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| issue date = 11/04/2015
| issue date = 11/04/2015
| title = Supplement to Staff Assessment of Response to 10 CFR 50.54(f) Information Request - Flood Causing Mechanisms Reevaluation
| title = Supplement to Staff Assessment of Response to 10 CFR 50.54(f) Information Request - Flood Causing Mechanisms Reevaluation
| author name = Govan T V
| author name = Govan T
| author affiliation = NRC/NRR/JLD
| author affiliation = NRC/NRR/JLD
| addressee name = Hanson B C
| addressee name = Hanson B
| addressee affiliation = Exelon Generation Co, LLC
| addressee affiliation = Exelon Generation Co, LLC
| docket = 05000237, 05000249
| docket = 05000237, 05000249
Line 18: Line 18:


=Text=
=Text=
{{#Wiki_filter:UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 Mr. Bryan C. Hanson Senior Vice President Exelon Generation Company, LLC 4300 Winfield Rd Warrenville, IL 60555 November 4, 2015
{{#Wiki_filter:UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 November 4, 2015 Mr. Bryan C. Hanson Senior Vice President Exelon Generation Company, LLC 4300 Winfield Rd Warrenville, IL 60555


==SUBJECT:==
==SUBJECT:==
DRESDEN NUCLEAR POWER STATION, UNITS 2 AND 3-SUPPLEMENT TO STAFF ASSESSMENT OF RESPONSE TO 10 CFR 50.54(f) INFORMATION REQUEST -FLOOD-CAUSING MECHANISMS REEVALUATION (TAC NOS. MF1795 AND MF1796)  
DRESDEN NUCLEAR POWER STATION, UNITS 2 AND 3-SUPPLEMENT TO STAFF ASSESSMENT OF RESPONSE TO 10 CFR 50.54(f) INFORMATION REQUEST - FLOOD-CAUSING MECHANISMS REEVALUATION (TAC NOS. MF1795 AND MF1796)


==Dear Mr. Hanson:==
==Dear Mr. Hanson:==
The purpose of this letter is to transmit the supplement to the U.S. Nuclear Regulatory Commission (NRC) staff's assessment of Dresden Nuclear Power Station, Units 2 and 3 (Dresden) reevaluated flood hazard information that was issued to you by a letter dated March 31, 2015 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML 15072A007).
The supplement updates the original staff assessment to address changes in the NRC's approach to next 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 NRC 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 *10, 2013 (ADAMS Accession No. ML 13135A120), Exelon Generation Company, LLC (the ilcensee) responded to this request for Dresden. In response to NRC staff questions, this response was supplemented by letter dated May 19, 2014 (ADAMS Accession No. ML 15092A821).
* The NRC 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 TAC Nos. MF1795 and MF1796. . The enclosed supplement to the staff assessment updates the NRC staff's conclusions in accordance with the flood hazard reevaluation approach described in NRC 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
* Requirements Memorandum (ADAMS Accession No. ML 15209A682) to COMSECY-15-0019 (ADAMS Accession No. ML 15153A104) that described the NRC's mitigating strategies and flooding hazard reevaluation action plan.
B. Hanson As documented in the NRC staff assessment and the enclosed supplement, the NRC 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 Dresden. Further, the NRC staff continues to assert that 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 and upstream dam failures 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) 415-6197 or e-mail at Tekia.Govan@nrc.gov.
Docket Nos. 50-237 and 50-249


==Enclosure:==
The purpose of this letter is to transmit the supplement to the U.S. Nuclear Regulatory Commission (NRC) staff's assessment of Dresden Nuclear Power Station, Units 2 and 3 (Dresden) reevaluated flood hazard information that was issued to you by a letter dated March 31, 2015 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML15072A007). The supplement updates the original staff assessment to address changes in the NRC's approach to next 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. ML12053A340), the NRC 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 *10, 2013 (ADAMS Accession No. ML13135A120), Exelon Generation Company, LLC (the ilcensee) responded to this request for Dresden. In response to NRC staff questions, this response was supplemented by letter dated May 19, 2014 (ADAMS Accession No. ML15092A821).
* The NRC 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 TAC Nos. MF1795 and MF1796.                                          .
The enclosed supplement to the staff assessment updates the NRC staff's conclusions in accordance with the flood hazard reevaluation approach described in NRC letter dated September 1, 2015 (ADAMS Accession No. ML15174A257), '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
* Requirements Memorandum (ADAMS Accession No. ML15209A682) to COMSECY-15-0019 (ADAMS Accession No. ML15153A104) that described the NRC's mitigating strategies and flooding hazard reevaluation action plan.


Supplement to Staff Assessment of Flood Hazard Reevaluation Report cc w/encl: Distribution via Listserv Tekia Govan, 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
B. Hanson                                          As documented in the NRC staff assessment and the enclosed supplement, the NRC 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 Dresden. Further, the NRC staff continues to assert that 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 and upstream dam failures 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) 415-6197 or e-mail at Tekia.Govan@nrc.gov.
Tekia Govan, Project Manager Hazards Management Branch Japan Lessons-Learned Division Office of Nuclear Reactor Regulation Docket Nos. 50-237 and 50-249


===2.1 RELATED===
==Enclosure:==
TO THE FUKUSHIMA DAl-ICHI NUCLEAR POWER PLANT ACCIDENT DRESDEN NUCLEAR POWER STATION, UNITS 2 AND 3 DOCKET NOS. 50-237 AND 50-249
 
==1.0 INTRODUCTION==
 
This document is a supplement to the U.S. Nuclear Regulatory Commission (NRC) staff assessment that was transmitted by letter dated March 31, 2015 (NRC, 2015a), to Exelon Generation Company, LLC (the licensee) for Dresden Nuclear Power Station, Units 2 and 3 (Dresden, DNPS). With the exceptions of the Table 3.1.2-1 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, 2015b) to COMSECY 0019 (NRC, 2015c), which described the NRC's mitigating strategies and flooding hazard reevaluation action plan. Table 3.1.2-1 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===
Supplement to Staff Assessment of Flood Hazard Reevaluation Report cc w/encl: Distribution via Listserv


BACKGROUND
SUPPLEMENT TO STAFF ASSESSMENT BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO FLOODING HAZARD REEVALUATION REPORT NEAR-TERM TASK FORCE RECOMMENDATION 2.1 RELATED TO THE FUKUSHIMA DAl-ICHI NUCLEAR POWER PLANT ACCIDENT DRESDEN NUCLEAR POWER STATION, UNITS 2 AND 3 DOCKET NOS. 50-237 AND 50-249


===2.1 Applicable===
==1.0    INTRODUCTION==


Regulatory Requirements There are no changes or updates to this section of the NRC staff assessment.  
This document is a supplement to the U.S. Nuclear Regulatory Commission (NRC) staff assessment that was transmitted by letter dated March 31, 2015 (NRC, 2015a), to Exelon Generation Company, LLC (the licensee) for Dresden Nuclear Power Station, Units 2 and 3 (Dresden, DNPS). With the exceptions of the Table 3.1.2-1 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, 2015b) to COMSECY 0019 (NRC, 2015c), which described the NRC's mitigating strategies and flooding hazard reevaluation action plan. Table 3.1.2-1 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 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 1O 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 practice. If the reevaluated flood-causing mechanisms are not bounded by the current plant design-basis flood hazard, an integrated assessment may be necessary.
Enclosure


===2.2 Enclosure===
2.2.1   Flood-Causing Mechanisms There are no changes or updates to this section of the NRC staff assessment.
 
2.2.2   Associated Effects There are no changes or updates to this section of the NRC staff assessment.
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 1 O 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 practice.
2.2.3   Combined Effects Flood There are no changes or updates to this section of the NRC staff assessment.
If the reevaluated flood-causing mechanisms are not bounded by the current plant design-basis flood hazard, an integrated assessment may be necessary.
2.2.4   Flood Event Duration There are no changes or updates to this section of the NRC staff assessment.
Enclosure  2.2.1 Flood-Causing Mechanisms There are no changes or updates to this section of the NRC staff assessment.  
2.2.5   Actions Following the Flooding Hazard Reevaluation Report (FHRR)
 
For the sites where the reevaluated probable maximum flood elevation is not bounded by the current design-basis probable maximum flood elevation for all flood-causing mechanisms, the 50.54(f) letter requests licensees and construction permit holders to:
====2.2.2 Associated====
 
Effects There are no changes or updates to this section of the NRC staff assessment.  
 
====2.2.3 Combined====
Effects Flood There are no changes or updates to this section of the NRC staff assessment.  
 
====2.2.4 Flood====
Event Duration There are no changes or updates to this section of the NRC staff assessment.  
 
====2.2.5 Actions====
Following the Flooding Hazard Reevaluation Report (FHRR) For the sites where the reevaluated probable maximum flood elevation is not bounded by the current design-basis probable maximum flood elevation for all flood-causing mechanisms, the 50.54(f) letter 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.
* Submit an Interim Action Plan with the FHRR documenting actions planned or already taken to address the reevaluated hazard.
* Perform an integrated assessment subsequent to the FHRR to (a) evaluate the effectiveness of the current licensing 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.
* Perform an integrated assessment subsequent to the FHRR to (a) evaluate the effectiveness of the current licensing basis (i.e., flood protection and mitigation systems),
After issuance of the 50.54(f) letter, the NRC 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.
(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.
The NRC letter dated September 1, 2015 (NRC, 2015d) describes the changes in the NRC's approach to the flood hazard reevaluations.
After issuance of the 50.54(f) letter, the NRC 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 NRC letter dated September 1, 2015 (NRC, 2015d) describes the changes in the NRC's approach to the flood hazard reevaluations.
If the reevaluated flood hazard is bounded by the current design-basis flood hazard for all causing mechanisms at the site, licensees are not required to perform an integrated assessment or focused evaluation(s) at this time. 3.0 TECHNICAL EVALUATION There are no changes or updates to this section of the NRC staff assessment. 3.1 Site Information There are no changes or updates to this section of the NRC staff assessment.
If the reevaluated flood hazard is bounded by the current design-basis flood hazard for all flood-causing mechanisms at the site, licensees are not required to perform an integrated assessment or focused evaluation(s) at this time.
3.2 LIP and Associated Site Drainage There are no changes or updates to this section of the NRC staff assessment.  


====3.2.1 Model====
==3.0      TECHNICAL EVALUATION==
Inputs and Assumptions There are no changes or updates to this section of the NRC staff assessment.


====3.2.2 Probable====
There are no changes or updates to this section of the NRC staff assessment.
Maximum Precipitation The licensee's analysis developed the 1-hour, 1-square mile probable maximum precipitation (PMP) event distribution using Hydrometerological Reports (HMRs) 51 and 52 (National Oceanic and Atmospheric Administration (NOAA), 1978 and 1982). The licensee stated that the local intense precipitation (LIP) event is defined by NUREG/CR-7046 (NRC, 2011 d) as the 1-hour, 1-square mile PMP event. The licensee extrapolated from the PMP depth contour map provided in HMR 52. The licensee developed the distribution of the 1-hour PMP from the 5-, 15-, 30-and 60-minute time intervals from HMR 52. The licensee computed a cumulative depth of the 1-hour, 1-square mile PMP of 17.97 inches (45.28 cm). The NRC staff verified the HMR 51 and HMR 52 computations based on the location of the DNPS, and concluded the PMP depths are appropriate.
The NRC staff notes that a reasonable estimate of the site's LIP PMP is the application of an appropriate NOAA HMR estimate for any rainfall duration used in NUREG/CR-7046, regardless of temporal distribution of the rainfall.
The licensee obtained 1-sq. mile PMP depths for durations ranging between 5-minutes to 1-hour using HMR-51 and HMR-52. Therefore, the NRC staff confirmed that the licensee selected appropriate rainfall rate values to satisfy the 50.54(f) information request. Based on the NRC staff's reviews of FHRRs to date, the NRC staff observed that, when using transient rainfall runoff models, PMP events having longer than 1-hour durations may result in higher LIP flood elevations and longer periods of inundation than the 1-hour event. NRC staff also noted that LIP events deriving from PMPs having relatively short durations may result in limiting warning time and may likewise result in consequential LIP flood elevation (e.g., flood elevations above the openings to plant structures).
3.2.3 Site Topography There are no changes or updates to this section of the NRC staff assessment.
3.2.4 Site Land Cover There are no changes or updates to this section of the NRC staff assessment. 3.2.5 FL0-20 Results The licensee computed the water surface elevations, maximum flooding depths, maximum velocity, maximum resultant impact loading (forces due to moving water), and maximum resultant static load (height of water) using the FL0-20 model. The maximum flood elevation of 518.1 ft (157.9 m) occurs at several Category 1 structures.
The flooding depths range between 0.1 ft (0.03 m) (Reactor Building, Unit 3) to 1. 7 ft (0.5 m) (Turbine Building, Unit 2), which produce a maximum static load of 85.0 lbs. per ft (126.5 kg-force perm). The velocities are generally less than 1 foot per second (ft/s; 0.3 meters per second (m/s)), with resultant impact loads of 5 lbs. per ft (7.5 kg-force perm) or less. The licensee noted that based on the sensitivity analysis of the grid size and Manning's n values (n-values), an accuracy of+/-0.1 ft (0.03 m) should be taken when considering the flood elevations.
The NRC staff reviewed the FL0-20 model input and output files provided by the licensee.
In the course of the review, NRC staff performed a mass balance verification and concluded that the runoff amount passing through the model boundary is less than the amount of rainfall.
The difference in mass could result in a potential underestimation of the maximum water levels around the site. The licensee is requested to resolve numerical modeling issue associated with the LIP flood analyses, specifically the reduction of volume of outflow runoff as compared to the inflow precipitation, accounting for infiltration.
This issue may relate to runoff from rooftops being removed from the numerical model domain rather than discharging to the ground surface near the structure or an adjacent area. 3.2.6 Site-Specific Probable Maximum Precipitation There are no changes or updates to this section of the NRC staff assessment.
3.2. 7 NRC Staff 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 for LIP and associated site drainage 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 modelling issues that are discussed in Section 3.2.5 should be addressed in the focused evaluation, if the licensee uses the same FL0-20 model in its 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 NRG staff assessment.  


===3.4 Failure===
3.1      Site Information There are no changes or updates to this section of the NRC staff assessment.
of Dams and Onsite Water Control/Storage Structures There are no changes or updates to this section of the NRG staff assessment.  
3.2      LIP and Associated Site Drainage There are no changes or updates to this section of the NRC staff assessment.
3.2.1    Model Inputs and Assumptions There are no changes or updates to this section of the NRC staff assessment.
3.2.2        Probable Maximum Precipitation The licensee's analysis developed the 1-hour, 1-square mile probable maximum precipitation (PMP) event distribution using Hydrometerological Reports (HMRs) 51 and 52 (National Oceanic and Atmospheric Administration (NOAA), 1978 and 1982). The licensee stated that the local intense precipitation (LIP) event is defined by NUREG/CR-7046 (NRC, 2011 d) as the 1-hour, 1-square mile PMP event. The licensee extrapolated from the PMP depth contour map provided in HMR 52. The licensee developed the distribution of the 1-hour PMP from the 5-, 15-
, 30- and 60-minute time intervals from HMR 52. The licensee computed a cumulative depth of the 1-hour, 1-square mile PMP of 17.97 inches (45.28 cm). The NRC staff verified the HMR 51 and HMR 52 computations based on the location of the DNPS, and concluded the PMP depths are appropriate.
The NRC staff notes that a reasonable estimate of the site's LIP PMP is the application of an appropriate NOAA HMR estimate for any rainfall duration used in NUREG/CR-7046, regardless of temporal distribution of the rainfall. The licensee obtained 1-sq. mile PMP depths for durations ranging between 5-minutes to 1-hour using HMR-51 and HMR-52. Therefore, the NRC staff confirmed that the licensee selected appropriate rainfall rate values to satisfy the 50.54(f) information request.
Based on the NRC staff's reviews of FHRRs to date, the NRC staff observed that, when using transient rainfall runoff models, PMP events having longer than 1-hour durations may result in higher LIP flood elevations and longer periods of inundation than the 1-hour event. NRC staff also noted that LIP events deriving from PMPs having relatively short durations may result in limiting warning time and may likewise result in consequential LIP flood elevation (e.g., flood elevations above the openings to plant structures).
3.2.3    Site Topography There are no changes or updates to this section of the NRC staff assessment.
3.2.4    Site Land Cover There are no changes or updates to this section of the NRC staff assessment.


====3.4.1 Critical====
3.2.5    FL0-20 Results The licensee computed the water surface elevations, maximum flooding depths, maximum velocity, maximum resultant impact loading (forces due to moving water), and maximum resultant static load (height of water) using the FL0-20 model. The maximum flood elevation of 518.1 ft (157.9 m) occurs at several Category 1 structures. The flooding depths range between 0.1 ft (0.03 m) (Reactor Building, Unit 3) to 1. 7 ft (0.5 m) (Turbine Building, Unit 2), which produce a maximum static load of 85.0 lbs. per ft (126.5 kg-force perm). The velocities are generally less than 1 foot per second (ft/s; 0.3 meters per second (m/s)), with resultant impact loads of 5 lbs. per ft (7.5 kg-force perm) or less.
Dam Evaluation and Selection There are no changes or updates to this section of the NRG staff assessment.  
The licensee noted that based on the sensitivity analysis of the grid size and Manning's n values (n-values), an accuracy of+/- 0.1 ft (0.03 m) should be taken when considering the flood elevations.
The NRC staff reviewed the FL0-20 model input and output files provided by the licensee. In the course of the review, NRC staff performed a mass balance verification and concluded that the runoff amount passing through the model boundary is less than the amount of rainfall. The difference in mass could result in a potential underestimation of the maximum water levels around the site. The licensee is requested to resolve numerical modeling issue associated with the LIP flood analyses, specifically the reduction of volume of outflow runoff as compared to the inflow precipitation, accounting for infiltration. This issue may relate to runoff from rooftops being removed from the numerical model domain rather than discharging to the ground surface near the structure or an adjacent area.
3.2.6    Site-Specific Probable Maximum Precipitation There are no changes or updates to this section of the NRC staff assessment.
3.2. 7  NRC Staff 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 for LIP and associated site drainage 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 modelling issues that are discussed in Section 3.2.5 should be addressed in the focused evaluation, if the licensee uses the same FL0-20 model in its 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.


====3.4.2 Upstream====
3.3    Streams and Rivers There are no changes or updates to this section of the NRG staff assessment.
Dam Failure Mechanism Summary There are no changes or updates to this section of the NRG staff assessment.  
3.4    Failure of Dams and Onsite Water Control/Storage Structures There are no changes or updates to this section of the NRG staff assessment.
3.4.1  Critical Dam Evaluation and Selection There are no changes or updates to this section of the NRG staff assessment.
3.4.2   Upstream Dam Failure Mechanism Summary There are no changes or updates to this section of the NRG staff assessment.
3.4.3  Hydrologic Dam Failure Analysis There are no changes or updates to this section of the NRG staff assessment.
3.4.4  Seismic Upstream Dam Failure There are no changes or updates to this section of the NRG staff assessment.
3.4.5  Upstream Dam Failure Timing and Duration There are no changes or updates to this section of the NRG staff assessment.
3.4.6  Other Associated Effects of Upstream Dam Failure 3.4. 7  NRG Staff Conclusion The NRC staff reviewed the assumptions and approach used in the flooding from upstream dams analysis, and concluded the approach appropriate and assumptions to be reasonable.
The NRG staff verified that the models were consistent with results presented in the FHRR and that in cases where the one model output was used as an input to another model, the two data sets were exactly the same. The NRG staff verified the references used, to ensure that they met standard engineering practices and NRG guidance. The NRG staff confirmed the licensee's conclusion that the reevaluated flood hazard for failure of dams and onsite water control or storage structures is not bounded by the current design-basis flood hazard. Therefore, the treatment of the upstream dam failures should be addressed in the integrated assessment or focused evaluation consistent with the process and guidance discussed in COMSECY-15-0019 (NRG, 2015c).


====3.4.3 Hydrologic====
3.5    Storm Surge There are no changes or updates to this section of the NRC staff assessment.
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 BOUNDED BY THE COB The NRC staff confirms that the reevaluated hazard results for LIP and dam failures are not bounded by the current design-basis flood hazard. Therefore, the NRC staff anticipates that the licensee will perform additional assessments (i.e., integrated assessment or focused evaluation) of plant response for DNPS, as described in NRC letter dated September 1, 2015 (NRC, 2015d).
The NRC staff reviewed the following flood hazard parameters needed to perform the additional assessments or evaluations of plant response:
* Flood event duration, including warning time and intermediate water surface elevations that trigger actions by plant personnel, as defined in Japan Lessons-Learned Directorate (JLD) Interim Staff Guidance (ISG) JLD-ISG-2012-05. Flood event durations for the flood-causing mechanisms identified above are shown in Table 4.0-1.
* Flood height and associated effects, as defined in JLD-ISG-2012-05. Flood height and associated effects for the flood-causing mechanisms identified above are shown in Table 4.0-2.
Based upon the preceding analysis, the NRC staff confirmed that the reevaluated flood hazard information 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 NRC staff).


Dam Failure Analysis There are no changes or updates to this section of the NRG staff assessment.  
==5.0    CONCLUSION==


====3.4.4 Seismic====
The NRC staff has reviewed the information provided for the reevaluated flood-causing mechanisms of DNPS, Units. 2 and 3. Based on its review, the NRC staff concludes that the licensee conducted the hazard reevaluation using present-day methodologies and regulatory guidance used by the NRC staff in connection with ESP and COL reviews.
Upstream Dam Failure There are no changes or updates to this section of the NRG staff assessment.  
Based on the preceding analysis, the NRC staff confirmed that the licensee responded appropriately to Enclosure 2, Required Response 2, of the 50.54(f) letter. In reaching this determination, the NRC staff confirmed the licensee's conclusions that (a) the reevaluated flood hazard results for LIP and upstream dam failure are not bounded by the current design-basis flood hazard, (b) additional assessments of plant response will be performed for the local intense precipitation and upstream dam failure 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 (NRC, 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 NRC staff).
The NRC staff has no additional information needs at this time with respect to the FHRR.


====3.4.5 Upstream====
==6.0      REFERENCES==
Dam Failure Timing and Duration There are no changes or updates to this section of the NRG staff assessment.


====3.4.6 Other====
6.1      U.S. Nuclear Regulatory Commission (NRC) Documents and Publications:
Associated Effects of Upstream Dam Failure 3.4. 7 NRG Staff Conclusion The NRC staff reviewed the assumptions and approach used in the flooding from upstream dams analysis, and concluded the approach appropriate and assumptions to be reasonable.
NRC (U.S. Nuclear Regulatory Commission), 2015a, letter from Victor Hall (NRC), to Bryan Hanson, Senior Vice President, Exelon Generation Company, LLC, " Dresden Nuclear Power Station, Units 2 and 3 - Staff Assessment of Response to 10 CFR 50.54(f)
The NRG staff verified that the models were consistent with results presented in the FHRR and that in cases where the one model output was used as an input to another model, the two data sets were exactly the same. The NRG staff verified the references used, to ensure that they met standard engineering practices and NRG guidance.
Information Request- Flood-Causing Mechanism Reevaluation" (TAC Nos. MF1795 AND MF1796) March 31, 2015, ADAMS Accession No. ML15072A007.
The NRG staff confirmed the licensee's conclusion that the reevaluated flood hazard for failure of dams and onsite water control or storage structures is not bounded by the current design-basis flood hazard. Therefore, the treatment of the upstream dam failures should be addressed in the integrated assessment or focused evaluation consistent with the process and guidance discussed in COMSECY-15-0019 (NRG, 2015c). 3.5 Storm Surge There are no changes or updates to this section of the NRC staff assessment.  
NRC (U.S. Nuclear Regulatory Commission), 2015b, "SRM - COMSECY-15-0019 - Closure Plan for the Reevaluation Of Flooding Hazards for Operating Nuclear Power Plants,''
COMSECY-15-0019, July 28, 2015, ADAMS Accession No. ML15209A682.
NRC (U.S. Nuclear Regulatory Commission), 2015c, "Closure Plan for the Reevaluation of Flooding Hazards For Operating Nuclear Power Plants," COMSECY-15-0019, June 30, 2015, ADAMS Accession No. ML15153A104.
NRC (U.S. Nuclear Regulatory Commission), 2015d, letter from William M. Dean, Director, to Power Reactor Licensees, "Coordination of Requests for Information Regarding Flooding Hazard Reevaluations and Mitigating Strategies for Beyond-Design-Basis External Events",
ADAMS Accession No. ML15174A257.
6.2      Codes and Standards There are no additions to the references in this section.
6.3      Other


===3.6 Seiche===
==References:==
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===
There are no additions to the references in this section.


FLOOD HEIGHT. EVENT DURATION AND ASSOCIATED EFFECTS FOR HAZARDS NOT BOUNDED BY THE COB The NRC staff confirms that the reevaluated hazard results for LIP and dam failures are not bounded by the current design-basis flood hazard. Therefore, the NRC staff anticipates that the licensee will perform additional assessments (i.e., integrated assessment or focused evaluation) of plant response for DNPS, as described in NRC letter dated September 1, 2015 (NRC, 2015d). The NRC staff reviewed the following flood hazard parameters needed to perform the additional assessments or evaluations of plant response:
                                                                              .. C urrent Des1gn T a bl e 3121  -                . Bas1s
* Flood event duration, including warning time and intermediate water surface elevations that trigger actions by plant personnel, as defined in Japan Lessons-Learned Directorate (JLD) Interim Staff Guidance (ISG) JLD-ISG-2012-05.
                                                          . Fl 00 d Hazard s Current Design Stillwater Associated          Basis Flood Flooding          Elevation Effects              Elevation    Reference Mechanism              ft {m) ft {m)
Flood event durations for the flood-causing mechanisms identified above are shown in Table 4.0-1.
NGVD29 NGVD29 Exelon, LIP and Associated          517.5 Not discussed        517.5 (157.7)  2013b Drainage                  (157.7)
* Flood height and associated effects, as defined in JLD-ISG-2012-05.
Section 5 525.0      3.0 ft (0.9 m)                      FHRR, Streams and Rivers                                            528.0 (160.9)
Flood height and associated effects for the flood-causing mechanisms identified above are shown in Table 4.0-2. Based upon the preceding analysis, the NRC staff confirmed that the reevaluated flood hazard information 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 NRC staff). 
(160.0)       (wind wave)                         Section 2.a Failure of Dams and Onsite Water                508.2                                            FHRR, Not discussed          508.2 (154.9)
Control/Storage          (154.9)                                           Section 2.a Structures FHRR, Storm Surge                          Not applicable to DNPS site.
Section 2.a FHRR, Seiche                                Not applicable to DNPS site.
Section 2.a FHRR, Tsunami                              Not applicable to DNPS site.
Section 2.a FHRR, Ice-Induced                        No elevation provided in UFSAR.
Section 2.a Channel Migrations          No elevation provided in UFSAR; discussed in    FHRR, or Diversions            reference to control of locks and dams by USAGE. Section 2.a


==5.0 CONCLUSION==
Table 4.0-1: Flood Event Duration for Flood-Causing Mechanisms Not Bounded by COB Hazard Time Available Flood-Causing                            Duration of          Time for Water to for Preparation Mechanism                              Inundation of Site  Recede from Site for Flood Event LIP and Associated          See Section 3.2    See Section 3.2      See Sections 3.2 Drainage Failure of Dams 1 day, 12 hours,    6 days, 15 hours and Onsite Water                                              11 days, 8 hours (Table and 45 minutes      (Table 6.1, Control/Storage                                                6.1, Barstow, 2014b)
(Kaegi, 2013)      Barstow, 2014b)
Structures Table 4.0-2: Reevaluated Flood Hazards for Flood-Causing Mechanisms Not Bounded by COB Hazard Stillwater                                Reevaluated Flood-Causing      Elevation,      Associated Effects          Flood Reference Mechanism            ft (m)                                  Hazard, ft NGVD29                                    (m) NGVD29 LIP and            Varies with    None                      Varies with      Exelon, 2013b, Associated          maximum of                                maximum of      Section 4 Drainage            518.1                                      518.1 (157.9)
(157.9)
Failure of Dams    Varies with                                                FHRR, Section Varies with a maximum      Varies with a and Onsite Water    a maximum                                                  3, Table 1, and of 4.2 (1.3) (Wind wave    maximum of Control/Storage    of 524.8                                                    Section 4, and wave runup)            529.0 (161.2)
Structures          (160.0)                                                    Table 6


The NRC staff has reviewed the information provided for the reevaluated flood-causing mechanisms of DNPS, Units. 2 and 3. Based on its review, the NRC staff concludes that the licensee conducted the hazard reevaluation using present-day methodologies and regulatory guidance used by the NRC staff in connection with ESP and COL reviews. Based on the preceding analysis, the NRC staff confirmed that the licensee responded appropriately to Enclosure 2, Required Response 2, of the 50.54(f) letter. In reaching this determination, the NRC staff confirmed the licensee's conclusions that (a) the reevaluated flood hazard results for LIP and upstream dam failure are not bounded by the current design-basis flood hazard, (b) additional assessments of plant response will be performed for the local intense precipitation and upstream dam failure 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 (NRC, 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 NRC staff). The NRC staff has no additional information needs at this time with respect to the FHRR. 
Table 4.0-3 Associated Effects Inputs Not Bounded by COB Hazard Flooding Mechanism Failure of Dams and Associated Effects              Local Intense                  Onsite Water Factor                  Precipitation                Control/Storage Structures Hydrodynamic loading at      Minimal, not bounding          117 .2 psf (572.2 kg-plant grade                (FHRR, Section. 4.2)          force/m 2) (FHRR, Table 4)
Debris loading at plant      None                          Varies with maximum of grade                                                      3,387 lbs (1,536 kg) at Unit 2 Turbine Building (Barstow, 2014b)
Sediment loading at plant    None                          Minimal effect (Barstow, grade                                                      2014b)
Sediment deposition and     None                          Minimal effect (Barstow, erosion                                                    2014b)
Concurrent conditions,      None                          Hail, strong winds, and including adverse weather                                  tornadoes (Barstow, 2014b)
Groundwater ingress          None                          Bounded (Barstow, 2014b)
Other pertinent factors      None                          Examined barge collision (e.g., waterborne                                          (Barstow, 2014b) projectiles)


==6.0 REFERENCES==
Table 5.0-1: Integrated Assessment Open Items Deleted
 
6.1 U.S. Nuclear Regulatory Commission (NRC) Documents and Publications:
NRC (U.S. Nuclear Regulatory Commission), 2015a, letter from Victor Hall (NRC), to Bryan Hanson, Senior Vice President, Exelon Generation Company, LLC, " Dresden Nuclear Power Station, Units 2 and 3 -Staff Assessment of Response to 10 CFR 50.54(f) Information Request-Flood-Causing Mechanism Reevaluation" (TAC Nos. MF1795 AND MF1796) March 31, 2015, ADAMS Accession No. ML 15072A007.
NRC (U.S. Nuclear Regulatory Commission), 2015b, "SRM -COMSECY-15-0019
-Closure Plan for the Reevaluation Of Flooding Hazards for Operating Nuclear Power Plants,''
COMSECY-15-0019, July 28, 2015, ADAMS Accession No. ML 15209A682.
NRC (U.S. Nuclear Regulatory Commission), 2015c, "Closure Plan for the Reevaluation of Flooding Hazards For Operating Nuclear Power Plants," COMSECY-15-0019, June 30, 2015, ADAMS Accession No. ML 15153A104.
NRC (U.S. Nuclear Regulatory Commission), 2015d, letter from William M. Dean, Director, to Power Reactor Licensees, "Coordination of Requests for Information Regarding Flooding Hazard Reevaluations and Mitigating Strategies for Beyond-Design-Basis External Events", ADAMS Accession No. ML 15174A257.
 
===6.2 Codes===
and Standards There are no additions to the references in this section. 6.3 Other
 
==References:==
 
There are no additions to the references in this section. T bl 3121 C t D . B . Fl d H d a e .. -urren es1gn as1s 00 azar s Stillwater Current Design Flooding Elevation Associated Basis Flood Effects Elevation Reference Mechanism ft {m) ft {m) NGVD29 NGVD29 LIP and Associated 517.5 Exelon, Drainage (157.7) Not discussed 517.5 (157.7) 2013b Section 5 Streams and Rivers 525.0 3.0 ft (0.9 m) 528.0 (160.9) FHRR, (160.0) (wind wave) Section 2.a Failure of Dams and Onsite Water 508.2 Not discussed 508.2 (154.9) FHRR, Control/Storage (154.9) Section 2.a Structures Storm Surge Not applicable to DNPS site. FHRR, Section 2.a Seiche Not applicable to DNPS site. FHRR, Section 2.a Tsunami Not applicable to DNPS site. FHRR, Section 2.a Ice-Induced No elevation provided in UFSAR. FHRR, Section 2.a Channel Migrations No elevation provided in UFSAR; discussed in FHRR, or Diversions reference to control of locks and dams by USAGE. Section 2.a  Table 4.0-1: Flood Event Duration for Flood-Causing Mechanisms Not Bounded by COB Hazard Flood-Causing Time Available Duration of Time for Water to for Preparation Mechanism for Flood Event Inundation of Site Recede from Site LIP and Associated See Section 3.2 See Section 3.2 See Sections 3.2 Drainage Failure of Dams 1 day, 12 hours, 6 days, 15 hours and Onsite Water 11 days, 8 hours (Table Control/Storage and 45 minutes (Table 6.1, 6.1, Barstow, 2014b) Structures (Kaegi, 2013) Barstow, 2014b) Table 4.0-2: Reevaluated Flood Hazards for Flood-Causing Mechanisms Not Bounded by COB Hazard Stillwater Reevaluated Flood-Causing Elevation, Associated Effects Flood Reference Mechanism ft (m) Hazard, ft NGVD29 (m) NGVD29 LIP and Varies with None Varies with Exelon, 2013b, Associated maximum of maximum of Section 4 Drainage 518.1 518.1 (157.9) (157.9) Failure of Dams Varies with Varies with a maximum Varies with a FHRR, Section and Onsite Water a maximum of 4.2 (1.3) (Wind wave maximum of 3, Table 1, and Control/Storage of 524.8 Section 4, Structures (160.0) and wave runup) 529.0 (161.2) Table 6  Table 4.0-3 Associated Effects Inputs Not Bounded by COB Hazard Flooding Mechanism Failure of Dams and Associated Effects Local Intense Onsite Water Factor Precipitation Control/Storage Structures Hydrodynamic loading at Minimal, not bounding 117 .2 psf (572.2 kg-plant grade (FHRR, Section. 4.2) force/m 2) (FHRR, Table 4) Debris loading at plant None Varies with maximum of grade 3,387 lbs (1,536 kg) at Unit 2 Turbine Building (Barstow, 2014b) Sediment loading at plant None Minimal effect (Barstow, grade 2014b) Sediment deposition and None Minimal effect (Barstow, erosion 2014b) Concurrent conditions, None Hail, strong winds, and including adverse weather tornadoes (Barstow, 2014b) Groundwater ingress None Bounded (Barstow, 2014b) Other pertinent factors None Examined barge collision (e.g., waterborne (Barstow, 2014b) projectiles)  Table 5.0-1: Integrated Assessment Open Items Deleted B. Hanson As documented in the NRC staff assessment and the enclosed supplement, the NRC 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 Dresden. Further, the NRC staff continues to assert that 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 and upstream dam failures 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) 415-6197 or e-mail at Tekia. Govan@nrc.gov.
Sincerely, IRA/ Tekia Govan, Project Manager Hazards Management Branch Japan Lessons-Learned Division Office of Nuclear Reactor Regulation Docket Nos. 50-237 and 50-249
 
==Enclosure:==


Supplement to Staff Assessment of Flood Hazard Reevaluation Report cc w/encl: Distribution via Listserv DISTRIBUTION:
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Latest revision as of 08:00, 5 February 2020

Supplement to Staff Assessment of Response to 10 CFR 50.54(f) Information Request - Flood Causing Mechanisms Reevaluation
ML15307A056
Person / Time
Site: Dresden  Constellation icon.png
Issue date: 11/04/2015
From: Tekia Govan
Japan Lessons-Learned Division
To: Bryan Hanson
Exelon Generation Co
Govan, Tekia, NRR/JLD 415-6197
References
TAC MF1795, TAC MF1796
Download: ML15307A056 (15)
v • d • e


Text

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 November 4, 2015 Mr. Bryan C. Hanson Senior Vice President Exelon Generation Company, LLC 4300 Winfield Rd Warrenville, IL 60555

SUBJECT:

DRESDEN NUCLEAR POWER STATION, UNITS 2 AND 3-SUPPLEMENT TO STAFF ASSESSMENT OF RESPONSE TO 10 CFR 50.54(f) INFORMATION REQUEST - FLOOD-CAUSING MECHANISMS REEVALUATION (TAC NOS. MF1795 AND MF1796)

Dear Mr. Hanson:

The purpose of this letter is to transmit the supplement to the U.S. Nuclear Regulatory Commission (NRC) staff's assessment of Dresden Nuclear Power Station, Units 2 and 3 (Dresden) reevaluated flood hazard information that was issued to you by a letter dated March 31, 2015 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML15072A007). The supplement updates the original staff assessment to address changes in the NRC's approach to next 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. ML12053A340), the NRC 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 *10, 2013 (ADAMS Accession No. ML13135A120), Exelon Generation Company, LLC (the ilcensee) responded to this request for Dresden. In response to NRC staff questions, this response was supplemented by letter dated May 19, 2014 (ADAMS Accession No. ML15092A821).

  • The NRC 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 TAC Nos. MF1795 and MF1796. .

The enclosed supplement to the staff assessment updates the NRC staff's conclusions in accordance with the flood hazard reevaluation approach described in NRC letter dated September 1, 2015 (ADAMS Accession No. ML15174A257), '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

B. Hanson As documented in the NRC staff assessment and the enclosed supplement, the NRC 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 Dresden. Further, the NRC staff continues to assert that 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 and upstream dam failures 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) 415-6197 or e-mail at Tekia.Govan@nrc.gov.

Tekia Govan, Project Manager Hazards Management Branch Japan Lessons-Learned Division Office of Nuclear Reactor Regulation Docket Nos. 50-237 and 50-249

Enclosure:

Supplement to Staff Assessment of Flood Hazard Reevaluation Report cc w/encl: Distribution via Listserv

SUPPLEMENT TO STAFF ASSESSMENT BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO FLOODING HAZARD REEVALUATION REPORT NEAR-TERM TASK FORCE RECOMMENDATION 2.1 RELATED TO THE FUKUSHIMA DAl-ICHI NUCLEAR POWER PLANT ACCIDENT DRESDEN NUCLEAR POWER STATION, UNITS 2 AND 3 DOCKET NOS. 50-237 AND 50-249

1.0 INTRODUCTION

This document is a supplement to the U.S. Nuclear Regulatory Commission (NRC) staff assessment that was transmitted by letter dated March 31, 2015 (NRC, 2015a), to Exelon Generation Company, LLC (the licensee) for Dresden Nuclear Power Station, Units 2 and 3 (Dresden, DNPS). With the exceptions of the Table 3.1.2-1 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, 2015b) to COMSECY 0019 (NRC, 2015c), which described the NRC's mitigating strategies and flooding hazard reevaluation action plan. Table 3.1.2-1 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 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 1O 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 practice. If the reevaluated flood-causing mechanisms are not bounded by the current plant design-basis flood hazard, an integrated assessment may be necessary.

Enclosure

2.2.1 Flood-Causing Mechanisms There are no changes or updates to this section of the NRC staff assessment.

2.2.2 Associated Effects There are no changes or updates to this section of the NRC staff assessment.

2.2.3 Combined Effects Flood There are no changes or updates to this section of the NRC staff assessment.

2.2.4 Flood Event Duration There are no changes or updates to this section of the NRC staff assessment.

2.2.5 Actions Following the Flooding Hazard Reevaluation Report (FHRR)

For the sites where the reevaluated probable maximum flood elevation is not bounded by the current design-basis probable maximum flood elevation for all flood-causing mechanisms, the 50.54(f) letter 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.
  • Perform an integrated assessment subsequent to the FHRR to (a) evaluate the effectiveness of the current licensing 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 NRC 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 NRC letter dated September 1, 2015 (NRC, 2015d) describes the changes in the NRC's approach to the flood hazard reevaluations.

If the reevaluated flood hazard is bounded by the current design-basis flood hazard for all flood-causing mechanisms at the site, licensees are not required to perform an integrated assessment or focused evaluation(s) at this time.

3.0 TECHNICAL EVALUATION

There are no changes or updates to this section of the NRC staff assessment.

3.1 Site Information There are no changes or updates to this section of the NRC staff assessment.

3.2 LIP and Associated Site Drainage There are no changes or updates to this section of the NRC staff assessment.

3.2.1 Model Inputs and Assumptions There are no changes or updates to this section of the NRC staff assessment.

3.2.2 Probable Maximum Precipitation The licensee's analysis developed the 1-hour, 1-square mile probable maximum precipitation (PMP) event distribution using Hydrometerological Reports (HMRs) 51 and 52 (National Oceanic and Atmospheric Administration (NOAA), 1978 and 1982). The licensee stated that the local intense precipitation (LIP) event is defined by NUREG/CR-7046 (NRC, 2011 d) as the 1-hour, 1-square mile PMP event. The licensee extrapolated from the PMP depth contour map provided in HMR 52. The licensee developed the distribution of the 1-hour PMP from the 5-, 15-

, 30- and 60-minute time intervals from HMR 52. The licensee computed a cumulative depth of the 1-hour, 1-square mile PMP of 17.97 inches (45.28 cm). The NRC staff verified the HMR 51 and HMR 52 computations based on the location of the DNPS, and concluded the PMP depths are appropriate.

The NRC staff notes that a reasonable estimate of the site's LIP PMP is the application of an appropriate NOAA HMR estimate for any rainfall duration used in NUREG/CR-7046, regardless of temporal distribution of the rainfall. The licensee obtained 1-sq. mile PMP depths for durations ranging between 5-minutes to 1-hour using HMR-51 and HMR-52. Therefore, the NRC staff confirmed that the licensee selected appropriate rainfall rate values to satisfy the 50.54(f) information request.

Based on the NRC staff's reviews of FHRRs to date, the NRC staff observed that, when using transient rainfall runoff models, PMP events having longer than 1-hour durations may result in higher LIP flood elevations and longer periods of inundation than the 1-hour event. NRC staff also noted that LIP events deriving from PMPs having relatively short durations may result in limiting warning time and may likewise result in consequential LIP flood elevation (e.g., flood elevations above the openings to plant structures).

3.2.3 Site Topography There are no changes or updates to this section of the NRC staff assessment.

3.2.4 Site Land Cover There are no changes or updates to this section of the NRC staff assessment.

3.2.5 FL0-20 Results The licensee computed the water surface elevations, maximum flooding depths, maximum velocity, maximum resultant impact loading (forces due to moving water), and maximum resultant static load (height of water) using the FL0-20 model. The maximum flood elevation of 518.1 ft (157.9 m) occurs at several Category 1 structures. The flooding depths range between 0.1 ft (0.03 m) (Reactor Building, Unit 3) to 1. 7 ft (0.5 m) (Turbine Building, Unit 2), which produce a maximum static load of 85.0 lbs. per ft (126.5 kg-force perm). The velocities are generally less than 1 foot per second (ft/s; 0.3 meters per second (m/s)), with resultant impact loads of 5 lbs. per ft (7.5 kg-force perm) or less.

The licensee noted that based on the sensitivity analysis of the grid size and Manning's n values (n-values), an accuracy of+/- 0.1 ft (0.03 m) should be taken when considering the flood elevations.

The NRC staff reviewed the FL0-20 model input and output files provided by the licensee. In the course of the review, NRC staff performed a mass balance verification and concluded that the runoff amount passing through the model boundary is less than the amount of rainfall. The difference in mass could result in a potential underestimation of the maximum water levels around the site. The licensee is requested to resolve numerical modeling issue associated with the LIP flood analyses, specifically the reduction of volume of outflow runoff as compared to the inflow precipitation, accounting for infiltration. This issue may relate to runoff from rooftops being removed from the numerical model domain rather than discharging to the ground surface near the structure or an adjacent area.

3.2.6 Site-Specific Probable Maximum Precipitation There are no changes or updates to this section of the NRC staff assessment.

3.2. 7 NRC Staff 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 for LIP and associated site drainage 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 modelling issues that are discussed in Section 3.2.5 should be addressed in the focused evaluation, if the licensee uses the same FL0-20 model in its 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 NRG staff assessment.

3.4 Failure of Dams and Onsite Water Control/Storage Structures There are no changes or updates to this section of the NRG staff assessment.

3.4.1 Critical Dam Evaluation and Selection There are no changes or updates to this section of the NRG staff assessment.

3.4.2 Upstream Dam Failure Mechanism Summary There are no changes or updates to this section of the NRG staff assessment.

3.4.3 Hydrologic Dam Failure Analysis There are no changes or updates to this section of the NRG staff assessment.

3.4.4 Seismic Upstream Dam Failure There are no changes or updates to this section of the NRG staff assessment.

3.4.5 Upstream Dam Failure Timing and Duration There are no changes or updates to this section of the NRG staff assessment.

3.4.6 Other Associated Effects of Upstream Dam Failure 3.4. 7 NRG Staff Conclusion The NRC staff reviewed the assumptions and approach used in the flooding from upstream dams analysis, and concluded the approach appropriate and assumptions to be reasonable.

The NRG staff verified that the models were consistent with results presented in the FHRR and that in cases where the one model output was used as an input to another model, the two data sets were exactly the same. The NRG staff verified the references used, to ensure that they met standard engineering practices and NRG guidance. The NRG staff confirmed the licensee's conclusion that the reevaluated flood hazard for failure of dams and onsite water control or storage structures is not bounded by the current design-basis flood hazard. Therefore, the treatment of the upstream dam failures should be addressed in the integrated assessment or focused evaluation consistent with the process and guidance discussed in COMSECY-15-0019 (NRG, 2015c).

3.5 Storm Surge There are no changes or updates to this section of the NRC staff assessment.

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 BOUNDED BY THE COB The NRC staff confirms that the reevaluated hazard results for LIP and dam failures are not bounded by the current design-basis flood hazard. Therefore, the NRC staff anticipates that the licensee will perform additional assessments (i.e., integrated assessment or focused evaluation) of plant response for DNPS, as described in NRC letter dated September 1, 2015 (NRC, 2015d).

The NRC staff reviewed the following flood hazard parameters needed to perform the additional assessments or evaluations of plant response:

  • Flood event duration, including warning time and intermediate water surface elevations that trigger actions by plant personnel, as defined in Japan Lessons-Learned Directorate (JLD) Interim Staff Guidance (ISG) JLD-ISG-2012-05. Flood event durations for the flood-causing mechanisms identified above are shown in Table 4.0-1.
  • Flood height and associated effects, as defined in JLD-ISG-2012-05. Flood height and associated effects for the flood-causing mechanisms identified above are shown in Table 4.0-2.

Based upon the preceding analysis, the NRC staff confirmed that the reevaluated flood hazard information 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 NRC staff).

5.0 CONCLUSION

The NRC staff has reviewed the information provided for the reevaluated flood-causing mechanisms of DNPS, Units. 2 and 3. Based on its review, the NRC staff concludes that the licensee conducted the hazard reevaluation using present-day methodologies and regulatory guidance used by the NRC staff in connection with ESP and COL reviews.

Based on the preceding analysis, the NRC staff confirmed that the licensee responded appropriately to Enclosure 2, Required Response 2, of the 50.54(f) letter. In reaching this determination, the NRC staff confirmed the licensee's conclusions that (a) the reevaluated flood hazard results for LIP and upstream dam failure are not bounded by the current design-basis flood hazard, (b) additional assessments of plant response will be performed for the local intense precipitation and upstream dam failure 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 (NRC, 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 NRC staff).

The NRC staff has no additional information needs at this time with respect to the FHRR.

6.0 REFERENCES

6.1 U.S. Nuclear Regulatory Commission (NRC) Documents and Publications:

NRC (U.S. Nuclear Regulatory Commission), 2015a, letter from Victor Hall (NRC), to Bryan Hanson, Senior Vice President, Exelon Generation Company, LLC, " Dresden Nuclear Power Station, Units 2 and 3 - Staff Assessment of Response to 10 CFR 50.54(f)

Information Request- Flood-Causing Mechanism Reevaluation" (TAC Nos. MF1795 AND MF1796) March 31, 2015, ADAMS Accession No. ML15072A007.

NRC (U.S. Nuclear Regulatory Commission), 2015b, "SRM - COMSECY-15-0019 - Closure Plan for the Reevaluation Of Flooding Hazards for Operating Nuclear Power Plants,

COMSECY-15-0019, July 28, 2015, ADAMS Accession No. ML15209A682.

NRC (U.S. Nuclear Regulatory Commission), 2015c, "Closure Plan for the Reevaluation of Flooding Hazards For Operating Nuclear Power Plants," COMSECY-15-0019, June 30, 2015, ADAMS Accession No. ML15153A104.

NRC (U.S. Nuclear Regulatory Commission), 2015d, letter from William M. Dean, Director, to Power Reactor Licensees, "Coordination of Requests for Information Regarding Flooding Hazard Reevaluations and Mitigating Strategies for Beyond-Design-Basis External Events",

ADAMS Accession No. ML15174A257.

6.2 Codes and Standards There are no additions to the references in this section.

6.3 Other

References:

There are no additions to the references in this section.

.. C urrent Des1gn T a bl e 3121 - . Bas1s

. Fl 00 d Hazard s Current Design Stillwater Associated Basis Flood Flooding Elevation Effects Elevation Reference Mechanism ft {m) ft {m)

NGVD29 NGVD29 Exelon, LIP and Associated 517.5 Not discussed 517.5 (157.7) 2013b Drainage (157.7)

Section 5 525.0 3.0 ft (0.9 m) FHRR, Streams and Rivers 528.0 (160.9)

(160.0) (wind wave) Section 2.a Failure of Dams and Onsite Water 508.2 FHRR, Not discussed 508.2 (154.9)

Control/Storage (154.9) Section 2.a Structures FHRR, Storm Surge Not applicable to DNPS site.

Section 2.a FHRR, Seiche Not applicable to DNPS site.

Section 2.a FHRR, Tsunami Not applicable to DNPS site.

Section 2.a FHRR, Ice-Induced No elevation provided in UFSAR.

Section 2.a Channel Migrations No elevation provided in UFSAR; discussed in FHRR, or Diversions reference to control of locks and dams by USAGE. Section 2.a

Table 4.0-1: Flood Event Duration for Flood-Causing Mechanisms Not Bounded by COB Hazard Time Available Flood-Causing Duration of Time for Water to for Preparation Mechanism Inundation of Site Recede from Site for Flood Event LIP and Associated See Section 3.2 See Section 3.2 See Sections 3.2 Drainage Failure of Dams 1 day, 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />, 6 days, 15 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br /> and Onsite Water 11 days, 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> (Table and 45 minutes (Table 6.1, Control/Storage 6.1, Barstow, 2014b)

(Kaegi, 2013) Barstow, 2014b)

Structures Table 4.0-2: Reevaluated Flood Hazards for Flood-Causing Mechanisms Not Bounded by COB Hazard Stillwater Reevaluated Flood-Causing Elevation, Associated Effects Flood Reference Mechanism ft (m) Hazard, ft NGVD29 (m) NGVD29 LIP and Varies with None Varies with Exelon, 2013b, Associated maximum of maximum of Section 4 Drainage 518.1 518.1 (157.9)

(157.9)

Failure of Dams Varies with FHRR, Section Varies with a maximum Varies with a and Onsite Water a maximum 3, Table 1, and of 4.2 (1.3) (Wind wave maximum of Control/Storage of 524.8 Section 4, and wave runup) 529.0 (161.2)

Structures (160.0) Table 6

Table 4.0-3 Associated Effects Inputs Not Bounded by COB Hazard Flooding Mechanism Failure of Dams and Associated Effects Local Intense Onsite Water Factor Precipitation Control/Storage Structures Hydrodynamic loading at Minimal, not bounding 117 .2 psf (572.2 kg-plant grade (FHRR, Section. 4.2) force/m 2) (FHRR, Table 4)

Debris loading at plant None Varies with maximum of grade 3,387 lbs (1,536 kg) at Unit 2 Turbine Building (Barstow, 2014b)

Sediment loading at plant None Minimal effect (Barstow, grade 2014b)

Sediment deposition and None Minimal effect (Barstow, erosion 2014b)

Concurrent conditions, None Hail, strong winds, and including adverse weather tornadoes (Barstow, 2014b)

Groundwater ingress None Bounded (Barstow, 2014b)

Other pertinent factors None Examined barge collision (e.g., waterborne (Barstow, 2014b) projectiles)

Table 5.0-1: Integrated Assessment Open Items Deleted

ML15307A056 *via email OFFICE NRR/JLD/JHMB/PM NRR/JLD/LA NRR/JLD/JHMB/PM NAME MMarshall* TGovan Slent DATE 11 /2/2015 11/4/2015 11 /4/2015 OFFICE NRO/DSEA/RHM/BC NRR/JLD/JHMB/BC NRR/JLD/JHMB/PM NAME CCook* MShams* TGovan DATE 10/30/2015 11/2/2015 11 /4/2015

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