RS-14-122, Response to Request for Additional Information Regarding Fukushima Lessons Learned - Flooding Hazard Reevaluation Report - Redacted

Response to Request for Additional Information Regarding Fukushima Lessons Learned - Flooding Hazard Reevaluation Report - Redacted

ML15092A821
Person / Time
Site:	Dresden
Issue date:	05/19/2014
From:	Kaegi G Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
Hall V, NRR/JLD, 415-2915
References
RS-14-122
Download: ML15092A821 (49)

Text

Exelon Generation 10 CFR 50.54(f)

RS-14-122 May 19, 2014 U.S. Nuclear Regulatory Commission ATTN : Document Control Desk Washington, DC 20555-0001 Dresden Nuclear Power Station , Units 2 and 3 Renewed Facility Operating License Nos. DPR-19 and DPR-25 NRC Docket Nos. 50-237 and 50-249

Subject:

Response to Request for Additional Information Regarding Fukushima Lessons Learned - Flood Hazard Reevaluation Report

References:

1. Exelon Generation Company, LLC Letter to USN RC, Response to March 12, 2012 Request for Information Enclosure 2, Recommendation 2.1, Flooding, Required Response 2, Flooding Hazard Reevaluation Report, dated May 10, 2013 (RS-13-11 O)

2. NRC Letter, Request for Information Pursuant to Title 1O 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

3. NRC Request for Additional Information Regarding Fukushima Lessons Learned - Flood Hazard Reevaluation Report, dated April 9, 2014 In Reference 1, Exelon Generation Company, LLC (EGC) provided the Dresden Nuclear Power Station, Units 2 and 3, Flooding Hazard Reevaluation Report in response to the March 12, 2012 Request for Information Enclosure 2, Recommendation 2.1, Flooding, Required Response 2, (Reference 2).

The purpose of this letter is to provide the response to the NRC request for additional information (RAI) (Reference 3) regarding the Dresden Nuclear Power Station, Units 2 and 3 Flooding Hazard Reevaluation Report. Enclosure 1 provides the response to each NRC RAI. Enclosures 2 and 3 provide the model input and output files as requested in NRC RAI Nos. 3 and 9.

Additionally, the Dresden Nuclear Power Station, Units 2 and 3 Local Intense Precipitation (LIP)

Evaluation initially provided in Reference 1 has been revised to incorporate new site-specific LIP information. Enclosure 4 provides the updated LIP evaluation.

U.S. Nuclear Regulatory Commission Response to Request for Additional Information (Flooding Hazard Reevaluation Report)

May 19, 2014 Page 2 This letter contains no new regulatory commitments. If you have any questions regarding this report, please contact Ron Gaston at (630) 657-3359.

I declare under penalty of perjury that the foregoing is true and correct. Executed on the 191h day of May 2014.

Respectfully submitted, Director - Licensing & Regulatory Affairs Exelon Generation Company, LLC

Enclosures:

1. Dresden Nuclear Power Station, Units 2 and 3 - Response to Request for Additional Information Regarding Fukushima Lessons Learned - Flood Hazard Reevaluation Report

2. CD-R labeled: Dresden Nuclear Power Station, Units 2 and 3, Enclosure 2 of RS-14-122, FL0-2D Model Input and Output Files Document Components:

LIP-DRE-001 Rev 2_FL0-2D Model.zip Summary of Record Transmittal for LIP-DRE-001 Rev. 2 FL0-2D Model-DRE_HMO_No-AquaDam 5-6-14.pdf Summary of Record Transmittal for LIP-DRE-001 Rev. 2 FL0-2D Model-DRE_MAX_No-AquaDam 5-6-14.pdf Summary of Record Transmittal for LIP-DRE-001 Rev. 2 FL0-2D Model-DRE_Stillwater_No-AquaDam 5-6-14.pdf Summary of Record Transmittal for LIP-DRE-001 Rev. 2 FL0-2D Model 5-6-14.pdf WGW-DRE-003 Rev O_FL0-2D Model_Partial Dam Break.zip

3. CD-R labeled: Dresden Nuclear Power Station, Units 2 and 3, Enclosure 3, DVD #1 and DVD #2, of RS-14-122, HMR-52, HEC-HMS, & HEC-RAS Model Input & Output Files Document Components DVD #1:

Flood in Streams and Rivers.zip (HEC-HMS, HEC-RAS, HMR-52)

Document Components DVD #2:

Fastest Arrival Time.zip (HEC-HMS, HEC-RAS, HMR-52)

Highest WSE.zip (HEC-HMS, HEC-RAS)

Seismin No Rainfall.zip (HEC-HMS, HEC-RAS)

4. Dresden Nuclear Power Station, Units 2 and 3 - Local Intense Precipitation Evaluation Report, Revision 4

U.S. Nuclear Regulatory Commission Response to Request for Additional Information (Flooding Hazard Reevaluation Report)

May 19, 2014 Page 3 cc: Director, Office of Nuclear Reactor Regulation NRC Regional Administrator - Region Ill NRC Senior Resident Inspector - Dresden Nuclear Power Station, Units 2 and 3 NRC Project Manager, NRR - Dresden Nuclear Power Station, Units 2 and 3 Mr. Rob F. Kuntz, NRR/JLD/JPMB, NRC Mr. Blake Purrnell, NRR/DORULPL3-2 Illinois Emergency Management Agency - Division of Nuclear Safety

Enclosure 1 Dresden Nuclear Power Station, Units 2 and 3-Response to Request for Additional Information Regarding Fukushima Lessons Learned - Flood Hazard Reevaluation Report (43 pages)

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 1 of 43 RAI 1: Site Information

Background:

The FHRR states that the maximum peak stillwater elevation during the probable maximum flood (PMF) is 525 feet (ft) mean sea level (MSL, National Geodetic Vertical Datum of 1929 (NGVD 29)) at the site based on the current licensing basis (CLB). However, the local intense precipitation evaluation report states that the maximum peak stillwater elevation during the PMF is 524.5 ft MSL at the site based on the 1982 Systematic Evaluation Program.

Request: Clarify which stillwater elevation is the CLB flooding height and, if necessary, revise Table 5 of the FHRR.

Response

The NRC Systematic Evaluation Program (SEP) (aka the Franklin report) determined the stillwater elevation to be 524.5 ft MSL. However, UFSAR Section 2.4.3 states that for the PMF "the peak discharge of 490,000 cfs would result in a stillwater elevation at the site of about 525 ft MSL". The UFSAR references the SEP and uses the 525 ft MSL elevation.

RAI 2: Local Intense Precipitation and Associated Drainage

Background:

The most conservative case (Case 3 of NRC NUREG/CR-7046, "Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America," November 2011, ADAMS Accession No. ML11321A195) in which all drainage is non-functioning is modeled with significant site drainage occurring via the cooling water canals. The canals, which are integral to the site drainage appear to be unobstructed, which does not conform to Case 3. In addition, the analysis uses the 1-square-mile probable maximum precipitation (PMP) with no discussion of PMP impacts on areas directly adjacent to the 29-acre site, such as potential offsite runoff and debris flows into the canal.

Request: Describe the potential effect of debris and additional flow from offsite on the cooling water canal boundary conditions during the design event to support the assumptions of the most conservative scenario.

Response

To evaluate the potential effect of debris and additional flow from offsite drainage areas, available LiDAR data was used to delineate the contributing drainage areas, evaluate the off-site topographic relief, and evaluate the available storage areas assuming the cooling water canals are blocked. The power block, where safety-related structures are located, is bounded by the cooling water canals to the north and to the west. The cooling water canals to the west flow in opposite directions: the outer canal flows in the northerly direction from the Dresden Cooling Lake to the Illinois River; and the inner canal flows in the southerly direction to the Dresden Cooling Lake. The canals are separated by a dividing berm with an elevation of 513 ft MSL (Reference 1). As shown in Figure 2.1, runoff from the off-site drainage area (Drainage Area 2) would drain towards the outer canal. Runoff from the majority of the power block (Drainage Area 1) where safety-related structures are located would drain towards the inner canal, while the remaining portions of the power block (Drainage Area 3) would drain directly to the Kankakee River.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 2 of 43

__. Canal Flow Direction

=-==-=- Dividing Berm Drainage Area 1

. . Drainage Area 2 Drainage Area 3 5 ft Topo Contour (NAVD88) 0 0.25 0.5 Miles Figure 2.1: Site Drainage Areas

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 3 of 43 The potential effect of debris blockage was evaluated by developing stage-storage curves for each drainage area. The water surface elevations in the cooling water canals were based on LiDAR data; however, they were checked against available drawings. While the water levels change depending on how many pumps are in use, the normal water surface elevations for the outer and inner canals at the intersection with the N Dresden Road are 508 ft MSL and 501.65 ft MSL, respectively (Reference 1). The existing topography and the cooling water canals provide significant storage for runoff during the LIP event in case the cooling water canals are blocked per Case 3 of NUREG/CR-7046. The stage-storage relationships for the power block (Drainage Area 1) and for the off-site drainage area (Drainage Area 2) are provided in Tables 2.1 and 2.2.

The stage-storage calculations indicate that even if all rainfall were instantaneously transformed to runoff with no losses or routing considered and the cooling water canals were blocked with debris, the topography of the site and the cooling water canals provide sufficient storage to maintain water levels below plant grade. Furthermore, the potential maximum water levels due to the accumulated runoff from the off-site drainage area (Drainage Area 2) would be lower than the elevation of the dividing berm. As a result, runoff from the off~site drainage area (Drainage Area 2) would not contribute to water levels in the inner cooling water canal and the storage capacity of the inner cooling water canal would not be impacted. Therefore, runoff from the power block (Drainage Area 1) would flow freely towards the inner cooling water canal. This indicates that the boundary conditions of the FL0-2D model would not be affected by debris blockage of the cooling water canals or runoff from contributing drainage areas and the LIP model accurately reflects site conditions during the LIP event.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 4 of 43 Ta bl e 2..

1 Stage-Storage CI a cu Iat1on

. f or Dramage

. A rea 1 Description : Drainage Area 1 Drainage Area (acres) : 118.5 1-hr/1-sq-mi PMP Depth (inches): 13.9 Total Rainfall Volume (acre-ft) : 137.3 Stage at Total Rainfall Volume (ft MSL) : 510.3 Available Storage up to Plant Grade 517.0 ft MSL (acre-ft): 399.2 Stage (ft NAVD88) Stage (ft MSL) Area (sq ft) Area (acres) Storage (cu ft) Storage (acre-ft) 503.5 503.8 1,518 0.0 0 0.0 504.0 504.3 213,893 4.9 38,173 0.9 504.5 504.8 550,616 12.6 243,527 5.6 505.0 505.3 690,043 15.8 550,185 12.6 505.5 505.8 794,944 18.2 918,312 21.1 506.0 506.3 859,668 19.7 1,329,589 30.5 506.5 506.8 947,012 21 .7 1,778,465 40.8 507.0 507.3 998,943 22.9 2,265,638 52.0 507.5 507.8 1, 154,609 26.5 2,800,716 64.3 508.0 508.3 1, 196,283 27.5 3,388,408 77.8 508.5 508.8 1,226,779 28.2 3,993,313 91 .7 509.0 509.3 1,256,140 28.8 4,613,125 105.9 509.5 509.8 1,289,670 29.6 5,248,555 120.5 510.0 510.3 1,321 ,666 30.3 5,900,629 135.5 510.5 510.8 1,350,684 31 .0 6,567,577 150.8 511.0 511 .3 1,380,490 31.7 7,249,770 166.4 511.5 511 .8 1,411 ,968 32.4 7,948,047 182.5 512.0 512.3 1,447,822 33.2 8,663,224 198.9 512.5 512.8 1,492,232 34.3 9,398,647 215.8 513.0 513.3 1,545,877 35.5 10,159,169 233.2 513.5 513.8 1,617,079 37.1 10,950,562 251 .4 514.0 514.3 1,714,288 39.4 11,783,607 270.5 514.5 514.8 1,802,009 41.4 12,663,982 290.7 515.0 515.3 1,913,805 43.9 13,592, 136 312.0 515.5 515.8 2,035,233 46.7 14,581,572 334.7 516.0 516.3 2,190,637 50.3 15,634,070 358.9 516.5 516.8 2,607,103 59.9 16,813,645 386.0 516.7 517.0 2,846,146 65.3 17,387,682 399.2 517.2 517.5 3,217,031 73.9 18,915,321 434.2

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 5 of 43 Tabl e 2..2 S taQe-s toraQe CI a cu Iat1on

. f or Dramage

. A rea 2 Description : Drainage Area 2 Drainage Area (acres) : 174.3 1-hr/1-sq-mi PMP Depth (inches): 13.9 Total Rainfall Volume (acre-ft) : 201.9 Stage at Total Rainfall Volume (ft MSL) : 512.9 Available Storage up to Plant Grade 517.0 ft MSL (acre-ft) : 477.0 Stage (ft NAVD88) Stage (ft MSL) Area (sq ft) Area (acres) Storage (cu ft) Storage (acre-ft) 503.5 503.8 0 0.0 0 0.0 504.0 504.3 0 0.0 0 0.0 504.5 504.8 317,974 7.3 63, 121 1.4 505.0 505.3 443,227 10.2 250,076 5.7 505.5 505.8 489,390 11.2 478,962 11.0 506.0 506.3 533,792 12.3 729,547 16.7 506.5 506.8 616,134 14.1 1,017,738 23.4 507.0 507.3 657,147 15.1 1,337,394 30.7 507.5 507.8 688,361 15.8 1,671,256 38.4 508.0 508.3 853,954 19.6 2,046,339 47.0 508.5 508.8 903,085 20.7 2,484,801 57.0 509.0 509.3 984,771 22.6 2,954,861 67.8 509.5 509.8 1,109,303 25.5 3,485,421 80.0 510.0 510.3 1,204,548 27.7 4,061,843 93.2 510.5 510.8 1,525,589 35.0 4,720,288 108.4 511.0 511.3 1,820,511 41.8 5,570,162 127.9 511.5 511.8 1,905,591 43.7 6,500,810 149.2 512.0 512.3 1,992,490 45.7 7,473,039 171.6 512.5 512.8 2,094,899 48.1 8,494,813 195.0 513.0 513.3 2,228,032 51.1 9,575,564 219.8 513.5 513.8 2,387,481 54.8 10,730,252 246.3 514.0 514.3 2,565,124 58.9 11,970,124 274.8 514.5 514.8 2,769.831 63.6 13,306,308 305.5 515.0 515.3 2,976,793 68.3 14,743,823 338.5 515.5 515.8 3,246,409 74.5 16,294,377 374.1 516.0 516.3 3,651,210 83.8 18,019,418 413.7 516.5 516.8 3,978,606 91.3 19,927,605 457.5 516.7 517.0 4,138,234 95.0 20,779,720 477.0 517.2 517.5 4,478,389 102.8 22,936,859 526.6 References

1. Exelon drawing S-17 (1998), Circulating Water Flume Plan & Profile, Sheet 1.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 6 of 43 RAI 3: Local Intense Precipitation and Associated Drainage

Background:

The FHRR does not describe the calibration and sensitivity analysis for the FL0-20 two-dimensional transient flow computer model.

Request: Describe the effect of the uncertainty in the FL0-2D model regarding sensitivity of water surface elevations to Manning's roughness coefficients (n-values). Provide the input and output files for the FL0-2D model for the local intense precipitation that results in the highest water surface elevations discussed in the FHRR.

Response

Sensitivity of water surface elevations to Manning's roughness coefficients (n-values) is addressed in Section 4 of the May 1O, 2013, Flood Hazard Reevaluation Report submittal, (Dresden Nuclear Power Station, Units 2 and 3, Local Intense Precipitation Evaluation Report, Revision 3), which states:

Results provided in this report are direct outputs from the FL0-20 model. The FL0-20 model reports results to the hundredth of a foot. However, based on the sensitivity analysis of grid size and Manning's n values, an accuracy of+/- 0. 1 foot should be taken into consideration when evaluating the reported results.

More specifically, the difference in water surface elevations between the upper and lower range of Manning's n-values (below), taken from page 22 of the FL0-2D Reference Manual, is

+/- 0.03 feet (Section 7.0 of the Dresden LIP Calculation Package, LIP-DRE-001 ).

• Bermuda and dense grass, dense vegetation: 0.17-0.48

• Shrubs and forest litter, pasture: 0.30-0.40

• Asphalt, concrete, and buildings: 0.02-0.05 The input and output files for the FL0-20 model for the local intense precipitation are provided in Enclosure 2.

RAI 4: Stream and River Flooding

Background:

The Alternative 2 PMF flow calculated in the FHRR is approximately 20 percent less than the PMFpeak flow calculated by the hydrologic model (HEC-1) in the CLB. More significantly, the Alternative 1 PMF flow of 318,000 cubic feet per second (cfs) for the all-season PMP is approximately 35 percent less than CLB all-season PMP flow. In Table 5 of the FHRR, the licensee listed comparisons between two hydrologic computer models, the FHRR HEC-HMS (Hydrologic Modeling System) and CLB HEC-1 Flood Hydrograph Package, that show significantly increased levels of imperviousness and slight reductions in drainage area.

Request: Describe in detail the decrease in peak flow in light of increasing impervious area and slight reduction in basin area including relevant HEC-1 parameters and HEC-1 output from the CLB.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 7 of 43

Response

The modeling performed for the current licensing basis (CLB) is documented in the Franklin Research Center, Technical Evaluation Report, Hydrological Considerations, Commonwealth Edison Company, Dresden Unit 2. Full detailed modeling inputs and outputs associated with the CLB are not available. The CLB indicates the watershed was subdivided into 13 sub-basins. However, the exact configuration and distribution of sub-basins throughout the watershed is unknown.

The CLB only evaluated an all-season PMP event, analogous to the Alternative 1 combination defined by NUREG/CR-7046. The CLB peak flows for the Des Plaines River and Kankakee River are provided to be 145,000 cfs and 375,000 cfs, respectively. The flow hydrographs are combined in a steady-state hydraulic model just upstream of the site, with limited coverage of approximately 1.4 miles. The peaks are not aligned in time and combine for a total peak flow of 490,000 cfs at the site.

The flood hazard reevaluation report (FHRR) supporting modeling subdivides the watershed into 23 subbasins. The hydrologic model results are obtained at several locations and used as input into a more extensive unsteady-state hydraulic model that extends over 47 miles.

For direct comparison, the Alternative 1 peak flow for the Des Plaines is subdivided into four hydrographs. The summation of the peak flows total 210,000 cfs. However, the peaks are not aligned in time. Peak flow for the Kankakee is 198,800 cfs and is also not aligned with any of the Des Plaines peaks. Routed through the more sophisticated unsteady-state modeling, the peak flow at the site is 318,000 cfs.

The Alternative 2 peak flow for the Des Plaines is also subdivided into four hydrographs. The summation of the peak flows total 192,200 cfs. However, the peaks are not aligned in time.

Peak flow for the Kankakee is 256,600 cfs and is also not aligned with any of the Des Plaines peaks. Routed through the more sophisticated unsteady-state modeling, the peak flow at the site is 380,000 cfs.

It appears that the increase in percent impervious area may have impacted the Des Plaines watershed response. The CLB peak flow is based on 18 percent impervious, while the FHRR peak flow is based on a range of 22 to 63 percent impervious for a relatively similar watershed area. Collectively, the summation of peak flows from the FHRR exceeds the CLB peak flow for the Des Plaines watershed. However, the slight increase from less than 1 percent CLB percent impervious to a range of 2 to 6 percent impervious does not appear to have had an effect on the watershed response for the Kankakee. Hydrologic modeling details of the CLB, including sub-basin arrangement, storm positioning, individual routing parameters, losses, etc., are not known.

It is estimated that the configuration of the FHRR hydrologic model better accounts for timing between individual tributaries based on the number of sub-basin areas incorporated.

In summary, although watershed conditions have changed, the FHRR supporting models are calibrated to a more refined scale in the hydrologic model (e.g., 23 sub-basins vs. 13 sub-basins) and utilize more sophisticated modeling for the hydraulics (e.g., over 47 miles of unsteady-state vs. 1.4 miles of steady-state). As a result the timing of individual contributing areas is not aligned and peak flows are different than the CLB. For reference, the peak flows of the individual contributing flow hydrographs for the HEC-RAS model are provided in Table 4.1.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 8 of 43 Table 4.1 - River Reach Peak Flows River Reach/Location Alternative 1 Alternative 2 Peak Flow (cfs} Peak Flow (cfs}

Kankakee River 198,000 256,600 Des Plaines River at Lock ort 90,500

~~~--~~-'-~~-+~~--'~~----i 112,700 Du Pa e River 33,500

~~~--~~-'-~~-+~~~~~----i 31,600 Hicko Creek 35,600

~~~--+-~~-'-~~---~~~~~---t 18, 700 Local Site Areas and Des Plaines Downstream of Lockport 50,400

~~~--~~-'-~~-+~~~~~----i 29,200 Routed and Combined PMF Result at Site 318,000 380,000 Note peak flow results from Calculation DRE12-0018 Attachment 2 RAI 5: Stream and River Flooding

Background:

For the calibration of HEC-RAS (River Analysis System) computer model, there are possibly multiple sets of input values for pool inflow and gate operation that could achieve the same water surface elevation at the dam pool Request: Describe the effect of pool inflow and gate operation uncertainty on the HEC-RAS model results.

Response

The locks and dams and gate operations were modeled as defined by their respective U.S.

Army Corps of Engineers Master Water Control Manuals. To assess the uncertainty of pool inflow and gate operation, the record HEC-RAS model (Calculation DRE12-0018) is modified to account for alternative gate operations. The sensitivity Water Surface Elevation (WSE) results are provided for the site Area of Interest (AOI) corresponding to the WSE for the cross sections at the site AOI.

Sensitivity is performed for two alternative pool inflow/gate operations at the Dresden lock and Dam, located immediately downstream of the site. All other parameters are unchanged from the original model. The two alternatives are:

A) All gates closed alternative. This is a highly conservative alternative that assumes all the inline structure (dam) gates are closed completely and do not operate. This is expected to provide the maximum WSE at the site.

B) 30% of gates closed alternative. This alternative assumes blockage or failure of a portion of the inline structure (dam) gates by reducing the original total horizontal gates width by 30%. This is a conservative alternative that is expected to provide WSE results less than alternative A above.

Two new plans corresponding to the sensitivity alternatives are added to the existing model plans, and run for the most critical case scenario, which is the cool-season probable maximum flood (PMF) with snowmelt. The sensitivity is assessed as a measure of change to the resulting total flows and WSE. Original results and the results from the two sensitivity alternatives are summarized in Tables 5.1, 5.2, and 5.3, respectively. The plan descriptions and names are:

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 9 of 43

• For the original model (provided to client) : Snowmelt

• For Alternative A: Snowmelt_allgatesclosed

• For Alternative B: Snowmelt_30%gatesclosed Table 5 .1 - Flow and WSE from the Original Model Results at the Site AOI (Plan: Snow melt)

Model Q Model WSEL (ft.)

River Station Profile Plan Total (cfs) NGVD29 135306 MaxWS Snowmelt 372794 524.88 135194 MaxWS Snowmelt 372793 524.85 135082 MaxWS Snowmelt 378789 524.80 134970 MaxWS Snowmelt 378694 524.76 134858 MaxWS Snowmelt 378789 524.74 134746 MaxWS Snowmelt 378789 524.71 134634 MaxWS Snowmelt 378693 524.69 134522 MaxWS Snowmelt 378693 524.63 134410 MaxWS Snowmelt 378693 524.62 Table 5.2 - Flow and WSE from Alternative A {All Gates Closed) Model Results at the Site AOI (P Ian: Snowmelt_allaatesclosed)

River Model Q Model WSEL (ft.)

Profile Plan Station Total (cfs) NGVD29 135306 MaxWS Snowmelt allgatesclosed 373186 524.91 135194 MaxWS Snowmelt_allaatesclosed 373246 524.88 135082 MaxWS Snowmelt_allqatesclosed 379224 524.83 134970 MaxWS Snowmelt allgatesclosed 379223 524.80 134858 MaxWS Snowmelt allgatesclosed 379223 524.78 134746 MaxWS Snowmelt allaatesclosed 379110 524.74 134634 MaxWS

• Snowmelt allaatesclosed 379223 524.72 134522 MaxWS Snowmelt allgatesclosed 379222 524.67 134410 MaxWS Snowmelt allaatesclosed 379222 524.65 Table 5.3- Flow and WSE from Alternative B (30%Gates Closed) Model Results at the Site AOI (P Ian: Snowmelt_30%aatesclosed)

River Model Q Model WSEL (ft.)

Profile Plan Station Total (cfs} NGVD29 135306 MaxWS Snowmelt 30%gatesclosed 373001 524.89 135194 MaxWS Snowmelt 30%gatesclosed 373001 524.86 135082 MaxWS Snowmelt 30%aatesclosed 378979 524.81 134970 MaxWS Snowmelt_30%Qatesclosed 378855 524.77 134858 MaxWS Snowmelt 30%gatesclosed 378979 524.75 134746 MaxWS Snowmelt 30%gatesclosed 378855 524.72 134634 MaxWS Snowmelt_30%aatesclosed 378855 524.70 134522 MaxWS Snowmelt 30%gatesclosed 378745 524.64 134410 MaxWS Snowmelt_30%aatesclosed 378855 524.63

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 10 of 43 Results for alternative A (all gates closed alternative) indicate a slight increase in the total flows (373185.5 cfs) compared to the original model total flow (372793.6 cfs). This resulted in a slight increase in WSE (524.91 ft) compared to the original model WSE (524.88 ft). This results in total flow difference of 391.9 cfs, and a WSE difference of 0.03 ft.

Results for alternative B (30% gates closed alternative) indicate a slight increase in the total flows (373001.4 cfs) compared to the original model total flow (372793.6 cfs). This resulted in a slight increase in WSE (524.89 ft) compared to the original model WSE (524.88 ft). This results in total flow difference of 207.8 cfs, and a WSE difference of 0.01 ft.

It can be concluded that resulting WSE and total flow at the site AOI are not sensitive to the gate operations at the dam for the critical case scenario.

RAI 6: Failure of Dams and Water Control Structures

Background:

On the basis of results from the HEC-HMS model, the scenarios of the critical storm centers CMSC17, DPSC5, and DPSC2 with front temporal distribution (no antecedent storm) have faster arrival times down to 9.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> to reach the 509 ft NGVD elevation. These scenarios could logically represent the possible shortest flood arrival time (fastest flood response time per FHRR) in the coincident dam failures scenarios.

Request: Provide an evaluation of those scenarios that show the shortest arrival time in the following analyses (e.g., DRE12-0018): (1) Alternative 2 with critical storm center CMSC09, front temporal distribution and no antecedent storm, and (2) Alternative 2 with critical storm center CMSC17 front temporal distribution and no antecedent storm with coincident dam failure.

Describe the process used for determining shortest arrival times and how it impacts the level of conservatism. In addition, provide a table comparing results which includes assumptions about spatial distribution/storm center, antecedent conditions, and temporal distribution for each of the alternatives.

Response

The scenarios resulting in a shortest arrival times to the benchmark elevations of 509 ft NGVD29 and 517 ft NGVD29 are identified in Calculation DRE12-0018 (Table 8.2), Dresden Riverine Hydraulics Analysis HEC-RAS. The scenarios with the fastest arrival times are the flood from the all-season probable maximum precipitation (PMP), with frontal temporal distribution and no antecedent storm (Alternative 1 without the antecedent storm) occurring at storm centers DPSC2, DPSC5, CMSC17 and CMSC09. The arrival times corresponding to the storm centers are 9.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, 9.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, 9.58 hours6.712963e-4 days <br />0.0161 hours <br />9.589947e-5 weeks <br />2.2069e-5 months <br /> and 9.83 hours9.606481e-4 days <br />0.0231 hours <br />1.372354e-4 weeks <br />3.15815e-5 months <br />, respectively.

Note, that the arrival times in Table 8.2 (Calculation DRE12-0018) are determined preliminarily from HEC-HMS PMF models based on the flows corresponding to the benchmark elevations (from the current licensing basis). This is performed in order to identify potentially critical storm centers and scenarios. The arrival times from HEC-HMS models do not necessarily reflect actual times to the benchmark elevations. The actual arrival times are determined for the identified potentially critical scenarios in HEC-RAS models and the critical timing results are summarized in Table 8.4 (Calculation DRE12-0018).

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 11 of 43 The PMP occurring at storm center CMSC17 arrives at the site essentially at the same time as the PMPs occurring at storm centers DPSC2 and DPSC5 arrive at the site. However, the CMSC17 PMP has significantly higher precipitation volumes. Storm centers DPSC2 and DPSCS are concentrated on the Des Plaines watershed, while the CMSC17 storm center includes both the Kankakee and Des Plaines watersheds. Although the results of storm centers DPSC2 and DPSC5 reach the 509 ft benchmark elevation, the results don't reach the 517 ft benchmark elevation. Therefore, the PMPs with storm centers at DPSC2 and DPSC5 are removed from further analysis. The PMP occurring at storm center CMSC09 has a similar arrival time but higher precipitation volumes compared to the PMP occurring at storm center CMSC17. Therefore, PMPs from both storm centers are examined further.

The additional scenario descriptions examined are provided below:

Scenario 1:

All-season PMP with a storm center at CMSC09 Frontal temporal distribution No antecedent storm Coincident hydrologic dam failure Scenario 2:

All-season PMP with a storm center at CMSC17 Frontal temporal distribution No antecedent storm Coincident hydrologic dam failure The watershed of the Dresden NGS consists of two watersheds, the Kankakee River watershed and the Des Plaines River watershed. All the upstream dams identified as potentially critical are located in the Des Plaines River watershed. Flow volumes are larger from the Kankakee River watershed due to the watershed size and the location of the critical storm center.

The hydrologic dam failure is applied using the domino-like failure scenario. The most critical failure times are identified using an iterative approach to ensure the flows resulting from upstream dam failures arrive at the site 1) coincident with each other and 2) at the same time as the peak flows from the Kankakee River.

This is the most conservative approach to estimate the shortest arrival time at the site. The dam failure times are adjusted to occur at earlier times in order for peak failure flows to coincide with the rainfall runoff peak flows. This approach maximizes the flows at the site and results in the water level reaching the benchmark elevations faster.

The results of this evaluation are presented in Table 6.1 and compared to the results from calculations DRE12-0018 (Table 8.4) and DRE12-0015 (Table 8.0.1 ).

The header for Scenario A in Table 6.1 is revised from CMSC09, as shown in calculation DRE12-0018 (Table 8.4), to DPSC05 due to a typo.

REDACTED WITHHELD FROM PUBLIC DISLOSURE PER 10 CFR 2.390

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 13 of 43 RAI 7: Failure of Dams and Water Control Structures

Background:

The FHRR does not discuss calibration or sensitivity analyses with respect to the two dimensional hydraulic computer models RiverFlo-2D and FL0-2D. Parameters are different in those models from those used in the calibrated HEC-RAS model, most notably a reduction in Manning's n-values.

Request: Describe the sensitivity of water surface elevations to Manning's n-values in the RiverFlo-2D and FL0-2D models.

Response

The development of Manning's n-values used in the FHR RiverFL0-2D model was based on the understanding that typical Manning's n-values are intended for one-dimensional (1-d) flow analysis techniques (e.g. Manning's equation, HEC-RAS, etc.) and, therefore, inherently account for two-dimensional (2-d} flow and hydraulic effects associated with energy losses.

When used in 2-d models, Manning's n-values need to be reduced to account for the momentum exchange within a cross section. According to the RiverFL0-2D model documentation, this reduction can be as high as 30% compared to 1-d models. The methodology selected for development of the initial Manning's n-values for the FHR was first presented by Cowan (Reference 1) and takes into consideration the following parameters individually: material, channel irregularity, variation of channel cross section, relative effects of obstructions, vegetation, and the degree of meandering.

Equation 1 Where n0 =Value of the material involved; n, = Degree of irregularity; n2 = Variations of channel cross sections; n3 = Relative effect of obstructions; n4 =Vegetation; and m =Degree of meandering.

The USGS Water Supply Paper 2339 (Reference 2) expanded upon the above-mentioned work and included a procedure for estimating the overbank roughness. This approach is similar to the methodology for estimating the roughness in the channel; however the adjustment values were adapted to reflect conditions in the overbanks. In the overbanks, the meandering factor (ms) and variation in the shape and size of the floodplain cross section (n 2) are not considered.

The USGS study is geared towards the estimation of 1-d roughness parameters; however, it can be adapted for 2-d models.

The RiverFL0-2D model calculates velocity in both the X and Y directions at each node of the surface mesh representing the topography using the "Weighted Residual Shallow Water Equations." As such, the model inherently accounts for channel irregularity, surface irregularity, and channel alignments, which would otherwise be accounted for by the irregularity parameter (n,). Therefore, this parameter can be set equal to O in the calculation of a Manning's n-value with only the material (surface roughness), obstruction and vegetation parameters remaining.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 14 of 43 Obstruction and vegetation parameters, n3and n4 , respectively, were estimated according to the USGS study, as shown in Table 7.1.

Table 7 .1: Adjustment for factors that affect roughness of floodplains (USGS 1989) n value Flood-plain conditions Example adjustment Smooth 0.000 Compares to the smoothest, flanest flood plain attainable in a given bed material.

Minor 0.001-0.005 Is a flood plain slightly irregular in shape. A few rises and dips or sloughs Degree of may be visible on the flood plain.

irregularity (n 1 ) Moderate 0.006-0.010 Has more rises and dips. S.loughs and hummocks may occur.

Severe 0.011-0.020 Rood plain very irregular in shape. Many rises and dips or sloughs are visible.

Irregular ground surfaces in pastureland and furrows perpendicular to the flow are also included.

Vatiation of flood-plain 0.0 Not applicable.

cross section (n2)

Negligible 0.000-0.004 Few scattered obstructions. which include debris deposits, stumps, exposed Effect of roots, logs, or isolated boulders, occupy less than 5 percent of the cross-obstructions sectional area.

(113) Minor 0.005-0.019 Obstructions occupy less than 15 percent of the cross-sectional area.

Appreciable 0.020--0.030 Obstructions occupy from I 5 to SO percent of the cross-sectional area.

Small 0.001--0.010 Dense growth of flexible turf grass, such as Bennuda, or weeds growing where the average depth of flow is at least two times the height of the vegetation, or supple tree seedlings such as willow, cottonwood, arrowweed, or saltcedar growing where the average depth of flow is at least three times the height of the vegetation.

Medium 0.011-0.025 Turf grac;s growing where the average depth of flow is from one to two times the height of the vegetation. or moderately dense stemmy grass. weeds, or iree seedlings growing where the average depth of flow is from two to three rimes the height of the vegetation; brushy, moderately dense vegetation, similar lo 1- to 2-year-old willow trees in the dormanl season.

Large 0.025-0.050 Turf grass growing where the average depth of flow is about equal to the height Amount of of the vegetation, or 8- to I 0-year-old willow or cottonwood trees intergrown vegetation (11 4) with some weeds and brush (none of the vegetation in foliage) where the hydraulic radius exceeds 2 fl, or mature row crops such as small vegetables, or mature field crops where depth of flow is at least twice the height of the vegetation.

Very large 0.050--0. 100 Turf grass growing where the average depth of flow is less than half the height of the vegetation, or moderate to dense brush, or heavy stand of timber with few down trees and little undergrowth where depth of flow is below branches, or mature field crops where depth of flow is less than the height of the vegetation.

Extreme 0.100-0.200 Dense bushy willow, mesquite, and saltcedar (all vegetation in full foliage), or heavy stand of timber, few down 1rees. depth of flow reaching branches.

Degree of 1.0 Not applicable.

meander (m)

The Manning's n-values were calculated by evaluating the obstruction (n3) and vegetation (n 4 }

parameters separately. For example, the pasture/hay cover type would have negligible obstructions, since the land would be cleared in the preparation of the crop. This cover type would have a medium adjustment due to the amount of vegetation and the average depth of flow over the floodplain. In addition, to ensure that the obstruction parameter is applied in a consistent manner, the obstruction value (n 3) was compared to photographs provided in the USGS study. These photographs provided documentation of the estimated floodplain

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 15 of 43 roughness parameters for field conditions, and allowed for comparison with field conditions to verify computed n-values. The Manning's n-values derived for the 2-d model are generally (exceptions being shrubs and pasture) 0 to 15% lower than the typical n-values used for 1-d analyses and well within the 30% suggested in the RiverFL0-20 manual. As such, the n-values used in the 2-d analysis should be considered conservative. To evaluate the sensitivity of water surface elevations to Manning's n-values, the FHR RiverFL0-20 model was re-run with Manning's n-values increased by 10% and 15%. The calculated Manning's n-values used in the FHR and in the sensitivity analysis are shown below in Table 7.2.

Table 7.2: Manning's n-values Used in the RiverFL0-2D Model FHA n-value Difference n-values n-values NLCD Normal NLCD Code Description in RiverFLO- from increased increased Code n-value

• 2D Model "Normal" by10% by15%

21 Developed, Open Space 0.035 0.030 -14% 0.033 0.035 22 Developed, Low Intensity 0.035 0.030 -14% 0.033 0:035 23 Developed, Medium Intensity 0.035 0.030 -14% 0.033 0.035 24 Developed, High Intensity 0.035 0.035 0% 0.039 0.040 31 Barren Land (Rock, Sand, Clay) 0.030 0.030 0% 0.033 0.035 41 Deciduous Forrest 0.100 0.085 -15% 0.094 0.098 42 Evergreen Forrest 0.100 0.085 -15% 0.094 0.098 43 Mixed Forrest 0.100 0.085 -15% 0.094 0.098 52 Shrub/Scrub 0.100 0.075 -25% 0.083 0.086 71 Grassland/Herbaceous 0.035 0.030 -14% 0.033 0.035 81 Pasture/Hay 0.040 0.030 -25% 0.033 0.035 82 Cultivated Crops 0.040 0.040 0% 0.044 0.046 90 Woody Wetlands 0.070 0.075 7% 0.083 0.086 95 Emergent Herbaceous Wetlands 0.100 0.090 -10% 0.099 0.104

• Normal value as presented in Chow (Reference 3)

The results of the sensitivity analysis indicate that the difference in water surface elevations is minimal and ranges from 0.18 ft to 0.24 ft for the 10% increase in n-values, and from 0.25 ft to 0.32 ft for the 15% increase in n-values. The percent increase in flooding depth ranges from 2.5% to 6.2%. Locations of selected points of interest used in the sensitivity analysis are shown in Figure 7.1. Detailed results are provided in Table 7.3.

References

1. Cowan, W.L. (1956). Estimating hydraulic roughness coefficients: Agricultural Engineering, 377, pp. 473-475.

2. Arcement, G.J., Jr. and V.R. Schneider (1989). Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains. US Geological Survey Water Supply Paper 2339, 38 p. *

3. V.T. Chow {1959). Open Channel Hydraulics, McGraw Hill.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 16 of 43 Legend

• Points of Interest c:::::J RiverFL02D Mesh Elements 0 100 200 Feet Figure 7.1: Location of Selected RiverFL0-20 Points of Interest

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Enclosure 1 Page 17 of 43 Table 7.3: Sensitivity Analysis at Selected RiverFL0-2D Points of Interest RiverFLO- FHR Manning's n-values 10% Increase in Manning's n-values 15% Increase in Manning's n-values 2D Building Node Node Node Node Node Node Node Node Node Point of Interest Reference Increase Percent Increase Percent Entrance Max Max Max Max Max Max Max Max Max Location Node in WSEL Increase in WSEL Increase ID Velocity Depth WSEL Velocity Depth WSEL Velocity Depth WSEL Element ID (ft/s) (ft) (ft MSL) (ft/s) (ft) (ft MSL) (ft) (%) (ft/s) (ft) (ft MSL) (ft) (%)

East Unit 3 Turbine Bldg Door 136 35 2.57 7.03 524.00 2.58 7.21 524.18 0.18 2.60% 1.74 7.28 524.25 0.25 3.53%

East Unit 3 Turbine Bldg Door 135 88 4.39 7.49 523.80 4.10 7.68 523.99 0.19 2.52"!0 3.77 7.75 524.06 0.26 3.45%

East Unit 3 Turbine Bldg Door 135 687 2.59 7.56 524.03 2.56 7.76 524.22 0.19 2.54% 2.43 7.82 524.29 0.26 3.42%

East Unit 3 Turbine Bldg Door 136 896 1.40 7.09 523.91 1.26 7.28 524.09 0.18 2.59% 0.85 7.34 524.16 0.25 3.55%

North East Unit 3 Turbine Bldg Bay 137 681 2.13 7.09 523.91 2.11 7.27 524.09 0.18 2.58% 1.24 7.34 524.16 0.25 3.60%

North Unit 2 Turbine Bldg Door 140 2631 2.09 7.11 523.94 2.19 7.29 524.12 0.19 2.60% 2.22 7.36 524.18 0.25 3.49%

South East Unit 3 Turbine Bldg Door 133 2155 0.87 7.15 524.22 0.88 7.36 524.43 0.21 2.89% 0.89 7.43 524.50 0.28 3.90%

South East Unit 3 Turbine Bldg Bay 132 2156 0.98 7.26 524.15 1.01 7.46 524.36 0.21 2.84% 1.02 7.53 524.43 0.28 3.83%

South East Unit 3 Turbine Bldg Door 133 2615 1.24 7.96 524.24 1.26 8.17 524.45 0.21 2.61% 1.26 8.24 524.52 0.28 3.52%

South East Unit 3 Turbine Bldg Door 134 2617 2.04 7.01 524.20 2.11 7.22 524.40 0.20 2.91% 2.03 7.29 524.47 0.27 3.91%

South Side Administration Bldg Door 141 3121 0.34 8.28 524.84 0.28 8.51 525.07 0.23 2.78% 0.26 8.59 525 .15 0.31 3.72"/o South Side Administration Bldg Door 141 3123 0.56 8.15 524.87 0.44 8.38 525.10 0.23 2.80% 0.31 8.46 525.18 0.30 3.74%

South Side Unit 2 Reactor Bldg Door 126 292 0.47 7.40 524.67 0.48 7.63 524.90 0.23 3.04% 0.47 7.70 524.98 0.30 4.08%

South Side Unit 2 Reactor Bldg Door 128 294 0.61 7.32 524.74 0.61 7.54 524.96 0.23 3.10% 0.55 7.62 525.04 0.30 4.14%

South Side Unit 2 Reactor Bldg Bay 127 296 1.19 7.51 524.67 1.20 7.73 524.89 0.23 3.00% 1.16 7.81 524.97 0.30 4.01%

South Side Unit 2 Reactor Bldg Door 125 297 0.53 7.53 524.70 0.54 7.76 524.92 0.23 3.00% 0.51 7.83 525.00 0.30 4.01%

South Side Unit 3 Reactor Bldg Door 131 1403 2.27 7.70 524.16 2.27 7.91 524.37 0.21 2.74% 2.25 7.99 524.44 0.29 3.73%

South Side Unit 3 Reactor Bldg Door 130 17596 4.15 7.66 524.42 4.09 7.87 524.64 0.22 2.81% 4.04 7.95 524.72 0.29 3.81%

South Side Unit 3 Reactor Bldg Bay 129 17598 4.25 7.34 524.09 4.19 7.56 524.31 0.22 2.97% 4.14 7.64 524.39 0.30 4.06%

South Side Unit 3 Reactor Bldg Bay 129 17600 4.45 7.62 524.33 4.40 7.84 524.55 0.22 2.90% 4.35 7.92 524.63 0.30 3.94%

South Side Unit 2 Turbine Bldg Bayl24 304 O.Q7 7.04 524.70 0.02 7.27 524.93 0.23 3.21% 0.01 7.34 525.00 0.30 4.29%

East ISFSI NA 546 2.55 6.48 524.77 2.40 6.73 525.01 0.24 3.73% 2.34 6.81 525.09 0.32 4.98%

West ISFSI NA 27936 5.76 4.94 523.78 5.49 5.16 524.00 0.22 4.52% 5.35 5.24 524.08 0.30 6.16%

West ISFSI NA 27991 5.03 5.16 524.13 4.83 5.38 524.35 0.22 4.24% 4.71 5.46 524.43 0.30 5.78%

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 18 of 43 RAI 8: Failure of Dams and Water Control Structures

Background:

The PMF coincident with dam failure may result in transport of sediments and debris carried by flood water. The FHRR does not discuss the effect of transport of sediment near the Dresden Nuclear Power Station site during the PMF and dam failure events.

Request: Describe the evaluation of transport of sediment near the site, including potential changes in water surf ace elevation and velocity at various structures at the site due to sediment transport.

Response

Erosion evaluation Critical structures - namely, Reactor Building Units 2 and 3; Turbine Building Units 2 and 3; and HPCI and Diesel Generator Building - are located in asphalt/concrete areas. The surrounding areas are characterized by gravel/riprap.

Maximum velocities for the governing PMF scenario (PMF with upstream dam failures and wind-induced waves) in the areas surrounding the critical structures are generally below 6 ft/s, except for a few localized areas further away from the critical structures. No erosion is expected for asphalt/concrete, which can remain stable in flow velocities at least an order of magnitude greater than flow velocities during the governing PMF scenario. In addition, the recommended maximum permissible mean channel velocity for fine gravel per US ACOE Engineering Manual EM 1110-2-1601 is 6 ft/sec. Gravel mobilization could potentially occur in gravel areas further away from the critical structures; however, no entrainment and transport in suspension is expected to occur for this range of flow velocities.

Sediment deposition evaluation The ratio of shear velocity U* (a measure of the shear stress exerted by the flow on the ground, which is responsible for the turbulence that keeps sediment in suspension) to sediment settling velocity Vs (which depends on the sediment characteristics, namely grain size and weight) determines the primary mode of sediment transport. Bed load (i.e., sediment transported at the ground level) is dominant for values of u.f vs less than about 0.4. A transition zone called mixed load is found where 0.4 < u.fvs < 2.5 in which both the bed load and the suspended load contribute to the total load. Where u.f vs> 2.5, most of the sediment load is transported in suspension (Reference 1).

Sediment settling velocity Vs was computed using the following equation developed for natural sediment:

Equation 2 (Reference 2) v 5 =R 1 ~RgD where g is the acceleration of gravity (equal to 9.81 m/s2 ), Dis the sediment grain size, and R is the sediment submerged specific gravity, defined as:

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 19 of 43 Equation 3 (Reference 2)

R = Ps -p...,

P,.,

where Ps and Pw are mass density of sediment and water, respectively. The term R, was computed using the following equation:

Equation 4 (Reference 2) 2 R1 =exp {-b, + b2 ln (Re P)-b3 [In (Re P)] -b~ [ln(Re P)] + b5 [In (Re P)]

3 4

}

where b 1 = 2.891394, b 2 = 0.95296, b3 = 0.056835, b4 = 0.002892, and bs = 0.000245, and:

Equation 5 (Reference 2)

Re = ..jRii5 D p v where v is the kinematic viscosity of water (equal to 1 x 10-6 m2/s at temperature of 20 °C}.

For water depth Hand flow velocity V, the shear velocity U* was computed as follows:

Equation 6 (Reference 2)

~

u.=

vP:

where rb is the shear stress exerted by the flow on the ground, computed as:

Equation 7 (Reference 2)

Tb= PwCJV2 where the friction coefficient c, was computed as follows:

Equation 8 (Reference 3) 1 C - gn~

• 1 - H1/3 where n is Manning's roughness coefficient.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 20 of 43 Settling velocity was computed for a representative 50% finer, or median grain size (050) sediment grain size of 0.05 mm, corresponding to coarse silt. This value was obtained from a sediment grain size distribution curve (Figure 8.1) developed for the measured suspended sediment at Wilmington, IL (located about 5 miles upstream of Dresden) during a high flow event on March 13, 1979 (Reference 4).

100 June 13, 19r~~ ~ I ~

90 April 25. 1979 80

\ 1 rl I I II 1--.

-- ff March 13, 1979 Aprfl 12. 1979

\ l'lli..--

.... 70

c

~ \ ~

~ 60

~  !'--._ ..........

ffi z:

LL.

....u~

0::

w Q..

so 40 30

~

-- '"""' .... I""-..

ItMarch r----

13. 1979 20 10 0

1.0 0.1 0.01 o.oon Sand/fine Split; *-er GRAIN SIZE, 11D SAND SILT OR CLAY Figure 8.1: Particle Size Characteristics of Suspended Sediment at Wilmington, IL (Reference 4)

The submerged weight of sediments (i.e., difference of weight of sediment and water) in the Kankakee River was estimated by the USGS (Reference 5) to be approximately 93 lb1/ft 3 3

(14,609.1 N/m3). This corresponds to submerged sediment density of 1,489.2 kg/m and 3

sediment density Ps of 2489.2 kg/m . Using Equation 3, the submerged specific gravity, R, was calculated to be 1.49. This specific gravity value was used in the analysis and compares well with the typical value 1.65 assumed for natural sediment (Reference 2).

Therefore, using 0 =0.05 mm and R = 1.49 in Equation 2 results in a calculated sediment settling velocity V5 of 0.20 cm/s (0.002 mis).

Computation of shear velocity and an evaluation of the ratio between shear velocity and settling velocity were performed at the time step corresponding to the maximum water surface elevation, which is the most critical in terms of impact of sediment accumulation on water surface elevations. Table 8.1 reports the values of u./v5 tor selected representative locations, computed with Manning's n of 0.035 corresponding to asphalt and concrete.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Enclosure 1 Page 21of43 Table 8.1: Computed values of u./vs Node Velocity Node RiverFLO Dresden at Maximum Maximum c, t'b U* ulv.

Location -2D Node Depth Depth Door ID ID (ft/s) (mis) (ft) (m) (-) (Pa) (m/s) (-)

East Unit 3 Turbine Bldg 35 Door 136 1.61 0.49 7.03 2.14 0.0093 2.24 0.047 23.8 East Unit 3 Turbine Bldg 88 Door 135 3.71 1.13 7.49 2.28 0.0091 11 .67 0.108 54.3 East Unit 3 Turbine Bldg 687 Door 135 2.32 0.71 7.56 2.31 0.0091 4.53 0.067 33.9 East Unit 3 Turbine Bldg 896 Door 136 0.78 0.24 7.09 2.16 0.0093 0.53 0.023 11 .5 North East Unit 3 Turbine Bldg 681 Bayl37 0.98 0.30 7.09 2.16 0.0093 0.83 0.029 14.5 North Unit 2 Turbine Bldg 2631 Door 140 2.05 0.62 7.11 2.17 0.0093 3.63 0.060 30.3 South East Unit 3 Turbine Bldg 2155 Door 133 0.87 0.26 7.15 2.18 0.0093 0.64 0.025 12.8 South East Unit 3 Turbine Bldg 2156 Bayl32 0.98 0.30 7.26 2.21 0.0092 0.82 0.029 14.4 South East Unit 3 Turbine Bldg 2615 Door 133 1.24 0.38 7.96 2.43 0.0089 1.27 0.036 17.9 South East Unit 3 Turbine Bldg 2617 Door 134 2.02 0.62 7.01 2.14 0.0093 3.53 0.059 29.9 South Side Administration Bldg 3121 Door 141 0.21 0.06 8.28 2.52 0.0088 0.04 0.006 3.0 South Side Administration Bldg 3123 Door 141 0.32 0.10 8.15 2.49 0.0089 0.09 0.009 4.7 South Side Unit 2 Reactor Bldg 292 Door 126 0.47 0.14 7.40 2.26 0.0092 0.19 0.014 6.9 South Side Unit 2 Reactor Bldg 294 Door 128 0.56 0.17 7.32 2.23 0.0092 0.27 0.016 8.2 South Side Unit 2 Reactor Bldg 296 Bayl27 1.17 0.36 7.51 2.29 0.0091 1.16 0.034 17.1 South Side Unit 2 Reactor Bldg 297 Door 125 0.51 0.16 7.53 2.30 0.0091 0.22 0.015 7.4 South Side Unit 3 Reactor Bldg 1403 Door 131 2.27 0.69 7.70 2.35 0.0090 4.32 0.066 33.1 South Side Unit 3 Reactor Bldg 17596 Door 130 4.15 1.26 7.66 2.33 0.0091 14.47 0.120 60.5 South Side Unit 3 Reactor Bldg 17598 Bayl29 4.25 1.29 7.34 2.24 0.0092 15.40 0.124 62.4 South Side Unit 3 Reactor Bldg 17600 Bayl29 4.45 1.36 7.62 2.32 0.0091 16.69 0.129 65.0 South Side Unit 2 Turbine Bldg 304 Bayl24 0.01 0.00 7.04 2.15 0.0093 0.00 0.000 0.1 The results of this analysis indicate that, at representative locations throughout the power block (Figure 7.1), the primary mode of sediment transport is in suspension. This implies that no significant sediment deposition is expected to occur during peak flows. It should also be noted that that the grain size considered in this analysis is representative of the main channel of the Kankakee River, where velocities are higher and larger sediment particles can be transported.

The sediment expected in the overbank area will likely be finer, which further decreases the likelihood of sedimentation during peak flows.

References

1. Julien, P. Y. (2009). Fluvial Transport of Suspended Solids. Encyclopedia of Inland Waters (2009), vol. 1, pp. 681-683.

2. Garcia, M. H. (2008). Sedimentation Engineering: Processes, Measurements, Modeling, and Practice. ASCE Manuals and Reports on Engineering Practice No. 11 O, 1132 pp.

3. Roelvink, J. A. (2011 ). A Guide to Modeling Coastal Morphology. World Scientific, SCIENCE, pp. 250.

4. Bhowmik, N. G., Bonini, A. P., Bogner, W. C., and R. P. Byrne (1980). Hydraulics of Flow and Sediment Transport in the Kankakee River in Illinois. ISWS/Rl-98/80, Report of Investigation 98, State of Illinois, Illinois Institute of Natural Resources, Illinois State Water Survey, Champaign.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 22 of 43

5. Terrio, P. J. and J.E. Nazimek (1997). Changes in Cross-Section Geometry and Channel Volume in Two Reaches of the Kankakee River in Illinois, 1959-94. U.S.

Geological Survey, Water-Resources Investigations Report 96-4261, Prepared in cooperation with the Kankakee County Soil and Water Conservation District, Urbana, Illinois.

RAI 9: Failure of Dams and Water Control Structures/Stream and River Flooding

Background:

The licensee modeled numerous flooding scenarios to determine several flood causing mechanisms that would trigger an integrated assessment. Detailed flooding characteristics are needed to evaluate the minimum warning times that would cover the range of flooding scenarios for each flood causing mechanism for use in the integrated assessment.

Request: Identify the conservative flooding scenarios from both the flooding in streams and rivers mechanism and the dam breaches and failures mechanism analyzed to determine the following criteria:

• Fastest arrival time to reach the critical water surface elevations 509 ft NGVD 29 from the beginning of the flooding event;

• Fastest arrival time to reach the critical water surface elevations 517 ft NGVD 29 from the beginning of the flooding event; and

• Highest water surface elevation at the site For all 6 scenarios, provide the input and output files for the computer models HMR-52 (National Weather Service's Hydrometerological Report 52), HEC-HMS and HEC-RAS. Provide the input and output files for FL0-2D model for the riverine flooding with dam failure and concurrent two-year wind event scenario. Identify and describe the fastest arrival time to reach the critical water surface elevations 509 ft NGVD 29 and peak water surface elevation subsequent to the seismic event that triggers dam failure without the addition of the one-half PMF.

Response

For the Flooding in Streams and Rivers multiple flood scenarios were analyzed to determine the following:

1. The shortest time to reach the critical elevation at the site of 509 ft NGVD29 (See Note 1)

2. The shortest time to reach the critical elevation at the site of 517 ft NGVD29

3. The highest water surface elevation at the site The same flood criteria were assessed for the PMF coincident with upstream dam failures:

1. The shortest time to reach the critical elevation at the site of 509 ft NGVD29

2. The shortest time to reach the critical elevation at the site of 517 ft NGVD29

3. The highest water surface elevation at the site The detailed results for the critical scenarios are presented in Table 6.1, Response to RAI No.6.

It should be noted that the flooding scenarios with coincident dam failures bound the flooding scenarios without dam failures both tor critical timing and maximum water elevation. Therefore, additional analyses for the flooding without dam failures were not performed and all the results for the Flooding in Streams and Rivers {Table 9.1) are collected from the analyses performed as

REDACTED WITHHELD FROM PUBLIC DISLOSURE PER 10 CFR 2.390

REDACTED WITHHELD FROM PUBLIC DISLOSURE PER 10 CFR 2.390

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 25 of 43 Table 9.2 - Computer Models Corresponding to Conservative Flooding Scenarios Computer Scenario Computer Model Name Software

1. Flooding in Streams and Rivers 1a HMR-52

---

*---*------*---* ---------- Flood in Streams and Rivers/HMR-52/DPStorm5 --------*

__ !j_~_g_:.!:!~_$________________ £!2.9-9. in S!_r~~f!.l~_~nq Riv_~rs/HEC-HMS/D PSQ§__,,f ron!~M ~~r:iJy______

HEC-RAS Flood in Streams and Rivers/HEC-RAS

J!i&ecT*:=-__::=:=~=-~~::~ =~:JEes-cfen~B_~y_i§loiJ£tti~C~:::~===:==~:=~~~~~=~~~=~===~==-==::==*

__ _E!?J! ___ -------*----* ______ --**----- ______.Q_E_§.9.Q9-=E!~!1_t=fY_c.i__f.3!!._~~-q_~_qf!_r2_L(£E}_:}J________________ **-** *-----*-*-*--------- _*-* *- .______

---

~~C?J!!.~rt_Jf!?_ __________________f.ME=.E!_fY_~~--U1.!_Q)_ ---------*-------*- -***-----------**--*-- ***-*--*-----*------------------- _____

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Output Dresden_DPSCOS_Front_NoAntecedent. dss 1b and 1c HEC-HMS Flood in Streams and Rivers /HEC-HMS/Alt. 2 PMF

-H-EC:R-AS______________ - Floodin-Streams and Rivers/HEC: RAS ------------*----*-----------*

....-................-...---*----***--*-**--*----***--***-...--**~*---* ------*----*****-----*-*--*--*----*-------*---*-..**----*------*----***-*-------*------*---..*--*----*------***---*---*-..**--*----*---*----*-----*

__f!!.9.Lf!_9_f___,_______________f?_!.f!_Sden_R~VISION FINA~---*-----------------------------*------*

__ f:'-?_'! __ ______________ --*- _... _______$.!!Q'!.!f!!~!.U:P§.?L________ -* ---------- ---------------------------- ----------*---------------*-------

__. Qt7_Q!!!.~_rry_ijf§!______________ __,_J:~£=E.!_fYAb_Lg_!_QL____,________________________________________________________

__,__1:!_12~J~9-<:!Y.E!.g~£!1.~.--*** --**** *

• _$_'.1-QY.!.'!!~!t_Ly_~_l?) __ . _____. _____________ ***-*****- --------*-* * *** --**. * -* * * *- ****--*****--- *----***- * **** * * * * * * ***-* * *-*

Output Dresden_ Snowmelt. dss

2. PMF with Dam Breaches and Failures (HEC-RAS Modeling) 2a and 2b HMR-52

-*----~

Dam Failures/Fastest Arrival Time/HMR-52/CMSC17 HEC-HMS Dam Failures/Fastest Arrival Time/HEC-HMS/

---

*--*------------------*---* __Ct\!~_g_! 7__Cri!i_g_~!~~~nariQ_______________________________________________

HEC-RAS Dam Failures/Fastest Arrival Time/HEC-RAS/HEC-RAS CMSC1 7

_J'roject Dresden_ DamBreak

--* E@'!_ ------------------**-**--- --* _______E!!!JJ!_~--~<j_cJ.~<J_Z?. _'29-1!!.~__(p.Q~L---------------------**-------*-- *-------- *------------------

---

~_§!_9.!!!._l!.!rt__(ile ----**------* _____0~'!2?!5!.~!5.=_?.!.?_!.'dC?_f!.~_fL!_LgQ_!L____________ ___________________________________________________

_ '!!!~~eady Flow File _ Run No.! - added 72 h_rs (.uO_J)_ _ _______________________

Output Run No. 7_added72hours.dss 2c __ !j_~_g_:tlM§_______ Da!}1_£ailures/Hig!!~~1Y'JSE!lj~_Q:_tl_MS/ Combined_Model_R_uf'!§____

__ !j_§_g_:~-~_§__________________________[;)am £~!':'!:~~!:!!~1~.~~!__'f.Y._?§1._l::f_~~:E~_~_§________________,_ _________________________

__f!.Pif!_9L___,________________f}_!!!_~<}_f?n_O?__r!}B!_~?k ____________________________,______________________

___ E'-9-'! ____ ----*----*---- _----- *- ___fiL!!!.JQ.:__EiL!!..!~+/-./jY.e9.Lf}_:Q?_!!!.~LP._!?l ______ _____ --*---- **--------*--------*---- *----*- ___

---

~~_<2!!!.~!Y!!!~----------- __,___Q9-'!!f?!.~?J.!5.=_?.!?__'!_g_g!!.~_<L!_LgQ_!L____________________________ _____________________

__1:!_12~t.~9-<:!.Y..£1E_~EH~ --- ______fiu._'!_!:!_q_:!__Q _:-'3.Y!J.f!..+/-._/jyp_g!J2Q?..'!!~LY..!§L ___ __________ ***-----*-------------- _

Output Run No.11 .dss

4. Seismic Dam Failure without 112 PMF Dam Failures/Seismic Failure No Rainfall/HEC-HMS/

4a HEC-HMS Seismic_RAI

---

*----*---------------*-------*--- ..___________gg_Q!:.~~~~=?eisi:nJ_C2=~gg~jf.! ___________________________________________

Dam Failures/ Seismic Failure No Rainfall /HEC-RAS/Dresden HEC-RAS HEC-RAS Seismic

-* Project Dresden_DamBreak -------------*-----

Pl?_f1 No Rain+ Dam Breaks1 LJ!.01)__________,______________

Geometry_ file _ . ___ ____Q!JmBreak_f!_randon x_Q. 7_(_...g_0_, 1)______

Unsteady Flow File NoRain+DamBrks 1 (. u01) --------*-----------*-----

Output Seismic_NoRain.dss

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 26 of 43 RAI 1O: Channel Migrations or Diversions

Background:

The FHRR states that control structures and flows that might play a role in channel migration are outside of the licensee's authority and do not describe available information on the risk of channel migrations or diversions.

Request: Describe in detail the available morphological information, such as historical topographic maps and information on erosive quality of channel material (both adjacent rivers and the on-site channels for cooling water), consistent with NRC NUREG/CR-7046.

Response

Dresden Nuclear Power Station (DNPS) is located at the confluence of the Des Plaines and Kankakee rivers that forms the Illinois River, as shown in Figure 10.1. DNPS is also located just upstream of the Dresden Island Lock and Dam, which is owned and operated by the U.S. Army Corps of Engineers (USACE) . The nominal ground elevation of DNPS is about 516 feet mean sea level (msl) datum at the location of the principal structures of Units 2 and 3, and the design plant grade is 517 feet msl. The finished floor elevation of the plant structure is 517.5 feet msl.

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17' O" Figure 10.1 DNPS Location (Minooka, IL USGS Quadrangle)

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 27 of 43 Historical Channel Diversion A comparison of a historical United States Geological Survey (USGS) topographic map with a current topographic map did not yield evidence suggesting there have been significant historical diversions of both Des Plaines and Kankakee Rivers near DNPS over the last century. The comparison is based on the channel banks and their potential for migration and/or diversion throughout the years toward the site. Topographic (7.5 Min. Quadrangle) maps for Wilmington, IL for Kankakee River, and Channahon, IL for the Des Plaines River were utilized for this purpose at different historical years, and were obtained from the USGS Store.

Note that the USGS map of 2012 is the latest map available for both Kankakee River and Des Plaines River. Thus, the 2012 map is used as the current location of the river portion of interest channel and overbanks. For comparison purposes, the 2012 topographic map is used as a reference map for the rest of the topographic maps. The assessment is based on comparing the continuity of the river over the period.

Des Plaines River A review of the historical topographic maps did not yield evidence suggesting that there have been significant historical diversions of the Des Plaines River near DNPS over the last 58 years (1954-2012).

Historical maps for the years of 1954, 1993, 1999, and 2012, are downloaded from The USGS Store. The topographic maps indicate that the continuity of the Des Plaines River in 1954 and 1999 is similar to the present-day path, indicating that no significant migration/diversion occurred since 1954 up to date. The topographic maps for Des Plaines River (Channahon, IL USGS quadrangle) for years 1954, 1993, 1999, and 2012 are provided in Figures 10.2 through 10.5, respectively.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 28 of 43 CHANNAHON 0\J AORANOLE

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Figure 10.2: Des Plaines River 1954 (Channahon, IL USGS Quadrangle)

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 29 of 43 CHA.N"'fA l'iON QUA DRA NGLE UNIT£!> STATES

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Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 30 of 43

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Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 31of43

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Figure 10.5 - Des Plaines River 2012 (Channahon, IL USGS Quadrangle)

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 32 of 43 Kankakee River A review of the historical topographic maps did not yield evidence suggesting there have been significant historical diversions of the Kankakee River near DN PS over the last 120 years ( 1892

- 2012).

Historical maps for the years of 1892, 1918, 1954, and 2012, are downloaded from The USGS Store. The topographic maps indicate that the general path of the Kankakee River in 1892, 1918, and 1954 is similar to the present-day path, indicating that no significant migration/diversion occurred since 1892 up to date. The topographic maps for Kankakee River (Wilmington, IL USGS quadrangle) for years 1892, 1918, 1954, and 2012 are provided in Figures 10.6 through 10.9, respectively.

Kankakee River runs along the eastern and southern edges of the cooling lake, which is protected by earthen dikes. The normal pool elevation of the cooling lake is higher than the normal water surface elevation in the Kankakee River. A channel diversion towards the cooling lake may result in failure of dikes. As indicated in the UFSAR, the failure of the cooling pond dikes will not impact the plant because of topographic relief.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 33 of 43 1

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Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 34 of 43

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Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 35 of 43

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Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 36 of 43

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Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 37 of 43 Landslide No evidence of landslide-induced channel diversion near DNPS was found based on the sources reviewed. The following information is summarized from the Dresden UFSAR - "the authority to control the Dresden Island Lock and Dam, and therefore, the level of the pool impounded by it, is vested in the USAGE. Should the dam become damaged, one method of maintaining the pool level would be through the use of increased water diversion from Lake Michigan. This would be accomplished with the permission of the U.S. Supreme Court."

Additionally, the USGS compiled a report and data information in GIS on landslide incidence and susceptibility for the contiguous United States. Susceptibility to a landslide is classified as high, medium or low based on the probable degree of response of soil and rock to cutting or loading of slopes or from abnormally high precipitation. Landslide incidence is classified as high (greater than 15-percent of the area is susceptible to landsliding), medium (1.5- to 15-percent of the area is susceptible to landsliding), and low (less than 1.5-percent of the area is susceptible to landsliding). As shown in Figure 10.1 O, DNPS is located within an area considered to have a moderate susceptibility/low landslide incidence.

Therefore, channel migration is not considered to be a potential contributor to flooding at DNPS.

A review of historical data and site information indicates that the Kankakee River and Des Plaines River has not exhibited a tendency to meander towards the DNPS. Channel diversion impacts at DNPS are not anticipated to occur as a result of landslide because of the low landslide potential in the area as well as the relatively flat surrounding floodplain areas.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 38 of 43 89° 20' 38' West 86° 52' 17" West 42°09'41" North Lambert Azimuthal Equal-Area 87° 08' 06" West Projection http://nationalatlas.gov Miles 10 20 30 01-Apr-14 02:34PM Boundaries States Source: U S Ge2l ogjca! Suryey li'SZJ St.tu Counties Source: U S Geol og1ca1 Suryey eszl Countfu Geol ogy Landslide Incidence and Susceptibility Source: U S. Geol og1cal Suryey Layer partially covered by another layer L.1 ndslld* Incidence .1nd Su1Hptlblltty L.1nd1Udt lnoldenct D Low (lu:s th*n 1.0 ~ of uu lnvohttd)

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Response to Request for Additional Information (Flood Hazard Reevaluation Report)

• Page 39 of 43 RAI 11: Hazard Input for the Integrated Assessment - Flood Event Duration Parameters

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. Flood scenario parameters from the flood hazard reevaluation serve as the input to the integrated assessment. To support efficient and effective evaluations under the integrated assessment, NRC staff will review flood scenario parameters as part of the flood hazard reevaluation and document results of the review as part of the NRC staff assessment of the flood hazard reevaluation.

Request: Provide the applicable flood event duration parameters (see definition and Figure 6 of the NRC interim staff guidance document JLD-ISG-2012-05, "Guidance for Performing an Integrated Assessment," November 2012, ADAMS Accession No. ML12311A214}), associated with mechanisms that trigger an integrated assessment using the results of the flood hazard reevaluation. This includes (as applicable) 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 for the mechanisms that are not bounded by the current design basis. The licensee is also requested to provide the basis or source of information 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/or timing information derived from the hazard analysis.

Response

Flood event duration parameters are determined for the scenario critical to each specific duration parameter. Therefore, the combined duration parameters are not necessarily derived from a single scenario. The results are provided based on the HEC-RAS modeling and the final Water Surface Elevation (WSE) at the site Area of Interest (AOI). This corresponds to the cross section 135306 WSE at the Dresden site AOI.

The basis for the critical scenarios and their corresponding critical timing is provided in the Flood Hazard Re-Evaluation Report (FHHR) and in the Request for Additional Information (RAI) 6 response. Table 6.1 in the RAI 6 response identifies different scenarios and their durations above the benchmark elevations (506 ft NGVD29, 509 ft NGVD29, and 517 ft NGVD29}. The scenarios resulting in the shortest arrival times to the benchmark elevations of 506 ft NGVD29, 509 ft NGVD29, and 517 ft NGVD29 are described in detail in the RAI 6 response, and are not repeated here. As identified by JLD-ISG-2012-05, the critical warning time, inundation time, and recession time, considering multiple scenarios, are provided in Figure 11.1. Note that the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> critical rainfall storm event has a frontal temporal distribution. The bulk volume of the rain will occur in the first few hours of the total duration. Therefore, the benchmark elevations (506 ft, 509 ft, 517 ft) above normal pool (505 ft) are achieved relatively quickly.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Pa e 40 of 43 t~-~-~-*-°'"'--*--~;;~----6d-15h_r_0m_1n____ ._"~~~- _j Site Preparation Period of Reeesslon of for Flood Event Inundation water horn site Od 10hr5mln Od 23hr 25mln 14d O'lr 50mln Beginning of Weather and Conditions are Water completely Arrival of flood Water begins to RalnfaD RalnfaD met for entry into receded from site waters on site recede from site Continuous flood procedures and plant in safe 517 ft 517 ft Monitoring or notification of and stable state Trtger bnpending flood thatcanbe Elevation 509 ft maintained 506ft indefinitely 509 ft Figure 11.1 - Flood Event Duration Parameters Identified By JLDISG- 2012-05 for the Combined Effect Flood Scenario If weather forecasts predict rainfall equal to or greater than 2 inches in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> or the river level at the Crib House intake canal is equal to or greater than 506 feet, monitoring actions are triggered following the procedures described in Attachment C, "Monitoring River Information and Rainfall Forecasts," document DOA 0010-04.

The water level at the Crib House is measured and river level trends at upstream locations are monitored. Various actions are taken at intermediate levels between 506 feet and 509 feet.

However, if the water level reaches 509 feet and is predicted to continue to rise, shutdown procedures are initiated. Alternatively, if the water level at the Crib House is predicted to be equal to or greater than 509 feet anytime in the next 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, shutdown procedures are initiated.

In addition to the timing information derived from the hazard analysis, as previously discussed, relevant forecasting methods are summarized as the following:

• Early forecasts of flood conditions are provided by the National Oceanic and Atmospheric Administration (NOAA) National Weather Service. Notification of predicted heavy rainfall is provided by Murray and Trettel. However, forecasts of rising river level are no longer provided, and must be predicted based on rainfall forecasts and status of the gates at Dresden Lock and Dam.

• The U.S. Geological Survey (USGS) flow gages, may be used to assess flow data and stage measurements throughout the watershed.

• The U.S. Army Corps of Engineers (USAGE) flow gages, may be used to assess flow data and stage measurements at upstream and downstream locks and dams.

• Communication with the U.S. Army Corps of Engineers incorporating operations of upstream and downstream locks and dams, including emergency notification procedures.

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 41of43 RAI 12: Hazard Input for the Integrated Assessment - 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. Flood scenario parameters from the flood hazard reevaluation serve as the input to the integrated assessment. To support efficient and effective evaluations under the integrated assessment, NRC staff will review flood scenario parameters as part of the flood hazard reevaluation and document results of the review as part of the staff assessment of the flood hazard reevaluation .

Request: Provide the flood height and associated effects (as defined in Section 9 of JLD-ISG-2012-05) that are not described in the flood hazard reevaluation report for mechan isms that trigger an integrated assessment. This includes the following quantified information for each mechanism (as applicable) :

• Hydrodynamic loading , including debris;

• Effects caused by sediment deposition and erosion (e.g ., flow velocities, scour) ,

• Concurrent site conditions , including adverse weather; and

• Groundwater ingress

Response

a. Hydrodynamic/debris loading: Hydrodynamic and debris impact loading during the governing PMF scenario were evaluated in Calculation WGW-DRE-003. The summary of the results is provided in Table 12.1. Please note that since the hydrodynamic pressure calculation is independent of the flood depth, the 2-dimensional (2-d) model grid elements corresponding to maximum velocity and hydrodynamic pressure are not necessarily those characterized by the maximum hydrodynamic force (i.e ., the maximum hydrodynamic pressure and the maximum hydrodynamic force may occur at different locations and time steps).

Table 12.1: Summary of Hydrodynamic Loads on Critical Structures Maximum Maximum Ma ximum Hydrodynamic Structure Name Hydrodynamic Force Debris Impact Load Pressure psf (lb/ft) (lb)

Reactor Building, Unit l 8L.53 129.24 2,790.76 Reactor Building, Unit 2 71.61 150.72 2,615.43 Reactor Building, Unit 3 37.19 80.69 1,884.86 Turbine Building, Unit l 11 7.20 238.80 3,624.83 Turbine Building, Unit 2 102.35 220.52 3,387.39 Turbine Building, Unit 3 93.62 197.44 3,239.66 ISFSI-East 20.88 16.18 8,827.68 ISFSI-West 50.73 3 12.54 13,759.0L HPCI & Diesel Generator 37.19 78.54 1,884.86

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 42 of 43 The Des Plaines River is a navigable water body and, therefore, the potential for a barge to impact critical structures during the PMF event was assessed. As shown in Figure 12.1 below, velocity vectors from the 2-d model indicate that inertia alone would direct floating barges around the plant, making any direct or indirect strike on critical structures (Reactor Building Unit 2 and Reactor Building Unit 3) not credible. Uncontrolled floating barges along the Des Plaines River would instead follow the path of higher velocities in the main channel of the Illinois River and bypass the site to the north. Furthermore, in the highly unlikely event that a barge would overtake resistance from inertia and swerve toward the site, the Reactor Building is shielded by structures that are not critical for safe shutdown of the plant, as shown in Figure 12.2.

000 - 020 0_20 . 0 50 OS0-100 1 00 . , 50 1 so. 2.00

' 200-250 250 - 300 300 - *00

+ < 00-500

+ 500 - 10 41 RivatFL020 Mesh Elenwnts Figure 12.1: Velocity Vectors around Dresden Generating Station

Response to Request for Additional Information (Flood Hazard Reevaluation Report)

Page 43 of 43 000

• 020 020-050 I 050

• 100 I 1oo . 1 50

• 150-200

• 200 - 250 250*300 300-

• 00

• H)O

• 5 00 5>00 . 10.41 RIVtrFL020 Mesh Ektments Figure 12.2: Velocity Vectors around Dresden Generating Station

b. Effects of sediment deposition and erosion: these effects are addressed in the response to RAI #8.

c. Concurrent site conditions: The meteorological events that could potentially result in significant rainfall of the LIP and PMP magnitude are squall lines, thunderstorms with capping inversion, and mesoscale convective systems. These meteorological events are typically accompanied by hail, strong winds, and even tornadoes. The flood hazard reevaluation calculations indicate that the site is also subject to flooding from a rain-on-snow event in the watershed, which can produce concurrent high winds, ice, and snow conditions on the site. The riverine PMF can be accompanied with debris loads, which may impact site accessibility once the flood waters recede.

d. Groundwater ingress: Effects on groundwater ingress during a flood is already addressed by Dresden's current licensing basis, which assumes extreme groundwater conditions at plant grade

Enclosure 2 CD-R labeled Dresden Nuclear Power Station, Units 2 and 3 Enclosure 2 of RS-14-122 FL0-20 Model Input and Output Files

Enclosure 3 CD-R labeled Dresden Nuclear Power Station, Units 2 and 3 Enclosure 3, DVD #1 and DVD #2, of RS-14-122 HMR-52, HEC-HMS, & HEC-RAS Model Input & Output Files