determined the residual heat removal pump would initiate in the low pressure coolant injection mode within 118 seconds to restore level, the residual heat removal pump would begin injecting water into the reactor within 153 seconds, and the reactor would experience a maximum peak clad temperature of 597 F. For the evaluation of the alternate scenario, the licensee relied upon guidance contained in NEI 00-01, Appendix G, to credit the automatic initiation of the residual heat removal pump in the low pressure coolant injection mode. The team noted that this assumption could be considered adequate for analyzing control room fire scenarios that were outside of the licensing basis, but would be considered inadequate for demonstrating compliance with control room fire scenarios that were required to be considered and mitigated since it credited the availability of electrical power as well as the automatic initiation of the residual heat removal pump. In either case, the team noted that the evaluation of the alternate scenario failed to account for the steps in the alternative shutdown procedure that directed operators to open the breaker for the residual heat removal pump prior to restoring electrical power and subsequently restarting the pump. The team performed a timed walkdown of the alternative shutdown procedure and determined that it took operators approximately 8 minutes to open the breaker for the residual heat removal pump and approximately 12 minutes to depressurize the reactor and restart the residual heat removal pump. The team was concerned that the licensees determination of the time available for operators to depressurize the reactor may be incorrect. Specifically, the team was concerned that the licensees determination that operators had 14.3 minutes available to take the required immediate actions prior to the reactor vessel level reaching -160 may be incorrect since the evaluation failed to assume any spurious actuations. As described in the previous concern, the licensee may be required to consider and mitigate the spurious actuation of a single safety relief valve, the automatic depressurization system, or all 20 safety relief valves. The team noted that the thermal hydraulic analysis did not evaluate the plant response to the spurious actuation of a single safety relief valve or the automatic depressurization system. The team required additional information in order to resolve this concern. Specifically, the team required a thermal hydraulic analysis that provided the plant response to the control room fire scenarios that were required to be considered and mitigated. This thermal hydraulic analysis will be used to determine the amount of time available for operators to restore electrical power, align the residual heat removal pump in the low pressure coolant injection mode, and prepare to depressurize the reactor prior to reaching a reactor vessel level of -160 . Concern 3: Isolation of the Safety Relief Valve Circuits The third issue of concern was associated with the isolation of the safety relief valve circuits. For control room fires, the licensees alternative shutdown strategy required operators to open two breakers (72-11A23 and 72-11B34) in order to ensure that the 14 non-credited safety relief valves were closed. The six credited safety relief valves were isolated from the control room via the use of transfer switches. The team was concerned 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 valve circuits also contained other 125 Vdc circuits that may remain energized during an alternative shutdown. The team was concerned that hot shorts from one of these circuits could prevent the closure of safety relief valves (if spuriously open) or could spuriously open the safety relief valves after the control room was isolated and control transferred from the control room to the remote shutdown panel. The team required additional information in order to resolve this concern. Specifically, the team required an evaluation of the remaining circuits in the control room panels that contain the safety relief valve circuits in order to determine if any of these other circuits remain energized during an alternative shutdown. The licensee entered these issues of concern into the corrective action program as Condition Report CR-GG-2014-03690. These issues of concern are being treated as an Unresolved Item 05000416/2014007-01, Possible Spurious Actuation of the Safety Relief Valves During Control Room Fire Scenarios. |
| Site: | Grand Gulf  |
|---|---|
| Report | IR 05000416/2014007 Section 1R05 |
| Date counted | Jun 30, 2014 (2014Q2) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.05 |
| Inspectors (proximate) | J Dixon J Mateychick N Okonkwo S Alferink S Gravesj Mateychicks Makor N Okonkwo E Uribe G Werner |
| INPO aspect | |
| ' | |
Finding - Grand Gulf - IR 05000416/2014007 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Finding List (Grand Gulf) @ 2014Q2
Self-Identified List (Grand Gulf)
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||