ML15296A377: Difference between revisions
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==Dear Mr. Gatlin:== | ==Dear Mr. Gatlin:== | ||
The purpose of this letter is to transmit a supplement to the U.S. Nuclear Regulatory Commission (NRG) staff's assessment for Virgil C. Summer Nuclear Station, Unit 1 (Summer) reevaluated flood hazard information that was issued to you by letter dated December 23, 2014 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML 14356A002). The supplement updates the original NRG staff assessment to address changes in the NRC's approach to the steps following the review of the flood hazard reevaluations as directed by the Commission. The letter also addresses the next steps associated with the mitigation strategies assessment with respect to the reevaluated flood hazards. By letter dated March 12, 2012 (ADAMS Accession No. ML 12053A340), the NRG issued a request for information pursuant to Title 10 of the Code of Federal Regulations, Section 50.54(f) (hereafter referred to as the 50.54(f) letter). The request was issued as part of implementing lessons learned from the accident at the Fukushima Dai-ichi nuclear power plant. Enclosure 2 to the 50.54(f) letter requested licensees to reevaluate flood-causing mechanisms using present-day methodologies and guidance. By letter dated March 12, 2013 (ADAMS Accession No. ML 13073A114), South Carolina Electric and Gas Company responded to this request for Summer. This response was supplemented by letters dated August 22, 2013, March 26, 2014, and November 24, 2014 (ADAMS Accession Nos. ML 13240A005, ML 14093A320, and ML 14329B257. By letter dated December 23, 2014, the NRG staff transmitted to the licensee a staff assessment of the information provided in the aforementioned letters. The NRG staff has completed its review of the information provided, as documented in the staff assessment and the enclosed supplement to the staff assessment. This closes out the NRC's efforts associated with CAC No. MF1112. The enclosed supplement to the staff assessment updates the NRG staff's conclusions in accordance with the flood hazard reevaluation approach described in NRG letter dated September 1, 2015 (ADAMS Accession No. ML 15174A257), concerning the coordination of requests for information regarding flooding hazard reevaluations and mitigating strategies for beyond-design-basis external events. This letter describes the changes in the NRC's approach to the flood hazard reevaluations that were approved by the Commission in its Staff T. Gatlin | The purpose of this letter is to transmit a supplement to the U.S. Nuclear Regulatory Commission (NRG) staff's assessment for Virgil C. Summer Nuclear Station, Unit 1 (Summer) reevaluated flood hazard information that was issued to you by letter dated December 23, 2014 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML 14356A002). The supplement updates the original NRG staff assessment to address changes in the NRC's approach to the steps following the review of the flood hazard reevaluations as directed by the Commission. The letter also addresses the next steps associated with the mitigation strategies assessment with respect to the reevaluated flood hazards. By letter dated March 12, 2012 (ADAMS Accession No. ML 12053A340), the NRG issued a request for information pursuant to Title 10 of the Code of Federal Regulations, Section 50.54(f) (hereafter referred to as the 50.54(f) letter). The request was issued as part of implementing lessons learned from the accident at the Fukushima Dai-ichi nuclear power plant. Enclosure 2 to the 50.54(f) letter requested licensees to reevaluate flood-causing mechanisms using present-day methodologies and guidance. By letter dated March 12, 2013 (ADAMS Accession No. ML 13073A114), South Carolina Electric and Gas Company responded to this request for Summer. This response was supplemented by letters dated August 22, 2013, March 26, 2014, and November 24, 2014 (ADAMS Accession Nos. ML 13240A005, ML 14093A320, and ML 14329B257. By letter dated December 23, 2014, the NRG staff transmitted to the licensee a staff assessment of the information provided in the aforementioned letters. The NRG staff has completed its review of the information provided, as documented in the staff assessment and the enclosed supplement to the staff assessment. This closes out the NRC's efforts associated with CAC No. MF1112. The enclosed supplement to the staff assessment updates the NRG staff's conclusions in accordance with the flood hazard reevaluation approach described in NRG letter dated September 1, 2015 (ADAMS Accession No. ML 15174A257), concerning the coordination of requests for information regarding flooding hazard reevaluations and mitigating strategies for beyond-design-basis external events. This letter describes the changes in the NRC's approach to the flood hazard reevaluations that were approved by the Commission in its Staff T. Gatlin Requirements Memorandum (ADAMS Accession No. ML15209A682) to COMSECY-15-0019 (ADAMS Accession No. ML 15153A104) that described the NRC's mitigating strategies and flooding hazard reevaluation action plan. As documented in the NRC staff assessment and the enclosed supplement, the staff has concluded that the licensee's reevaluated flood hazard information is suitable for the assessment of mitigation strategies developed in response to Order EA-12-049 (i.e., defines the mitigating strategies flood hazard information described in guidance documents currently being finalized by the industry and NRC staff) for Summer. Further, the licensee's reevaluated flood hazard information is suitable for other assessments associated with Near-Term Task Force Recommendation 2.1 "Flooding". The reevaluated flood hazard results for local intense precipitation, streams and rivers, and storm surge, were not bounded by the current design-basis flood hazard. In order to complete its response to Enclosure 2 to the 50.54(f) letter, the licensee is expected to submit a revised integrated assessment or focused evaluation(s), as appropriate, to address these reevaluated flood hazards, as described in the NRC's September 1, 2015, letter. If you have any questions, please contact me at (301) 415-6185 or email at Anthony.Minarik@nrc.gov. Docket No. 50-395 | ||
==Enclosure:== | ==Enclosure:== | ||
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==1.0 INTRODUCTION== | ==1.0 INTRODUCTION== | ||
This document is a supplement to the U.S. Nuclear Regulatory Commission (NRC) staff assessment that was transmitted by letter dated December 23, 2014 (NRC, 2014d), to South Carolina Electric and Gas Company (SCE&G, the licensee) for Virgil C. Summer Nuclear Station, Unit 1 (Summer, VCSNS). With the exceptions of the Table 3.1-1 and the Reference section, this supplement only contains the sections that were changed to resolve the open items and reflect the changes in the NRC's approach to the flood hazard reevaluations that were approved by the Commission in its Staff Requirements Memorandum (SRM) (NRC, 2015a) to COMSECY-15-0019 (NRC, 2015b), which described the NRC's mitigating strategies and flooding hazard reevaluation action plan. Table 3.1-1 at the end of the supplement is copied from the staff assessment for convenience. Instead of repeating the Reference section in its entirety, only the additions to the list of references are included in the supplement. 2.0 REGULATORY BACKGROUND There are no changes or updates to this section of the NRC staff assessment 2.1 Applicable Regulatory Requirements There are no changes or updates to this section of the NRC staff assessment. 2.2 Enclosure 2 to the 50.54(f) Letter By letter dated March 12, 2012 (NRC, 2012a) the NRC issued a request for information Pursuant to Title 10 of the Code of Federal Regulations (10 CFR), Section 50.54(f) (hereafter referred to as the 50.54(f) letter). The 50.54(f) letter requests all power reactor licensees and construction permit holders reevaluate all external flooding-causing mechanisms at each site. The reevaluation should apply present-day methods and regulatory guidance that are used by the NRC staff to conduct early site permit (ESP) and combined license (COL) reviews. This includes current techniques, software, and methods used in present-day standard engineering Enclosure practice. If the reevaluated flood-causing mechanisms are not bounded by the current plant design-basis flood hazard, an integrated assessment or focused evaluation may be necessary. 2.2.1 Flood-Causing Mechanisms There are no changes or updates to this section of the NRG staff assessment. 2.2.2 Associated Effects There are no changes or updates to this section of the NRG staff assessment. 2.2.3 Combined Effects Flood There are no changes or updates to this section of the NRG staff assessment. 2.2.4 Flood Event Duration There are no changes or updates to this section of the NRG staff assessment. 2.2.5 Actions Following the flooding hazard reevaluation report (FHRR) For the sites where the reevaluated flood probable maximum flood (PMF) elevation is not bounded by the current design-basis flood PMF elevation for all flood-causing mechanisms, the 50.54(f) letter (NRG, 2012a) requests licensees and construction permit holders to: | This document is a supplement to the U.S. Nuclear Regulatory Commission (NRC) staff assessment that was transmitted by letter dated December 23, 2014 (NRC, 2014d), to South Carolina Electric and Gas Company (SCE&G, the licensee) for Virgil C. Summer Nuclear Station, Unit 1 (Summer, VCSNS). With the exceptions of the Table 3.1-1 and the Reference section, this supplement only contains the sections that were changed to resolve the open items and reflect the changes in the NRC's approach to the flood hazard reevaluations that were approved by the Commission in its Staff Requirements Memorandum (SRM) (NRC, 2015a) to COMSECY-15-0019 (NRC, 2015b), which described the NRC's mitigating strategies and flooding hazard reevaluation action plan. Table 3.1-1 at the end of the supplement is copied from the staff assessment for convenience. Instead of repeating the Reference section in its entirety, only the additions to the list of references are included in the supplement. 2.0 REGULATORY BACKGROUND There are no changes or updates to this section of the NRC staff assessment 2.1 Applicable Regulatory Requirements There are no changes or updates to this section of the NRC staff assessment. 2.2 Enclosure 2 to the 50.54(f) Letter By letter dated March 12, 2012 (NRC, 2012a) the NRC issued a request for information Pursuant to Title 10 of the Code of Federal Regulations (10 CFR), Section 50.54(f) (hereafter referred to as the 50.54(f) letter). The 50.54(f) letter requests all power reactor licensees and construction permit holders reevaluate all external flooding-causing mechanisms at each site. The reevaluation should apply present-day methods and regulatory guidance that are used by the NRC staff to conduct early site permit (ESP) and combined license (COL) reviews. This includes current techniques, software, and methods used in present-day standard engineering Enclosure practice. If the reevaluated flood-causing mechanisms are not bounded by the current plant design-basis flood hazard, an integrated assessment or focused evaluation may be necessary. 2.2.1 Flood-Causing Mechanisms There are no changes or updates to this section of the NRG staff assessment. 2.2.2 Associated Effects There are no changes or updates to this section of the NRG staff assessment. 2.2.3 Combined Effects Flood There are no changes or updates to this section of the NRG staff assessment. 2.2.4 Flood Event Duration There are no changes or updates to this section of the NRG staff assessment. 2.2.5 Actions Following the flooding hazard reevaluation report (FHRR) For the sites where the reevaluated flood probable maximum flood (PMF) elevation is not bounded by the current design-basis flood PMF elevation for all flood-causing mechanisms, the 50.54(f) letter (NRG, 2012a) requests licensees and construction permit holders to: | ||
* Submit an Interim Action Plan with the FHRR documenting actions planned or already taken to address the reevaluated hazard(s). | * Submit an Interim Action Plan with the FHRR documenting actions planned or already taken to address the reevaluated hazard(s). | ||
* Perform an integrated assessment subsequent to the FHRR to (a) evaluate the effectiveness of the current design-basis (i.e., flood protection and mitigation systems), (b) identify plant-specific vulnerabilities, and (c) assess the effectiveness of existing or planned systems and procedures for protecting against and mitigating consequences of flooding for the flood event duration. After issuance of the 50.54(f) letter, the NRG changed the approach to the steps following the review of the flood hazard reevaluations, as directed by the Commission, to permit use of focused evaluations as an alternative to an integrated assessment. The NRG letter dated September 1, 2015 (NRG, 2015c), describes the changes in the NRC's approach to the flood hazard reevaluations If the reevaluated PMF elevation is bounded by the current design-basis PMF elevation for all flood-causing mechanisms at the site, licensees are not required to perform an integrated assessment or a focused evaluation at this time. | * Perform an integrated assessment subsequent to the FHRR to (a) evaluate the effectiveness of the current design-basis (i.e., flood protection and mitigation systems), (b) identify plant-specific vulnerabilities, and (c) assess the effectiveness of existing or planned systems and procedures for protecting against and mitigating consequences of flooding for the flood event duration. After issuance of the 50.54(f) letter, the NRG changed the approach to the steps following the review of the flood hazard reevaluations, as directed by the Commission, to permit use of focused evaluations as an alternative to an integrated assessment. The NRG letter dated September 1, 2015 (NRG, 2015c), describes the changes in the NRC's approach to the flood hazard reevaluations If the reevaluated PMF elevation is bounded by the current design-basis PMF elevation for all flood-causing mechanisms at the site, licensees are not required to perform an integrated assessment or a focused evaluation at this time. | ||
==3.0 TECHNICAL EVALUATION== | ==3.0 TECHNICAL EVALUATION== | ||
There are no changes or updates to this section of the NRG staff assessment. | There are no changes or updates to this section of the NRG staff assessment. 3.1 Site Information There are no changes or updates to this section of the NRG staff assessment. 3.1.1 Detailed Site Information There are no changes or updates to this section of the NRG staff assessment. 3.1.2 Design-Basis Flood Hazards1 There are no changes or updates to this section of the NRG staff assessment. 3.1.3 Flood-related Changes to the Licensing Basis There are no changes or updates to this section of the NRG staff assessment. 3.1.4 Changes to the Watershed and Local Area There are no changes or updates to this section of the NRG staff assessment. 3.1.5 Current Licensing Basis Flood Protection and Pertinent Flood Mitigation Features There are no changes or updates to this section of the NRG staff assessment. 3.1.6 Additional Site Details to Assess the Flood Hazard There are no changes or updates to this section of the NRG staff assessment. 3.1.7 Plant Walkdown Activities There are no changes or updates to this section of the NRG staff assessment. 3.2 Local Intense Precipitation and Associated Site Drainage There are no changes or updates to this section of the NRG staff assessment. 3.2. 1 Site Drainage and Elevations There are no changes or updates to this section of the NRG staff assessment. 1 In its FHRR, the licensee used the terms "design-basis" and "licensing basis" interchangeably. Because the references to current licensing basis were to various elevations that were specific to each flood hazard, the NRC staff assumes in this document that the licensee intended the term "current licensing basis" in its FHRR to refer to the "current design-basis" since this is what the 50.54(f) letter, Enclosure 2, requested. The NRC staff will thus use the term "current design-basis", as appropriate, throughout this document. 3.2.2 Local Intense Precipitation Depths There are no changes or updates to this section of the NRG staff assessment. 3.2.3 Modeling of Flood Levels To estimate runoff and perform hydrologic routing, the licensee used the FL0-2D software application (SGE&G, 2013a; FL0-2D Software, Inc. 2009). The licensee used the 1-mi2 (2.6-km2), 1-h duration rainfall value of 19.0 in (48.3 cm) as input to the FL0-2D application. Given the significant role that the FL0-2D model performs in the licensee's analysis of the PMF caused by local intense precipitation (LIP), the NRG staff requested that the licensee provide FL0-2D input files. The licensee provided a detailed description of the application of the FL0-2D model along with the model input files in its response to NRG's request for additional Information (RAI) (SGE&G, 2014b, RAls 1 and 2 responses). The NRG staff reviewed model input configurations and the manner in which results of the FL0-2D analysis were used to predict water-surface elevations. The VGSNS, Unit 1 FHRR (SGE&G, 2013a) stated that the roofs of safety-related buildings were designed to store up to 4 inches (10 cm) of precipitation. The licensee did not describe how or if this design feature was incorporated into the LIP analysis and did not characterize similar design features of non-safety-related buildings. The NRG staff reviewed the licensee's FL0-2D application and found that all precipitation falling on building roofs was retained on the roofs and did not enter the flow domain adjacent to the buildings. Water retained on building roofs would reduce the discharge adjacent to, and downstream of, the buildings. The portion of runoff from the plant site into the service water pond (SWP), with coincident wind setup and wave runup discussed later relative to streams and rivers in Section 3.3.4, is also impacted indirectly by the roof drainage issue in the FL0-2D model. The licensee's reevaluation yielded a PMF water-surface elevation of 436.6 ft to 437.5 ft (133.1 m to 133.4 m) (SGE&G, 2013a), which is higher than the current design-basis stillwater-surface elevation of 436.15 ft (132.94 m). Elevation 436.6 ft (133.1 m) is at the east side, and elevation 437.5 ft (133.4 m) is at the west side of the powerblock (SGE&G, 2014b). 3.2.4 Flood Event Duration The VGSNS, Unit 1 FHRR (SGE&G, 2013a) did not address flood warning time or the duration of inundation resulting from LIP flooding. The NRG staff requested additional information from the licensee (NRG, 2014b) to supplement its FHRR (SGE&G, 2013a). The licensee's response (SGE&G, 2014b, RAI 6 response) stated that existing modeling indicates that most ponded water drains from the site within 7 hours. Warning time, based on meteorological warnings, is expected to be more than 24 hours. The NRG staff notes that longer duration probable maximum precipitation (PMP) events that deliver greater precipitation volumes, such as the 72-h PMP, generate greater volumes of runoff. Shorter-duration PMP events that have higher rates of precipitation, such as the 1-hour PMP, may however result in much shorter warning times and higher water levels. The NRC staff notes that a reasonable estimate of the site's LIP PMP is application of an appropriate National Oceanic and Atmospheric Administration hydrometeorological report (HMR) estimate for any rainfall duration used in NUREG/CR-7046, regardless of temporal distribution of the rainfall. The licensee obtained 1-mi2 (2.6-km2}, 1-h duration PMP value using HMR-52. Therefore, the NRC staff confirmed that the licensee selected an appropriate rainfall rate value to satisfy the 50.54(f) information request. 3.2.5 Conclusion The NRC staff confirmed the licensee's conclusion that the reevaluated flood hazard for LIP and associated site drainage is not bounded by the current design-basis flood hazard. Therefore, the licensee is expected to submit a focused evaluation for LIP and associated site drainage consistent with the process outlined in COMSECY-15-0019 (NRC, 2015b) and associated guidance that will be issued. Under this approach, the NRC staff anticipates that licensees will perform and document a focused evaluation that evaluates the impact of the LIP hazard on the site and implements any necessary programmatic, procedural or plant modifications to address this hazard exceedance. The roof drainage and Service Water Pond issues that are discussed in Sections 3.2.3 and 3.3.4 should be addressed in the focused evaluation. The NRC staff anticipates that licensees will submit letters providing a summary of the evaluation and, if needed, regulatory commitments to implement and maintain appropriate programmatic, procedural or plant modifications to protect against the LIP hazard. 3.3 Streams and Rivers There are no changes or updates to this section of the NRC staff assessment. 3.3.1 Additional Information There are no changes or updates to this section of the NRC staff assessment. 3.3.2 Flooding Scenarios and Associated Effects There are no changes or updates to this section of the NRC staff assessment. 3.3.3 Monticello Reservoir There are no changes or to this section of the NRC staff assessment. 3.3.4 Service Water Pond In the VCSNS, Unit 1 FHRR (SCE&G, 2013a), the SWP is described as a Seismic Category 1 impoundment that serves as the Ultimate Heat Sink for VCSNS, Unit 1. The SWP is adjacent to Monticello Reservoir (Figure 3.3-2). The SWP is separated from Monticello Reservoir by two islands and three Seismic Category 1 dams, which have crest elevations of 438.0 ft (133.5 m) on three sides. The crest elevation of the West Embankment is 435.0 ft (132.6 m). The West Embankment adjoins the VCSNS, Unit 1 plant yard grade, and therefore represents the land elevation value critical for assessment of the PMF maximum water-surface elevation in the SWP. In its FHRR, the licensee stated that water is supplied to the SWP from Monticello Reservoir by a pipe configured with a butterfly isolation valve. This isolation valve is kept closed during normal operations. The FHRR states that the SWP normal pool elevation is 422.0 ft (128.6 m). Under normal operational conditions the pool elevation ranges from 420.5 ft to 425.0 ft (128.2 m to 129.5 m). The NRC staff confirmed that the description of the SWP is consistent with that provided in the VCSNS, Unit 1 UFSAR (SCE&G, 201 Oa). Given the control of the SWP pool elevation by the operation of the SWP isolation valve on the interconnecting pipe between the SWP and Monticello Reservoir, the NRC staff requested that the licensee provide a detailed description of conditions leading to the valve's operation, frequency of operation, and any assumptions related to the state of the isolation valve used in the reevaluation of the PMF for the SWP (NRC, 2014b). The licensee provided a detailed description of the operation of the SWP isolation valve, frequency of operation, and the assumptions used by the licensee related to the state of the valve in its analysis of the PMF (SCE&G, 2014b, RAI 4 response). The VCSNS, Unit 1 FHRR (SCE&G, 2013a), included a description of the runoff and course models associated with the PMF flooding in streams and rivers related to LIP runoff from the plant area as it drains into the SWP. In response to the NRC's RAI (NRC, 2014b), the licensee described its analysis of water levels in the SWP resulting from the PMP on the site plus associated effects (SCE&G, 2014b, RAI 5 response). The SWP receives a portion of the LIP runoff from the plant area as mentioned in Section 3.2.3 above, and the issue within the FL0-20 model, related to lack of roof drainage, impacts the portion of runoff from the plant site to the SWP. The VCSNS, Unit 1 FHRR (SCE&G, 2013a), referenced a full description of the coincident wave effects associated with the PMF flooding in streams and rivers, contained in the VCSNS, Unit 1 UFSAR (SCE&G, 201 Oa). The VCSNS, Unit 1 FHRR (SCE&G, 2013a), Section 4.1.2.2.2.2, stated that the VCSNS, Unit 1 UFSAR (SCE&G, 201 Oa) PMF included wind effects that would result in a SWP water-surface elevation of 433.6 ft (132.2 m). The FHRR maximum water-surface elevation including wind-wave activity estimates are based, in part, on the stillwater-surface elevation in the SWP. The VCSNS, Unit 1 FHRR (SCE&G, 2013a), Section 4.2.2.2, stated that the reevaluated PMF elevation for the SWP at the West Embankment was determined to be 428.3 ft (130.5 m). The licensee used the FL0-20 model to estimate LIP and runoff from the site into the SWP and the associated rise in the stillwater-surface elevation. 3.3.5 Broad River There are no changes or updates to this section of the NRC staff assessment. 3.3.6 Conclusion The NRC staff confirmed the licensee's conclusion (SCE&G 2014c) that the reevaluated hazard for flooding from streams and rivers is not bounded by the current design-basis flood hazard when combined with wind setup and wave runup; therefore, the licensee should include flooding from streams and rivers with wind setup and wave runup from Monticello Reservoir within the scope of the integrated assessment or focused evaluation consistent with the process and guidance discussed in COMSECY-15-0019 (NRC, 2015b). Information on flooding from streams and rivers that is specific to the data needs of the integrated assessment or focused evaluation is described in Section 4 of this staff assessment. 3.4 Failure of Dams and Onsite Water Control/Storage Structures There are no changes or updates to this section of the NRC staff assessment. 3.5 Storm Surge The VCSNS, Unit 1 FHRR (SCE&G, 2013a) reported that the reevaluated PMF elevation, including associated effects, for site flooding due to the wind setup and wave runup aspects of storm surge is 437.0 ft (133.2 m) when combined with the PMF in the streams and rivers analysis. This flood-causing mechanism is described in the licensee's current design-basis. This reevaluated PMF elevation exceeds the current design-basis PMF elevation for site flooding due to storm surge of 436.6 ft (133.1 m). The licensee stated that water level increases due to storm surge were included in the PMF determinations in the Monticello Reservoir and the SWP; no further analysis was performed specifically for storm surge without the PMP. The plant site is protected by the North Berm to elevation 438.0 ft (133.5 m). In summary, the NRC staff confirmed the licensee's conclusion that flooding from storm surge does not inundate the site. However, this hazard mechanism exceeds the current design-basis. Therefore, the treatment of the storm surge should be addressed in the integrated assessment or focused evaluation consistent with the process and guidance discussed in COMSECY-15-0019 (NRC, 2015c). 3.6 Seiche There are no changes or updates to this section of the NRC staff assessment. 3.7 Tsunami There are no changes or updates to this section of the NRC staff assessment. 3.8 Ice-Induced Flooding There are no changes or updates to this section of the NRC staff assessment. 3.9 Channel Migrations or Diversions There are no changes or updates to this section of the NRC staff assessment. 4.0 REEVALUATED FLOOD HEIGHT, EVENT DURATION AND ASSOCIATED EFFECTS FOR HAZARDS NOT BOUNDEDE BY THE COB The NRG staff confirms that for certain flooding mechanisms that the reevaluated hazard is not bounded by the current design-basis flood hazard. Therefore, the NRG staff concludes that an integrated assessment or focused evaluation(s) is necessary, and that it should consider the following flood-causing mechanisms: LIP, storm surge, and flooding in streams and rivers with wind setup and wave runup from Monticello Reservoir as a combined effect. The NRG staff reviewed the following flood hazard parameters needed to perform the additional assessments or evaluations of plant response: | ||
3.1 Site Information There are no changes or updates to this section of the NRG staff assessment. 3.1.1 Detailed Site Information There are no changes or updates to this section of the NRG staff assessment. 3.1.2 Design-Basis Flood Hazards1 There are no changes or updates to this section of the NRG staff assessment. 3.1.3 Flood-related Changes to the Licensing Basis There are no changes or updates to this section of the NRG staff assessment. 3.1.4 Changes to the Watershed and Local Area There are no changes or updates to this section of the NRG staff assessment. 3.1.5 Current Licensing Basis Flood Protection and Pertinent Flood Mitigation Features There are no changes or updates to this section of the NRG staff assessment. 3.1.6 Additional Site Details to Assess the Flood Hazard There are no changes or updates to this section of the NRG staff assessment. 3.1.7 Plant Walkdown Activities There are no changes or updates to this section of the NRG staff assessment. 3.2 Local Intense Precipitation and Associated Site Drainage There are no changes or updates to this section of the NRG staff assessment. 3.2. 1 Site Drainage and Elevations There are no changes or updates to this section of the NRG staff assessment. 1 In its FHRR, the licensee used the terms "design-basis" and "licensing basis" interchangeably. Because the references to current licensing basis were to various elevations that were specific to each flood hazard, the NRC staff assumes in this document that the licensee intended the term "current licensing basis" in its FHRR to refer to the "current design-basis" since this is what the 50.54(f) letter, Enclosure 2, requested. The NRC staff will thus use the term "current design-basis", as appropriate, throughout this document. | |||
3.2.2 Local Intense Precipitation Depths There are no changes or updates to this section of the NRG staff assessment. 3.2.3 Modeling of Flood Levels To estimate runoff and perform hydrologic routing, the licensee used the FL0-2D software application (SGE&G, 2013a; FL0-2D Software, Inc. 2009). The licensee used the 1-mi2 (2.6-km2), 1-h duration rainfall value of 19.0 in (48.3 cm) as input to the FL0-2D application. Given the significant role that the FL0-2D model performs in the licensee's analysis of the PMF caused by local intense precipitation (LIP), the NRG staff requested that the licensee provide FL0-2D input files. The licensee provided a detailed description of the application of the FL0-2D model along with the model input files in its response to NRG's request for additional Information (RAI) (SGE&G, 2014b, RAls 1 and 2 responses). The NRG staff reviewed model input configurations and the manner in which results of the FL0-2D analysis were used to predict water-surface elevations. The VGSNS, Unit 1 FHRR (SGE&G, 2013a) stated that the roofs of safety-related buildings were designed to store up to 4 inches (10 cm) of precipitation. The licensee did not describe how or if this design feature was incorporated into the LIP analysis and did not characterize similar design features of non-safety-related buildings. The NRG staff reviewed the licensee's FL0-2D application and found that all precipitation falling on building roofs was retained on the roofs and did not enter the flow domain adjacent to the buildings. Water retained on building roofs would reduce the discharge adjacent to, and downstream of, the buildings. The portion of runoff from the plant site into the service water pond (SWP), with coincident wind setup and wave runup discussed later relative to streams and rivers in Section 3.3.4, is also impacted indirectly by the roof drainage issue in the FL0-2D model. The licensee's reevaluation yielded a PMF water-surface elevation of 436.6 ft to 437.5 ft (133.1 m to 133.4 m) (SGE&G, 2013a), which is higher than the current design-basis stillwater-surface elevation of 436.15 ft (132.94 m). Elevation 436.6 ft (133.1 m) is at the east side, and elevation 437.5 ft (133.4 m) is at the west side of the powerblock (SGE&G, 2014b). 3.2.4 Flood Event Duration The VGSNS, Unit 1 FHRR (SGE&G, 2013a) did not address flood warning time or the duration of inundation resulting from LIP flooding. The NRG staff requested additional information from the licensee (NRG, 2014b) to supplement its FHRR (SGE&G, 2013a). The licensee's response (SGE&G, 2014b, RAI 6 response) stated that existing modeling indicates that most ponded water drains from the site within 7 hours. Warning time, based on meteorological warnings, is expected to be more than 24 hours. The NRG staff notes that longer duration probable maximum precipitation (PMP) events that deliver greater precipitation volumes, such as the 72-h PMP, generate greater volumes of runoff. Shorter-duration PMP events that have higher rates of precipitation, such as the 1-hour PMP, may however result in much shorter warning times and higher water levels. | |||
The NRC staff notes that a reasonable estimate of the site's LIP PMP is application of an appropriate National Oceanic and Atmospheric Administration hydrometeorological report (HMR) estimate for any rainfall duration used in NUREG/CR-7046, regardless of temporal distribution of the rainfall. The licensee obtained 1-mi2 (2.6-km2}, 1-h duration PMP value using HMR-52. Therefore, the NRC staff confirmed that the licensee selected an appropriate rainfall rate value to satisfy the 50.54(f) information request. 3.2.5 Conclusion The NRC staff confirmed the licensee's conclusion that the reevaluated flood hazard for LIP and associated site drainage is not bounded by the current design-basis flood hazard. Therefore, the licensee is expected to submit a focused evaluation for LIP and associated site drainage consistent with the process outlined in COMSECY-15-0019 (NRC, 2015b) and associated guidance that will be issued. Under this approach, the NRC staff anticipates that licensees will perform and document a focused evaluation that evaluates the impact of the LIP hazard on the site and implements any necessary programmatic, procedural or plant modifications to address this hazard exceedance. The roof drainage and Service Water Pond issues that are discussed in Sections 3.2.3 and 3.3.4 should be addressed in the focused evaluation. The NRC staff anticipates that licensees will submit letters providing a summary of the evaluation and, if needed, regulatory commitments to implement and maintain appropriate programmatic, procedural or plant modifications to protect against the LIP hazard. 3.3 Streams and Rivers There are no changes or updates to this section of the NRC staff assessment. 3.3.1 Additional Information There are no changes or updates to this section of the NRC staff assessment. 3.3.2 Flooding Scenarios and Associated Effects There are no changes or updates to this section of the NRC staff assessment. 3.3.3 Monticello Reservoir There are no changes or to this section of the NRC staff assessment. 3.3.4 Service Water Pond In the VCSNS, Unit 1 FHRR (SCE&G, 2013a), the SWP is described as a Seismic Category 1 impoundment that serves as the Ultimate Heat Sink for VCSNS, Unit 1. The SWP is adjacent to Monticello Reservoir (Figure 3.3-2). The SWP is separated from Monticello Reservoir by two islands and three Seismic Category 1 dams, which have crest elevations of 438.0 ft (133.5 m) on three sides. The crest elevation of the West Embankment is 435.0 ft (132.6 m). The West Embankment adjoins the VCSNS, Unit 1 plant yard grade, and therefore represents the land elevation value critical for assessment of the PMF maximum water-surface elevation in the SWP. In its FHRR, the licensee stated that water is supplied to the SWP from Monticello Reservoir by a pipe configured with a butterfly isolation valve. This isolation valve is kept closed during normal operations. The FHRR states that the SWP normal pool elevation is 422.0 ft (128.6 m). Under normal operational conditions the pool elevation ranges from 420.5 ft to 425.0 ft (128.2 m to 129.5 m). The NRC staff confirmed that the description of the SWP is consistent with that provided in the VCSNS, Unit 1 UFSAR (SCE&G, 201 Oa). Given the control of the SWP pool elevation by the operation of the SWP isolation valve on the interconnecting pipe between the SWP and Monticello Reservoir, the NRC staff requested that the licensee provide a detailed description of conditions leading to the valve's operation, frequency of operation, and any assumptions related to the state of the isolation valve used in the reevaluation of the PMF for the SWP (NRC, 2014b). The licensee provided a detailed description of the operation of the SWP isolation valve, frequency of operation, and the assumptions used by the licensee related to the state of the valve in its analysis of the PMF (SCE&G, 2014b, RAI 4 response). The VCSNS, Unit 1 FHRR (SCE&G, 2013a), included a description of the runoff and course models associated with the PMF flooding in streams and rivers related to LIP runoff from the plant area as it drains into the SWP. In response to the NRC's RAI (NRC, 2014b), the licensee described its analysis of water levels in the SWP resulting from the PMP on the site plus associated effects (SCE&G, 2014b, RAI 5 response). The SWP receives a portion of the LIP runoff from the plant area as mentioned in Section 3.2.3 above, and the issue within the FL0-20 model, related to lack of roof drainage, impacts the portion of runoff from the plant site to the SWP. The VCSNS, Unit 1 FHRR (SCE&G, 2013a), referenced a full description of the coincident wave effects associated with the PMF flooding in streams and rivers, contained in the VCSNS, Unit 1 UFSAR (SCE&G, 201 Oa). The VCSNS, Unit 1 FHRR (SCE&G, 2013a), Section 4.1.2.2.2.2, stated that the VCSNS, Unit 1 UFSAR (SCE&G, 201 Oa) PMF included wind effects that would result in a SWP water-surface elevation of 433.6 ft (132.2 m). The FHRR maximum water-surface elevation including wind-wave activity estimates are based, in part, on the stillwater-surface elevation in the SWP. The VCSNS, Unit 1 FHRR (SCE&G, 2013a), Section 4.2.2.2, stated that the reevaluated PMF elevation for the SWP at the West Embankment was determined to be 428.3 ft (130.5 m). The licensee used the FL0-20 model to estimate LIP and runoff from the site into the SWP and the associated rise in the stillwater-surface elevation. 3.3.5 Broad River There are no changes or updates to this section of the NRC staff assessment. 3.3.6 Conclusion The NRC staff confirmed the licensee's conclusion (SCE&G 2014c) that the reevaluated hazard for flooding from streams and rivers is not bounded by the current design-basis flood hazard when combined with wind setup and wave runup; therefore, the licensee should include flooding from streams and rivers with wind setup and wave runup from Monticello Reservoir within the scope of the integrated assessment or focused evaluation consistent with the process and guidance discussed in COMSECY-15-0019 (NRC, 2015b). Information on flooding from streams and rivers that is specific to the data needs of the integrated assessment or focused evaluation is described in Section 4 of this staff assessment. 3.4 Failure of Dams and Onsite Water Control/Storage Structures There are no changes or updates to this section of the NRC staff assessment. 3.5 Storm Surge The VCSNS, Unit 1 FHRR (SCE&G, 2013a) reported that the reevaluated PMF elevation, including associated effects, for site flooding due to the wind setup and wave runup aspects of storm surge is 437.0 ft (133.2 m) when combined with the PMF in the streams and rivers analysis. This flood-causing mechanism is described in the licensee's current design-basis. This reevaluated PMF elevation exceeds the current design-basis PMF elevation for site flooding due to storm surge of 436.6 ft (133.1 m). The licensee stated that water level increases due to storm surge were included in the PMF determinations in the Monticello Reservoir and the SWP; no further analysis was performed specifically for storm surge without the PMP. The plant site is protected by the North Berm to elevation 438.0 ft (133.5 m). In summary, the NRC staff confirmed the licensee's conclusion that flooding from storm surge does not inundate the site. However, this hazard mechanism exceeds the current design-basis. Therefore, the treatment of the storm surge should be addressed in the integrated assessment or focused evaluation consistent with the process and guidance discussed in COMSECY-15-0019 (NRC, 2015c). 3.6 Seiche There are no changes or updates to this section of the NRC staff assessment. 3.7 Tsunami There are no changes or updates to this section of the NRC staff assessment. 3.8 Ice-Induced Flooding There are no changes or updates to this section of the NRC staff assessment. 3.9 Channel Migrations or Diversions There are no changes or updates to this section of the NRC staff assessment. | |||
4.0 REEVALUATED FLOOD HEIGHT, EVENT DURATION AND ASSOCIATED EFFECTS FOR HAZARDS NOT BOUNDEDE BY THE COB The NRG staff confirms that for certain flooding mechanisms that the reevaluated hazard is not bounded by the current design-basis flood hazard. Therefore, the NRG staff concludes that an integrated assessment or focused evaluation(s) is necessary, and that it should consider the following flood-causing mechanisms: LIP, storm surge, and flooding in streams and rivers with wind setup and wave runup from Monticello Reservoir as a combined effect. The NRG staff reviewed the following flood hazard parameters needed to perform the additional assessments or evaluations of plant response: | |||
* Flood height and associated effects (see Table 4.0-2), as defined in JLD-ISG-2012-05 (NRG, 2012d) | * Flood height and associated effects (see Table 4.0-2), as defined in JLD-ISG-2012-05 (NRG, 2012d) | ||
* Flood event duration (see Table 4.0-1 ), including warning time and intermediate water surface elevations that trigger actions by plant personnel, as defined in JLD-ISG-2012-05 (NRG, 2012d) 4.1 Flood Height and Associated Effects The licensee estimated maximum water-surface elevations using its FL0-20 analysis (SCE&G, 2013a, 2014b). The NRG staff found that, in the licensee's FL0-20 application, precipitation was modeled as being retained on building roofs rather than discharged to the ground surface near the structure or an adjacent area, which could result in underestimation of the maximum water-surface elevation. Because the LIP flooding mechanism is being evaluated as part of a focused evaluation, the NRG staff determined that this numerical modeling issue should be resolved as part of the focused evaluation. The streams and rivers flood-causing mechanism was combined with the storm surge causing mechanism (wind setup and wave runup). This combined effect flood reevaluation is not bounded by the current design-basis, and results in an increase in the elevation of water impinging upon a flood protection structure (i.e., North Berm of the plant site at Monticello Reservoir). The NRG staff has observed that the increase in the combined effect flood has resulted in a reduction of margin that is quantitatively minor (0.4 ft (0.1 m)) and characterized by brief and intermittent impingement of waves on a passive low-head flood protection feature (a 3 ft (1 m) berm). 4.2 Flood Event Duration Section 3.2.4 of this staff assessment discusses flood event duration. The NRG staff notes that a reasonable estimate of the site's LIP PMP is the application of an appropriate National Oceanic and Atmospheric Administration hydrometeorological report (HMR) estimate for any rainfall duration used in NUREG/CR-7046, regardless of temporal distribution of the rainfall. The licensee obtained a 1-mii2 (2.6-km2), 1-h duration PMP value using HMR-52. Therefore, the NRG staff confirmed that the licensee selected an appropriate rainfall rate value to satisfy the 50.54(f) information request. | * Flood event duration (see Table 4.0-1 ), including warning time and intermediate water surface elevations that trigger actions by plant personnel, as defined in JLD-ISG-2012-05 (NRG, 2012d) 4.1 Flood Height and Associated Effects The licensee estimated maximum water-surface elevations using its FL0-20 analysis (SCE&G, 2013a, 2014b). The NRG staff found that, in the licensee's FL0-20 application, precipitation was modeled as being retained on building roofs rather than discharged to the ground surface near the structure or an adjacent area, which could result in underestimation of the maximum water-surface elevation. Because the LIP flooding mechanism is being evaluated as part of a focused evaluation, the NRG staff determined that this numerical modeling issue should be resolved as part of the focused evaluation. The streams and rivers flood-causing mechanism was combined with the storm surge causing mechanism (wind setup and wave runup). This combined effect flood reevaluation is not bounded by the current design-basis, and results in an increase in the elevation of water impinging upon a flood protection structure (i.e., North Berm of the plant site at Monticello Reservoir). The NRG staff has observed that the increase in the combined effect flood has resulted in a reduction of margin that is quantitatively minor (0.4 ft (0.1 m)) and characterized by brief and intermittent impingement of waves on a passive low-head flood protection feature (a 3 ft (1 m) berm). 4.2 Flood Event Duration Section 3.2.4 of this staff assessment discusses flood event duration. The NRG staff notes that a reasonable estimate of the site's LIP PMP is the application of an appropriate National Oceanic and Atmospheric Administration hydrometeorological report (HMR) estimate for any rainfall duration used in NUREG/CR-7046, regardless of temporal distribution of the rainfall. The licensee obtained a 1-mii2 (2.6-km2), 1-h duration PMP value using HMR-52. Therefore, the NRG staff confirmed that the licensee selected an appropriate rainfall rate value to satisfy the 50.54(f) information request. 4.3 Conclusion Based upon the preceding analysis, the NRG staff confirmed that the reevaluated flood hazard information defined in the sections above is appropriate input to other assessments or evaluations associated with Near-Term Task Force Recommendations, including the assessment of mitigation strategies developed in response to Order EA-12-049 (i.e., defines the mitigating strategies flood hazard information described in guidance documents currently being finalized by the industry and NRG staff). | ||
4.3 Conclusion Based upon the preceding analysis, the NRG staff confirmed that the reevaluated flood hazard information defined in the sections above is appropriate input to other assessments or evaluations associated with Near-Term Task Force Recommendations, including the assessment of mitigation strategies developed in response to Order EA-12-049 (i.e., defines the mitigating strategies flood hazard information described in guidance documents currently being finalized by the industry and NRG staff). | |||
==5.0 CONCLUSION== | ==5.0 CONCLUSION== | ||
The NRG staff has reviewed the information provided for the reevaluated flood-causing mechanisms of VCSNS, Unit 1. Based on its review, the NRG staff concludes that the licensee conducted the hazard reevaluation using present-day methodologies and regulatory guidance used by the NRG staff in connection with ESP and COL reviews. Based on the preceding analysis, the NRG staff confirmed that the licensee responded appropriately to Enclosure 2, Required Response 2, of the 50.54(f) letter, dated March 12, 2012. In reaching this determination, the NRG staff confirmed the licensee's conclusions that (a) the reevaluated flood hazard results for local intense precipitation, streams and rivers, and storm surge are not bounded by the current design-basis flood hazard, (b) additional assessments of plant response will be performed for the local intense precipitation streams and rivers, and storm surge flood-causing mechanisms, and (c) the reevaluated flood-causing mechanism information is appropriate input to additional assessments or evaluations of plant response, as described in the 50.54(f) letter and COMSECY-15-0019 (NRG, 2015b), including the assessment of mitigation strategies developed in response to Order EA-12-049 (i.e., defines the mitigating strategies flood hazard information described in guidance documents currently being finalized by the industry and NRG staff). The NRG staff has no additional information needs at this time with respect to the FHRR. | The NRG staff has reviewed the information provided for the reevaluated flood-causing mechanisms of VCSNS, Unit 1. Based on its review, the NRG staff concludes that the licensee conducted the hazard reevaluation using present-day methodologies and regulatory guidance used by the NRG staff in connection with ESP and COL reviews. Based on the preceding analysis, the NRG staff confirmed that the licensee responded appropriately to Enclosure 2, Required Response 2, of the 50.54(f) letter, dated March 12, 2012. In reaching this determination, the NRG staff confirmed the licensee's conclusions that (a) the reevaluated flood hazard results for local intense precipitation, streams and rivers, and storm surge are not bounded by the current design-basis flood hazard, (b) additional assessments of plant response will be performed for the local intense precipitation streams and rivers, and storm surge flood-causing mechanisms, and (c) the reevaluated flood-causing mechanism information is appropriate input to additional assessments or evaluations of plant response, as described in the 50.54(f) letter and COMSECY-15-0019 (NRG, 2015b), including the assessment of mitigation strategies developed in response to Order EA-12-049 (i.e., defines the mitigating strategies flood hazard information described in guidance documents currently being finalized by the industry and NRG staff). The NRG staff has no additional information needs at this time with respect to the FHRR. | ||
==6.0 REFERENCES== | |||
U.S. Nuclear Regulatory Commission (NRG) Documents and Publications: NRG (U.S. Nuclear Regulatory Commission), 2014d, letter from Robert F. Kuntz, NRG, to Thomas D. Gatlin, Vice President, "Virgil C. Summer Nuclear Station, Unit 1 -Staff Assessment of Response to 10 CFR 50.54(F) Information Request-Flood-Causing Mechanism Reevaluation (TAC No. MF1112), December 23, 2014, ADAMS Accession No. ML 14356A002. NRG (U.S. Nuclear Regulatory Commission), 2015a, SRM -COMSECY-15-0019-Closure Plan for the Reevaluation of Flooding Hazards for Operating Nuclear Power Plants,", July 28, 2015, ADAMS Accession No. ML 15209A682. NRG (U.S. Nuclear Regulatory Commission), 2015b, COMSECY-15-0019 "Closure Plan for the Reevaluation of Flooding Hazards for Operating Nuclear Power Plants,", June 30, 2015, ADAMS Accession No. ML 15153A 104. NRG (U.S. Nuclear Regulatory Commission), 2015c, letter from William M. Dean, Director, to Power Reactor Licensees," Coordination of Requests for Information for Flooding Hazard Reevaluations and Mitigating Strategies for Beyond Design Basis External Events", September 1, 2015, ADAMS Accession No. ML15174A257. Codes and Standards There are no additions to the references in this section. Other | |||
==References:== | ==References:== | ||
There are no additions to the references in this section. | There are no additions to the references in this section. Table 3.1-1: Current Design-Basis (COB) Flood Hazard Flooding COB Stillwater COB Associated COB Flood Level, Level, Reference Mechanism NGV029 Effects* NGV029 Local Intense 436.15 ft None 436.15 ft FHRR (SCE&G, 2013a) Precipitation (132.94 m) (132.94 m) based Sections 4.3.1 and and Associated based on on maximum hour 4.1.2.1.1. Drainage maximum hour within 6 hr PMP UFSAR (SCE&G, 201 Oa) within 6 hrs Section 2.4.3.1.3 PMP** Streams and 429.1 ft (130.8 7.5 ft (2.3 m) from 436.6 ft (133.1 m) FHRR (SCE&G, 2013a) Rivers m), including wave runup Section 4.1.2.2.1 (including wave 4.1 ft (1.25 m) UFSAR (SCE&G, 201 Oa) runup from from 48-hrs Section 2.4.3.6.2 Storm Surge) PMP on initial reservoir elevation of 425.0 ft (129.5 m) Failure of <290 ft No Impact <290 ft (88.4 m), FHRR (SCE&G, 2013a) Dams and (88.4 m) Identified Parr Shoals Section 4.2.2.4 Onsite Water No threat of Reservoir I Broad Control/Storage flooding at site River, Structures No threat of flooding at site Storm Surge See Streams See Streams and See Streams and Included in Streams and and Rivers for Rivers for Rivers for Rivers as combined effect combined combined effect combined effect effect Seiche No Impact No Impact No Impact FHRR (SCE&G, 2013a) Identified Identified Identified Section 4.2.2.6 UFSAR (SCE&G, 201 Oa) Section 2.4.5.5 Tsunami No Impact No Impact No Impact FHRR (SCE&G, 2013a) Identified Identified Identified Section 4.2.2.7 Ice-Induced No Impact No Impact No Impact FHRR (SCE&G, 2013a) Identified Identified Identified Section 4.2.2.8 Channel No Impact No Impact No Impact FHRR (SCE&G, 2013a) Migrations or Identified Identified Identified Section 4.2.2.9 Diversions .. | ||
* No associated effects are identified from debns, sediment depos1t1on or erosion, concurrent site conditions (including adverse weather other than the associated PMP), or groundwater ingress. ** 436.15 ft (132.94 m) stillwater level represents nominal plant grade of 435.0 ft (132.6 m) with floodwater depth of 1.15 ft (0.350 m). Table 4.0-1: Flood Event Duration for Reevaluated Flood-Causing Mechanisms Not Bounded by the Current Design Basis Hazard* Flood-Causing Site Preparation for Period of Site Recession of Total Event Mechanism Flood Event Inundation Water from Site Duration Local Intense >24 hr 7 hr 17 h >48 hours Precipitation and (Assuming all claimed (At Power Block (Resulting from 1-Associated warning time is used for West) h PMP) Drainage site preparation} Streams and > 24 hr warning time Oh Oh N/A Rivers (Implied by local intense (No site (No site (No site inundation (Monticello precipitation analysis; inundation; inundation) from 72-hrs PMP Reservoir Monticello Reservoir is licensee analysis including wind flooding) immediately adjacent to indicates that wind setup and wave site, and warning time setup and wave runup) would be same as for runup impinge on local intense North Berm flood precipitation) protection feature for 72 seconds) Storm Surge See Streams and Rivers See Streams and See streams and See streams and for combined effect Rivers for Rivers for Rivers for combined effect combined effect combined effect *Elements of flood event duration are shown in Figure 2.2-1. Table 4.0-2: Reevaluated Flood-Causing Mechanisms and Associated Effects Hazards Not Bounded by the Current Design Basis Hazard Reevaluated Stillwater Associated Effects** Reevaluated Flood Flood-Causing Elevation Hazard Elevation Reference Mechanism (NGVD29) Local Intense 436.6 ft to None 436.6 ft to 437.5 ft Maximum surface Precipitation and 437.5 ft (133.1 mto133.4m) water elevation Associated (133.1 m to (East side to west side during PMP at Drainage 133.4 m) of power block, from main plant 1-hr PMP of 19 in [48 buildings and cm]) doors -see (Water depths of 1.6 ft FHRR (SCE&G, 2013a, Section to 2.5 ft [0.49 m to 4.2.1.2) 0.76 m]) Streams and 431.07 ft 5.93 ft (1.81 m) 437.0 ft (133.2 m) at Maximum PMF Rivers (131.39 m) wind setup and wave North Berm elevation for (Resulting from (Includes run up (Remains below North Monticello 72-hr PMP) effects of Berm crest elevation of Reservoir at including wind 6.07 ft [1.85 438.0 ft [133.5 m] North Berm of setup and wave m] as direct NGVD29) plant -see runup from PMP and FHRR (SCE&G, Monticello basin runoff) 2013a, Section Reservoir 4.2.2.1) and RAI 3 response (SCE&G, 2014b) Storm Surge See Streams See Streams and See Streams and See Streams and and Rivers Rivers for combined Rivers for combined Rivers for for combined effect effect combined effect effect ** No associated effects are anticipated from debris, sediment deposition or erosion, concurrent site conditions (including adverse weather other than the associated PMP), or groundwater ingress (SCE&G, 2013a; SCE&G, 2014b) Table 5.0-1: Integrated Assessment Open Items Deleted | |||
* No associated effects are identified from debns, sediment depos1t1on or erosion, concurrent site conditions (including adverse weather other than the associated PMP), or groundwater ingress. ** 436.15 ft (132.94 m) stillwater level represents nominal plant grade of 435.0 ft (132.6 m) with floodwater depth of 1.15 ft (0.350 m). | |||
ML 15209A682) to COMSECY-15-0019 (ADAMS Accession No. ML 15153A104) that described the NRC's mitigating strategies and flooding hazard reevaluation action plan. As documented in the NRC staff assessment and the enclosed supplement, the staff has concluded that the licensee's reevaluated flood hazard information is suitable for the assessment of mitigation strategies developed in response to Order EA-12-049 (i.e., defines the mitigating strategies flood hazard information described in guidance documents currently being finalized by the industry and NRC staff) for Summer. Further, the licensee's reevaluated flood hazard information is suitable for other assessments associated with Near-Term Task Force Recommendation 2.1 "Flooding". The reevaluated flood hazard results for local intense precipitation, streams and rivers, and storm surge, were not bounded by the current design-basis flood hazard. In order to complete its response to Enclosure 2 to the 50.54(f) letter, the licensee is expected to submit a revised integrated assessment or focused evaluation(s), as appropriate, to address these reevaluated flood hazards, as described in the NRC's September 1, 2015, letter. If you have any questions, please contact me at (301) 415-6185 or email at Anthony.Minarik@nrc.gov. Docket No. 50-395 | |||
==Enclosure:== | ==Enclosure:== | ||
Staff Assessment of Flood Hazard Reevaluation Report cc w/encl: Distribution via Listserv DISTRIBUTION: see next page ADAMS Accession No.: ML15296A377 OFFICE NRR/JLD/JHMB/PM NAME MMarshall* DATE 10/21/2015 OFFICE NRO/DSEA/RHM1/BC NAME CCook* DATE 10/25/2015 Sincerely, IRA/ Anthony Minarik, Project Manager Hazards Management Branch Japan Lessons-Learned Division Office of Nuclear Reactor Regulation *via email NRR/JLD/JHMB/PM NRR/JLD/LA AMinarik SLent* 10/28/2015 10/30/2015 NRR/JLD/JHMB/BC NRR/JLD/JHMB/PM MShams* AMinarik 10/26/2015 11/3/2015 | Staff Assessment of Flood Hazard Reevaluation Report cc w/encl: Distribution via Listserv DISTRIBUTION: see next page ADAMS Accession No.: ML15296A377 OFFICE NRR/JLD/JHMB/PM NAME MMarshall* DATE 10/21/2015 OFFICE NRO/DSEA/RHM1/BC NAME CCook* DATE 10/25/2015 Sincerely, IRA/ Anthony Minarik, Project Manager Hazards Management Branch Japan Lessons-Learned Division Office of Nuclear Reactor Regulation *via email NRR/JLD/JHMB/PM NRR/JLD/LA AMinarik SLent* 10/28/2015 10/30/2015 NRR/JLD/JHMB/BC NRR/JLD/JHMB/PM MShams* AMinarik 10/26/2015 11/3/2015 Letter to Thomas D. Gatlin from Anthony Minarik dated November 3, 2015 | ||
==SUBJECT:== | ==SUBJECT:== | ||
VIRGIL C. SUMMER NUCLEAR STATION, UNIT 1 -STAFF ASSESSMENT OF RESPONSE TO 10 CFR 50.54(f) INFORMATION REQUEST-CAUSING MECHANISM REEVALUATION (CAC NO. MF1112) DISTRIBUTION: PUBLIC JHMB R/F RidsNroDsea Resource RidsNrrDorlLpl2-1 Resource RidsNrrSummer Resource RidsNrrLASLent Resource RidsOgcMailCenter Resource VHall, NRR BHarvey, NRO KErwin, NRO CCook, NRO ARivera-Varona, NRO SFlanders, NRO LQuinn-Willingham, NRO ACampbell, NRO AMinarik, NRR | VIRGIL C. SUMMER NUCLEAR STATION, UNIT 1 -STAFF ASSESSMENT OF RESPONSE TO 10 CFR 50.54(f) INFORMATION REQUEST-CAUSING MECHANISM REEVALUATION (CAC NO. MF1112) DISTRIBUTION: PUBLIC JHMB R/F RidsNroDsea Resource RidsNrrDorlLpl2-1 Resource RidsNrrSummer Resource RidsNrrLASLent Resource RidsOgcMailCenter Resource VHall, NRR BHarvey, NRO KErwin, NRO CCook, NRO ARivera-Varona, NRO SFlanders, NRO LQuinn-Willingham, NRO ACampbell, NRO AMinarik, NRR}} | ||
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Revision as of 01:12, 3 June 2018
| ML15296A377 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 11/03/2015 |
| From: | Minarik A J Japan Lessons-Learned Division |
| To: | Gatlin T D South Carolina Electric & Gas Co |
| Minarik, Anthony, NRR/JLD 415-6185 | |
| References | |
| CAC MF1112 | |
| Download: ML15296A377 (18) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 Mr. Thomas D. Gatlin, Vice President Nuclear Operations November 3, 2015 South Carolina Electric and Gas Company Virgil C. Summer Nuclear Station Post Office Box 88, Mail Code 800 Jenkinsville, SC 29065
SUBJECT:
VIRGIL C. SUMMER NUCLEAR STATION, UNIT 1 -SUPPLEMENT TO STAFF ASSESSMENT OF RESPONSE TO 10 CFR 50.54(f) INFORMATION REQUEST -FLOOD-CAUSING MECHANISMS REEVALUATION (CAC NO. MF1112)
Dear Mr. Gatlin:
The purpose of this letter is to transmit a supplement to the U.S. Nuclear Regulatory Commission (NRG) staff's assessment for Virgil C. Summer Nuclear Station, Unit 1 (Summer) reevaluated flood hazard information that was issued to you by letter dated December 23, 2014 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML 14356A002). The supplement updates the original NRG staff assessment to address changes in the NRC's approach to the steps following the review of the flood hazard reevaluations as directed by the Commission. The letter also addresses the next steps associated with the mitigation strategies assessment with respect to the reevaluated flood hazards. By letter dated March 12, 2012 (ADAMS Accession No. ML 12053A340), the NRG issued a request for information pursuant to Title 10 of the Code of Federal Regulations, Section 50.54(f) (hereafter referred to as the 50.54(f) letter). The request was issued as part of implementing lessons learned from the accident at the Fukushima Dai-ichi nuclear power plant. Enclosure 2 to the 50.54(f) letter requested licensees to reevaluate flood-causing mechanisms using present-day methodologies and guidance. By letter dated March 12, 2013 (ADAMS Accession No. ML 13073A114), South Carolina Electric and Gas Company responded to this request for Summer. This response was supplemented by letters dated August 22, 2013, March 26, 2014, and November 24, 2014 (ADAMS Accession Nos. ML 13240A005, ML 14093A320, and ML 14329B257. By letter dated December 23, 2014, the NRG staff transmitted to the licensee a staff assessment of the information provided in the aforementioned letters. The NRG staff has completed its review of the information provided, as documented in the staff assessment and the enclosed supplement to the staff assessment. This closes out the NRC's efforts associated with CAC No. MF1112. The enclosed supplement to the staff assessment updates the NRG staff's conclusions in accordance with the flood hazard reevaluation approach described in NRG letter dated September 1, 2015 (ADAMS Accession No. ML 15174A257), concerning the coordination of requests for information regarding flooding hazard reevaluations and mitigating strategies for beyond-design-basis external events. This letter describes the changes in the NRC's approach to the flood hazard reevaluations that were approved by the Commission in its Staff T. Gatlin Requirements Memorandum (ADAMS Accession No. ML15209A682) to COMSECY-15-0019 (ADAMS Accession No. ML 15153A104) that described the NRC's mitigating strategies and flooding hazard reevaluation action plan. As documented in the NRC staff assessment and the enclosed supplement, the staff has concluded that the licensee's reevaluated flood hazard information is suitable for the assessment of mitigation strategies developed in response to Order EA-12-049 (i.e., defines the mitigating strategies flood hazard information described in guidance documents currently being finalized by the industry and NRC staff) for Summer. Further, the licensee's reevaluated flood hazard information is suitable for other assessments associated with Near-Term Task Force Recommendation 2.1 "Flooding". The reevaluated flood hazard results for local intense precipitation, streams and rivers, and storm surge, were not bounded by the current design-basis flood hazard. In order to complete its response to Enclosure 2 to the 50.54(f) letter, the licensee is expected to submit a revised integrated assessment or focused evaluation(s), as appropriate, to address these reevaluated flood hazards, as described in the NRC's September 1, 2015, letter. If you have any questions, please contact me at (301) 415-6185 or email at Anthony.Minarik@nrc.gov. Docket No. 50-395
Enclosure:
Staff Assessment of Flood Hazard Reevaluation Report cc w/encl: Distribution via Listserv Anthony Minarik, Project Manager Hazards Management Branch Japan Lessons-Learned Division Office of Nuclear Reactor Regulation SUPPLEMENT TO STAFF ASSESSMENT BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO FLOODING HAZARD REEVALUATION REPORT NEAR-TERM TASK FORCE RECOMMENDATION 2.1 VIRGIL C. SUMMER NUCLEAR STATION, UNIT 1 DOCKET NO. 50-395
1.0 INTRODUCTION
This document is a supplement to the U.S. Nuclear Regulatory Commission (NRC) staff assessment that was transmitted by letter dated December 23, 2014 (NRC, 2014d), to South Carolina Electric and Gas Company (SCE&G, the licensee) for Virgil C. Summer Nuclear Station, Unit 1 (Summer, VCSNS). With the exceptions of the Table 3.1-1 and the Reference section, this supplement only contains the sections that were changed to resolve the open items and reflect the changes in the NRC's approach to the flood hazard reevaluations that were approved by the Commission in its Staff Requirements Memorandum (SRM) (NRC, 2015a) to COMSECY-15-0019 (NRC, 2015b), which described the NRC's mitigating strategies and flooding hazard reevaluation action plan. Table 3.1-1 at the end of the supplement is copied from the staff assessment for convenience. Instead of repeating the Reference section in its entirety, only the additions to the list of references are included in the supplement. 2.0 REGULATORY BACKGROUND There are no changes or updates to this section of the NRC staff assessment 2.1 Applicable Regulatory Requirements There are no changes or updates to this section of the NRC staff assessment. 2.2 Enclosure 2 to the 50.54(f) Letter By letter dated March 12, 2012 (NRC, 2012a) the NRC issued a request for information Pursuant to Title 10 of the Code of Federal Regulations (10 CFR), Section 50.54(f) (hereafter referred to as the 50.54(f) letter). The 50.54(f) letter requests all power reactor licensees and construction permit holders reevaluate all external flooding-causing mechanisms at each site. The reevaluation should apply present-day methods and regulatory guidance that are used by the NRC staff to conduct early site permit (ESP) and combined license (COL) reviews. This includes current techniques, software, and methods used in present-day standard engineering Enclosure practice. If the reevaluated flood-causing mechanisms are not bounded by the current plant design-basis flood hazard, an integrated assessment or focused evaluation may be necessary. 2.2.1 Flood-Causing Mechanisms There are no changes or updates to this section of the NRG staff assessment. 2.2.2 Associated Effects There are no changes or updates to this section of the NRG staff assessment. 2.2.3 Combined Effects Flood There are no changes or updates to this section of the NRG staff assessment. 2.2.4 Flood Event Duration There are no changes or updates to this section of the NRG staff assessment. 2.2.5 Actions Following the flooding hazard reevaluation report (FHRR) For the sites where the reevaluated flood probable maximum flood (PMF) elevation is not bounded by the current design-basis flood PMF elevation for all flood-causing mechanisms, the 50.54(f) letter (NRG, 2012a) requests licensees and construction permit holders to:
- Submit an Interim Action Plan with the FHRR documenting actions planned or already taken to address the reevaluated hazard(s).
- Perform an integrated assessment subsequent to the FHRR to (a) evaluate the effectiveness of the current design-basis (i.e., flood protection and mitigation systems), (b) identify plant-specific vulnerabilities, and (c) assess the effectiveness of existing or planned systems and procedures for protecting against and mitigating consequences of flooding for the flood event duration. After issuance of the 50.54(f) letter, the NRG changed the approach to the steps following the review of the flood hazard reevaluations, as directed by the Commission, to permit use of focused evaluations as an alternative to an integrated assessment. The NRG letter dated September 1, 2015 (NRG, 2015c), describes the changes in the NRC's approach to the flood hazard reevaluations If the reevaluated PMF elevation is bounded by the current design-basis PMF elevation for all flood-causing mechanisms at the site, licensees are not required to perform an integrated assessment or a focused evaluation at this time.
3.0 TECHNICAL EVALUATION
There are no changes or updates to this section of the NRG staff assessment. 3.1 Site Information There are no changes or updates to this section of the NRG staff assessment. 3.1.1 Detailed Site Information There are no changes or updates to this section of the NRG staff assessment. 3.1.2 Design-Basis Flood Hazards1 There are no changes or updates to this section of the NRG staff assessment. 3.1.3 Flood-related Changes to the Licensing Basis There are no changes or updates to this section of the NRG staff assessment. 3.1.4 Changes to the Watershed and Local Area There are no changes or updates to this section of the NRG staff assessment. 3.1.5 Current Licensing Basis Flood Protection and Pertinent Flood Mitigation Features There are no changes or updates to this section of the NRG staff assessment. 3.1.6 Additional Site Details to Assess the Flood Hazard There are no changes or updates to this section of the NRG staff assessment. 3.1.7 Plant Walkdown Activities There are no changes or updates to this section of the NRG staff assessment. 3.2 Local Intense Precipitation and Associated Site Drainage There are no changes or updates to this section of the NRG staff assessment. 3.2. 1 Site Drainage and Elevations There are no changes or updates to this section of the NRG staff assessment. 1 In its FHRR, the licensee used the terms "design-basis" and "licensing basis" interchangeably. Because the references to current licensing basis were to various elevations that were specific to each flood hazard, the NRC staff assumes in this document that the licensee intended the term "current licensing basis" in its FHRR to refer to the "current design-basis" since this is what the 50.54(f) letter, Enclosure 2, requested. The NRC staff will thus use the term "current design-basis", as appropriate, throughout this document. 3.2.2 Local Intense Precipitation Depths There are no changes or updates to this section of the NRG staff assessment. 3.2.3 Modeling of Flood Levels To estimate runoff and perform hydrologic routing, the licensee used the FL0-2D software application (SGE&G, 2013a; FL0-2D Software, Inc. 2009). The licensee used the 1-mi2 (2.6-km2), 1-h duration rainfall value of 19.0 in (48.3 cm) as input to the FL0-2D application. Given the significant role that the FL0-2D model performs in the licensee's analysis of the PMF caused by local intense precipitation (LIP), the NRG staff requested that the licensee provide FL0-2D input files. The licensee provided a detailed description of the application of the FL0-2D model along with the model input files in its response to NRG's request for additional Information (RAI) (SGE&G, 2014b, RAls 1 and 2 responses). The NRG staff reviewed model input configurations and the manner in which results of the FL0-2D analysis were used to predict water-surface elevations. The VGSNS, Unit 1 FHRR (SGE&G, 2013a) stated that the roofs of safety-related buildings were designed to store up to 4 inches (10 cm) of precipitation. The licensee did not describe how or if this design feature was incorporated into the LIP analysis and did not characterize similar design features of non-safety-related buildings. The NRG staff reviewed the licensee's FL0-2D application and found that all precipitation falling on building roofs was retained on the roofs and did not enter the flow domain adjacent to the buildings. Water retained on building roofs would reduce the discharge adjacent to, and downstream of, the buildings. The portion of runoff from the plant site into the service water pond (SWP), with coincident wind setup and wave runup discussed later relative to streams and rivers in Section 3.3.4, is also impacted indirectly by the roof drainage issue in the FL0-2D model. The licensee's reevaluation yielded a PMF water-surface elevation of 436.6 ft to 437.5 ft (133.1 m to 133.4 m) (SGE&G, 2013a), which is higher than the current design-basis stillwater-surface elevation of 436.15 ft (132.94 m). Elevation 436.6 ft (133.1 m) is at the east side, and elevation 437.5 ft (133.4 m) is at the west side of the powerblock (SGE&G, 2014b). 3.2.4 Flood Event Duration The VGSNS, Unit 1 FHRR (SGE&G, 2013a) did not address flood warning time or the duration of inundation resulting from LIP flooding. The NRG staff requested additional information from the licensee (NRG, 2014b) to supplement its FHRR (SGE&G, 2013a). The licensee's response (SGE&G, 2014b, RAI 6 response) stated that existing modeling indicates that most ponded water drains from the site within 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />. Warning time, based on meteorological warnings, is expected to be more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. The NRG staff notes that longer duration probable maximum precipitation (PMP) events that deliver greater precipitation volumes, such as the 72-h PMP, generate greater volumes of runoff. Shorter-duration PMP events that have higher rates of precipitation, such as the 1-hour PMP, may however result in much shorter warning times and higher water levels. The NRC staff notes that a reasonable estimate of the site's LIP PMP is application of an appropriate National Oceanic and Atmospheric Administration hydrometeorological report (HMR) estimate for any rainfall duration used in NUREG/CR-7046, regardless of temporal distribution of the rainfall. The licensee obtained 1-mi2 (2.6-km2}, 1-h duration PMP value using HMR-52. Therefore, the NRC staff confirmed that the licensee selected an appropriate rainfall rate value to satisfy the 50.54(f) information request. 3.2.5 Conclusion The NRC staff confirmed the licensee's conclusion that the reevaluated flood hazard for LIP and associated site drainage is not bounded by the current design-basis flood hazard. Therefore, the licensee is expected to submit a focused evaluation for LIP and associated site drainage consistent with the process outlined in COMSECY-15-0019 (NRC, 2015b) and associated guidance that will be issued. Under this approach, the NRC staff anticipates that licensees will perform and document a focused evaluation that evaluates the impact of the LIP hazard on the site and implements any necessary programmatic, procedural or plant modifications to address this hazard exceedance. The roof drainage and Service Water Pond issues that are discussed in Sections 3.2.3 and 3.3.4 should be addressed in the focused evaluation. The NRC staff anticipates that licensees will submit letters providing a summary of the evaluation and, if needed, regulatory commitments to implement and maintain appropriate programmatic, procedural or plant modifications to protect against the LIP hazard. 3.3 Streams and Rivers There are no changes or updates to this section of the NRC staff assessment. 3.3.1 Additional Information There are no changes or updates to this section of the NRC staff assessment. 3.3.2 Flooding Scenarios and Associated Effects There are no changes or updates to this section of the NRC staff assessment. 3.3.3 Monticello Reservoir There are no changes or to this section of the NRC staff assessment. 3.3.4 Service Water Pond In the VCSNS, Unit 1 FHRR (SCE&G, 2013a), the SWP is described as a Seismic Category 1 impoundment that serves as the Ultimate Heat Sink for VCSNS, Unit 1. The SWP is adjacent to Monticello Reservoir (Figure 3.3-2). The SWP is separated from Monticello Reservoir by two islands and three Seismic Category 1 dams, which have crest elevations of 438.0 ft (133.5 m) on three sides. The crest elevation of the West Embankment is 435.0 ft (132.6 m). The West Embankment adjoins the VCSNS, Unit 1 plant yard grade, and therefore represents the land elevation value critical for assessment of the PMF maximum water-surface elevation in the SWP. In its FHRR, the licensee stated that water is supplied to the SWP from Monticello Reservoir by a pipe configured with a butterfly isolation valve. This isolation valve is kept closed during normal operations. The FHRR states that the SWP normal pool elevation is 422.0 ft (128.6 m). Under normal operational conditions the pool elevation ranges from 420.5 ft to 425.0 ft (128.2 m to 129.5 m). The NRC staff confirmed that the description of the SWP is consistent with that provided in the VCSNS, Unit 1 UFSAR (SCE&G, 201 Oa). Given the control of the SWP pool elevation by the operation of the SWP isolation valve on the interconnecting pipe between the SWP and Monticello Reservoir, the NRC staff requested that the licensee provide a detailed description of conditions leading to the valve's operation, frequency of operation, and any assumptions related to the state of the isolation valve used in the reevaluation of the PMF for the SWP (NRC, 2014b). The licensee provided a detailed description of the operation of the SWP isolation valve, frequency of operation, and the assumptions used by the licensee related to the state of the valve in its analysis of the PMF (SCE&G, 2014b, RAI 4 response). The VCSNS, Unit 1 FHRR (SCE&G, 2013a), included a description of the runoff and course models associated with the PMF flooding in streams and rivers related to LIP runoff from the plant area as it drains into the SWP. In response to the NRC's RAI (NRC, 2014b), the licensee described its analysis of water levels in the SWP resulting from the PMP on the site plus associated effects (SCE&G, 2014b, RAI 5 response). The SWP receives a portion of the LIP runoff from the plant area as mentioned in Section 3.2.3 above, and the issue within the FL0-20 model, related to lack of roof drainage, impacts the portion of runoff from the plant site to the SWP. The VCSNS, Unit 1 FHRR (SCE&G, 2013a), referenced a full description of the coincident wave effects associated with the PMF flooding in streams and rivers, contained in the VCSNS, Unit 1 UFSAR (SCE&G, 201 Oa). The VCSNS, Unit 1 FHRR (SCE&G, 2013a), Section 4.1.2.2.2.2, stated that the VCSNS, Unit 1 UFSAR (SCE&G, 201 Oa) PMF included wind effects that would result in a SWP water-surface elevation of 433.6 ft (132.2 m). The FHRR maximum water-surface elevation including wind-wave activity estimates are based, in part, on the stillwater-surface elevation in the SWP. The VCSNS, Unit 1 FHRR (SCE&G, 2013a), Section 4.2.2.2, stated that the reevaluated PMF elevation for the SWP at the West Embankment was determined to be 428.3 ft (130.5 m). The licensee used the FL0-20 model to estimate LIP and runoff from the site into the SWP and the associated rise in the stillwater-surface elevation. 3.3.5 Broad River There are no changes or updates to this section of the NRC staff assessment. 3.3.6 Conclusion The NRC staff confirmed the licensee's conclusion (SCE&G 2014c) that the reevaluated hazard for flooding from streams and rivers is not bounded by the current design-basis flood hazard when combined with wind setup and wave runup; therefore, the licensee should include flooding from streams and rivers with wind setup and wave runup from Monticello Reservoir within the scope of the integrated assessment or focused evaluation consistent with the process and guidance discussed in COMSECY-15-0019 (NRC, 2015b). Information on flooding from streams and rivers that is specific to the data needs of the integrated assessment or focused evaluation is described in Section 4 of this staff assessment. 3.4 Failure of Dams and Onsite Water Control/Storage Structures There are no changes or updates to this section of the NRC staff assessment. 3.5 Storm Surge The VCSNS, Unit 1 FHRR (SCE&G, 2013a) reported that the reevaluated PMF elevation, including associated effects, for site flooding due to the wind setup and wave runup aspects of storm surge is 437.0 ft (133.2 m) when combined with the PMF in the streams and rivers analysis. This flood-causing mechanism is described in the licensee's current design-basis. This reevaluated PMF elevation exceeds the current design-basis PMF elevation for site flooding due to storm surge of 436.6 ft (133.1 m). The licensee stated that water level increases due to storm surge were included in the PMF determinations in the Monticello Reservoir and the SWP; no further analysis was performed specifically for storm surge without the PMP. The plant site is protected by the North Berm to elevation 438.0 ft (133.5 m). In summary, the NRC staff confirmed the licensee's conclusion that flooding from storm surge does not inundate the site. However, this hazard mechanism exceeds the current design-basis. Therefore, the treatment of the storm surge should be addressed in the integrated assessment or focused evaluation consistent with the process and guidance discussed in COMSECY-15-0019 (NRC, 2015c). 3.6 Seiche There are no changes or updates to this section of the NRC staff assessment. 3.7 Tsunami There are no changes or updates to this section of the NRC staff assessment. 3.8 Ice-Induced Flooding There are no changes or updates to this section of the NRC staff assessment. 3.9 Channel Migrations or Diversions There are no changes or updates to this section of the NRC staff assessment. 4.0 REEVALUATED FLOOD HEIGHT, EVENT DURATION AND ASSOCIATED EFFECTS FOR HAZARDS NOT BOUNDEDE BY THE COB The NRG staff confirms that for certain flooding mechanisms that the reevaluated hazard is not bounded by the current design-basis flood hazard. Therefore, the NRG staff concludes that an integrated assessment or focused evaluation(s) is necessary, and that it should consider the following flood-causing mechanisms: LIP, storm surge, and flooding in streams and rivers with wind setup and wave runup from Monticello Reservoir as a combined effect. The NRG staff reviewed the following flood hazard parameters needed to perform the additional assessments or evaluations of plant response:
- Flood height and associated effects (see Table 4.0-2), as defined in JLD-ISG-2012-05 (NRG, 2012d)
- Flood event duration (see Table 4.0-1 ), including warning time and intermediate water surface elevations that trigger actions by plant personnel, as defined in JLD-ISG-2012-05 (NRG, 2012d) 4.1 Flood Height and Associated Effects The licensee estimated maximum water-surface elevations using its FL0-20 analysis (SCE&G, 2013a, 2014b). The NRG staff found that, in the licensee's FL0-20 application, precipitation was modeled as being retained on building roofs rather than discharged to the ground surface near the structure or an adjacent area, which could result in underestimation of the maximum water-surface elevation. Because the LIP flooding mechanism is being evaluated as part of a focused evaluation, the NRG staff determined that this numerical modeling issue should be resolved as part of the focused evaluation. The streams and rivers flood-causing mechanism was combined with the storm surge causing mechanism (wind setup and wave runup). This combined effect flood reevaluation is not bounded by the current design-basis, and results in an increase in the elevation of water impinging upon a flood protection structure (i.e., North Berm of the plant site at Monticello Reservoir). The NRG staff has observed that the increase in the combined effect flood has resulted in a reduction of margin that is quantitatively minor (0.4 ft (0.1 m)) and characterized by brief and intermittent impingement of waves on a passive low-head flood protection feature (a 3 ft (1 m) berm). 4.2 Flood Event Duration Section 3.2.4 of this staff assessment discusses flood event duration. The NRG staff notes that a reasonable estimate of the site's LIP PMP is the application of an appropriate National Oceanic and Atmospheric Administration hydrometeorological report (HMR) estimate for any rainfall duration used in NUREG/CR-7046, regardless of temporal distribution of the rainfall. The licensee obtained a 1-mii2 (2.6-km2), 1-h duration PMP value using HMR-52. Therefore, the NRG staff confirmed that the licensee selected an appropriate rainfall rate value to satisfy the 50.54(f) information request. 4.3 Conclusion Based upon the preceding analysis, the NRG staff confirmed that the reevaluated flood hazard information defined in the sections above is appropriate input to other assessments or evaluations associated with Near-Term Task Force Recommendations, including the assessment of mitigation strategies developed in response to Order EA-12-049 (i.e., defines the mitigating strategies flood hazard information described in guidance documents currently being finalized by the industry and NRG staff).
5.0 CONCLUSION
The NRG staff has reviewed the information provided for the reevaluated flood-causing mechanisms of VCSNS, Unit 1. Based on its review, the NRG staff concludes that the licensee conducted the hazard reevaluation using present-day methodologies and regulatory guidance used by the NRG staff in connection with ESP and COL reviews. Based on the preceding analysis, the NRG staff confirmed that the licensee responded appropriately to Enclosure 2, Required Response 2, of the 50.54(f) letter, dated March 12, 2012. In reaching this determination, the NRG staff confirmed the licensee's conclusions that (a) the reevaluated flood hazard results for local intense precipitation, streams and rivers, and storm surge are not bounded by the current design-basis flood hazard, (b) additional assessments of plant response will be performed for the local intense precipitation streams and rivers, and storm surge flood-causing mechanisms, and (c) the reevaluated flood-causing mechanism information is appropriate input to additional assessments or evaluations of plant response, as described in the 50.54(f) letter and COMSECY-15-0019 (NRG, 2015b), including the assessment of mitigation strategies developed in response to Order EA-12-049 (i.e., defines the mitigating strategies flood hazard information described in guidance documents currently being finalized by the industry and NRG staff). The NRG staff has no additional information needs at this time with respect to the FHRR.
6.0 REFERENCES
U.S. Nuclear Regulatory Commission (NRG) Documents and Publications: NRG (U.S. Nuclear Regulatory Commission), 2014d, letter from Robert F. Kuntz, NRG, to Thomas D. Gatlin, Vice President, "Virgil C. Summer Nuclear Station, Unit 1 -Staff Assessment of Response to 10 CFR 50.54(F) Information Request-Flood-Causing Mechanism Reevaluation (TAC No. MF1112), December 23, 2014, ADAMS Accession No. ML 14356A002. NRG (U.S. Nuclear Regulatory Commission), 2015a, SRM -COMSECY-15-0019-Closure Plan for the Reevaluation of Flooding Hazards for Operating Nuclear Power Plants,", July 28, 2015, ADAMS Accession No. ML 15209A682. NRG (U.S. Nuclear Regulatory Commission), 2015b, COMSECY-15-0019 "Closure Plan for the Reevaluation of Flooding Hazards for Operating Nuclear Power Plants,", June 30, 2015, ADAMS Accession No. ML 15153A 104. NRG (U.S. Nuclear Regulatory Commission), 2015c, letter from William M. Dean, Director, to Power Reactor Licensees," Coordination of Requests for Information for Flooding Hazard Reevaluations and Mitigating Strategies for Beyond Design Basis External Events", September 1, 2015, ADAMS Accession No. ML15174A257. Codes and Standards There are no additions to the references in this section. Other
References:
There are no additions to the references in this section. Table 3.1-1: Current Design-Basis (COB) Flood Hazard Flooding COB Stillwater COB Associated COB Flood Level, Level, Reference Mechanism NGV029 Effects* NGV029 Local Intense 436.15 ft None 436.15 ft FHRR (SCE&G, 2013a) Precipitation (132.94 m) (132.94 m) based Sections 4.3.1 and and Associated based on on maximum hour 4.1.2.1.1. Drainage maximum hour within 6 hr PMP UFSAR (SCE&G, 201 Oa) within 6 hrs Section 2.4.3.1.3 PMP** Streams and 429.1 ft (130.8 7.5 ft (2.3 m) from 436.6 ft (133.1 m) FHRR (SCE&G, 2013a) Rivers m), including wave runup Section 4.1.2.2.1 (including wave 4.1 ft (1.25 m) UFSAR (SCE&G, 201 Oa) runup from from 48-hrs Section 2.4.3.6.2 Storm Surge) PMP on initial reservoir elevation of 425.0 ft (129.5 m) Failure of <290 ft No Impact <290 ft (88.4 m), FHRR (SCE&G, 2013a) Dams and (88.4 m) Identified Parr Shoals Section 4.2.2.4 Onsite Water No threat of Reservoir I Broad Control/Storage flooding at site River, Structures No threat of flooding at site Storm Surge See Streams See Streams and See Streams and Included in Streams and and Rivers for Rivers for Rivers for Rivers as combined effect combined combined effect combined effect effect Seiche No Impact No Impact No Impact FHRR (SCE&G, 2013a) Identified Identified Identified Section 4.2.2.6 UFSAR (SCE&G, 201 Oa) Section 2.4.5.5 Tsunami No Impact No Impact No Impact FHRR (SCE&G, 2013a) Identified Identified Identified Section 4.2.2.7 Ice-Induced No Impact No Impact No Impact FHRR (SCE&G, 2013a) Identified Identified Identified Section 4.2.2.8 Channel No Impact No Impact No Impact FHRR (SCE&G, 2013a) Migrations or Identified Identified Identified Section 4.2.2.9 Diversions ..
- No associated effects are identified from debns, sediment depos1t1on or erosion, concurrent site conditions (including adverse weather other than the associated PMP), or groundwater ingress. ** 436.15 ft (132.94 m) stillwater level represents nominal plant grade of 435.0 ft (132.6 m) with floodwater depth of 1.15 ft (0.350 m). Table 4.0-1: Flood Event Duration for Reevaluated Flood-Causing Mechanisms Not Bounded by the Current Design Basis Hazard* Flood-Causing Site Preparation for Period of Site Recession of Total Event Mechanism Flood Event Inundation Water from Site Duration Local Intense >24 hr 7 hr 17 h >48 hours Precipitation and (Assuming all claimed (At Power Block (Resulting from 1-Associated warning time is used for West) h PMP) Drainage site preparation} Streams and > 24 hr warning time Oh Oh N/A Rivers (Implied by local intense (No site (No site (No site inundation (Monticello precipitation analysis; inundation; inundation) from 72-hrs PMP Reservoir Monticello Reservoir is licensee analysis including wind flooding) immediately adjacent to indicates that wind setup and wave site, and warning time setup and wave runup) would be same as for runup impinge on local intense North Berm flood precipitation) protection feature for 72 seconds) Storm Surge See Streams and Rivers See Streams and See streams and See streams and for combined effect Rivers for Rivers for Rivers for combined effect combined effect combined effect *Elements of flood event duration are shown in Figure 2.2-1. Table 4.0-2: Reevaluated Flood-Causing Mechanisms and Associated Effects Hazards Not Bounded by the Current Design Basis Hazard Reevaluated Stillwater Associated Effects** Reevaluated Flood Flood-Causing Elevation Hazard Elevation Reference Mechanism (NGVD29) Local Intense 436.6 ft to None 436.6 ft to 437.5 ft Maximum surface Precipitation and 437.5 ft (133.1 mto133.4m) water elevation Associated (133.1 m to (East side to west side during PMP at Drainage 133.4 m) of power block, from main plant 1-hr PMP of 19 in [48 buildings and cm]) doors -see (Water depths of 1.6 ft FHRR (SCE&G, 2013a, Section to 2.5 ft [0.49 m to 4.2.1.2) 0.76 m]) Streams and 431.07 ft 5.93 ft (1.81 m) 437.0 ft (133.2 m) at Maximum PMF Rivers (131.39 m) wind setup and wave North Berm elevation for (Resulting from (Includes run up (Remains below North Monticello 72-hr PMP) effects of Berm crest elevation of Reservoir at including wind 6.07 ft [1.85 438.0 ft [133.5 m] North Berm of setup and wave m] as direct NGVD29) plant -see runup from PMP and FHRR (SCE&G, Monticello basin runoff) 2013a, Section Reservoir 4.2.2.1) and RAI 3 response (SCE&G, 2014b) Storm Surge See Streams See Streams and See Streams and See Streams and and Rivers Rivers for combined Rivers for combined Rivers for for combined effect effect combined effect effect ** No associated effects are anticipated from debris, sediment deposition or erosion, concurrent site conditions (including adverse weather other than the associated PMP), or groundwater ingress (SCE&G, 2013a; SCE&G, 2014b) Table 5.0-1: Integrated Assessment Open Items Deleted
ML 15209A682) to COMSECY-15-0019 (ADAMS Accession No. ML 15153A104) that described the NRC's mitigating strategies and flooding hazard reevaluation action plan. As documented in the NRC staff assessment and the enclosed supplement, the staff has concluded that the licensee's reevaluated flood hazard information is suitable for the assessment of mitigation strategies developed in response to Order EA-12-049 (i.e., defines the mitigating strategies flood hazard information described in guidance documents currently being finalized by the industry and NRC staff) for Summer. Further, the licensee's reevaluated flood hazard information is suitable for other assessments associated with Near-Term Task Force Recommendation 2.1 "Flooding". The reevaluated flood hazard results for local intense precipitation, streams and rivers, and storm surge, were not bounded by the current design-basis flood hazard. In order to complete its response to Enclosure 2 to the 50.54(f) letter, the licensee is expected to submit a revised integrated assessment or focused evaluation(s), as appropriate, to address these reevaluated flood hazards, as described in the NRC's September 1, 2015, letter. If you have any questions, please contact me at (301) 415-6185 or email at Anthony.Minarik@nrc.gov. Docket No. 50-395
Enclosure:
Staff Assessment of Flood Hazard Reevaluation Report cc w/encl: Distribution via Listserv DISTRIBUTION: see next page ADAMS Accession No.: ML15296A377 OFFICE NRR/JLD/JHMB/PM NAME MMarshall* DATE 10/21/2015 OFFICE NRO/DSEA/RHM1/BC NAME CCook* DATE 10/25/2015 Sincerely, IRA/ Anthony Minarik, Project Manager Hazards Management Branch Japan Lessons-Learned Division Office of Nuclear Reactor Regulation *via email NRR/JLD/JHMB/PM NRR/JLD/LA AMinarik SLent* 10/28/2015 10/30/2015 NRR/JLD/JHMB/BC NRR/JLD/JHMB/PM MShams* AMinarik 10/26/2015 11/3/2015 Letter to Thomas D. Gatlin from Anthony Minarik dated November 3, 2015
SUBJECT:
VIRGIL C. SUMMER NUCLEAR STATION, UNIT 1 -STAFF ASSESSMENT OF RESPONSE TO 10 CFR 50.54(f) INFORMATION REQUEST-CAUSING MECHANISM REEVALUATION (CAC NO. MF1112) DISTRIBUTION: PUBLIC JHMB R/F RidsNroDsea Resource RidsNrrDorlLpl2-1 Resource RidsNrrSummer Resource RidsNrrLASLent Resource RidsOgcMailCenter Resource VHall, NRR BHarvey, NRO KErwin, NRO CCook, NRO ARivera-Varona, NRO SFlanders, NRO LQuinn-Willingham, NRO ACampbell, NRO AMinarik, NRR