View source for CP-201500740, Submittal of Request for Additional Information Regarding Fukushima Lessons Learned - Flooding Hazard Reanalysis Report

{{Adams
| number = ML15278A306
| issue date = 09/22/2015
| title = Submittal of Request for Additional Information Regarding Fukushima Lessons Learned - Flooding Hazard Reanalysis Report
| author name = Flores R, Mccool T
| author affiliation = Luminant Generation Co, LLC, Luminant Power
| addressee name = 
| addressee affiliation = NRC/Document Control Desk, NRC/NRR
| docket = 05000445, 05000446
| license number = 
| contact person = 
| case reference number = CP-201500740, TAC MF1099, TAC MF1100, TXX-15111
| document type = Letter type:CP, Response to Request for Additional Information (RAI)
| page count = 29
| project = TAC:MF1099, TAC:MF1100
| stage = Request
}}

=Text=
{{#Wiki_filter:SRafael                                   Senior Vice  President Flores                         P 0 Box 1002Power Luminant
                                                                  & Chief Nuclear Officer                 6322 North FM 56 Luminant                                                          Rafael.Flores@Luminant.com              Glen Rose, TX 76043 T 254 897 5590 C 817 559 0403 F 254 897 6652 CP-201500740                                                                              10 CFR 50.54(f)
TXX-15111 September 22, 2015 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001

==SUBJECT:==
Comanche Peak Nuclear Power Plant (CPNPP) Docket Nos. 50-445 And 50-446 Submittal of Request for Additional Information Regarding Fukushima Lessons Learned - Flooding Hazard Reanalysis Report (TAC NOS. MF1099 and MF1100)

==References:==
: 1. Luminant Generation Company LLC's Letter TXX-13053, Response to March 12, 2012, Request for Information Enclosure 2, Recommendation 2.1, Flooding Hazard Reevaluation Report, of the Near-Term Task Force Review of Insights from the Fukushima Dai-Ichi Accident, dated March 12, 2013, Accession No. ML13074A058
: 2. NRC Letter, Request for Additional Information Regarding Fukushima Lessons Learned
                     - Flooding Hazard Reanalysis Report, dated March 7, 2014, Accession No.
ML14059A188
: 3. Luminant Generation Company LLC's Letter TXX-14048, Comanche Peak Nuclear Power Plant (CPNPP) Docket Nos. 50-445 And 50-446 Submittal of Requested Information Regarding Fukushima Lessons Learned -. Flooding Hazard Reanalysis Report (TAC NOS. MF1099 and MF1100) dated April 4, 2014, Accession No.
ML14100A049
: 4. Luminant Generation Company LLC's Letter TXX-14094, Comanche Peak Nuclear Power Plant (CPNPP) Docket Nos. 50-445 And 50-446 Submittal of Fukushima Lessons Learned - Flood Hazard Reevaluation Report Supplement 1 (TAC NOS. MiF1099 and MF1100) dated August 14, 2014, Accession No. ML14245A136
: 5. NRC email dated May 4, 2015, Request for Additional Information: Comanche Peak Flooding Hazard Reevaluation Report (TAC Nos. MF1099 and MF1100), Accession No.
ML15124A890

==Dear Sir or Madam:==

On March 12, 2013, Luminant Generation Company LLC (Luminant Power) submitted Comanche Peak Nuclear Power Plant's (CPNPP) Flooding Hazard Reevaluation Report (Reference 1). References 3 and 4 submitted additional information to the NRC in response to Reference 2. As a result of additional communications between the NRC staff and Luminant during and subsequent to a flooding audit meeting on July 6, 2015, it was agreed that a response to Reference 5 would be provided by September 22, 2015. This submittal is providing additional information in response to Reference 5 (see Attachment to TXX-15111).

U. S. Nuclear Regulatory Commission TXX-15111 Page 2 of 3 09/22/2015 Calculations F-O5 and F-06 have been uploaded to the NRC e-portal. Related input /output data is provided in the Enclosure to TXX-15111 (publicly available).
This letter contains no regulatory commitments.
If there are any questions regarding this plan, please contact Mr. Carl B. Corbin at (254) 897-0121 carl.corbin@luminant.com.
I state under penalty of perjury that the foregoing is true and correct.
Executed on September 22, 2015.
Sincerely, Luminant Generation Company LLC Rafael Flores By             :__*___
Thomas P. McCool Vice President Engineering and Support Attachment          Comanche Peak Nuclear Power Plant (CPNPP) Response to Request for Additional Information Regarding Flooding Hazard Reevaluation Report Enclosure           Input / Output data for Paul C. RIZZO Associates, "Local Intense Precipitation Refined Analysis," Calculation 14-5213 F-O5, Revision 0, September 2015 and Paul C. RIZZO Associates (RIZZO), "Turbine Building Inflow Analysis," Calculation 14-5213 F-06, Revision 0, September 2015 (publicly available).

U. S. Nuclear Regulatory Commission TXX-15111 Page 3 of 3 09/22/2015 c-         William M. Dean, Director, Office of Nuclear Reactor Regulation
* Marc L. Dapas, Region IV*
Jessica A. Kratchman, NRR/JLD/ PMB*
Balwant K. Singal, NRR*
Victor Hall, NRR Resident Inspectors, Comanche Peak Nuclear Power*
* Without Enclosure

Attachment to TXX-15111        CPNPP Response to Request for Additional Information Page 1 of 26                       Regarding Flooding Hazard Reevaluation Report NRC Introduction to Request for Additional Information By letter dated March 12, 2013, Luminant Generation Company (the licensee) submitted its flood hazard reevaluation report (FHRR) for Comanche Peak Nuclear Power Plant Units 1 and 2 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML13074A058). By letter dated March 7, 2014, the U.S. Nuclear Regulatory Commission (NRC) staff provided a request for additional information (RAI) regarding the above referenced FHRR (ADAMS Accession No. ML14059A188). The licensee responded to this RAI by letter dated April 4, 2014 (ADAMS Accession No. ML14100A049). By letter dated August 14, 2014, the licensee supplemented its FHRR to address a calculation error (ADAMS Accession No. ML14245A136). The staff determined that additional information, as requested below, is necessary to complete its assessment of the licensee's FHRR.
NRC RAI 2-1: Local Intense Precipitation (LIP) - Choice of Methods and Technical Rationale

==Background:==
The response to the previously issued RA1 6 included input and outputfiles for the Hydrologic EngineeringCenter River Analysis System (HEC-RAS) model used for modeling the local intense precipitation (LIP) event.
The discretizationin the model appears insufficient to adequately predictflow depth and direction in critical areas around safety structures. This is evident in the elevations of the storage areas, and the routing of the flow:
Regarding elevations, the off-channel storage area capability within HE C-RA S estimates water surface elevationfor each catchment based on a stage-volume curve. The model computes water surface elevationfor each catchment and time-step, and it is noted that a single water surface elevation is computed for each catchment and time-step.
Hozoever, the ground surface within an individual catchment contains variable elevations. The lack of resolution in the model limits the model's capability to predictflow depth and direction in critical areas around site structures important to safety.
Regarding the routing of the flow, water accumulates in areas of lowest topographicprofile before exiting the catchment at the lowest elevation along the circumscribingcross sections. As such, the LIP model predictsflooding only within the lowest-profile sections of a catchment. Thus, the model does not adequately representflooding and routing across the entire catchment, especially in areas of higher elevation. As a result, the model could not accurately predict the depth offlow and direction offlood routing since the flooding and point of discharge are located only in zones of low topographicprofile.
For example, Figure 7-1 from CalculationF-03 shows a peak water surface elevation of 807.24ft NA VD88 in the largest catchment located to the southwest. Flooding woithin this catchment is limited to the lowest elevation portion along the lower-right corner of the catchment. However, water could potentially pond in the upland area of this large catchment, some of which is at a ground elevation of over 850 ft. Precipitationfalling onto this high elevation area has the potential to flow east to the powerblock area due to localized topographic channeling and gradient. As a result, the model may not adequately simulate temporal and spatialflood routing characteristicswithin and between the catchments, which could impact plant safety features.
Request: Provide an updated modeling evaluation of the LIP flood event that includes improved discretization of the plant area that is capable of better simulating temporal and spatialflow effects across the site and in the vicinity of safety structures. Provide electronic versions of any associated modeling input and output files.
Evaluate LIP flood-water inundation into the Unit 2 TB sump and condenser pit, per the previous RAI 9, based on the results from the revised model. Evaluate a variety of different LIP storm durationsand distributions (see RAI 2-2). Provide a quantitative analysis of any flooding that occurs within the Electrical and Control Building due to water conveyance via the Unit 2 TB equipment ramp.

Attachment to TXX-15111    CPNPP Response to Request for Additional Information Page 2 of 26                  Regarding Flooding Hazard Reevaluation Report Nw6 of
                                                                                   ~mst Flow
* ina     01too~s sbfq Rulate4~~w..E           a *s toGS 1ropogrqahlAPINA*A           1sto2       0 I&#xb6;22SoF~e J4gh 656568             2to          -w iU,
                                      -     l,. 715 Figure 7-1 from Calculation F LIP flood modeling results

Attachment to TXX-15111     CPNPP Response to Request for Additional Information Page 3 of 26                   Regarding Flooding Hazard Reevaluation Report CPNPP Response to RAT 2-1:
A refined Local Intense Precipitation (LIP) model has been developed with improved discretization of the Comanche Peak Nuclear Power Plant (CPNPP) area. The purpose of the improved discretization is to provide a better characterization of the temporal and spatial flow effects on the CPNPP site, with particular focus given to the flow near safety-related Structures, Systems, and Components (SSCs).
The overall modeling approach of routing flow over weirs between onsite catchments using the HEC-HMS and HEC-RAS computer programs is similar to the approach presented in the CPNPP Flood Hazard Reevaluation Report (FHRR) Supplement 1 (Ref. [1]). However, the updated models include a higher catchment resolution in areas near safety-related SSCs, providing a more detailed representation of flood levels and flows. Additionally, the catchment delineation is refined based on topographic features, including consideration of upland areas away from the powerblock. Several linear features (reaches) were also added to the model to characterize the flow of water away from the site through drainage channels.
The updated model indicated flood levels that were broadly comparable with the previous results (as discussed below). The peak flood levels adjacent to safety-related SSCs are lower than the door threshold elevations (minimum of 810.5 ft Mean Sea Level (MSL)). Hence there is no ingress of floodwater directly to safety-related SSCs.
While no floodwater enters safety-related SSCs directly, it was important to reevaluate the potential for flow to enter the Unit 2 Turbine Building via the west side equipment ramp and the associated impacts (if any) on safety-related SSCs. The evaluation of potential flow into the Unit 2 Turbine Building was revised using the updated HEC-HMS and HEC-RAS models. Based on the response to RAI 2-2, the LIP event duration and temporal distribution reported in the CPNPP FHRR Supplement 1 (Ref. [1]) is sufficient for addressing the requirements of Recommendation 2.1 (Ref. [2]). However, a 72-hour duration Probable Maximum Precipitation (PMP) event was considered for this updated analysis in addition to the 6-hour event previously considered. This approach is consistent with the previous treatment of inflow to the Turbine Building in RAI 9 (Ref. [3]).
As indicated below, the updated analysis concluded that there is no ingress of water into the Electrical and Control Building due to water conveyance via the Unit 2 Turbine Building equipment ramp. The peak water level inside the Turbine Building due to on-site flooding is stored at an elevation below the top of the basement slab, precluding any flow of water into the Electrical and Control Building.
The updated model configurations and associated results are described briefly in the following subsections. More details regarding the updated site drainage analysis can be found in RIZZO Calculation 14-5213 F-OS (Ref. [4]). More details regarding the updated Turbine Building inflow analysis can be found in RIZZO Calculation 14-5213 F-06 (Ref. [5]). Calculations 14-5213 F-O5 and 14-5213 F-06 have been uploaded to the NRC e-portal for NRC review. The electronic versions of associated model input and output files are provided in Enclosure to TXX-15111.
Updated Site Delineation and HEC-HMS Model A refined delineation of the CPNPP site was developed to support the updated rainfall-runoff model. The delineation was performed with the aid of ArcHydro Tools 10.1 (Ref. [6]). The runoff from each catchment was computed using the HEC-HMS computer program version 3.5 (Ref. [7]).
The updated HEC-HMS rainfall-runoff model (Figure 2-1-1) includes 66 catchments, which provides increased resolution over the 31 catchment model used as the basis for the CPNPP FHRR Supplement 1 (Ref. [1]). The increased number of catchments provides a more detailed representation of flood ponding and flood routing. The number of catchments was determined with consideration for providing the best

Attachment to TXX-15111        CPNPP Response to Request for Additional Information Page 4 of 26                      Regarding Flooding Hazard Reevaluation Report representation of flooding on the site based on the same topographic data of the site used in FHRR Supplement 1 (Ref. [1]).
Two scenarios were considered for the HEC-HMS model: a 6-hour LIP event, and a 72-hour PMP event.
Both scenarios include characterization of rainfall losses using the Natural Resources Conservation Service (NRCS) Curve Number method (Ref. [81). The lag time for runoff transformation is computed following the methodology of TR-55 (Ref. [8]).
Updated HEC-RAS Model The runoff rates computed using HEC-HMS were used as input for the updated hydraulic model (Figure 2-1-2) using HEC-RAS version 4.1 (Ref. [9]). The overall flood routing methodology for the updated HEC-RAS model is consistent with the analysis documented in the FHRR Supplement 1 (Ref. [1]). However, the increased resolution of catchments near safety-related SSCs, and the addition of several reaches to the model provide an improved characterization of the site hydraulics during the flooding events evaluated.
The simulations performed for this analysis include the large diameter culverts discussed in the response to RAI 4 (Ref. [31), which are assumed to remain unblocked during the flood events evaluated. Refer to Calculation 14-5213 F-05 (Ref. [4]) for additional details about the HEC-RAS model setup.
Updated Flooding Results The results for the updated HEC-HMS and HEC-RAS models are summarized as follows:
* Figure 2-1-1 presents the updated HEC-HMS model and supersedes Figure 3-3 of the FHRR Supplement 1 (Ref. [1]).
* Figure 2-1-2 presents the updated HEC-RAS model used for Calculation 14-5213 F-O5 (Ref. [4]).
* Table 2-1-1 summarizes the HEC-HMS runoff results for each catchment for the 6-hour LIP and supersedes Table 3-1 in the FHRR Supplement 1 (Ref. [1]) as the results of record for the CPNPP site runoff rates. Table 2-1-1 also supersedes Table 9-2 presented in the response to RAI 9 (Ref.
[3]).
* Table 2-1-2 summarizes the HEC-HMS runoff results for each catchment for the 72-hour PMP and supersedes Table 9-4 presented in the response to RAI 9 (Ref. [3]).
* Table 2-1-3 reports the peak ponding levels (HEC-RAS results) near important plant structures and supersedes Table 3-2 of the FHRR Supplement 1 (Ref. [1]) as the water levels of record for the Recommendation 2.1 Flooding analysis at the CPNPP site.
* Figure 2-1-3 shows an inundation map of the CPNPP site for the 6-hour LIP and supersedes Figure 3-4 of the FHRR Supplement 1 (Ref. [1]).
* Table 2-14 presents a comparison between the Current Licensing Basis (CLB) flood levels and reevaluated flood levels for all flooding mechanisms with the new LIP results included. This supersedes Table 3-3 of the FHRR Supplement 1 (Ref. [1]).
* Figures 2-1-4 and 2-1-5 show the flood event duration timelines for the new 6-hour LIP and 72-hour PMP analyses. Together, these two figures supersede Figure 20-1 presented in the response to RAI 20 (Ref. [3]). Figures 2-1-4 and 2-1-5 show the timeline with regard to flood levels rising and beginning to recede as illustrated in Figure 6 of JLD-ISG-2012-05 (Ref. [10]). It is important to note that there is no inundation above elevation 810.5 ft MSL adjacent to safety-related SSCs (Table 2-1-3), so the plant remains in a safe and stable state for the entire duration of the flooding event.

Attachment to TXX-15111            CPNPP Response to Request for Additional Information Page 5 of 26                            Regarding Flooding Hazard Reevaluation Report LEGEND Figure 2-1-1 SUB8BASIN SCHEMATIC OF UPDATED HEC-HMS MODEL FOR                                        C27'0 SUBBASIN NUMBER SITE DRAINAGE ANALYSIS                                          3j RESERVOIR Note: HEC-HMS is only used for calculation of rainfall-runoff HYDRAUUC CONNECTION hydrographs. The flows are subsequently routed across the site using HEC-RAS. Sink-I and Sink-2 are used as dummy resereoirs for the subbasins to discharge into                                    SUBBASIN DEUNEATIO N Figure 2-1-1 Schematic of Updated HEC-HMS Model for Site Drainage Analysis

Attachment to TXX-15111    CPNPP Response to Request for Additional Information Page 6 of 26                   Regarding Flooding Hazard Reevaluation Report 2-1-2 for Site Drainage Analysis FigureModel Schematic of Updated HEC-RAS

Attachment to TXX-15111     CPNPP Response to Request for Additional Information Page 7 of 26                   Regarding Flooding Hazard Reevaluation Report TABLE 2-1-1 HEC-HMS SIMULATED PEAK RUNOFF FROM CATCHMENTS USING THE 6-HOUR LIP Peak                                               Peak Catchment        Drainage Area         DhCatchment                          Drainage Area  Dicag D(surmie)                  (cfs)I               ID            (square miles)  (cfs~
C269           0.012489             432.41               C311              0.000209      7.43 C270           0.025851             895.04               C312              0.001049     37.28 C280           0.008869             307.07               C313              0.000373     13.25 C281           0.004200             145.42               C314              0.000430     15.28 C282           0.003468             123.23               C315              0.000934     33.19 C283           0.000817              28.39               C316              0.000318     11.30 C284           0.000379              13.47               C317              0.002239     77.90 C285           0.000760              27.01               C318              0.000715     25.41 C286           0.001366              43.99               C319              0.000440     15.64 C287           0.000834              29.64               C320              0.000533     18.94 C288           0.001851              65.77               C321              0.000326     11.58 C289          0.001814              57.79               C322              0.004804     149.43 C290          0.000625              22.21               C323              0.000480     17.06 C291          0.001125              39.98               C324              0.004349     154.54 C292          0.001823              64.78               C325              0.000753     26.76 C293          0.001258              44.70               C326              0.000211      7.50 C294          0.001110              39.44               C327              0.000608     21.61 C295          0.000842              29.92               C328              0.000188      6.68 C296          0.001245              44.24               C329              0.003657     129.95 C297          0.000282              10.02               C330              0.000904     32.12 C298           0.000620              22.03               C331              0.000246      8.74 C299          0.000629              22.35               C332              0.001253     44.52 C300          0.000833              29.60               C333              0.000579     20.57 C301          0.000540              19.19               C334              0.001174     41.72 C302          0.000340              12.08               C335              0.000397     14.11 C303          0.001505              53.48               C336              0.000751     26.69 C304          0.000691               24.55              C337              0.000554     19.69 C305          0.001051              37.35               C338              0.000292     10.38 C306          0.000731               25.98              C339              0.000398     14.14 C307          0.000907               32.23              C340              0.000296     10.52 C308          0.000072               2.57               C341              0.000275      9.77 C309          0.000599               21.29              C342              0.000453     16.10 C310          0.001061               37.70              C343              0.000130      4.62 Notes: 1 Peak discharge is reported in cubic feet per second (cfs).

Attachment to TXX-1 5111       CPNPP Response to Request for Additional Information Page 8 of 26                      Regarding Flooding Hazard Reevaluation Report TABLE 2-1-2 HEC-HMS SIMULATED PEAK RUNOFF FROM CATCHMENTS USING THE 72-HOUR PMP Peak                                               Peak Catchment         Drainage Area          Dic~e             Catchment          Drainage Area   Dicag ID           (square miles)           (cfs)                ID            (square miles)  (cfs~
C269              0.012489              175.06              C311              0.000209      2.93 C270              0.025851              362.35              C312              0.001049     14.71 C280              0.008869              124.32              C313              0.000373      5.23 C281              0.004200              58.87               C314              0.000430      6.03 C282              0.003468               48.62              C315              0.000934     13.09 C283              0.000817               11.45              C316              0.000318      4.46 C284              0.000379               5.31               C317              0.002239     31.38 C285              0.000760               10.65              C318              0.000715     10.02 C286              0.001366               19.12              C319              0.000440      6.17 C287              0.000834               11.69              C320              0.000533      7.47 C288              0.001851               25.95              C321              0.000326      4.57 C289              0.001814               25.39              C322              0.004804     67.20 C290              0.000625               8.76               C323              0.000480      6.73 C291              0.001125               15.77              C324              0.004349     60.97 C292             0.001823               25.56              C325              0.000753      10.56 C293              0.001258               17.64              C326              0.000211      2.96 C294              0.001110               15.56              C327              0.000608      8.52 C295             0.000842               11.80              C328              0.000188      2.64 C296             0.001245               17.45              C329              0.003657      51.27 C297             0.000282                3.95              C330              0.000904      12.67 C298             0.000620                8.69              C331              0.000246      3.45 C299             0.000629                8.82              C332              0.001253      17.57 C300             0.000833               11.68              C333              0.000579      8.12 C301             0.000540                7.57              C334              0.001174      16.46 C302             0.000340                4.77              C335              0.000397      5.57 C303             0.001505               21.10              C336              0.000751      10.53 C304             0.000691                9.69              C337              0.000554       7.77 C305             0.001051               14.73              C338              0.000292      4.09 C306             0.000731               10.25              C339              0.000398      5.58 C307             0.000907               12.72              C340              0.000296      4.15 C308             0.000072                1.02              C341              0.000275      3.86 C309             0.000599                8.40              C342              0.000453       6.35 C310             0.001061               14.87              C343              0.000130       1.82 Notes: I Peak discharge is reported in cubic feet per second (cfs).

Attachment to TXX-15111       CPNPP Response to Request for Additional Information Page 9 of 26                      Regarding Flooding Hazard Reevaluation Report TABLE 2-1-3 PEAK WATER LEVELS AND PONDING DEPTHS DUE TO 6-HOUR LIP AND 72-HOUR
________PMP                               _____
Updated           FHRR           Updated         Updated      Change from Peak Water Supplement           Peak Water            Peak       Peak Water ystms, Struturs,Safty-         Levl 6           1 eak          Level 72-        Pnding     Level Reported Stucud     s Systoemts,     Saelty-?       Level 6-P Water Level          Hour PMP Depth for 6-             in FHRR an opnnt eae?                orLP 6-Hour LIP                              Hour LIP I') Supplement 1 (ft MSL)        (ft MSL)         (ft MSL)            (ft)              (ft)
Condensate Storage             Yes           809.71          809.53          809.63            0.00             0.18 Tank 1                                                            ____             ____          _____
Condensate Storage             Yes           810.26          810.33           810.18           0.26             -0.07 Tank 2              ___           ____                             __       __
Reactor Water Makeup             Yes           809.33          809.53           809.34           0.00             -0.20 Storage Tank 1           ___                                            _____
Reactor Water Makeup             Yes           810.24          810.33           810.16           0.24             -0.09 Storage Tank 2           ___                                            _____
Auxiliary Building            Yes            N/A             N/A             N/A              N/A              N/A Containment Unit 1             Yes           809.69          809.86           809.54            0.00            -0.17 Containment Unit 2             Yes           810.42          810.12           810.36            0.42             0.30 Diesel Building 1 East          Yes           809.69          809.53           809.54            0.00             0.16 Diesel Building 1 West            Yes          809.33          809.53           809.35            0.00            -0.20 Diesel Building 2 East           Yes          810.42          810.33           810.36            0.42             0.09 Diesel Building 2 West            Yes          810.26          810.33           810.18            0.26            -0.07 ElcrcladCnrl            Yes         N/A    (2)      N/A    (21       N/A   (2)         N/A              N/A Building                          _______________                                                 ______
Fuel Building               Yes          810.19          810.12           810.11            0.19             0.07 Refueling Water               Yes          809.69          809.53           809.54            0.00             0.16 Storage Tank_ 1_____                                                                                  ______
Refueling Water               Yes          810.42          810.33           810.36            0.42             0.09 Storage Tank 2                          __________                                      _____           ______
Safeguard Building I             Yes          809.33          809.53           809.35            0.00             -0.20 Safeguard Building 2             Yes          810.26          810.33           810.18            0.26            -0.07 Sevc ae   nae           Yes         795.86  (3)    795.86   (3)     795.86   (3)       N/A              N/A Structure                                                                                       _______
Switchgear Building 1            No           810.28          809.18           810.15            0.28             1.10 Switchgear Building 2            No           810.22          810.33           810.14            0.22            -0.11 Turbine Building 1             No           810.56          810.34           810.50            0.56             0.22 Turbine Building 2             No           810.13          810.34           810.06            0.13             -0.21 Notes:
(1) The peak ponding depth is calculated based on the plant grade elevation of 810 ft MSL.
(2) The potential for floodwater to reach the Electrical and Control Building through ingress to the Unit 2 Turbine Building was evaluated. Water would not enter the Electrical and Control Building because the penetrations between the Turbine Building and the Electrical and Control Building are above the elevation of potential ponding in the Turbine Building.
(3) Refer to the FHRR Supplement 1 (Ref. [1]) for the basis for this value. The water level at the Service Water Intake Structure was not revised for the updated analyses presented herein.

CPNPP Response to Request for Additional Information Regarding Flooding Hazard Reevaluation Report 1I__        Condensate Stonage Tank 1 Figure 2-1-3 2 ___       Condensate Stonage Tank 2 3      Reactor Water Makeup Storage Tank 1
  -  4  __  Reactor Water Makeup Storagle Tank 2 5                 Aiotary B'*ingi                                  Inundation          Map of the 6 ___            Cordainmiart Ukit 1                      Reevaluated            Flood Level Due to 7                ConainmenrtLUit 2                        6-Hour Local Intense Precipitation
   -                   Diesel Buildingl 1 g__              D iesel Bukling 2 10         Electrical and Control Building Legend 11                   Fuel Buildirg
                                                 *--     Model Catchments              Non-Safety Related Structures 12        Refueling Water Storage Tank 1 13 14 Refusling Water Storagle Tank 2 Safeguard Builing 1 Inundated Area       [J       Safety Related Structures 15              Safagusrd Building 2 16         Service Water hltake Stn~cture Black labels show peak water levels in ft MSL 17             Swuthear Buildng 1 18             Switoh~ear Builing 2 19                Turbine Building 1           0            250               500                                    1,000 Feet I      I        i        l 20                Tudrine Building 2 21       Safe Shualown Impoundmnt Dam Referanca(s): Source: Earl, Digital Gtobe, GeoEye, i-cubed. Earthatar.
Figure 2-1-3 Inundation Map of the Reevaluated Flood Level Due to 6-Hour Local Intense Precipitation

Attachment to TXX-15111        CPNPP Response to Request for Additional Information Page 11 of 26                       Regarding Flooding Hazard Reevaluation Report TABLE 2-1-4 COMPARISON OF CURRENT AND REEVALUATED FLOOD LEVELS Flooding                    Reevaluated Water Level (ft MSL)                 Current Licensing Basis (')
Mechanism         ___________________                                              Water Level (ft MSL)
Local Intense                                  810.42 (2)                                     N/A Prciittin         (peak level adjacent to a safety-related SSC)            (39.1 in. given as 48-hour PreciptationPMP                                                      for 64 mij2 area) (3) 792.64 (Within SCR, upstream of 9CR Dam: PMF Scenario)                                       789.7 (Within 5CR) 792.66 (Within SCR, east side of SSI Dam: PMF Scenario)                          790.5 (Within SSI) 792.69 (Within SSI: PMF Scenario) 794.89794.7 (PMF Scenario + 794.8Ru-po                  WSsd)       (Within SCR Including Wave n CWS sde)Run-up)
WaveRunup River Flooding                                   794.56                                        791.3 (PMF Scenario + Wave Run-up on SSI Dam from                (Within SCR at the SSI Dam SCR side)                           Including Wave Run-up) 794.23 (PMF Scenario + Wave Run-up on SSI Dam from 991 side) 793.33                                        790.5 (PMF Scenario + Wave Run-up on SWIS                  (Within   the 991; Wave Run-embankment)                                   up  Negligible) 795.75 (PMF Scenario + Wave Run-up on SWIS vertical face)
Reevaluated Water Levels due to Dam Failure                 N/A (700.00 is reported DamFoodngailre ncluingRun-p o 9C and991are                    downstream of the Squaw Dam Foodngailre ncluingRun-p o SC andSSIare                     Creek Dam, which is not Flooding             bounded by the above River Flooding Water                 analogous to reevaluated Levelswater                      levels at the Site)
Storm Surge and                                    N/A                                           N/A Seiche Flooding_____________________
Tsunami Flooding                                      N/A                                           N/A Ice Flooding                                    N/A                                           N/A Channel Diversion                                     N/A                                           N/A Flooding        ______________________                                      _____________

Attachment to TXX-1 5111              CPNPP Response to Request for Additional Information Page 12 of 26                              Regarding Flooding Hazard Reevaluation Report TABLE 2-1-4 COMPARISON OF CURRENT AND REEVALUATED FLOOD LEVELS Flooding                            Reevaluated Water Level (ft MSL)                Current Licensing Basis (1)
Mechanism                _____________________                                        Water Level (ft MSL) 792.84 (Within 3CR, upstream of 3CR Dam: Combined Events Scenario) 792.85 (Within SCR, east side of SSI Dam: Combined Events Scenario) 792.89 Combined                        (Within SS1: Combined Events Scenario)                Combined Events not Events Flooding               795.09 (Combined Events Scenario + Wave Run-              Required to be Evaluated up on CWIS side) 794.75 (Combined Events Scenario + Wave Run-up on 331 Dam from SCR side) 794.43 (Combined Events Scenario + Wave Run-up on 331 Dam from 331 side) 793.53 (Combined Events + Wave Run-up on SWIS embankment) 795.95 (Combined Events + Wave Run-up on
________________           ~            SWIS vertical face)______________
Notes:
(1) There is no difference between the current design basis water elevations generated for the CPNPP site and the current licensing basis as reported in the CPNPP Final Safety Analysis Report for Units 1 & 2 (Ref. [291).
(2) This table is a reproduction of Table 3-3 in the CPNPP FHRR Supplement 1 (Ref. [1]), with edits only to the reevaluated water level for LIP.
(3) Current Licensing Basis PMP event considers a plant grade elevation of 810 ft MSL immediately adjacent to safety related plant structures having exterior entrances at elevation 810.5 ft MSL and no ponding >&#x17d;810.5 ft MSL.

Attachment to TXX-15111    CPNPP Response to Request for Additional Information Page 13 of 26                 Regarding Flooding Hazard Reevaluation Report Flood event duration = 360 minutes I                   _  _  _  _
                                                                                         >1 Water Level Rise Begins                   Water Level Recession Begins (Time = 1 minutes)                        (Time     =   12 minutes)
Figure 2-1-4 Water Level Rise and Recession for Peak Water Levels Near Safety-related SSCs Hour LIP Flood event duration = 72 hours II                                                                              U I                                                                               -i
                                                          &#xa3;l            ,
Water Level                                   Water Level Recession Begins Rise Begins (Time   =  0.9 hours)                         (Time = 39.3 hours)
Figure 2-1-5 Water Level Rise and Recession for Peak Water Levels Near Safety-related SSCs Hour PMP

Attachment to TXX-15111        CPNPP Response to Request for Additional Information Page 14 of 26                     Regarding Flooding Hazard Reevaluation Report Updated Turbine Building Inflow Analysis The Turbine Building inflow analysis was updated using the same methodology applied for developing the response to RAI 9 (Ref. [3]). In overview, this approach involves:
* Representing the Turbine Building as an additional storage area in the HEC-RAS model, with a stage volume relationship (Table 2-1-5) developed from site civil drawings. The stage volume relationship has not been modified from the relationship presented in the response to RAl 9 (Ref.
[3]), because it is an attribute of the physical plant structures and is not affected by the updated hydrologic/hydraulic models.
* Characterizing the flow down the equipment ramp into the Turbine Building within HEC-RAS as wefr flow from the adjoining catchment.
The resulting inflow rates to the Turbine Building and stages within the Turbine Building are reported in Table 2-1-6, which supersedes Tables 9-3 and 9-5 of the response to RAT 9 (Ref. [3]). Note that the critical elevation above which flow could enter the Electrical and Control Building is 778 ft MSL, which is approximately 17 feet above the highest flood level expected within the Turbine Building (Table 2-1-6).
TABLE 2-1-S STAGE VOLUME RELATIONSHIP FOR THE TURBINE BUILDING Stage (ft MSL) 1           Volume (acre-ft) 755.25                           0.00 758.33                           0.27 758.75                           0.32 759.00                           0.36 778.00                           4.61 778.20                          10.30 803.00                           60.82 TABLE 2-1-6 HEC-RAS SIMULATED PEAK STAGES WITHIN TURBINE BUILDING Peak Flow Into          T   Total Flow Volume Simulation                Peak Stage (ft MSL)
Turbine Building (cfs)            {(acre-ft)
Into Turbine Building 6-Hour LIP                   760.75                       15.80                         0.75 72-Hour PMP                    760.93                       12.83                         0.79 It should be noted that additional LIP storm durations and distributions were not considered in the updated analyses, in accordance with the following response to RAl 2-2.

Attachment to TXX-15T11          CPNPP Response to Request for Additional Information Page 15 of 26                       Regarding Flooding Hazard Reevaluation Report NRC RAI 2-2: LIP - Event Duration and Distribution

==Background:==
The LIP analysis methodology included a 6-hour duration event, beginning with the 1-hour, 1-mi 2 PMP arrangedin a front-loaded (or descending) distribution. This approachmay not capture the potentially most conservative and boundingflood condition resultingfrom precipitation events of different magnitude, duration, and timing.
Request: Providejustification that the LIP analysis presented in the FHRR is bounding in terms of warning time, flood depth, and flood duration. This justification can include sensitivity analysis of LIP event duration to consider localized (1-mi2) PMP events up to 72 hours in duration (e.g., 1-, 6-, 12-, 24-, 48-, 72-hour PMPs) and various rainfall distributions (e.g., center-loaded and others in addition to a front-loaded distribution). The evaluations could identiflj potentially bounding scenarioswith respect to flood height, event duration, and associated effects.
Provide electronic versions of any associated modeling input and output files for the sensitivity runs.
CPNPP Response to RAI 2-2:
NUREG/CR-7046 (Ref. [11]) Section 3.2 states that:
Local intense precipitation is a measure of the extreme precipitation at a given location.
The duration of the event and the support area are needed to qualify an extreme precipitation event fully. Generally, the amount of extreme precipitation decreases with increasing duration and increasing area.
The PMP values for areas of the United States east of the 105th meridian are presented in HMRs 51 (Schreiner and Riedel11978) (Ref. [12]) and 52 (Hansen et al. 1982) (Ref. [13]).
The 1-hr, 2.56-km 2 (1-mi2) PMP was derived using single station observations of extreme precipitation, coupled with theoretical methods for moisture maximization, transposition, and envelopment. HMR 52 recommnended that no increase in PMP values for areas smaller than 2.56 km 2 (1 mi2) should be considered over the 1-hr, 2.56- km 2 (1-ni 2) PMP. The local intense precipitation is, therefore, deemed equivalent to the 1-hr, 2.56-km 2 (1-mi2 ) PMP at the location of the site.
The Comanche Peak Nuclear Power Plant (CPNPP) Local Intense Precipitation (LIP) analysis was performed in accordance with the definition of a LIP event provided in NUREG/ CR-7046. NUJREG/ CR-7046 does not refer to evaluating 6-, 12-, 24-, 48-, and 72-hour PMP events or evaluating different temporal distributions within the storms. CPNPP followed the example in NUREG/CR-7046 Appendix B, which used a single temporal distribution and (for added realism) extended the precipitation out to 6 hours.
A 1-hr, 1-mi 2 LIP event centered over the CPNPP site would fully encompass the contributing drainage area of the CPNPP Units 1 and 2 (6.4 acres; (Ref.[1])). For longer duration events, the use of a 1-mi 2 PMP would be reasonable (e.g., 6-hr, 1-mi2). However, because NUREG/CR-7046 defines the LIP as a 1-hr event, the use of any 6-hr event (e.g., corresponding to a 10-mi2 area as in Ref. [1]) is conservative and the addition of longer durations and larger storm events (12-, 24-, 48-, and 72-hour) is not warranted. This approach is in accordance with the definition of the LIP event per NUREG/CR-7046, as described above.
In addition, because of the rainfall intensity during the first hour of the storm event, the amount of precipitable water available for a longer duration storm event would be minimal compared to the first or sixth hour. Therefore, any increase in maximum flood levels due to a longer duration storm event is unlikely.

Attachment to TXX-15111     CPNPP Response to Request for Additional Information Page 16 of 26                   Regarding Flooding Hazard Reevaluation Report The event temporal distribution was developed in accordance with HMR 52 (Ref. [13]), which provides a set of multiplication factors for the 5-min, 15-ndn, 30-min and 6-hour time intervals relative to the 1-hr, 1-nmi2 PMP depths. While HMR 52 does not specifically state the time intervals be arranged in this particular order, with the typical west-east flow across North America, the type of storm set-up that would provide an LIP would likely be a mesoscale convective system (such as a squall line for example).
Using the conceptual model of this type of system (Ref. [14]) the initial precipitation is associated with the mature cells and a zone of convergence and as such will be very intense. The storm motion and nature of the system would then see a decrease in the precipitation after the initial burst as the rear trailing stratiform region with the cold pool moves over the area. This type of meteorological system fits with the front loaded distribution.
By extending the rainfall to 6 hours, the LIP analysis documented in the CPNPP FHRR (Ref. [15]) and a sdupplement to the FHRR (Ref. [1]) is more realistic than the LIP definition as given in Section 3.2 of NUREG/CR-7046 and is sufficient for meeting the requirements of the 50.54(f) letter (Ref. [2]). Although consideration of additional durations is not necessary for LIP analysis on the site, a 72-hour PMP was considered in relation to maximum inflow volume to the Turbine Building, as detailed in the previous response to RAI 2-1.

Attachment to TXX(-15111       CPNPP Response to Request for Additional Information Page 17 of 26                      Regarding Flooding Hazard Reevaluation Report NRC RAL 2-3: Stream and River Flooding            - Model Documentation, Choice of Methods, and Technical Rationale

==Background:==
The response to the previously issued RAI111 stated: "Calibration to a single return period was considered suf-ficientfor the purposes of modelingflow in the streams upstream and downstream of the SCR, because the modeling of the PMFflood stage of importance is dominated by storage and outflow effects rather than flow in the upstream and dowonstream channels." However there was no technical justification to support the conclusion.
After reviewing the results it is apparent that, the 5CR water level is highly sensitive to upstream modeling assumptions, even when fidly crediting the SCR Dam dischargestructures.
Considering the fact that the effects of upstream and downst ream flows influence storage and outflow during aflood event, calibration of the hydrological model is essential to providing appropriatepredictions of water surface elevations duringflooding events. In this respect, it is necessary to ensure the model performs well under various hydrologicflow/flooding conditions, and, as such, multiple observations should be used to determine model calibrationaccuracy when available.
Due to unavailability offlow datafor the calibration of Squaw Creek flow predictions, model calibration relied on regression equations developed by the Texas Department of Transportation (TxDOT, 2011). Calibrationof the HEC-HMS hydrological model included adjustment of the channel Manning's roughness coefficient and Snyder's peaking and basin coefficients to match the regression equation resultsfor a 100-year return periodflood. The source documentation includes a regression equation for a 500-year return periodflood. However this equation woas not used as a part of the calibration. Using the 500-year return period precipitation could result in lower peak flowo compared with the 500-year regression equation, especially under more extreme flooding conditions The HEC-HMS model includes site-specific inputs and is based on hydrologic processes wohile the empirical equation (TxDOT, 2011) includes only afew site-specific parameters (annual precipitation,stream slope, and watershed area, and a regression coefficient). In addition, the results indicate that the calculatedpeak flow from the regression equation is sensitive to the three calibrationparameters.
Reus: Justify the use of empirically derived regression equation resultsfor a 100-year return periodflood for calibrationwithout having considered potential uncertainty and sensitivity in the empirical equation or calibration to or extrapolation beyond the 100-year return period event. Perform additional calibrationand validation to verify the hydrological model againsthigher returnperiodfloods (including calibratingto the regression equationfor the 500-year return period; or beyond using extrapolation)to ensure the model conservatively estimates discharge across variable precipitation and higherflow conditions. In addition, consider calibratingthe HEC-HMS model using other watersheds in the area that are hydrologically comparable to the SCR watershed. Provide electronic versions of any associated modeling input and output files for the revised calculations.
CPNPP Response to RAI 2-3:
The 100-year regression equation was selected for the calibration of the watershed model based on the R-squared value of the various regression equations included in the Texas Department of Transportation Hydraulic Design Manual (Ref. [16]; Table 4-4). The R-squared value for the 100-year regression equation (0.86) is similar to the R-squared value for the 50-year regression equation (0.87). However, the 500-year regression equation has a lower R-squared value of 0.81, reflecting a decrease in confidence for the 500-year equation. Calibration to the 500-year regression equation was not considered appropriate due to the increased uncertainty inherent to that equation. Extrapolating to even lower probability events would only lead to a further increase of uncertainty.

Attachment to TXX-15111      CPNPF Response to Request for Additional Information Page 18 of 26                   Regarding Flooding Hazard Reevaluation Report It is important to note that the analysis was not a Probabilistic Flood Hazard Assessment (PFHA), so the return period of the event is not the primary focus of the analysis. The intent was to choose a relatively extreme event with enough information available to perform a calibration. The 100-year event was selected as the most appropriate for this purpose. Note that a governmental entity (i.e., Texas Department of Transportation) produced the estimate of the 100-year return period flood. Moreover, the 100-year flood is the national standard used by the NFJP and all Federal agencies for the purposes of requfring the purchase of flood insurance and regulating new development.
Calibrating the HEC-HMS model to similar watersheds in the area was not considered necessary due to the availability of the regression equation for the site area and its ability to support a basic calibration of the model. If an adequate record of streamflow is not available at or near the project site, a Log-Pearson Type Ill (LPIII) distribution cannot be developed, for instance, using Bulletin #17B procedures (Ref. [17]).
An alternative for estimating the needed flow is to use a regression equation. Regression equations are used to transfer flood characteristics from gauged to ungauged sites through the use of watershed and climatic characteristics as explanatory or predictor variables. USGS has developed such regression equations for natural basins throughout the State of Texas. This approach has been applied before by the USACE (i.e., checking and comparing the HMS results to regression equations as in Ref. [18]).

Attachment to TXX.-15111       CPNPP Response to Request for Additional Information Page 19 of 26                     Regarding Flooding Hazard Reevaluation Report NRC RAI 2-4: Stream and River Flooding - Model Documentation, Choice of Methods, and Technical Rationale

==Background:==
Supplement 1 to the FHRR provided updates to the river modeling, including corrections to the originalhydrologic model calibration, as well as refined hydraulic model configurations to better simulate the system. The changes resulted in an overall increase in the stillwaterelevation for the PMF and combined events scenarios, which in turn increased the fetch length at critical CPNPPlocations (for example, the fetch length on the cooling water intake structure side increased to 18,386ft from a previous value of 15,113 ft) used in the wave runup calculations. As a result, the wave runup and wind setup associated effects could be impacted.
Although the fetch lengths (Section 3.2.2 .2.4) and stillwater elevations (Table 3-3) reportedin the FHRR Supplement 1 are higher than the reported values in the original FHRR, the combined events flooding elevations (including wave run-up) reported in Table 3-3 are lower than the previously reported values at each critical location, except the seroice water intake structure verticalface.
Request: Provide technical justification to describe why the combined events elevations with runup are lower despite increases in flood stillwater elevation andfetch length. Provide a description of associated effects such as wave runup and wind setup that supports the combined events results in order to confirm the results provided in the FHRR Supplement 1 in Table 3-3. Provide updated calculationpackages, similar to the documentation and Excel file originally accompanying the F-13 material.
CPNPP Response to RAT 2-4:
The updated wind-wave and run-up analysis corresponding to the updated combined events analysis is documented in RIZZO Calculation 14-5213 F-3 (Ref. [19]). The updated wind-wave and run-up analysis incorporated refinements to the analysis in accordance With the Hierarchical Hazard Assessment (HHA) approach, which resulted in lower run-up elevations even though the stillwater elevation had increased.
The refinements to the analysis are summarized as follows:
* The empirical coefficients on the run-up equations applied (Equations 5 and 7 of RIZZO Calculation 14-5213 F-3 (Ref. [19]) were modified to reflect "mean" relationships for run-up.
These "mean" run-up relationships were obtained from the United States Army Corps of Engineers (USACE) Coastal Engineering Manual (CEM) (Ref. [20]; Equations VI-5-6 and VI-5-7) and provide "best-estimate" values for the run-up levels at the CPNPP site. The decision to refine the run-up analysis to use the "mean" relationships resulted from a review of the conservatisms associated with the original Fl-RR~analysis. Inclusion of small "safety factors" in each analysis aspect can add up to a considerable level of overall conservatism. The refined analysis reflects the choice for "best-estimate" values in order to avoid compounding conservatisms.
o The roughness of the shoreline was accounted for using the roughness reduction factor in the run-up equations. The roughness reduction factor was assigned for each beach face (e.g., rip-rap) based on Table VI-5-3 in the CEM (Ref. [20]). The inclusion of the roughness reduction factor adds an additional layer of realism to the wave run-up analysis that was not included in RIZZO Calculation 12-4891 F-13 (Ref. [21]) or RIZZO Calculation 12-4891 F-24 (Ref. [22]).
The HI-A refined run-up analysis for Supplement 1 (Ref. [1]) results in lower run-up levels than those of RIZZO Calculation 12-4891 F-13 (Ref. [21]), which lead to lower overall peak flood levels even though the stillwater elevation had increased.

Attachment to TXX-15111         CPNPP Response to Request for Additional Information Page 20 of 26                      Regarding Flooding Hazard Reevaluation Report NRC RAI 2-5: Stream and River Flooding             - Model Documentation, Choice of Methods, and Technical Rationale

==Background:==
Supplement I to the FHRR provides a description of the methodology used to evaluate stream and river and combined events flooding and outlines the major results. However, the supplement does not provide specific details on how the calculations were revised, and does not provide calibration results. Supplement 1 also does not contain updated calculationspackages or input and outputfiles.
Req~uest: Clarify what changes were made in Supplement 1, including whether the calculations are based on a revised set of assumptions. If different assumptions were used, provide those assumptions and the updated calculation packages that describe the stream and river flooding and combined events flooding analyses.
CPNPP Response to RAI 2-5:
A summary of the changes to Supplement 1 to the FHRR:
* Correct use of regression equation used to determine 100 year flow rate (not read correctly from source document). This impacted previous PMF analysis calculations supporting the original FHRR (i.e., Calculations 12-4891 F-10 (Ref. [23]), 12-4891 F-12 (Ref. [241), 12-4891 F-19 (Ref. [25]),
and 12-4891 F-22 (Ref. [26])).
* New Calculation 12-4891 F-24 (Ref. [22]) was performed to the corrected value resulting in revised PMF flow rates for Squaw Creek, revised water surface elevations within Squaw Creek Reservoir (SCR) and the SSI, and revised wind setup and wave runup.
o Calculation 12-4891 F-10 (Ref. [23]) - the 100-year flood increased by approximately 25,000 cfs, which resulted in revised calibrated values of Manning's Roughness Coefficients reported in Calculation 12-4891 F-24 (Ref. [22])
o Calculations 12-4891 F-12 (Ref. [24]) and 12-4891 F-19 (Ref. [25]) - previous results bounded the new calculation 12-4891 F-24 (Ref. [22])
o Calculation 12-4891 F-22 (Ref. [26]) - water surface elevations for SCR were approximately one foot lower than the revised elevations generated by Calculation 12-4891 F-24 (Ref. [22])
* Due to increased water surface elevations of Calculation 12-4891 F-24 (Ref. [22]) several parameters were addressed in the FHRR Supplement 1, based on the HI-A approach:
o Calculation 14-5213 F-2 (Ref. [30]) provides an updated analysis including refinement of weir coefficients based on the head at the SCR dam during the flood. The geometry of the emergency spillway was also refined to consider the as-built shallow sloping of the side walls that allow traffic to drive over the spillway.
o Calculation 14-5213 F-i (Ref. [31]) determines that the 500-year inflow to the SCR is less than the inflow from 40% of the probable maximum precipitation (PMP) event. ANS 2.8 (Ref. [28]), provides the option to use the lesser of the 40% PMP and the 500-year precipitation, as the antecedent storm prior to the full probable maximum precipitation (PMP). Calculation 14-5213 F-2 (Ref. [30]) therefore utilizes the 500-year flood as the antecedent storm for the combined events analysis.
The changes in FHRR Supplement 1 (Ref. [1]) were identified by a change bar in the right hand margin.
Additionally, a track-changes version showing additions / deletions has been uploaded to the NRC e-portal.
The assumptions remain unchanged for the FHRR Supplement 1 (Ref. [1]) supporting calculations listed above. However, Calculation 14-5213 F-3, "Revised Wind Wave Analysis for PMF and Combined Events" (Ref. [19]), also used to support the FI-RR Supplement 1, is based on an updated set of assumptions, as discussed in the response to RAI 2-4.

Attachment to TXX-15111   CPNPP Response to Request for Additional Information Page 21 of 26                Regarding Flooding Hazard Reevaluation Report The new calculations supporting the FHIRR Supplement 1 (Ref. [1]) have been uploaded to the NRC e-portal. The associated input / output files for the new calculations were provided in Enclosure 1 to Ref. [27].

Attachment to TXX-15111         CPNPP Response to Request for Additional Information Page 22 of 26                       Regarding Flooding Hazard Reevaluation Report NRC RAI 2-6: Hazard Input for the Integrated Assessment                 - Flood Event Duration Parameters, Flood Height and Associated Effects

==Background:==
Enclosure 2 of the 50.54(f) letter requests the licensee to perform an integratedassessment of the plant's response to the reevaluated hazard if the reevaluatedflood hazard is not bounded by the current design basis.
The FHRR should include all of theflood hazard information needed to understand the flood hazard and associated effects that will be an input to the integrated assessment; including the flood durationparametersfor LIP (see definition and Figure 6 of the NRC interim staff guidance document JLlJ-ISG-2012-05, 'Guidancefor Performing an Integrated Assessment," dated November 2012 (ADAMS Accession No. ML12311A214).
Request: Provide the applicableflood event duration parametersassociatedwith LIP using the results of the flood hazard reevaluation. This includes the warning time the site will have to preparefor the event (e.g., the time between notification of an impending flood event and arrival of floodwaters on site) and the period of time the site is inundated. Provide the basis for the flood event duration, which may include a description of relevantforecasting methods (e.g., productsfrom local, regional, or national weatherforecasting centers) and timing information derived from the hazard analysis.
Provide the flood height and associatedeffects that are not described in the FHRR for mechanisms that trigger an integrated assessment. This includes the following quantified informationfor each mechanism, as applicable:
* wind waves and run-up effects
* hydrodynamic loading, including debris a   effects caused by sediment deposition and erosion
* concurrent site conditions, including adverse weather conditions
* groundwateringress
* other pertinentfactors CPNPP Response to RAI 2-6:
As described in CPNPP's Response to RAI 2-1, the updated model provided improved discretization and a better characterization of the temporal and spatial flow effects on the CPNPP site, with particular focus given to the flow near safety-related Structures, Systems, and Components (SSCs). The resultant updated LIP flood levels were broadly comparable with the previous results reported in the CPNPP FHRR Supplement 1 (Ref. [1]). As provided on Table 2-1-3 and shown on Figure 2-1-3, the peak flood levels adjacent to safety-related SSCs still remain lower than the door threshold elevations (minimum of 810.5 ft MSL). Hence, there is no ingress or inundation of floodwater directly to safety-related SSCs so the plant remains in a safe and stable state for the entire duration of the flooding event.
Flood event duration timelines attributed to the updated 6-hour LIP and 72-hour PMP' analyses are provided on Figures 2-1-4 and 2-1-5, respectively. These timelines show the duration that it takes the updated flood levels to rise and recede similar to that illustrated in Figure 6 of JLD-ISG-2012-05 (Ref.
[10]). Figures 2-1-4 and 2-1-5 supersedes Figure 20-1 previously presented in Response to RAT 20 (Ref.
[3]). Consistent with the rainfall event temporal distributions applicable for the CPNPP site as described in RAT 2-2, the provided flood event duration timelines and the fact that there is no consequential flooding ingress or inundation concerns to safety related SSCs, rain event triggers and warning time mechanisms are not required to be established. Thus, given the applicable rainfall event and update LIP results, there is no requirement to implement any flood protection or mitigation measures.
CPNPP FHRR Supplement 1 (Ref. [1]) and CPNPP response to RAT 8 (Ref. [3]) previously reported that the water levels in the Squaw Creek Reservoir (5CR) and the Safe Shutdown Impoundment (SSI) due to the local intense precipitation event are lower than the water levels in the SCR and SSI due to the regional river flooding (and combined events) on the Squaw Creek watershed. Therefore, the applicable associated effects for water levels coincident to the regional PMF were considered bounding for the

Attachment to TXX-15111      CPNPP Response to Request for Additional Information Page 23 of 26                  Regarding Flooding Hazard Reevaluation Report determination of water levels on the safety-related SSCs. The result of the updated LIP analysis does not invalidate this conclusion.
Thus, the flood height and applicable associated effects attributed to the controlling regional PMF is not changed from that previously reported in response to RA1 21 (Ref. [31) due to the updated LIP analysis.
The flooding mechanisms of hydrodynamic loading, debris loading and the effects caused by sediment deposition and erosion remain unchanged from that previously described in the response RAI121 (Ref.
[3]) and are not credible mechanisms to consider for the updated LIP spatial flow effects and resulting flood heights for the onsite catchments surrounding the safety related structures. In addition, the controlling hazards are all precipitation hazards. Therefore, rainfall is considered concurrent with the hazard. As previously described in the response RAI121 (Ref. [3]), Table 3-5 of the CPNPP FHRR Supplement 1 (Ref. [1]) lists the maximum 2-year wind speed that would be concurrent with the flood hazards and identifies that lightning and hail may be expected to occur during the hazards. In regard to the LIP and the associated site drainage wind waves and run-up effects and groundwater ingress, the following is provided.
Wind Waves and Run-Up Effects As reported in RAI 2-1 and shown in Table 2-1-3 above, the updated model indicated LIP flood levels that were comparable with the previous results reported in the CPNPP FHRR Supplement 1 (Ref. [1]) with the peak flood levels adjacent to safety-related SSCs still lower than the door threshold elevations (minimum of 810.5 ft Mean Sea Level (MSL)). The most limiting updated flood level is 810.42 ft MSL which is only 0.09 ft greater than the peak ponding level reported adjacent to a safety related building in CPNPP FHRR Supplement 1 (Ref. [1]). As previously reported in CPNPP response to RAI 8 (Ref. [3]) and for the same reasons provided, the updated ponding depths on the powerblock area do not support the growth of wind waves. Hence, there is no ingress of LIP related floodwater into building areas containing safety related SSCs due to wind waves and run-up effects.
Groundwater Ingress The updated LIP water levels adjacent to the safety-related Service Water Intake Structure (SWIS) and as shown in Table 2-1-3 is unchanged from that previously reported in CPNPP FHRR Supplement 1 (Ref. [1]
and addressed in response to RAT121 (Ref. [3]). Therefore, there is no change to groundwater impact to the SWIS as previously described in the CPNPP FHRR Supplement 1 (Ref. [1]).
The updated LIP water levels adjacent to the balance of the safety-related building as shown in Table 2 3 decreased for some areas and increased for others with a maximum peak level of 810.42 ft MSL as noted on Table 2-1-4. Similar to that described in CPNPP FHIRR Supplement 1 (Ref. [11 and addressed in response to RAI 21 (Ref. [3]), the groundwater hydrostatic pressure on the lowest safety-related building elevation (being 773') would increase by the ratio of (810.42-810) / (810-773) = 1.14%. This additional stress level (with all other contributing loads unaffected) in the affected walls & floor base mats are still considered negligible and well within the design margin provided in the structural integrity analyses for all applicable exterior building wall and floor base mat structural elements.
In addition to the potential groundwater effects that are still applicable as previously described in pages 45 through 48 of CPNPP FHRR Supplement 1 (Ref. [1] and identified in response to RA1 21 (Ref. [3]), the safety related structures are surrounded by a relatively impervious cover which should limit infiltration of precipitation and perched water seepage to a uminmum. Any minute groundwater intrusion attributed to the updated LIP ponding levels which are still marginally higher than plant grade, would (1) be bounded by the existing design basis internal building flooding analyses and (2) be well within the design rating & operating capacity of the applicable building sumps such that there is reasonable assurance safety related SSCs are fully capable of performing their specified safety functions.

Attachment to TXX-15111      CPNFP Response to Request for Additional Information Page 24 of 26                   Regarding Flooding Hazard Reevaluation Report RAI 2-7: Hazard Input for the Integrated Assessment - Comparison of Reevaluated Flood Hazard with Current Design Basis

==Background:==
The FHRR provides a comparison of the reevaluatedflood hazards with the current licensing basis (CLB) instead of the current design basis as requiredin the 5054f letter. Table 3-3 of the FHRR has a tabulated summary of this comparison.
Request: Clarifjy the comparison of the reevaluatedflood hazard to the current design bases.
CPNPP Response to RAI 2-7:
There is no difference between the current design basis water elevations generated for the CPNPP site and the current licensing basis as reported in the CPNPP Final Safety Analysis Report for Units 1 & 2 (Ref. [29]).
Consistent with NRC Letter, Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3, and 9.3, of the Near-Term Task Force Review of Insights from the Fukushima Dai-Ichi Accident; dated March 12, 2012, (ADAMS Accession No.
ML12073A348), and specifically Recommendation 2.1: Flooding; Item 1.a.ii, the flood elevations attributed to all applicable flood causing mechanisms at the CPNPP site and represented in the FI-RR Supplement 1 (Ref. [1]), Tables 2-4 and 3-3 as the Current Licensing Basis water elevations are the same as the current design basis water levels.

Attachment to TXX-15111    CPNPP'Response to Request for Additional Information Page 25 of 26                 Regarding Flooding Hazard Reevaluation Report REFERENCES
[1] Luminant Generation Company LLC (Luminant) Letter TXX-14094, "Comanche Peak Nuclear Power Plant (CPNPP) Docket Nos. 50-445 and 50-446 Submittal of Fukushima Lessons Learned - Flood Hazard Reevaluation Report Supplement 1 (TAC NOS. MF1099 and MF 1100)," NRC Agencywide Documents Access and Management System (ADAMS)
Accession No. ML14245A136, August 14, 2014.
[2] United States Nuclear Regulatory Commission (NRC), "Request for Information Pursuant to Title 10 of the Code of Federal Regulations 50.54(f) Regarding Recommendations 2.1, 2.3, and 9.3, of the Near-Term Task Force Review of Insights From the Fukushima Dai-Ichi Accident," Letter to All Power Reactor Licensees and Holders of Construction Permits in Active or Deferred Status, NRC Agencywide Documents Access and Management System (ADAMS) Accession No. ML12053A340, March 12, 2012.
[3] Luminant Generation Company LLC (Luminant) Letter TXX-14048, "Comanche Peak Nuclear Power Plant (CPNPP) Docket Nos. 50-445 And 50-446 Submittal of Requested Information Regarding Fukushima Lessons Learned - Flooding Hazard Reanalysis Report (TAC NOS. MF 1099 and MF 1100)," NRC Agencywide Documents Access and Management System (ADAMS) Accession No. ML14100A049, April 4, 2014.
[4]   Paul C. RIZZO Associates, "Local Intense Precipitation Refined Analysis," Calculation 14-5213 F-05, Revision 0, September 2015.
[5]   Paul C. RIZZO Associates (RIZZO), "Turbine Building Inflow Analysis," Calculation 14-5213 F-06, Revision 0, September 2015.
[6]   ESRI, "Arc Hydro for ArcGIS 10.1," Version 10.1, May 13, 2013.
[7]   United States Army Corps of Engineers (USACE), Hydrologic Engineering Center (JIEC),
Hydrologic Modeling System HEC-HMS, Technical Reference Manual, March 2000.
[8]   United States Department of Agriculture (USDA), "Urban Hydrology for Small Watersheds," Technical Release 55 (TR-55), Second Edition, 1986.
[9]   United States Army Corps of Engineers (USACE), Hydrologic Engineering Center (HEC),
HEC-RAS River Analysis System, Hydraulic Reference Manual, Version 4.1, January 2010.
[ 10] United States Nuclear Regulatory Commission (NRC), "Guidance for Performing the Integrated Assessment for External Flooding," JLD-ISG-2012-05, Revision 0, Agencywide Documents Access and Management System (ADAMS) Accession No. ML12311A214, November 30, 2012.
[11 ] United States Nuclear Regulatory Commission (NRC), "Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America," Washington D.C., NUREG/CR-7046, PNNL-20091, NRC Job Code N6575, Agencywide Documents Access and Management System (ADAMS) Accession No. ML11321A195, November 2011.
[12]  National Weather Service (NWS), "Hydrometeorological Report No. 51, Probable Maximum Precipitation Estimates, United States, East of the 105th Meridian," June 1978, Reprinted 1980.
[13]  National Weather Service (NWS), "Hydrometeorological Report No. 52, Application of Probable Maximum Precipitation Estimates - United States East of the 105th Meridian,"
August 1982.
[14] R. A. Houze Jr, "Mesoscale Convective Systems," Review of Geophysics, vol. 42, 2004.

Attachment to TXX-15111   CPNPP Response to Request for Additional Information Page 26 of 26                Regarding Flooding Hazard Reevaluation Report
[ 15] Luminant Generation Company LLC (Luminant) Letter TXX-13053, "Response to March 12, 2012, Request for Information Enclosure 2, Recommendation 2.1, Flood Hazard Reevaluation Report, of the Near-Term Task Force Review of Insights from the Fukushima Dai-Ichi Accident," NRC Agencywide Documents Access and Management System (ADAMS) Accession No. ML13074A058, March 12, 2013.
[16] Texas Department of Transportation (TxDOT), "Hydraulic Design Manual," October 2011.
[17] United States Geological Survey (USGS), "Guidelines for Determining Flood Flow Frequency," Bulletin # 17B, 1982.
[18] United States Army Corps of Engineers (USACE), "Description of the Hydrologic Engineering Center's Hydrologic Modeling System (HEC-HMS) and Application to Watershed Studies," ERDC/TN SMART-04-3, September 2004.
[19] Paul C. RIZZO Associates (RIZZO), "Revised Wind Wave Analysis for PMF and Combined Events," Calculation 14-52 13 F-3, Revision 0, June 25, 2014.
[20] United States Army Corps of Engineers (USACE), "Coastal Engineering Manual,"
Engineering Manual 1110-2-1100, (in 6 volumes), Updated 2008.
[21] Paul C. RIZZO Associates (RIZZO), "Wind Waves and Runup on SSCs due to PMF,"
Calculation 12-4891 F-13, Revision 0, January 11, 2013.
[22] Paul C. RIZZO Associates (RIZZO), "CPNPP Sensitivity Analyses Supporting CPAR 013-14," Calculation 12-4891 F-24, Revision 1, November 12, 2014.
[23] Paul C. RIZZO Associates (RIZZO), "PMF Estimate for Squaw Creek," Calculation 12-4891 F-10, Revision 1, February 11, 2013.
[24] Paul C. RIZZO Associates (RIZZO), "Water Level Estimation Due to PMF," Calculation 12-4891 F-12, Revision 0, January 11, 2013.
[25] Paul C. RIZZO Associates (RIZZO), "Analysis of Combined Events,", Calculation 12-489 1 F-19, Revision 0, January 11, 2013.
[26] Paul C. RIZZO Associates (RIZZO), "PMF Estimation for SSI," Calculation 12-4891 F-22, Revision 0, January 11, 2013.
[27] Luminant Generation Company LLC (Luminant) Letter TXX-141 16, "Comanche Peak Nuclear Power Plant (CPNPP) Docket Nos. 50-445 and 50-446 Submittal of Requested Information Regarding Fukushima Lessons Learned               -  Flood Hazard Reevaluation Report Supplement 1 (TAC NOS. MF1099 and MF1100)," dated September 18, 2014.
[28]  ANS, 1992, American Nuclear Society (ANS), American National Standard for Determining Design Basis Flooding at Power Reactor Sites. Prepared by the American Nuclear Society Standards Committee Working Group ANS-2.8, La Grange Park, Illinois, 1992.
[29]  Luminant Generation Company, LLC (Luminant), "Comanche Peak Nuclear Power Plant, Final Safety Analysis Report (FSAR Units 1&2)," Amendment 106, Glen Rose, Texas, 2014.
[30]  Paul C. RIZZO Associates (RIZZO), "HEC-RAS Model for PMF and Combined Events,"
Calculation 14-52 13 F-2, Revision 0, June 25, 2014.
[31]  Paul C. RIZZO Associates (RIZZO), "HEC-HiMS Model for PMF and Combined Events,"
Calculation 14-5213 F-i, Revision 0, June 18, 2014.}}