05000416/FIN-2012008-07
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Finding | |
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| Title | Potential Internal Flooding Caused by Circulation Water System Failure |
| Description | The inspectors reviewed Calculation M6.3.051, Circulating Water System-Calculate Revised Plant Flooding Elevations Due to Aux Cooling Tower, Revision B, to verify that the postulated failure of circulating water system components in the turbine building would not affect safety-related equipment required for achieving safe shutdown. This calculation assumes that the entire inventory of the circulating water system, 13.4 million gallons, is released into the Unit 1 turbine building due to a circulating water system failure and determines the resulting flood elevations. The calculation does not consider postulated flood flow rates; it is a steady state calculation based on the total circulating water system inventory being contained within the plant buildings. The calculation includes an assumption that the Unit 2 turbine building volume would be available to accommodate floodwater because the passage/corridor between the Unit 1 and Unit 2 turbine buildings is not watertight. In addition, the maximum flood elevation is calculated based on the volume of the radwaste building being available to accommodate floodwater. The sliding door between the Unit 1 turbine building and the radwaste building is not addressed in the calculation. Based on these assumptions, the calculation determines that the bounding flood elevation is 104.0 feet, and that the flood will not reach safety-related equipment located in the control building at elevation 111 feet. The calculation also determines that the bounding flood elevation would reach 111.4 feet in the control building if the volume of the Unit 2 turbine building were not considered. These calculated flood elevations do not include the additional volume contributed by 23,200 gallon per minute makeup from the plant service water system to the circulating water system. The calculation concludes that operator action to stop the makeup flow within 70 minutes is acceptable due to the margin available in the calculation. The inspectors questioned the assumptions of this calculation; especia!ly the assumption that buildings by passageways that are not watertight flood coincidently with each other. The inspectors asked if the expected leak rate between Unit 1 turbine building, Unit 2 turbine building, and the radwaste building through large sliding doors would be sufficient to limit the maximum flood elevation in the control building which is connected to the Unit 1 turbine building with a conventional door. During the inspection, the licensee performed Calculation M6.3.051-001, Circulating Water Systems - Calculate Revised Unit 1 Turbine Building and Unit 1 Control Building Flooding Elevations, Revision O. This calculation was performed to address the inspectors\\\' questions documented in Condition Report CR-GGN-2012-9424. This calculation was a transient analysis of the flood level considering the closed sliding doors between the Unit 1 turbine building and the Unit 2 turbine building and the Unit 1 turbine building and radwaste building. The calculation considered the gaps around the closed doors, and included the contribution of the makeup flow from the plant service water system to the circulating water system. However, Calculation M6.3.051-001, revision 0 was based on a limited flowrate from an expansion boot failure in the circulating water system. The calculation used the methodology of NRC Branch Technical Position MEB 3-1 to predict the maximum flow from a failed circulating water system expansion joint. Applying the MEB 3-1 methodology to the 10-foot diameter expansion joint results in a postulated crack of feet long and i-inch wide. This crack results in a calculated flowrate of approximately 15,500 gpm. Based on this limited flowrate, the calculation determined that the maximum flood elevation would be approximately 104 feet. The inspectors question the applicability of NRC Branch Technical Position MEB 3-1 to nonsafety-related expansion joints and asked the licensee to determine the maximum flood flowrate that would not exceed a flood elevation of 111 feet. In response to these questions, the licensee performed an informal analysis and determined that a flowrate of approximately 75,000 gpm or greater would result in exceeding a flood elevation in the Unit 1 turbine building, potentially communicating with the control building. The licensee also stated that they considered the application of the MEB 3-1 methodology to the expansion joints to be consistent with their licensing basis (UFSAR Section 3.6a.2.1) and that a gross failure of the expansion joint is highly unlikely since the expansion joint in reinforced with steel belts and leakage would be through a local defect. They also stated that the metal shield covering the expansion joints would serve to limit flow from the expansion joint failure, but did not provide the expected flowrate from a large failure of an expansion joint within the metal shield. The inspectors performed a review of licensing basis documentation related to flooding resulting from failures of circulating water components and did not identify any specific value for the maximum flood flowrate or the maximum postulated failure size in an expansion joint. Grand Gulf Nuclear Station Update Safety Analysis Report, Section 10.4.5.3, describes the potential of the entire volume of the circulating water system flooding the Unit 1 turbine building, discusses a potential gross failure in the circulating water system, and describes the maximum circulating water system flowrate but does not specifically address the maximum postulated flood flowrate from a circulating water system failure. The inspectors determined that design basis calculation M6.3.051, Revision B did not adequately verify that the postulated failure of circulating water system components in the turbine building would not affect safety-related equipment required for achieving safe shutdown. This steady state calculation did not consider the effects of closed doors on the maximum flood level in the control building. Calculation M6.3.051-001, Revision 0 was a transient analysis that did address the effects of the closed doors. However, this calculation was based on calculating a limited flood flowrate by applying the methodology of NRC Branch Technical Position MEB 3-1 to non safety-related circulating water system expansion joints. The inspectors were not able to determine if this methodology was consistent with the licensing basis during the period of the inspection. Resolution of this issue will require determining the maximum flowrate resulting from the postulated failure of a circulating water system component in the turbine building and verifying that the resulting flood elevation will not affect safetyrelated equipment required for achieving safe shutdown. The inspectors have discussed this design and licensing basis issue with NRC staff in the Office of Nuclear Reactor Regulation. Due to complexity of establishing the appropriate design and licensing bases for this issue, this item is considered unresolved pending further NRC review to determine if a finding exists. This will be tracked as URI 05000416/2012008-07, Internal Flooding Caused by Circulation Water System Failure. |
| Site: | Grand Gulf  |
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| Report | IR 05000416/2012008 Section 1R21 |
| Date counted | Sep 30, 2012 (2012Q3) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | I Anchondo N Okonkwo J Braistedd Proulxg George J Laughlin L Carson N Okonkwo R Kumana R Smith S Achen S Makor B Rice |
| INPO aspect | |
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Finding - Grand Gulf - IR 05000416/2012008 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Grand Gulf) @ 2012Q3
Self-Identified List (Grand Gulf)
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