ML24149A133

Pre-Submittal Meeting for Proposed License Amendment Regarding Hydrologic Analysis Updates Public Portion Meeting Slides

ML24149A133
Person / Time
Site:	Browns Ferry
Issue date:	05/29/2024
From:	Tennessee Valley Authority
To:	Kimberly Green Office of Nuclear Reactor Regulation
Green K
Shared Package
ML24149A143	List: ML24149A133
References
EPID L-2024-LRM-0053
Download: ML24149A133 (53)

Text

Browns Ferry Nuclear Plant Pre-Submittal Meeting for Proposed License Amendment Regarding Hydrologic Analysis Updates May 29, 2024

Introduction Background and History Probable Maximum Precipitation (PMP) Update Translation of Digital Rainfall Depth-Area-Duration (DAD) Data to Inflows in the Simulation Model Browns Ferry Nuclear (BFN) License Amendment Request (LAR) - Key Updates Regulatory Precedent Schedule for Submittal Summary Noon Break Closed Portion Agenda l BFN-Hydrological Analysis Updates License Amendments 2

Tennessee Valley Watershed BFN - Hydrological Analysis Updates License Amendment l 3 Watershed is complex, draining over 27,000 square miles above Wheeler Dam Major Tributaries French Broad, Little Tennessee and Hiwassee Rivers originating in the Blue Ridge mountains northeast of the plant site Holston and Clinch Rivers originating in Virginia northeast of the plant site Elk River originating in Tennessee, northeast of the site and entering the Tennessee River downstream of Browns Ferry Browns Ferry Nuclear Plant Sequoyah and Watts Bar Nuclear Plants TVA River Management provides operational control of the reservoirs in an integrated fashion Flood Control Navigation Hydropower

- Water Quality

- Water Supply

- Recreation

=

Background===

The BFN Design Basis hydrological model, described in Section 2.4, Appendix 2.4A, of the Updated Final Safety Analysis Report (UFSAR), utilizes historical TVA unsteady flow software (SOCH*)

with limited capability, requiring manual interfaces between multiple river model segments and dam rating curve balancing in the performance of hydrological analysis.

BFN Condition Report (CR) identifies TVA issue with spillway dam stability assumptions, requiring update of the design basis model.

TVA decision made to utilize the US Army Corps of Engineer (USACE) HEC-RAS** modeling software for the update and to apply Topical Report (TR) PMP for consistency with Sequoyah Nuclear Plant (SQN) and Watts Bar Nuclear (WBN) LARs PMP assumptions.

• SOCH - Simulated Open Channel Hydraulics ** HEC-RAS - Hydraulics Engineering Center-River Analysis Software BFN - Hydrological Analysis Updates License Amendment l 4

Resolution Plan BFN-Hydrological Analysis Updates License Amendments l 5 Update the BFN historical SOCH hydrological model software to USACEs HEC-RAS river flow modeling software, an industry standard. Apply the same HEC-RAS models (except Chickamauga Dam boundary assumptions) used for SQN and WBN LARs hydrological analysis updates.

Utilize the PMPs defined in TR TVA-NPG-AWA16-A, replacing the BFN design basis HMR PMPs, for evaluation of local and river flooding impacts at the BFN site.

Reassess global stability of critical dams above the site using TVA Dam Safety dam stability criteria and apply results in the HEC-RAS model. Limit earth supported spillway flow to original design capacities.

Update wind data supporting wind speed calculations and revise wind wave characteristics.

Include in the UFSAR an analysis of upstream seismically-induced dam failures and a fastest rising warning time evaluation for site flooding above grade.

Browns Ferry Nuclear Plant Application of Gridded Rainfall used in UFSAR Hydrologic Analysis Updates May 29, 2024

TVA Projects and River Network Model BFN - Hydrological Analysis Updates License Amendments l 7

TVA Projects and River Network Model BFN - Hydrological Analysis Updates License Amendments l 8

PMP: Topical Report TVA-NPG-AWA16-A Topical Report NRC issued a Safety Evaluation Report (ML19010A274)

TVA used Topical Report for PMP rainfall in WBN and SQN LAR analyses Topical PMP Defines rainfall depth-area-duration data for three storm types (local, general and tropical) on gridded network over TVA drainage basin Application Methodology Methodologies used for areal and temporal distributions, effective rainfall and runoff hydrograph development same as used for TVA SQN and WBN sites (SQN LAR has been approved by NRC; WBN LAR currently under NRC review)

BFN - Hydrological Analysis Updates License Amendments l 9

TVA Model - Unit Hydrograph Sub-Basins BFN - Hydrological Analysis Updates License Amendments l 10 Browns Ferry NP

Gridded Rainfall Network BFN - Hydrological Analysis Updates License Amendments l 11 Browns Ferry NP

Areal Distribution of Gridded Rainfall l

PMP Nesting Criteria Areas of Interest (AOIs) split into successive primary and secondary areas.

Primary AOI scenario may be a single projects set of one or more sub-basins, or any combination of adjacent projects set of sub-basins meeting simplifying assumptions.

Secondary (larger) AOIs must include all primary AOI sub-basins (i.e., embedded PMP).

All simulations end with the boundary condition AOI at Wheeler with all upstream sub-basins included.

Adjustments made based on individual sub-basin at each duration as part of total AOI PMP volume.

BFN - Hydrological Analysis Updates License Amendments l 12

Primary and Multiple Secondary Basin Nesting 13 GUH_01-Controlling BFN PMP BFN - Hydrological Analysis Updates License Amendments l

Temporal Distribution of Gridded Rainfall l

BFN - Hydrological Analysis Updates License Amendments l 14 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0

1 2

3 4

5 6

7 8

9 10 11 12 13 14 15 16 0

6 12 18 24 30 36 42 48 54 60 66 72 78 84 90 96 102 108 114 120 126 132 138 144 150 156 162 168 174 180 186 192 198 204 210 216 Incremental Rainfall (in inches)

Cumulative Rainfall (in inches)

Time from Start (in hours)

GUH_01 Rainfall above WEH Applied Incrementall Effective Incremental Cumulative Applied Cumulative Effective

Areal Distribution of Gridded Rainfall l

GUH_01-Controlling BFN PMP BFN - Hydrological Analysis Updates License Amendments l 15

Application of Gridded Rainfall l

Nested total rainfall depths developed for 1-, 6-, 12-, 18-,

24-, 48-and 72-hour durations for application to each TVA sub-basin.

Previously used HMR guidance 3-day antecedent, 3-day dry period and 3-day main sequence was adopted.

Antecedent scaled to 40% of PMP applied in same areal distribution.

BFN - Hydrological Analysis Updates License Amendments l 16

Simulations Using Gridded Rainfall Summary Gridded rainfall allows more detailed analysis of PMP events.

Documented simulations were performed with PMP depths at and above Wheeler Dam.

Simulations are directly comparable PMF candidates at locations where applied rainfall meets DAD volume.

Additional preliminary and follow-up simulations were performed to analyze seasonality, warning time and downstream projects.

BFN - Hydrological Analysis Updates License Amendments l 17

Browns Ferry Nuclear Plant Proposed License Amendment Request -- Key Updates May 29, 2024

BFN LAR - Key Updates BFN UFSARs Section 2.4, Appendix 2.4A, proposed changes reflect updates to the hydrologic analysis, including:

Changed PMP bases document and application of precipitation to the upstream drainage basins Revised process of determining the controlling PMP storms and resulting PMF.

Changed hydrological modeling software to HEC-RAS, utilizing the same basic model as SQN/WBN.

Updated the criteria applied in the determination of dam stability and applied revised stability results in the model.

Updated of overland wind speed data.

Added the seismically-induced dam failure flooding analysis.

Revised the warning time hydrologic basis for rainfall on rivers and streams and seismically-induced dam failure floods.

BFN - Hydrological Analysis Updates License Amendments l 19

BFN LAR - Key Update 1 BFN - Hydrological Analysis Updates License Amendments l 20 Changed PMP bases document and process of determining the controlling PMP storms Updated the design basis PMP used in the BFN site local intense precipitation (LIP) drainage analyses and the rivers and streams rainfall flooding hydrological analyses to use Topical Report TVA-NPG-AWA16-A PMPs (storms through 2014 considered)

Replaces the unique PMP storms defined for TVA rivers and streams in HMR 41 (1965), HMR 47 (1973) and HMR 56 (1986).

Defined digital rainfall Depth-Area-Duration data for 2.5 sq. mi. by 2.5 sq. mi. grid across the Tennessee Valley in lieu of fixed or movable unique isoheytal rainfall patterns.

BFN LAR - Key Update 2 BFN - Hydrological Analysis Updates License Amendments l 21 Changed the methodology for determining the controlling PMP storms and the resulting PMF.

Due to the Topical Report digital rainfall DAD data format, the method used to determine the rainfall pattern in PMP storm development has been updated.

Multiple PMPs considered varying precipitation depths on different sub-basin groupings are analyzed to determine worst case flood depths at the BFN site.

Applied concept of embedded primary PMP with multiple secondary PMPs included for peripheral rainfall.

Controlling Sub-Basin Grouping for BFN

BFN LAR - Key Update 3 BFN - Hydrological Analysis Updates License Amendments l 22 Changed the river and streams unsteady flow hydrological modeling software to the USACEs HEC-RAS for determining controlling BFN PMF Eliminated the use of TVA specific modeling software and replaced with an industry standard software package.

Updated the discussion of PMF on streams and rivers to reflect updated methodologies applied in HEC-HMS and HEC-RAS software elements for determining the LIP maximum depth and PMF for streams and rivers.

Applied the same model (except boundary conditions) utilized in the SQN and WBN hydrological analysis update LARs.

Updated other BFN analyses to HEC-RAS (warning time and seismically-induced dam failure analysis).

BFN LAR - Key Updates 4 and 5 BFN - Hydrological Analysis Updates License Amendments l 23 Updated structural criteria used in the determination of global dam stability and stability for earth-supported spillways.

Applied TVA Dam Safety criteria and factors of safety, generally conforming to USACE standards.

Limited flow at earth-supported spillways to the original design flow for the spillway.

Updated overland wind speed data to include more recent (2015) information Meteorological wind speed data updated to include more current wind data and data sources from airports surrounding the site (primarily Huntsville, Alabama airport).

Critical fetch and wind wave effects are updated for critical structures.

BFN LAR - Key Update 6 BFN - Hydrological Analysis Updates License Amendments l 24 Added the seismically-induced dam failure flooding analysis to the BFN UFSAR Applied general guidance provided in JLD-ISG-2013-01, Guidance for Assessment of Flooding Hazards Due to Dam Failure, for the seismic dam failures combined with flooding analysis.

Applied the methodology in the development of 25-year flood inflows which aligns with more recent work performed for the TVA Clinch River Early Site Permit Application. Inflows for a 500-year storm are created to replace the one-half PMF inflows used in combination with the less severe seismic hazard, as defined in JLD-ISG-2013-01.

BFN LAR - Key Update 6 (cont.)

BFN - Hydrological Analysis Updates License Amendments l 25 Added the seismically-induced dam failure flooding analysis (continued)

Applied the stability analysis methods and the factors of safety required for stability of concrete and embankment dam structures which align with TVA Dam Safety design.

Used the hydrological unsteady flow modeling software HEC-RAS in the simulation of seismically induced dam failures combined with flooding.

Applied same model (with minor boundary changes at Chickamauga Dam) used for the analysis of this event in the SQN/WBN LARs and in the evaluation of the PMF impacts at BFN.

BFN LAR - Key Update 7 BFN - Hydrological Analysis Updates License Amendments l 26 Updated the warning time hydrologic basis for rainfall on rivers and streams (Note: seismically-induced dam failure floods do not reach plant grade).

The PMP used in the warning time analysis for each site applies Tropical Report TVA-NPG-AWA16-A.

Analysis of fastest rising PMF storms assures sufficient time is available to perform required activities before the site floods.

The PMF warning scheme is revised to establish a target site flooding elevation of 563.5 ft. (below 565.0 ft plant grade) to accommodate wind wave impacts and wave breaks.

Regulatory Precedent

• TVA submitted a license amendment request for SQN to revise the SQN Units 1 and 2 UFSAR to reflect the results from a new hydrologic analysis (Letter from TVA to NRC, Revision to the Application to Revise Sequoyah Nuclear Plant, Units 1 and 2 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, (TS-19-02) (EPID L-2020-LLA-0004), dated April 11, 2023 [ML23101A179, ML23101A180]).

• The NRC approved that request in a letter from the NRC to TVA, Sequoyah Nuclear Plant, Units 1 and 2 - Issuance of Amendments Nos.

367 and 361 Regarding Changes to the Hydrologic Analysis, dated March 26, 2024 [ML24040A205, ML24040A206].

• The scope of that precedent is the same as the scope of this request.

BFN - Hydrological Analysis Updates License Amendments 27

Schedule for Submittal BFN - Hydrological Analysis Updates License Amendments l 28

• The BFN LAR is currently scheduled to be submitted to the NRC by September 2024.

Summary The PMP for BFN is being updated to reflect use of NRC-approved Topical Report TVA-NPG-AWA16-A.

Due to the Topical Report digital rainfall depth-area-duration data format, the method used to determine the rainfall pattern in PMP storm development is being updated.

BFN UFSAR is being updated to include seismically-induced dam failure flooding and to evaluate available warning time for site flooding.

TVA plans to submit BFN LAR to request changes to hydrologic analyses in September 2024.

BFN - Hydrological Analysis Updates License Amendments l 29

Backup Slides

Areal Distribution of Gridded Rainfall l 32 API Weeks Week #

Dates Week #

Dates Week #

Dates Week #

Dates 1

1 - 7 Jan 14 2 - 8 Apr 27 2 - 8 Jul 40 1 - 7 Oct 2

8 - 14 Jan 15 9 - 15 Apr 28 9 - 15 Jul 41 8 - 14 Oct 3

15 - 21 Jan 16 16 - 22 Apr 29 16 - 22 Jul 42 15 - 21 Oct 4

22 - 28 Jan 17 23 - 29 Apr 30 23 - 30 Jul 43 22 - 28 Oct 5

29 Jan - 5 Feb 18 30 Apr - 6 May 31 31 Jul - 5 Aug 44 29 Oct - 4 Nov 6

6 - 11 Feb 19 7 - 13 May 32 6 - 12 Aug 45 5 - 11 Nov 7

12 - 18 Feb 20 14 - 20 May 33 13 - 19 Aug 46 12 - 18 Nov 8

19 - 25 Feb 21 21 - 27 May 34 20 - 26 Aug 47 19 - 25 Nov 9

26 Feb - 4 Mar 22 28 May - 3 Jun 35 27 Aug - 2 Sep 48 26 Nov - 2 Dec 10 5 - 11 Mar 23 4 - 10 Jun 36 3 - 9 Sep 49 3 - 9 Dec 11 12 - 18 Mar 24 11 - 17 Jun 37 10 - 16 Sep 50 10 - 16 Dec 12 19 - 25 Mar 25 18 - 24 Jun 38 17 - 23 Sep 51 17 - 23 Dec 13 26 Mar - 1 Apr 26 25 Jun - 1 Jul 39 24 - 30 Sep 52 24 - 31 Dec

Areal Distribution of Gridded Rainfall SPAS 12081 Warner Park, TN 30Apr2010 l 33 Duration Area 1

6 12 24 48 72 0.4 4.63 15.31 17.77 18.39 19.66 19.71 1

4.58 15.06 17.52 18.12 19.38 19.45 10 4.44 14.98 17.31 18.06 19.15 19.43 25 4.29 14.66 17.08 17.8 19.05 19.24 50 4.04 14.12 16.7 17.33 18.82 19.01 100 3.72 13.21 15.9 16.63 18.51 18.71 150 3.58 12.62 15.37 16.07 18.35 18.48 200 3.43 12.18 14.99 15.78 18.11 18.32 300 3.16 11.56 14.47 15.28 17.85 18.05 400 2.97 11.07 14.08 14.91 17.65 17.85 500 2.81 10.63 13.52 14.61 17.4 17.67 1,000 2.27 8.99 12.55 13.5 16.86 17.05 2,000 1.79 7.41 11.11 12.62 16.14 16.37 5,000 1.38 5.23 9.24 10.93 14.79 15 10,000 0.99 3.76 7.39 8.64 13 13.13 20,000 0.66 2.93 5.44 7.16 11.04 11.15 50,000 0.32 1.58 3.19 4.59 7.63 7.75 53,736 0.3 1.57 2.97 4.34 7.25 7.38

Areal Distribution of Gridded Rainfall l 34

Inflow Development l 35 Excess rainfall convoluted using unit hydrograph (UH) methodology. UHs validated using large historical events.

Seasonal constant baseflow added to sub-basin hydrographs.

Where necessary, runoff hydrographs routed to corresponding channel routing model inflow points.

Channel routing used current geometry, calibrated to large historical events.

Areal Distribution of Gridded Rainfall l 36 Local Intense Precipitation Used previous HMR-56 basis rainfall hydrographs scaled to topical report depths.

1hr 1-sq.-mi. PMP Sites NWS AWA WBN 16.76 13.81 SQN 16.21 13.81 BFN 16.47 11.60

Areal Distribution of Gridded Rainfall l 37 Candidate Event Data Development

Areal Distribution of Gridded Rainfall l 38 Simulation Automation

Areal Distribution of Gridded Rainfall GIS software tool provided in Topical Report, but use must be reviewed by the NRC in a site-specific LAR (NRC review of the software tool has been performed for SQN in the NRC SER (3 24) of the 2023 SQN Hydrology LAR. The tool is identical in the hydrological update LARs for SQN (approved) / WBN (submitted)

/ BFN (planned)).

Tool retrieves transpositionable event data for selected storm type at the point under review.

Interpolates depth from each event specific Depth-Area-Duration (DAD) curve for selected area and durations.

Applies point specific adjustments to selected point value.

Reviews all transpositionable events and selects the maximum event depth found at each point under review for that duration.

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Areal Distribution of Gridded Rainfall l

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Areal Distribution of Gridded Rainfall l

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Areal Distribution of Gridded Rainfall l

l 42

Areal Distribution of Gridded Rainfall -

HMR(current) vs Topical Report (proposed)

Methodology Similarities Both apply depth using GIS surface.

Volumes are calculated within irregular sub-basin boundary areas and applied uniformly over each sub-basin.

Methodology Differences Historical method prescribes storm event rainfall pattern regardless of location characteristics.

Historical patterns do not provide means for analysis of smaller area PMP embedded within larger area PMP.

Proposed gridded rainfall application based on allowable event transposition adjusted for specific location and elevation.

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Areal Distribution of Gridded Rainfall l

Gridded Rainfall Advantages Evaluated historical storm event data through 2014.

Avoids subjectivity in over/under application of rainfall on different sized watersheds based on assumed pattern.

Provides more detailed analysis by application of PMP rainfall based on transpositionable events to specific points.

Allows different project PMP comparison beyond project watershed for more effective review of embedded events.

Facilitates application of rainfall depth for embedded PMP events.

Follows a repeatable process to determine precipitation depth.

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Areal Distribution of Gridded Rainfall l

Selection of a given Area of Interest (AOI) above a project defines the location of PMP depth from combined grid point DAD curves.

DAD curve is area specific and does not prescribe rainfall outside AOI.

Due to possibility of postulated failures during project PMPs, must assess impact of embedded PMP events.

Analysis of embedded PMP events requires development of a PMP event nesting methodology.

Objective:

Application of PMP rainfall volume in an AOI that meets the defined grid point DAD curve data for PMP volume.

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Areal Distribution of Gridded Rainfall l

Due to number of possible PMP combinations for 61 modeled sub-basins above Wheeler Dam Simplifying Assumptions for PMP Nesting Criteria:

AOIs may be defined only by selecting adjacent sub-basins.

AOIs may be defined only by the set of sub-basins between TVA river projects.

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Areal Distribution of Gridded Rainfall Example Calculation:

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Areal Distribution of Gridded Rainfall l

FP = Fort Patrick Henry CR - Cherokee Sub-basin #

9 10 11 12 13 14&15 for AOI Sub-basin Area (sq.mi.) 703.25 468.25 667.67 62.77 668.89 854.63 Total Area (sq.mi.)

PMP Depth (inches) 72hr Duration FP PMP Depths (inches) 14.30 18.74 14.34 15.07 1,901.94 15.43 72hr Duration CR PMP Depths (inches) 13.75 17.92 13.78 14.54 13.25 14.76 3,425.46 14.49 Raw 72hr Duration PMP at CR (inches) 14.30 18.74 14.34 15.07 13.25 14.76 3,425.46 14.84 Adjustment Factor = 0.95 72hr Duration Adjusted PMP at CR (inches) 14.30 18.74 14.34 15.07 12.52 13.95 3,425.46 14.49 Sub-basin #

9 10 11 12 13 14&15 Sub-basin Area (sq.mi.)

703.25 468.25 667.67 62.77 668.89 854.63 Volume (in sq.mi.-in.)

72hr Duration FP PMP Depths (inches) 14.30 18.74 14.34 15.07 29,350.73 72hr Duration CR PMP Depths (inches) 13.75 17.92 13.78 14.54 13.25 14.76 49,645.21 Unadjusted 72hr Duration CR Depths (inches) 13.25 14.76 21,470.85 Adjustment Factor (49645.2055 - 29350.7335) / 21470.8532 = 0.95 48

Areal Distribution of Gridded Rainfall l

Comparison with Previous HMR Based PMPs System 72hr PMP Above Projects (in inches) 49 Project Area (sq.mi.)

21400DSM WEH_05 GUH_01 Douglas 4,543 17.33 8.21 12.20 Cherokee 352 13.31 7.97 11.18 Fort Loudoun 9,549 16.84 8.18 11.67 Fontana 1,571 19.77 10.50 21.33 Tellico 2,626 20.01 10.81 18.22 Ft. Loudoun - Tellico 12,175 17.52 8.75 13.09 Norris 2,913 13.64 7.43 8.25 Watts Bar 17,294 16.78 8.69 11.75 Above Hiwassee 968 18.45 12.28 22.17 Above Apalachia 1,018 18.43 12.31 21.59 Above Blue Ridge 232 21.32 13.17 23.77 Chickamauga 20,781 16.73 9.20 12.06 Nickajack 21,853 16.49 9.43 11.99 Guntersville 24,452 15.77 9.93 11.83 Wheeler 29,593 14.14 11.47 11.47

Temporal Distribution of Gridded Rainfall Topical report historical event data used to develop temporal distributions.

Event data were statistically analyzed to develop storm type specific temporal distributions.

Four General event and one Tropical event temporal distributions were developed.

One antecedent event distribution was applied.

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Temporal Distribution of Gridded Rainfall l

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Seasonality of Simulated Events Topical Report: three likely storm types; Local, General &

Tropical Storm types are independent (i.e., do not mix)

Local limited to watersheds 500 sq.-mi. or less Tropical and General event types reviewed for seasonality Reviewed appropriate storm type run over weeks 11-23 Results used as guide for PMF simulation start dates Losses applied seasonally using Antecedent Precipitation Index (API) methodology.

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