ML15278A306

Submittal of Request for Additional Information Regarding Fukushima Lessons Learned - Flooding Hazard Reanalysis Report

ML15278A306
Person / Time
Site:	Comanche Peak
Issue date:	09/22/2015
From:	Flores R, McCool T P Luminant Generation Co, Luminant Power
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
CP-201500740, TAC MF1099, TAC MF1100, TXX-15111
Download: ML15278A306 (29)

Text

SRafael Flores Luminant Power Senior Vice President P 0 Box 1002& Chief Nuclear Officer 6322 North FM 56 Rafael.Flores@Luminant.com Glen Rose, TX 76043 Luminant 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 output files for the Hydrologic Engineering Center River Analysis System (HE C-RAS) model used for modeling the local intense precipitation (LIP) event.The discretization in the model appears insufficient to adequately predict flow 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 elevation for each catchment based on a stage-volume curve. The model computes water surface elevation for 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 predict flow depth and direction in critical areas around site structures important to safety.Regarding the routing of the flow, water accumulates in areas of lowest topographic profile before exiting the catchment at the lowest elevation along the circumscribing cross sections.

As such, the LIP model predicts flooding only within the lowest-profile sections of a catchment.

Thus, the model does not adequately represent flooding and routing across the entire catchment, especially in areas of higher elevation.

As a result, the model could not accurately predict the depth of flow and direction of flood routing since the flooding and point of discharge are located only in zones of low topographic profile.For example, Figure 7-1 from Calculation F-03 shows a peak water surface elevation of 807.24 ft 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 cat chment, some of which is at a ground elevation of over 850 ft. Precipitation falling 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 spatial flood routing characteristics within 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 spatial flow 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 durations and 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 Page 2 of 26 CPNPP Response to Request for Additional Information Regarding Flooding Hazard Reevaluation Report Nw6 of~mst Flowina 01too~s sbfq Rulate4~~w..E a toGS 1sto2 0 I ¶22SoF~e J4gh 656568 2to -w iU,-l,. 715 Figure 7-1 from Calculation F-03 -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 Page 5 of 26 CPNPP Response to Request for Additional Information Regarding Flooding Hazard Reevaluation Report LEGEND Figure 2-1-1 SUB8BASIN SCHEMATIC OF UPDATED HEC-HMS MODEL FOR SITE DRAINAGE ANALYSIS Note: HEC-HMS is only used for calculation of rainfall-runoff 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 C27'0 SUBBASIN NUMBER 3j RESERVOIR HYDRAUUC CONNECTION SUBBASIN DEUNEATIO N Figure 2-1-1 Schematic of Updated HEC-HMS Model for Site Drainage Analysis Attachment to TXX-15111 Page 6 of 26 CPNPP Response to Request for Additional Information Regarding Flooding Hazard Reevaluation Report Figure 2-1-2 Schematic of Updated HEC-RAS Model for Site Drainage Analysis Attachment to TXX-15111 Page 7 of 26 CPNPP Response to Request for Additional Information 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 Page 8 of 26 CPNPP Response to Request for Additional Information 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 Page 9 of 26 CPNPP Response to Request for Additional Information 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 Struturs, ystms, 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 2 ___ Condensate Stonage Tank 2 3 Reactor Water Makeup Storage Tank 1-4 __ Reactor Water Makeup Storagle Tank 2 5 Aiotary 6 ___ Cordainmiart Ukit 1 7 ConainmenrtLUit 2-Diesel Buildingl 1 g__ D iesel Bukling 2 10 Electrical and Control Building 11 Fuel Buildirg 12 Refueling Water Storage Tank 1 13 Refusling Water Storagle Tank 2 14 Safeguard Builing 1 15 Safagusrd Building 2 16 Service Water hltake Stn~cture 17 Swuthear Buildng 1 18 Switoh~ear Builing 2 19 Turbine Building 1 20 Tudrine Building 2 21 Safe Shualown Impoundmnt Dam Figure 2-1-3 Inundation Map of the Reevaluated Flood Level Due to 6-Hour Local Intense Precipitation LegendModel Catchments Non-Safety Related Structures Inundated Area [J Safety Related Structures Black labels show peak water levels in ft MSL 0 250 I I 500 i l1,000 Feet 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 Page 11 of 26 CPNPP Response to Request for Additional Information 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 + 794.8 Ru-po WSsd) (Within SCR Including Wave WaveRunup n CWS sde)Run-up)

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 Dam ailre Foodng ncluingRun-p o 9C and991are downstream of the Squaw Dam ailre Foodng 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 Page 12 of 26 CPNPP Response to Request for Additional Information 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 >810.5 ft MSL.

Attachment to TXX-15111 Page 13 of 26 CPNPP Response to Request for Additional Information Regarding Flooding Hazard Reevaluation Report Flood event duration = 360 minutes I _ _ _ _>1 Water Level Rise Begins (Time = 1 minutes)Water Level Recession Begins (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 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> II U I -i Water Level (Time = 0.9£l , Rise Begins hours)Water Level Recession Begins (Time = 39.3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />)Figure 2-1-5 Water Level Rise and Recession for Peak Water Levels Near Safety-related SSCs Hour PMP Attachment to TXX-15111 Page 14 of 26 CPNPP Response to Request for Additional Information 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 1 Volume (ft MSL) (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 Turbine Building Into Turbine Building (ft MSL) (cfs) {(acre-ft) 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 arranged in a front-loaded (or descending) distribution.

This approach may not capture the potentially most conservative and bounding flood condition resulting from precipitation events of different magnitude, duration, and timing.Request: Provide justification 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-mi 2) PMP events up to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> 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 scenarios with 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-mi 2) 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 mi 2) 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-mi 2) 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 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.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-nmi 2 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 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, 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-ficient for the purposes of modeling flow in the streams upstream and downstream of the SCR, because the modeling of the PMF flood 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 discharge structures.

Considering the fact that the effects of upstream and downst ream flows influence storage and outflow during a flood event, calibration of the hydrological model is essential to providing appropriate predictions of water surface elevations during flooding events. In this respect, it is necessary to ensure the model performs well under various hydrologic flow/flooding conditions, and, as such, multiple observations should be used to determine model calibration accuracy when available.

Due to unavailability of flow data for the calibration of Squaw Creek flow predictions, model calibration relied on regression equations developed by the Texas Department of Transportation (TxDOT, 2011). Calibration of 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 results for a 100-year return period flood. The source documentation includes a regression equation for a 500-year return period flood. 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 a few site-specific parameters (annual precipitation, stream slope, and watershed area, and a regression coefficient).

In addition, the results indicate that the calculated peak flow from the regression equation is sensitive to the three calibration parameters.

Reus: Justify the use of empirically derived regression equation results for a 100-year return period flood for calibration without having considered potential uncertainty and sensitivity in the empirical equation or calibration to or extrapolation beyond the 100-year return period event. Perform additional calibration and validation to verify the hydrological model against higher return period floods (including calibrating to the regression equation for the 500-year return period; or beyond using extrapolation) to ensure the model conservatively estimates discharge across variable precipitation and higher flow conditions.

In addition, consider calibrating the 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 original hydrologic model calibration, as well as refined hydraulic model configurations to better simulate the system. The changes resulted in an overall increase in the stillwater elevation for the PMF and combined events scenarios, which in turn increased the fetch length at critical CPNPP locations (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) reported in 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 vertical face.Request: Provide technical justification to describe why the combined events elevations with runup are lower despite increases in flood stillwater elevation and fetch 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 calculation packages, 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 calculations packages or input and output files.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 Page 21 of 26 CPNPP Response to Request for Additional Information 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 integrated assessment of the plant's response to the reevaluated hazard if the reevaluated flood hazard is not bounded by the current design basis.The FHRR should include all of the flood hazard information needed to understand the flood hazard and associated effects that will be an input to the integrated assessment; including the flood duration parameters for LIP (see definition and Figure 6 of the NRC interim staff guidance document JLlJ-ISG-2012-05, 'Guidance for Performing an Integrated Assessment," dated November 2012 (ADAMS Accession No. ML12311A214).

Request: Provide the applicable flood event duration parameters associated with LIP using the results of the flood hazard reevaluation.

This includes the warning time the site will have to prepare for 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 relevant forecasting methods (e.g., products from local, regional, or national weather forecasting centers) and timing information derived from the hazard analysis.Provide the flood height and associated effects that are not described in the FHRR for mechanisms that trigger an integrated assessment.

This includes the following quantified information for 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

• groundwater ingress* other pertinent factors 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 2-1-3 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 reevaluated flood hazards with the current licensing basis (CLB) instead of the current design basis as required in the 5054f letter. Table 3-3 of the FHRR has a tabulated summary of this comparison.

Request: Clarifjy the comparison of the reevaluated flood 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. ML1231 1A214, 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. ML1 132 1A195, 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.