ML24040A206

Issuance of Amendment Nos. 367 and 361 Regarding Changes to the Hydrologic Analyses (EPID L-2020-LLA-0004) - Public

ML24040A206
Person / Time
Site:	Sequoyah
Issue date:	03/26/2024
From:	Perry Buckberg Plant Licensing Branch II
To:	Jim Barstow Tennessee Valley Authority
Wentzel M
Shared Package
ML24040A207	List: ML24040A206
References
EPID L-2020-LLA-0004
Download: ML24040A206 (1)

Text

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March 26, 2024

James Barstow Vice President, Nuclear Regulatory Affairs and Support Services Tennessee Valley Authority 1101 Market Street, LP 4A-C Chattanooga, TN 37402-2801

SUBJECT:

SEQUOYAH NUCLEAR PLANT, UNITS 1 AND 2 - ISSUANCE OF AMENDMENT NOS. 367 AND 361 RE GARDING CHANGES TO THE HYDROLOGIC ANALYSES (EPID L-2020-LLA-0004)

Dear James Barstow:

The U.S. Nuclear Regulatory Commission (NRC, the Commission) has issued the enclosed Amendment Nos. 367 and 361 to Renewed Facility Operating License Nos. DPR-77 and DPR-79, for the Sequoyah Nuclear Plant, Units 1 and 2 (Sequoyah), respectively. The amendments consist of changes to the Sequoyah Updated Final Safety Analysis Report (UFSAR) in response to the Tennessee Valley Authority application dated January 14, 2020 (Agencywide Documents Access and Management System Accession No. ML20016A396), as supplemented by letters dated February 18, 2020, May 14, 2020, August 12, 2020, November 10, 2020, June 15, 2021, August 13, 2021, December 21, 2021, August 22, 2022, and April 11, 2023 (ML20049H184, ML20135H067, ML20225A170, ML20328A093, ML21203A335, ML21239A115, ML22013A278, ML22251A139, and ML23101A178, respectively).

The amendments revise the Sequoyah UFSAR to reflect the results from the new hydrologic analysis.

The NRC staff has determined that Enclosure 3 contains Critical Electric Infrastructure Information (CEII). Accordingly, the NRC staff has prepared a redacted version, which is provided as Enclosure 4. CEII text will be contained within double brackets (( this is CEII text )).

to this letter contains CEII information. When separated from Enclosure 3, this document is decontrolled.

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J. Barstow A copy of the Safety Evaluation is also enclosed. Notice of Issuance will be included in the Commissions monthly Federal Register notice.

Sincerely,

/RA/

Perry H. Buckberg, Senior Project Manager Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation

Docket Nos. 50-327 and 50-328

Enclosures:

1. Amendment No. 367 to DPR-77

2. Amendment No. 361 to DPR-79

3. Safety Evaluation (CEII)

4. Safety Evaluation (Redacted Version)

cc: Listserv w/o Enclosure 3

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DOCKET NO. 50-327

SEQUOYAH NUCLEAR PLANT, UNIT 1

AMENDMENT TO RENEWED FACI LITY OPERATING LICENSE

Amendment No. 367 Renewed License No. DPR-77

1. The U.S. Nuclear Regulatory Commission (the Commission) has found that:

A. The application for amendment by the Tennessee Valley Authority (the licensee) dated January 14, 2020, as supplemented as supplemented by letters dated February 18, 2020, May 14, 2020, August 12, 2020, November 10, 2020, June 15, 2021, August 13, 2021, December 21, 2021, August 22, 2022, and April 11, 2023, complies with the standards and requirements of the Atomic Energy Act of 1954, as amended (the Act), and the Commissions rules and regulations set forth in Title 10 of the Code of Federal Regulations (10 CFR) Chapter I;

B. The facility will operate in conformity with the application, the provisions of the Act, and the rules and regulations of the Commission;

C. There is reasonable assurance (i) that the activities authorized by this amendment can be conducted without endangering the health and safety of the public, and (ii) that such activities will be conducted in compliance with the Commissions regulations;

D. The issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public; and

E. The issuance of this amendment is in accordance with 10 CFR Part 51 of the Commissions regulations and all applicable requirements have been satisfied.

Enclosure 1

2. Accordingly, by Amendment No. 367, Renewed Facility Operating License No. DPR-77, is hereby amended based on a revision to the Updated Final Safey Analysis Report related to new site hydrologic analysis changes, as described in the application as supplemented.

3. This license amendment is effective as of the date of its issuance and shall be implemented within 60 days.

FOR THE NUCLEAR REGULATORY COMMISSION

/RA/

David Wrona, Chief Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation

Date of Issuance: March 26, 2024

TENNESSEE VALLEY AUTHORITY

DOCKET NO. 50-328

SEQUOYAH NUCLEAR PLANT, UNIT 2

AMENDMENT TO RENEWED FACI LITY OPERATING LICENSE

Amendment No. 361 Renewed License No. DPR-79

1. The U.S. Nuclear Regulatory Commission (the Commission) has found that:

A. The application for amendment by the Tennessee Valley Authority (the licensee) dated January 14, 2020, as supplemented as supplemented by letters dated February 18, 2020, May 14, 2020, August 12, 2020, November 10, 2020, June 15, 2021, August 13, 2021, December 21, 2021, August 22, 2022, and April 11, 2023, complies with the standards and requirements of the Atomic Energy Act of 1954, as amended (the Act), and the Commissions rules and regulations set forth in Title 10 of the Code of Federal Regulations (10 CFR)

Chapter I;

B. The facility will operate in conformity with the application, the provisions of the Act, and the rules and regulations of the Commission;

C. There is reasonable assurance (i) that the activities authorized by this amendment can be conducted without endangering the health and safety of the public, and (ii) that such activities will be conducted in compliance with the Commissions regulations;

D. The issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public; and

E. The issuance of this amendment is in accordance with 10 CFR Part 51 of the Commissions regulations and all applicable requirements have been satisfied.

Enclosure 2

2. Accordingly, by Amendment No. 361, Renewed Facility Operating License No. DPR-79, is hereby amended based on a revision to the Updated Final Safey Analysis Report related to new site hydrologic analysis changes, as described in the application as supplemented.

3. This license amendment is effective as of the date of its issuance and shall be implemented within 60 days.

FOR THE NUCLEAR REGULATORY COMMISSION

/RA/

David Wrona, Chief Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation

Date of Issuance: March 26, 2024

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION

AMENDMENT NO. 367 TO RENEWED FACILITY OPERATING LICENSE NO. DPR-77

AMENDMENT NO. 361 TO RENEWED FACILITY OPERATING LICENSE NO. DPR-79

SEQUOYAH NUCLEAR PLANT, UNITS 1 AND 2

TENNESSEE VALLEY AUTHORITY

DOCKET NOS. 50-327 AND 50-328

1.0 INTRODUCTION

By letter dated January 14, 2020 (Agencywide Documents Access and Management System Accession No. ML20016A396), as supplemented by letters dated February 18, 2020, May 14, 2020, August 12, 2020, November 10, 2020, June 15, 2021, August 13, 2021, December 21, 2021, August 22, 2022, and April 11, 2023 (ML20049H184, ML20135H067, ML20225A170, ML20328A093, ML21203A335, ML21239A115, ML22013A278, ML22251A139, and ML23101A179, respectively), Tennessee Valley Authority (TVA) requested an amendment to the Renewed Facility Operating Licenses for Sequoyah Nuclear Plant, Units 1 and 2 (Sequoyah or SQN), to revise the Updated Final Safety Analysis Report (UFSAR) to reflect the results from a new hydrologic analysis.

To support its review, the Nuclear Regulatory Commission (NRC) staff requested additional information from the on April 14, 2020, July 1, 2020, and September 14, 2020 (ML20106F104, ML20189A211, and ML20261H417 respectively). The licensees supplemental response letters, including the license amendment request (LAR) revision dated April 11, 2023, provided additional information that clarified the application, did not expand the scope of the application as originally noticed, and did not change the NRC staffs original proposed no significant hazards consideration determination as published in the Federal Register on May 5, 2020 (85 FR 26723).

This safety evaluation is separated into two portions:

(1) Section 2 regards the Quality Assurance (QA) of the licensees activities used to qualify/dedicate the software tools described in the LAR, which were used to estimate precipitation totals.

(2) Section 3 regards the Sequoyah site flooding based on the estimated precipitation totals.

Enclosure 4

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2.0 SOFTWARE DEDICATION

2.1 Introduction

As described in LAR Attachment A, Topical Report PMP [Probable Maximum Precipitation]

Evaluation Tool Description, to Enclosure 5 of the LAR, two software tools were used to develop the watershed average rainfall over the TVA sub-basins. These tools include the PMP Evaluation Tool and an automation tool that runs the Arc Geographic Information System (ArcGIS) and Quantum GIS (QGIS) software to create PMP depth estimates across various storm durations and areas at each TVA sub-basin. On February 18, 2020, TVA provided a supplement to the LAR (ML20049H184) to clarify descriptions of minor changes to the PMP Evaluation Tool.

Attachment A to Enclosure 5 of the January 14, 2020, LAR states that the PMP depth data produced from the PMP Evaluation Tool provides weighted average PMP depths over the TVA project sub-basins. The PMP Evaluation Tool described in TVA Topical Report TVA-NPG-AWA16-A, TVA Overall Basin Probable Maximum Precipitation and Local Intense Precipitation Analysis Calculation CDQ0000002016000041, Revision 1 (ML19010A212), was commercially dedicated. The NRC safety evaluati on report (SER) approving this Topical Report (ML19010A212) stated that the PMP Evaluation Tool was excluded from the NRC staffs safety evaluation.

On April 14, 2020, the NRC staff issued a Request for Additional Information (RAI)-IQVB-1 and RAI-IQVB-2 (ML20106F104) on the dedication activities performed on the PMP Evaluation Tool.

In its response dated May 14, 2020 (ML20135H067), the licensee submitted Barge Solutions LLCs (the third-party dedicator) SDR-16-01, Software Dedication Report [(SDR)] (proprietary).

In its August 13, 2021, partial RAI response (ML21239A115), the licensee submitted Barge Solutions LLCs SDR-16-01, Software Dedication Report-PMP Evaluation Tool Package, Revision 3 (proprietary). In response to needed SDR corrections, on December 21, 2021, the licensee submitted SDR Revision 4 (ML22013A278) (hereafter referred to as the PMP Evaluation Tool SDR) for the PMP Evaluation Tool (proprietary).

The NRC staffs evaluation of the PMP Evaluation Tool, ArcGIS and QGIS software, and the automated scripts used to run these tools are described in Section 2.3 of this SER.

2.2 Regulatory Basis

The regulations in Title 10 of the Code of Federal Regulations (10 CFR) Section 50.90, Application for amendment of license, construction permit or early site permit, set forth the NRCs regulatory requirements regarding amendment s of the license. Specifically, 10 CFR 50.34(b)(6)(ii) requires information to be provided regarding the managerial and administrative controls to be used to assure safe operation, including a discussion of how applicable requirements within Appendix B, Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants, to 10 CFR Part 50 are satisfied. The regulations in 10 CFR Part 21, Reporting of Defects and Noncompliance, Section 21.3, Definitions, defines the term dedication, in part as, an acceptance process undertaken to provide reasonable assurance that a commercial grade item to be used as a basic component will perform its intended safety and, in this respect deemed equivalent to an item designed and manufactured under a 10 CFR Part 50, Appendix B quality assurance program. In all cases, the dedication process must be

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conducted in accordance with applicable provisions of 10 CFR Part 50, Appendix B. The applicable criteria in Appendix B to 10 CFR Part 50 include:

Criterion II, Quality Assurance Program, which requires, in part, that the applicant shall identify the [structures, systems and components (SSCs)] to be covered by the quality assurance program...

Criterion III, Design Control, which requires, in part, that Measures shall also be established for the selection and review for suitability of application of materials, parts, equipment, and processes that are essential to the safety-related functions of

[SSCs]...The design control measures shall provide for verifying or checking the adequacy of design, such as by the performance of design reviews, by the use of alternate or simplified calculational methods, or by the performance of a suitable testing program.

Criterion VII, Control of Purchased Material, equipment, and Services, which requires, in part, that measures be established to assure that purchased material, equipment, and services, whether purchased directly or through contractors and subcontractors, conform to the procurement documents...Documentary evidence that material and equipment conform to the procurement requirements shall be available at the nuclear power plant or fuel reprocessing plant site prior to installation or use of such material and equipment.

Applicable regulatory guidance that provides one acceptable means to meet the above stated regulations include:

Regulatory Guide (RG) 1.28, Quality Assurance Program Criteria (Design and Construction), Revision 4, endorses, with clarifications and regulatory positions, the American Society of Mechanical Engineers (ASME) Nuclear Quality Assurance (NQA)-1-2008, Quality Assurance Requirements for Nuclear Facility Applications, standard and the NQA-1a-2009 Addenda. 1

RG 1.164, Dedication of Commercial-Grade Items for Use in Nuclear Power Plants, Revision 0, endorses, in part, the Electric Power Research Institute (EPRI) 3002002982, Revision 1 to EPRI NP-5652 and Technical Report (TR)-102260, Plant Engineering:

Guideline for the Acceptance of Commercial-Grade Items in Nuclear Safety-Related Applications, with respect to acceptance of commercial-grade dedication of items and services to be used as basic components for nuclear power plants.

RG 1.231, Acceptance of Commercial-Grade Design and Analysis Computer Programs Used in the Safety-Related Applications for Nuclear Power Plants, Revision 0, endorses Revision 1 of the EPRI TR-1025243, Plant Engineering: Guideline for the Acceptance of Commercial-Grade Design and Analysis Computer Programs Used in Nuclear Safety-Related Applications, with respec t to acceptance of commercial-grade design and analysis computer programs associated with basic components for nuclear power plants.

1 Topical Report TVA-NPG-AWA16-A, Revision 1 referenced NQA-1-2008 as the quality assurance standard used.

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EPRI TR-1025243, Revision 1, provides a methodology for safety classification of nonprocess computer programs that are not resident or embedded plant SSCs. This TR provides guidance for using commercial-grade dedication methodology to accept commercially procured computer program that performs a safety-related functions, including examples of critical characteristics for software and methods for verifying these critical characteristics that are consistent with the four methods described in EPRI 3002002982.

2.3 Technical Evaluation - Software Dedication

Different methods are used to validate the acceptability of software tools that are used to perform analyses to establish design requirements of safety-related SSCs. For this LAR, the hydrological analyses results derived from the output of the software tools, including the PMP Evaluation Tool, ArcGIS and QGIS, and the scripts used to run these tools, will be used to support establishing design requirements for flood protection of safety-related SSCs at Sequoyah. The summary of the methods used by the licensee to validate the acceptability of each of these software tools and the NRC staffs safety evaluation for each software tool are described below.

2.3.1 PMP Evaluation Tool

Attachment A to Enclosure 5 of the LAR states that the PMP Evaluation Tool computes the depth-area-duration (DAD) values for a defined watershed at grid points at set intervals of area and duration across the Tennessee Valley watershed above Kentucky Dam and the Great Falls project basin. The tool performs this computation using inputs of the storm type, storm duration and selected areas from a user and DAD table lookups. The DAD table values used were approved storm data values for the TVA sub-basins evaluated in the NRC staff SER for Topical Report TVA-NPG-AWA16-A, Revision 1. Attachment A to Enclosure 5 of the LAR states:

The PMP Evaluation Tool provided in the Topical Report Tool retrieves transpositionable event data for selected storm type at the point under review, interpolates the cumulative rainfall depth from each event specific DAD curve for selected area and duration, applies point specific adjustments to selected point value and selects the maximum cumulative rainfall depth found at the grid point under review for that duration. The resulting grid point rainfall depth data set may be a composite event with different storm events controlling at different durations.

This provides a bounding event data set at each grid point.

Attachment A to Enclosure 5 of the LAR states that the PMP Evaluation Tool was used, as provided, as part of commercial grade dedication, with some minor modifications. These modifications include removal of the capability to create raster files, removal of the script used to create TVA storm data, removal of the capability to extrapolate beyond the storm area range in each Storm Precipitation Analysis System (SPAS) event file, and password protecting the toolbox to prevent the user from modifying the tool. In the February 18, 2020, supplement to the LAR, the licensee stated, in part:

The dedication of the PMP Evaluation Tool was performed under the Barge Design Solutions (Barge) Nuclear Quality Assurance (QA) programThe Barge QA program has been audited and accepted by TVA and is on the TVA Acceptable Supplier List (ASL).

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In RAI-IQVB-2, the NRC staff requested that that the licensee provide information to demonstrate that the minor modifications made to the PMP Evaluation Tool do not invalidate the results of the dedication of this tool and that these modifications were conducted under TVAs quality assurance program. In the response to this RAI dated May 14, 2020, the licensee stated that the description in Attachment A to Enclosure 5 of the LAR is clarified by moving the statement, as part of the software dedication because these modifications were part of the dedication activities performed by Barge Solutions, as discussed in Section 2 of the PMP Evaluation Tool SDR.

Section 3 of the PMP Evaluation Tool SDR states that the PMP Evaluation Tool package has been classified as safety-related since it:

affects the SSCs of nuclear facilities because the script produces rainfall data used as an essential input to calculate water surface elevations at the facility.

The water surface elevation impacts the design criteria for the nuclear facility to prevent, for example, flooding or halting of pump systems.

Section 4 of the PMP Evaluation Tool SDR states that the methodology used to verify and validate the critical characteristics of the PMP Evaluation Tool include qualitative analysis of the PMP Evaluation Tool software script and quantitative analysis of the software output accuracy by comparison with manual calculations using a series of test cases. Table 1 in Attachment 5 to the SDR contains the critical characteristics for the PMP Evaluation Tool, the method for verifying each critical characteristic, the associated acceptance criteria, the objective evidence used for acceptance of the critical characteristic, and the results of the acceptance process.

Section 5 of the PMP Evaluation Tool SDR describes the failure modes and effects analysis approach and results and states that:

Consistent with EPRI 1025243 [Revision 1], reasonable assurance has been provided that the failure modes are not triggered when the software is used because (1) the broad range of special test areas is performed in an environmentequal to the environment in which the software is used, (2) the special test areas robustly challenge the type of calculations performed in the software and addresses the range of input values and (3) the independent calculations demonstrate the software achieves the desired accuracy.

Sections 6 and 7 of the PMP Evaluation Tool SDR describe the series of test case scenarios to verify and validate the functions of the PMP Evaluation Tool. Section 7 states, in part:

selected areas were chosen to represent areas both listedand between valuesin the DAD tables and to bracket known watershed area sizesSince area values in the DAD data sets vary, certain valueswere selected to ensure final values were interpolated between DAD values for all SPAS events.

The NRC staff evaluated the information in the LAR, as supplemented, applicable to the PMP Evaluation Tool to determine whether the dedication activities described meet the applicable criteria of Appendix B to 10 CFR Part 50. The NRC staff performed a Regulatory Audit of the licensees dedication activities for the PMP Evaluation Tool to verify that these activities were performed consistent with the descriptions provided in the LAR and to identify additional

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information that needs to be on the docket. The results of the NRC staffs audit are described in the staffs Audit Summary (ML24039A079). The NRC reviewed the licensees evaluation of the functions required of the PMP Evaluation Tool to determine whether these functions are safety-related. Based on the licensees determination that the PMP Evaluation Tool performs safety-related functions and therefore needs to be dedicated as a basic component, the NRC staff concluded the requirement for identification of SSCs that are covered by the licensees QA program in Criterion II of Appendix B to 10 CFR Part 50 has been met. The NRC staff reviewed the critical characteristics identified in Table 1 of Attachment 5 to the PMP Evaluation Tool SDR and the methodology used to verify these critic al characteristics. The NRC staff finds the identified critical characteristics and verification methodology conform to the guidance of EPRI TR-1025243, Plant Engineering: Guideline for the Acceptance of Commercial-Grade Design and Analysis Computer Programs Used in Nuclear Safety-Related Applications, as endorsed by RG 1.231, and therefore, the NRC staff concluded that the licensees process and results for identifying and verifying critical characteristics of the PMP Evaluation Tool meet the requirements of Criterion III of Appendix B to 10 CFR Part 50.

The NRC staff performed an analysis of the PMP Evaluation Tool with a select sample of test areas and verified that the PMP Evaluation Tool produces results which match the values in Topical Report TVA-NPG-AWA16-A with only slight differences in which interpolations of gridded PMP were included in the calculations. These slight differences were attributed to the NRC staffs analysis not fully replicating the process of interpolation performed by the PMP Evaluation Tool. The NRC staff sampled the manual computation results performed by the licensee and verified that the results of the sampled computations are consistent with the PMP Evaluation Tool output. Based on the results of the NRC staffs analysis and the review of licensees manual computations, the NRC staff concluded that the acceptance activities and results meet the requirements of Criterion VII of Appendix B to 10 CFR Part 50. Therefore, the NRC staff concluded that the dedication activities and results of the PMP Evaluation Tool are consistent with the definition of dedication in 10 CFR Part 21.

2.3.2 ArcGIS and QGIS

The ArcGIS and QGIS software libraries are used to create PMP depth estimates across various storm durations and areas at each TVA sub-basin. The licensee described the method used to calculate the watershed average rainfall over these model sub-basins in the response to RAI-IQVB-1 dated May 14, 2020. The licensee stated that calculations made within ArcGIS outside of the PMP Evaluation Tool were checked via alternate methodologies within QGIS by comparison of the average basin rainfall depth determined by the two different GIS methodologies (software libraries) for specific grid locations within the sub-basin area. The following steps describe how this comparison is performed:

1. An event-specific Python script runs the dedicated PMP Evaluation Tool to determine the gridded PMP Point Data for specific durations and storm types.

2. A different Python Script runs the ArcGIS tool using the gridded PMP Point Data calculated in Step 1 to produce the ArcGIS Sub basin Weighted Average PMP Depths.

3. Similarly, a Python script runs the QGIS tool using the gridded PMP Point Data calculated in Step 1.

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4. An Excel Macro is used to compare the results generated by ArcGIS in Step 2 and the results generated by QGIS in Step 3.

In the response dated May 14, 2020, Table 6.5.2, Differences between QGIS and ArcGIS computed Sub-basin Average Depths for all Duration and Storm Types, in Appendix A of Calculation No. CDQ0000002016000044, GIS PMP Event Depth Computation, Revision 0, shows the average and maximum differences calculated over all the durations and storm types encompassed in the hydrologic analysis for eac h TVA sub-basin. This table shows that the maximum differences between ArcGIS and QGISs calculated average depth at a particular grid point is 0.25 inches (0.74 percent) and an average of 0.13 inches (0.58 percent) for all grid points within a sub-basin among all the durations and storm types. Further, this table shows that the average differences between ArcGIS and QGISs calculated average depth at a particular grid point is 0.03 inches (0.21 percent) and an average of 0.00 inches 2 (-0.02 percent) for all grid points within a sub-basin among all the durations and storm types. The licensee concluded in Section 8 of Calculation No. CDQ0000002016000044, Revision 0, Appendix A that the differences between the computations performed using ArcGIS and QGIS are within the precision required for the hydrologic analysis.

The NRC staff evaluated the licensees method for validating the ArcGIS software library for compliance to Criterion III to Appendix B of 10 CFR Part 50. Criterion III states, in part, The design control measures shall provide for verifying or checking the adequacy of design, such as by the performance of design reviews, by the use of alternate or simplified calculational methods, or by the performance of a suitable testing program. In accordance with NQA-1-2008 and the 2009 Addenda, Part II, Subpart 2.7, Paragraph 202, The appropriate software engineering elements, described in para. 202 of this Subpart, shall define the control points and associated reviews. Reviews of software shall ensure compliance with the approved software design requirements. When review alone is not adequate to determine if requirements are met, alternate calculations shall be used, or tests shall be developed and integrated into the appropriate activities of the software development cycle. In RAI-IQVB-1, the NRC staff requested the licensee to provide information that demonstrates that ArcGIS and QGIS are sufficiently diverse such that the potential for these tools to produce the same erroneous outputs are significantly reduced. In its response to RAI-IQVB-1 dated May 14, 2020, the licensee identified the differences between ArcGIS and QGIS to demonstrate these two software tools are sufficiently diverse such that a common error in the software is unlikely and therefore, allows the output of one tool to validate the output of the other tool. These differences include:

ArcGIS is an industry standard software package for large scale geospatial analysis that was developed by Environmental Systems Research Institute (ESRI).

ESRI does not provide access or distribute the source code to the public or other developers without proper licensing. Alternatively, QGIS is an independently developed open-source software that utilizes contributions from multiple sources including non-governmental organizations and academic research. QGIS is also widely used in the industry and academia for large scale geospatial analysis.

The sub-basin average rainfall depths used for the PMP analysis were computed using two different calculation methods applied within the ArcGIS and QGIS software packages. ArcGIS uses polygon volume tool to process the triangular irregular network (TIN) surface to compute sub-basin average PMP depths. The

2 The difference is below the 2 decimal places precision used for the depth calculation.

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TIN surface is created using gridded shapefile data. QGIS utilizes grid statistics tool within QGIS to compute the sub-basin average PMP depths for a raster surface. The raster surface is created by using the nearest neighbor geoprocessing tool.

Based on these differences in the ArcGIS and QGIS software tool development and computational methodologies for the sub-basin average rainfall depths, the NRC staff determined that the likelihood of these tools producing an erroneous output due to a common error within the software is sufficiently low. Therefore, the NRC staff concluded the use of QGIS as an alternate calculational method to check the adequacy of the ArcGIS meets the requirement for establishing design control measures to verify the adequacy of design in Criterion III of Appendix B to 10 CFR Part 50.

2.3.3 Python Scripts for Running the PMP Evaluation Tool and ArcGIS and QGIS software libraries and Microsoft Excel Templates

Attachment A, Topical Report PMP Evaluation Tool Description, to LAR Enclosure 5, Evaluation of Proposed Changes, states that automation routines were developed utilizing both the Visual Basic Applications (VBA) and Python programming language to run the PMP Evaluation Tool and perform subsequent ArcGIS processing to produce weighted average PMP depths over model sub-basins. A template Microsoft Excel workbook (using VBA) was developed to execute the PMP Evaluation Tool, write a project PMP specific Python script for ArcGIS processing, execute the Python script and retrieve/store of ArcGIS computed weighted average depths. A separate Python script was developed to allow GIS processing of the gridded precipitation within the QGIS software environment. The licensee provided Figure 1 within its response to RAI-IQVB-1 dated May 14, 2020, to show the verification methods used for key elements in the development of the gridded PMP. This figure shows that the Python scripts used to generate the ArcGIS Base, run the PMP Evaluation Tool, and command the ArcGIS and QGIS were verified using design review. Calculation No. CDQ0000002016000044, Gridded Probable Maximum Precipitation Development, Revision 0, Computer Program Application, as it relates to Microsoft Excel, identifies that each template application is verified using hand calculations. The NRC staff evaluated the licens ees method for validating the Python scripts and Microsoft Excel templates created for automating the PMP computation process. Based on the use of design review under Barge Solutions QA program to verify the adequacy of these software tools and the qualification of Barge Solutions as an ASL for the licensee, the NRC staff concluded that the licensee has demonstrated that the requirement for establishing design control measures to verify the adequacy of design in Criterion III of Appendix B to 10 CFR Part 50 have been met for these tools.

2.4 Technical Conclusion - Software Dedication Evaluation

The NRC staff reviewed the licensees QA porti on of the software tools described in the submittal for the SQN Units 1 and 2 hydrology analyses LAR and its supplemental submittals, including the PMP Evaluation Tool SDR, Revision 4. The NRC staff determined the QA activities performed to qualify these tools for use in the SQN Units 1 and 2 hydrology analyses meet the applicable requirements of Appendix B to 10 CFR Pa rt 50. Specifically, the NRC staff concluded that the dedication activities and results of PMP Evaluation Tool meets the requirements of Criterion II, III, and VII of Appendix B to 10 CFR Part 50. The NRC staff also concluded that the design review activities performed for ArcGIS and the Python scripts and Excel templates

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created to automate the PMP computation process meet the requirements of Criterion III to 10 CFR Part 50.

3.0 SITE FLOODING

3.1 Introduction

The licensee proposed changes that fulfill Commitment Number 2 of the June 25, 2012, Confirmatory Action Letter (ML12165A527). The changes include the updated hydrologic analysis methods and results, the error correction in the calculation of overbank storage volume in the U.S. Army Corps of Engineers (USACE) Hydrology Engineering Center River Analysis System (HEC-RAS) simulation model, and the updates of the seismically induced dam failure flooding analysis consistent with the most recent relevant NRC guidance in NRC JLD-ISG-2013-01, Interim Staff Guidance for Assessment of Flooding Hazards Due to Dam Failure (ML13151A153).

3.2 Regulatory Evaluation

3.2.1 Hydrologic Analysis

The proposed amendments revise the Sequoyah UFSAR including methodology changes in the analysis of flooding hazards and changes in the results from the new analysis. The licensee used both the HEC-HMS and HEC-RAS models for the hydrologic and flood analyses. The HEC-RAS model was used to simulate the flood profiles in the Tennessee River watershed and the Sequoyah site resulting from PMP and local intense precipitation (LIP) events, as well as upstream dam breach outflows. The HEC-HMS model was used for hydrologic analysis to simulate the flow rates resulting from PMP events in the Tennessee River watershed.

Regarding the local flow rates from the LIP at Sequoyah, the licensee made the rainfall without rainfall loss and directly made surface runoff equal to the rainfall. In the LAR, the licensee indicated that the LIP event is not the controlling flood event and its associated flood elevations does not exceed the floor elevations of the plant. The licensee updated the flood hazard analysis (ML23101A180 (not publicly available)) with the NRC approved PMP and LIP included in topical report TVA-NPG-AWA16-A.

Pursuant to 10 CFR 50.59, Changes, tests and experiments, and 10 CFR 50.90 for revising the UFSAR, the licensee proposed a flood hazard protection scheme for Sequoyah and changed the targeted flood elevation for the initial warning prior to executing abnormal operating steps for the plant shutdown or other easily revocable steps. Based on observed rain amounts, the licensee proposed increasing the flood level from 703 feet (ft) to 705 ft before updating the flood forecast. However, for temporary protection of the safety-related structures, systems, and components (SSCs), the licensees equipment and facilities in flood mode either are designed for submerged operation or otherwise protected.

3.2.2 Summary of the Technical Changes

Details of revisions to proposed UFSAR Section 2.4, Hydrologic Engineering, are as follows:

(1) The licensee proposed to change flood hazard considerations and their design bases.

To determine the probable maximum flood (PMF), the licensee changed the design basis storms from the National Weather Service Hydrometeorological Reports (HMRs),

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HMR 41 and HMR 56, to the gridded probable maximum precipitation (PMP) of topical report TVA-NPG-AWA16-A, Revision 1.

(2) Regarding the flood hazard related to dam failures, the licensee updated dam stability analysis methods and factors of safety. The updates are aligned with the requirements of TVA Dam Safety Design and consistent with the guidance of the Federal Energy Regulatory Commission (FERC) and additional guidance from the USACE and U.S.

Bureau of Reclamation (USBR). The updates revised the flood simulation resulting from hypothetical dam failures induced by seismic events, which are aligned with and extended from RG 1.59 to current regulatory guidance provided in NRC JLD-ISG-2013-01. The licensee updated the flood simulation resulting from dam failures with seismic events using HEC-RAS model.

(3) The numerical model for flood profile simulation is changed. The licensee adopted the HEC-RAS model to replace TVAs legacy model, Simulated Open Channel Hydraulics (SOCH), for flood profile simulation.

(4) The numerical model used for hydrologic surface runoff simulation is changed. The licensee replaced the original sub-programs of the SOCH model with the USACE Hydrology Engineering Center Hydrologic Modeling System (HEC-HMS) model.

(5) The targeted river flood elevation used to establish the flood warning time is changed.

The timing of the power plant shutdown sequence is updated based on changes in flood hazard considerations.

(6) The wind wave heights at critical structures of the plant are updated based on additional wind data from airports and surrounding sites.

(7) The previous median antecedent precipitation index (API) value is replaced by weekly initial API values for computing rainfall excess or rainfall loss.

3.2.2.1 Executable Files for HEC-RAS and HEC-HMS Models of the LAR supplement dated August 22, 2022, contains a non-publicly available digital versatile disc (DVD) that provided executable files to the HEC-RAS model for the computed maximum flood elevation at the Sequoyah site and the HEC-HMS model for converting precipitations to surface runoff.

Since the NRC staff notes that the different versions of the HEC-RAS and HEC-HMS models could produce the computed numerical flood elevations in different accuracy within 0.1 foot, the NRC staff has therefore considered this limit in the technical review on updating licensees design basis flood elevation.

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In Table 2 of the Appendix to this safety evaluation, the NRC staff presents the key numerical differences between the current license bases and the licensees updates from current hydrologic and hydraulic analyses.

3.2.3 Regulations and Guidance

The following items are NRC regulatory requirements:

10 CFR 50, Appendix A, General Design Criteria for Nuclear Power Plants, GDC 2, is related to consideration of the most severe of the natural phenomena that have been historically reported for the site and surrounding area, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated.

10 CFR 50, Appendix B, Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants, is related to requirements related to an applicants quality assurance program.

The following items are related NRC regulatory guidance and other relevant NRC documents:

NUREG/CR-7046, Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America (ML11321A195), discusses the sensitivities of the rainfall-to-runoff transformation for peaking and lagging the unit hydrographs.

NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition, Section 2.4.1, Hydrologic Description, is related to the hydrological setting and data used for site safety analysis.

NUREG-0800, Section 2.4.2, Floods, is related to hydrologic engineering and flood analysis, and it also covers the potential effects of LIP.

NUREG-0800, Section 2.4.3, Probably Maximum Flood (PMF) on Streams and Rivers, is related to the hydrometeorological design basis that is developed to determine the extent of any flood protection required for those SSCs necessary to ensure the capability to shut down the reactor and maintain it in a safe shutdown condition.

NUREG-0800, Section 2.4.4, Potential Dam Failures, is related to hydrological design basis being developed to ensure that potential flood hazards to the safety-related SSCs resulting from the failures of water control structures at upstream, downstream, and onsite are considered.

NUREG-0800, Section 2.4.10, Flooding Protections Requirements, is related to flood protection on safety-related SSCs, and emergency procedures under flood hazard conditions.

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NUREG-0800, Section 2.4.14, Technical Specifications and Emergency Operation Requirements, is related to the actions specified in the technical specifications to shut down the plant and take appropriate emergency measures when the site is susceptible to flooding.

Regulatory Guide 1.59, Design Basis Floods for Nuclear Power Plants (ML003740388), as supplemented by current best practices, provides guidance for developing the hydrometeorological design basis to develop a critical flood.

Regulatory Guide 1.102, Flood Protection for Nuclear Power Plants (ML003740308),

describes acceptable flood protection to prevent the safety-related facilities from being adversely affected.

Japan Lessons-Learned Project Directorate, JLD-ISG -2013-01.

3.3 Technical Evaluation - Site Flooding

3.3.1 Methodology Changes

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The NRC staff reviewed the licensees updated PMP and the computational models of HEC-RAS and HEC-HMS (ML22251A167, non-publicly available). The NRC staff finds that the licensee developed the updated PMP from NRC approved topical report TVA-NPG-AWA16-A, and the licensee updated the hydrologic simulation from similar modeling used in the Clinch River Early Site Permit (CRESP) application in 2019 (ML17289B151, non-publicly available),

and the LAR that resulted in the 2015 Watts Bar Nuclear Plant, Unit 1 license amendment regarding changes to the hydrology analysis (ML15005A314, non-publicly available).

Based on NRCs approval of similar methodologies used in these previous licensing actions, as well as the NRC staffs detailed evaluation described in sub-sections 3.3.1.1 through 3.3.1.4 of this safety evaluation, the NRC staff determines that the methodology changes for PMP and the hydrologic simulations are acceptable.

3.3.1.1 Revised PMP Estimates

In the proposed revisions of UFSAR, the PMP has been updated as described in Section 2 of the safety evaluation and as mentioned in Table 1 of the Appendix to this safety evaluation. This updated PMP event can produce the worst potential flooding event to the Sequoyah site when compared to other flooding events, such as a flood wave created by hypothetical dam failures due to seismic events in areas upstream of the Sequoyah site.

The licensee previously determined the PMP using HMR 41 methodology to generate the controlling storm in a 21,400 square-mile area for the Sequoyah site. In the LAR, the licensee

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adopts the gridded PMP presented in Topical Report TVA-NPG-AWA16-A to update the previous PMP.

3.3.1.1.1 Application of PMP Evaluation Tool

Searching the worst flooding case generated by the gridded PMP, the licensee applied the PMP Evaluation Tool and combined it with a nesting methodology to create different primary and secondary storm areas above Chickamauga Dam. The Sequoyah site is at 13.5 miles upstream of the dam. The nesting methodology is TVAs method to convert the gridded PMP into the areal PMP in each basin of Tennessee River watershed.

In the nesting methodology, the licensee relocated a primary storm area to various upstream areas within the Tennessee River watershed above the dam. The primary storm area can be a selected sub-basin or a group of sub-basins above the dam. The licensee nested other sub-basins adjacent to the primary storm area as secondary storm areas and expanded the nested sub-basins outward in a series of other secondary storm areas to compose a total storm size.

Comparing the flooding results from the various cases that composed different primary and secondary storm areas, the licensee presents the worst flooding case. That is, the storm of locating the primary storm area in the Blue Ridge basin and its secondary storm areas in the sub-watersheds of the Hiwassee River, the Holston River, the French Broad, the Clinch River, the Little Tennessee River, and the Tennessee River.

The NRC staff reviewed the licensees gridded PMP distributions in various primary and secondary areas. The licensee relocated the primary storm area in the Hiwassee River watershed near to the Sequoyah site while considering storm types, rainfall spatial and temporal distributions, and seasonal rainfall losses. The NRC staff reviewed the licensees flood simulations for the various cases of primary and secondary storm areas and find that the licensees worst flooding case is reasonable.

The NRC staff finds that the licensee produced hundreds of storm cases using the nesting methodology. Therefore, the NRC staff determines that the licensee has searched sufficient PMP cases that could be used for multiple flood simulations at the Sequoyah site.

Based on above, the NRC staff concludes that the licensee has presented the methodology to search the most severe phenomena to meet 10 CFR 50, Appendix A, GDC 2.

3.3.1.1.2 Rainfall Temporal Distribution for PMP

Different peak rainfall intensity in early, middle, or late occurrences can produce different flood elevations at the Sequoyah site.

Prior to submitting the LAR, the temporal distribution of the PMP had a middle peak occurrence based on HMR 41. For the update, the licensee set rainfall temporal distribution in three different distributions for PMP events. They are the same as the early, middle, or late occurrences, but the licensee described them in different terms, front-, medial-, and back-loaded variations of the temporal rainfall distribution. Those distributions are based on multiple meteorologically feasible rainfall data and statistic methods as described in Volume 2 and 9 of the National Oceanic and Atmospheric Administration Atlas 14 (Bonnin, etc., 2006; Perica 2013).

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The NRC staff reviewed the temporal distributio ns as presented in the licensees HEC-HMS model. In the model, the NRC staff finds that the licensee has set the temporal distributions of the rainfall among different sub-basins and the distributions are consistent with the LAR. The NRC staff accepts the updated rainfall temporal distributions since the distributions are meteorologically feasible, as well as being developed with real rainfall data and established statistical methods.

3.3.1.2 Revised Flood Simulations

The licensees changes include using the HEC-HMS hydrologic model and the HEC-RAS flood simulation model. The HEC-HMS model transforms rain fall into surface runoff and stream flows.

The HEC-RAS model calculates flood profiles from unsteady stream flows, flood waves due to hydrological and seismic dam failures, and outflows regulated by reservoir operational rules for flood control. HEC-RAS and HEC-HMS are based on advance computational techniques and replace the legacy model, SOCH, developed in 1967, that was originally used for the flood simulation.

The licensee calibrated the hydrologic and flood simulation models based on two observed large flood events in the Tennessee River watershed. The NRC staff finds that the licensee set up the models that are based on existing stream cross sections with reasonable hydraulic and hydrologic parameters. Therefore, the NRC staff concludes that the licensees hydrologic and hydraulic models (HEC-HMS and HEC-RAS) used for the flood simulations are acceptable. The HEC-RAS and HEC-HMS models have been developed by the US Army Corps of Engineers Hydrologic Center and are commonly used in hydrologic engineering design. Therefore, the NRC staff determines that the models are acceptable.

The NRC staff reviewed the flood simulations and checked the changes of the PMF elevations resulting from the modern hydrologic and hydraulic models. Based on the Sequoyah flood elevation increase of 0.2 ft for the design basis flood, the NRC staff finds that the change in the design basis PMF elevation is within the margin of error for estimating water surface elevations from floods of this magnitude and therefore fi nds the PMF elevation to be acceptable.

3.3.1.3 Initial API Values to Estimate Effective Rainfall Depths Prior to submitting the LAR, the licensee applied antecedent precipitation index method (APIM) to determining rainfall excess (effective rainfall depth). The rainfall excess is equivalent to a surface runoff depth in the storm area. The APIM has an established muti-variable relationship that was developed from observed rainfall and stream data. TVA has used this multi-variable relationship for approximately 60 years to determine the effective rainfall depths as input data to estimate day-to-day stream flow rates for their flood control system. The muti-variables are a specific week of a year of a storm occurrence, a geographic location of interest area, and an API for the storm. Those three variables are key requirements in the APIM to determine an excess rainfall.

The APIM is a recursive model to estimate excess rainfall, and it needs an initial API value to start the modeling. The initial API value is to represent the initial condition of soil moisture content at the beginning of antecedent storm. To assign the initial values of API and to estimate the effective rainfall depths in different basins, the licensee previously adopted median API value derived from past observed records. The licensee applied the median API as the initial API value to the HMRs PMP events. Currently, the licensee proposed a different approach

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using weekly initial API values for different weeks of a year to replace the median API value.

The update is based on 18 years of National Weather Service (NWS) radar record from 1997 to 2015 and is applied to TVAs updated gridded PMP event. The updated initial API values are applicable for different basins in the Tennessee River watershed.

TVA developed the multi-variable relationship based on an empirical analysis. During NRC staffs regulatory audit (ML24039A083, non-publicly available), the licensees calculation packages were reviewed and the NRC staff finds that the LAR and the calculation packages were consistent. The NRC staff confirmed that the updated weekly initial API in the LAR is based on 18 years (1997 to 2015) of NWS record. The NRC staff confirmed that the multi-variable relationship has been applied to TVAs daily operation in their flood control system. The NRC staff accepts that the licensee has adopted the NWS data to make a reasonable change of the average weekly initial API for each basin in the Tennessee River watershed. Since the multi-variable relationship is the analysis results from realistic rainfall-runoff values, the NRC staff accepts that the updated initial API values in the APIM are applicable in TVAs daily dam operation that can affect the flood elevations at the Sequoyah site. Further, when comparing the previous median API, the NRC staff finds that the weekly initial API is more appropriate to represent the initial API in various seasons. With that comparison, the NRC staff determines that the weekly initial API values among the updated muti-variable relationship can be properly applied to a seasonal PMP event for estimating excess rainfall in the Tennessee River watershed.

In addition, the licensee has tested the multi-variable relationship with other methodologies in rainfall-runoff modeling and found that the APIM is a conservative method. To confirm the licensees APIM to be conservative, the NRC staff calculated the excess rainfall using the SCS runoff curve number methodology as an option provided in the HEC-HMS model. The NRC staff finds that the licensees excess rainfall is greater than the one estimated by the SCS runoff curve number approach. Based on the results of NRC staffs different approach, the NRC staff confirms that the licensees API method is cons ervative. Therefore, the NRC staff accepts the updated APIM for estimating effective rainfall depths.

3.3.1.4 Revised Computation of Surface Runoffs

In the LAR, the licensee applied the validated unit hydrograph to convolve effective rainfall depths into surface runoffs. The validated unit hydrographs were approved by NRC in a 2015 Watts Bar Nuclear Plant Unit 1 license am endment (ML15005A314, non-publicly available). The NRC staff determines that the NRC approved unit hydrographs are applicable to the Sequoyah site because Watts Bar Nuclear Plant and Sequoyah are in the same watershed and their locations are immediately upstream and downstream from each other.

In the LAR, the licensee adopted the HEC-HMS model to compute the surface runoffs with effective rainfall depths (ML22251A167, non-publicly available). As required by the model input, the licensee translated the previously validated unit hydrographs into the input format (ML22251A166, non-publicly available) of the HEC-HMS model. The input format is in terms of S-graphs as described in the HEC-HMS users manual, which are the accumulation flow volumes in percentage of the unit hydrographs. The NRC staff reviewed the licensees HEC-HMS model and finds that the S-graphs could reproduce the unit hydrographs. Therefore, the S-graphs have preserved the primary quality that made the S-graphs themselves equivalent to the unit hydrographs. The NRC staff accepts that the S-graphs could be used as the part of input to the model. Given the acceptable S-graphs used in the model and the other adequate

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hydrologic parameters as inputs to the HE C-HMS model, the NRC staff concludes that HEC-HMS model and the S-graphs can be used to develop the surface runoffs with effective rainfall depths of a PMP event.

3.3.2 Dam Failures

The failures of dams are additional factors to be considered in the flood hazard analysis for the Sequoyah site. The licensee postulated dam failures that may occur during a PMP event or during seismic events in the watershed. In the hypothetical cases of dam failures induced by the PMP event or seismic events, the licensee updated the dam failure analysis consistent with JLD-ISG-2013-01, Interim Staff Guidance for Assessment of Flooding Hazards Due to Dam Failure (ML13151A153). The licensee set the dams to fail when the analysis results are outside of the documented analysis of dam safety and stability.

The NRC staff finds that the licensee adjusted da m failure parameters for the dams in the flood simulations. The adjustments for the dams in the simulations include: (1) initial reservoir level, (2) reservoir inflows, (3) breach sectional configuration, and (4) the scripted instantaneous dam failure at specific timings or headwater levels. The NRC staff confirmed that the adjustments met the hydrological conditions of the dams. The NRC staff determines that the adjustments in the simulations are acceptable since the licens ee included the primary parameters to critically affect the breach outflows.

The NRC staff reviewed the dam failure effects on the flood profiles with the licensees dam failure parameters in the flood simulations. The NRC staff accepts the licensees analysis of dam failures in the flood simulations since the dam failure parameters align with NRC guidance and TVAs Dam Safety criteria.

3.3.2.1 Seismic Failures of Dams

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structures combined with a 500-year flood is a controlling flood event at the Sequoyah site. The NRC staff finds that the licensee adopted the 500-year flood that was in the same storm area in the Tennessee River watershed and was approved by NRC in 2019 as included in the CRESP (ML19162A166, non-publicly available). Therefore, the NRC staff determines that the 500-year flood in the LAR is applicable to the Sequoyah site.

In the LAR, the licensee set some, but not all, of the dams owned by TVA to fail during seismic events. The dams selected for failure were found to have a lower factor of safety outside accepted values or were not investigated by TVA. The NRC staff previously approved the same criteria including setting TVA dams for failures in the licensing action on CRESP. For additional conservatism, inflows from relevant dams found from the National Inventory of Dams that were not explicitly included in the model were added to the results.

3.3.2.2. Hydrologic Failures of Dams

In the updated dam safety analysis for the dynamic flow conditions induced by PMP events against the dams, the licensee used the dam stability acceptance criteria that are based on TVAs Dam Safety procedures. Flood control outlets such as spillways, spillway gates, lock gates, are limited to the original design flow capacity. Otherwise, for any exceedance over the design capacity, a failure is assumed.

The licensee analyzed global stability of concrete dams for the maximum headwater and corresponding tailwater levels that could occur in PMP events. The licensee stated that the sliding factor of safety is 1.3, as a minimum, when cohesion in the interface of the dam bottom is not considered. The acceptable factor of safety is 1.4 for embankments of dams for the PMP events. The licensee used TVAs Dam Safety procedures to perform global stability analysis for the embankments. The analysis included the maximum differences between the headwater and corresponding tailwater levels during various PMP events.

The NRC staff accepts the licensees global stab ility analysis for the dam safety because the licensee adopted TVA Dam Safety criteria to distinguish either the dams failure or stability under PMP events. The NRC staff determines the licensee s factors of safety were reasonable since those factors provide sufficient safety margin.

For the hydrological consideration, the NRC staff evaluated the dam failures combined with the PMP events. Using the licensees flood simulati on model, HEC-RAS, the NRC staff checked the timing of the dam failures. The NRC staff determines that the simulation outputs showed the instantaneous uprise hydrographs to match the dam breach moment. Based on the NRC staffs verification on the simulation outputs, the NRC staff concludes that the dam failures are adequately simulated by the model for the hydrological events.

3.3.2.3 Breach Outflows from Dam Failures

For the concrete dams in non-failure condition, the licensee defined the dam outflows that are controlled by dam rating curves and reservoir operational rules. For the concrete dams in failure condition, the licensee set a control flow section meet the adjacent upstream stream cross-section as the outflow cross-section. If the upstream stream cross-section is larger than the downstream sections, the control section is switched to downstream stream cross-section.

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The NRC staff reviewed the licensees breach outflows simulated by the HEC-RAS model.

Based on the modeling, the NRC staff determines that the licensees analysis using rating curves and operational rules for the control of outflows from concrete dams in non-failure conditions are adequate. Regarding the hypothetical failures, the NRC staff finds that the licensee setting the smaller cross-section at the dam between upstream and downstream as a control is reasonable since the smaller cross-section restricts the outflows from dam failures.

When the overtopping outflows occur for any earthen embankments or dams, the licensee assumed those structures to fail. The licensee defined earthen dams outflow of the failure section differently from concrete dams. The licensee set breach sections of earthen embankment dams are based on Von Thun and Gillette method (Von Thun, etc., 1990).

The NRC staff reviewed the licensees breach parame ters that define the outflow section of the failure of earthen embankments. The NRC staff conf irms that the licensees breach sections were developed from the Von Thun and Gillette method. Since the method is commonly used in the dam safety design and analysis to estimate the breach section for earthen dam failure outflows, the NRC staff accepts the breach sections determined by the method.

In the case of seismic failures of any dams listed in the National Inventory of Dams and not owned by TVA, the licensee used the Froehlich method (Froehlich, 1995) to determine the peak outflows.

The licensee assumed that the Chickamauga Dam, which is immediately downstream of Sequoyah, does not fail. This scenario leads to the worst-case site flooding due to heightened water surface elevations due to backwater effects.

The NRC staff reviewed the licensees unstead y flow rules that were embedded in the HEC-RAS model. The unsteady flow rules contain rating curves and reservoir operational rules that control dam discharges without dam failures combined with the dam failure parameters of the Von Thun and Gillette method that control earthen embankment breach outflows. The NRC staff reviewed the scripts of the unsteady flow rules and confirms that the licensee adequately set the unsteady flow rules in the HEC-RAS model to simulate the outflows with and without dam failures.

Based on the consistency between the rating curves generated by the model and the curves described in the LAR, the NRC staff concludes t hat the licensee applied the unsteady flow rules to adequately simulate the dam outflows.

The NRC staff accepts the licensees use of the Froehlich method to estimate the peak outflows for dam breach since those peak outflows are the results from a best-fit regression equation which is reasonably considered with the reservoir volume and reservoir water level.

The NRC staff accepts the licensees assumption of the Chickamauga Dam remaining in intact.

The NRC staff determines that the licensees assumption of non-failure of the Chickamauga Dam is reasonable since the assumption is conservative relative to the Sequoyah site.

The licensees methods for estimating the outflows of those failure dams are based on the same NRC approved methods previously addressed in the CRESP. Therefore, the NRC staff accepts the licensees methods in the LAR and resulting dam failure outflows and the dam failures.

However, the NRC staff finds that the estimated outflows of one-half of the 10 -4 annual

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exceedance probability ground motion and combined with a 500-year flood are bounded by PMP events.

3.3.3 Controlling Event for Site Flooding

In the updated hydrologic analysis for the site flooding, the licensee searched for a controlling event resulting from various hypothetical flood hazards. The sources of the hazards include LIP, PMP, and dam breach outflows due to dam failures.

3.3.3.1 LIP and PMP Updates In the LAR, the licensee indicated that the existing design basis flood elevation due to a LIP event is below 706 ft for the Sequoyah site. The LIP event was developed based on HMR 56 using storm intensities for the maximum 1-hour rainfall. To update the LIP flood, the licensee directly quoted the updated LIP event from Topical Report TVA-NPG-AWA16-A. Starting from the updated LIP, the licensee used the HEC-RAS model to simulate the local flood elevation.

The simulated flood elevations remain below the floor elevation of 706 ft at critical water intrusion locations of the Reactor, Auxiliary, Control, and Turbine Buildings.

For LIP events, the NRC staff finds that the licensee alternated the PMP temporal distributions as described in sub-section 3.3.1.1.2 of this safety evaluation to search for the maximum local flood elevation at the Sequoyah site using twelve 5-minute short time intervals for the shorter time period associated with a LIP event.

The NRC staff determines that the updated flood elevation resulting from the LIP update remains below 706 ft because the updated LIP rainfall depth in one hour and one square mile is reduced from the previous LIP rainfall depth of 16.21 inches to 13.81 inches.

In the LAR, the licensee indicated that the previous PMP event for the Tennessee River watershed area is a nine-day event with a three-day antecedent storm postulated to occur three days prior to a three-day PMP storm. The previous PMP was defined by HMR 41, and its rainfall depths were distributed in the watershed with isohyets described in HMR 41. The updated PMP in the LAR has no isohyets and is instead gridded rainfall depths within the watershed. The grid size is in a network of 0.025 by 0.025 decimal degrees in longitude and latitude of the geographic coordinate system as a grid cell, which has an approximate area of 2.5 square-miles for a total of 12,966 grid points above Wheeler Dam. The gridded PMP rainfall depths are derived based on Topical Report of TVA-NPG-AWA16-A. The updated average rainfall depth in the watershed area above Chickamauga Dam is 12.06 inches, which is reduced from the 16.25 inch depth in the previous PMP average that was developed based on HMR 41.

The NRC staff accepts the licensees updated PMP to replace the existing PMP since the licensee updated the PMP that is based on the NRC approved Topical Report of TVA-NPG-AWA16_A.

3.3.3.2 Site Flooding Due to PMP Update

The licensee demonstrated that flooding at the Sequoyah site using the updated, gridded PMP in the primary storm area above Blue Ridge Dam and the secondary larger area over multiple rainfall drainage areas above Wheeler Dam constitutes the controlling event. The updated

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controlling event of the site flooding is based on the flood simulation results of the updated PMP events.

The licensee has performed hundreds of flood simulations with various combinations of primary and secondary areas of the gridded PMP storms. In the simulations, the licensee included rainfall loss using API method to estimate rainfall loss and effective rainfall depth. The licensee used the unit hydrograph method to convert the effective rainfall depth to surface runoff. The licensee used the HEC-HMS model to calculate the surface runoff which is used as the input data to the HEC-RAS model for flood simulations. The NRC staff previously accepted both models for surface runoff and flood simulations that were used in the 2014 Watts Bar LAR. The licensee adopted the same modeling procedures with effective rainfall depths calculated from the updated PMP storms using the same HEC-RAS model to simulate the flood profiles of different surface runoff flows from the HEC-HMS model.

As input to the licensees HEC-HMS model, the licensee computed effective rainfall depths for PMP storms using the API method. The NRC staff accepts the API method because the multi-variable relationship used in the API method is based on observed data and the method is used in the daily river operations of the TVA water management system.

The NRC staff reviewed the input and output of flood simulation for the models and finds that the licensee has adequately simulated the flood profiles for the site with acceptable hydrologic parameters used in the modeling.

The licensee adjusted the unit hydrographs by increasing the peak 20% and decreasing the time-to-peak by one-third to reflect the non-linearity between the effective rainfall depth and surface runoff during an extreme large flood, such as a PMF event. The NRC staff finds that the retention effect of multiple dams caused non-significant impact on the PMF elevation at the Sequoyah site when the adjusted unit hydrograp h was considered in the flood simulations.

Therefore, the NRC staff concludes that the non -linearity does not have a notable impact on the PMF elevation at the Sequoyah site.

Based on above, the NRC staff concludes that the flood simulations with the updated PMP storm are reasonable, and the simulation results can be used to update the highest PMF elevation at the Sequoyah site.

3.3.3.3 Site Flooding due to Other Events

Both LIP events and seismic failure of upstream dams combined with a 500-year flood event are not the controlling Sequoyah site flooding events when compared to the PMP events.

3.3.3.3.1 Simulation Flood Elevation for LIP Event

The licensee utilized the local storm as LIP rainfall defined in Topical Report of TVA-NPG-AWA16-A that was approved by the NRC. Therefore, the NRC staff accepts that the licensee used the updated LIP for the Sequoyah site as input to the site-specific flood simulation based on HEC-RAS model.

The NRC staff noted that the licensees HEC-RA S model includes six interconnected storage areas for the western plant site and three interconnected storage areas for the eastern plant site. In the total plant area, the sub-drainage areas are spread in the south, north, west, and

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east portions of the surface drainage system of the Sequoyah site. The NRC staff finds that those interconnections with the storage areas, the spread sub-drainage areas, and the HEC-RAS model were reviewed and finds acceptable by the NRC staff for the Sequoyah flood hazard re-evaluation in 2016 (ML16188A274, not publicly available). Based on the previous analysis, the NRC staff accepts the model to be used in the LAR for updating the flood elevation of the LIP event.

The NRC staff finds that the hydraulic parameters used in the licensees HEC-RAS model for the LIP event are within acceptable ranges and finds that the licensee has adequately applied the LIP rainfall as input to the HEC-RAS model. Consequently, the NRC staff accepts the computational results that indicate the flood due to a LIP event is below the critical floor elevation 706 ft.

3.3.3.3.2 Simulation Flood Elevation for Seismic Events

The existing seismic failures of the upstream dams above the Sequoyah site were developed based on a postulated Safety-Shut Down earthquake (SSE) and Operating Basis earthquake (OBE). The updated seismic failures of the dams are based on probabilistic seismic hazard analysis (PSHA). Two seismic criteria are used in the update: one is the seismic hazard with an annual exceedance probability of 10 -4 combined with a 25-year flood, the other is the seismic hazard defined by one-half of the 10 -4 annual exceedance probability ground motion combined with a 500-year flood. The 25-year and 500-year floods have less magnitude when compared to a PMF resulting from a PMP storm. The licensee used the HEC-RAS model to simulate various cases of dam failures (PSHA) combined with a 500-year flood demonstrating that the highest resulting flood elevation at the Sequoyah site is 707.1 ft, which is 12.7 ft below the flood elevation of site flooding due to the PMP storm.

The NRC staff has reviewed the licensees calculat ions (Enclosure 2 to the August 22, 2022, LAR supplement) that support the results indicated in the LAR Revision 1 for a seismic event combined with a 500-year flood. The NRC staff determines the licensees estimated 500-year flood is reasonable since the flood was calculated from a statistic analysis and based on flood records of the stream gages in the watershed.

The NRC staff finds that the licensee utilized NRC JLD-ISG-2013-01 and followed TVA Dam Safety dam stability criteria to analyze the seismically induced dam failures combined with flooding simulations. The NRC staff finds that the licensee utilized NUREG-2115 along with Electric Power Research Institutes 2004/2006 ground motion prediction models and NUREG-6728 to adopt seismic source characterization for the dams and to define ground motion in vertical to horizontal ratios, respec tively. The NRC staff finds that the updated seismic analysis for the strength of dams is based on two-or three-dimensional finite element method.

Since the seismic events combined with a 25-year or 500-year flood event does not significantly and critically create a controlling event for the site flooding when compared to the PMF resulting from a PMP event, NRC staff did not review t he licensees finite element analysis and other structural stability analysis.

The NRC staff finds that the licensees flood hazard analysis for the seismic failures of dams had the similarity of NRCs approved seismic failure analysis for the CRESP. The NRC staff concludes that the flood hazard resulting from seismic events is acceptable based on the above evaluations and the evaluation addressed in Section 3.3.2.1 of this safety evaluation.

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3.3.4 Target Elevation Changes in Flood Warning

For rainfall induced stream floods, the proposed change of target elevation as a criterion to start flood warning is increased from 703 ft to 705 ft, which is the grade elevation of the Sequoyah site and is 1 foot below the floor elevation of 706 ft. However, the licensee does not change the required warning times for stages 1 and 2. Including multiple dam failures during hydrological events, the licensee used current updated hydrologic simulations to evaluate the minimum time of a flood wave arriving at the site and exceeding the grade elevation. The NRC staff finds that the licensees hydrologic flood wave simulations identified the shortest amount of time needed for the flood elevation to reach 705 ft. The NRC staff verified that this time is longer than the required warning time. Therefore, the NRC staff determines that the change of target elevation from 703 ft to 705 ft has no adverse impacts on the existing licensing basis and flood warning stages as defined currently in the UFSAR.

For floods induced by seismic failures of ups tream dams or any sudden dam failures, the NRC staff finds that the licensee provided stage 1 warning time for 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> to complete the required emergency procedures, and the flood wave arrival time to reach 705 ft would need 28.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />, which is longer than the 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> of stage 1 warning time. The NRC staff confirmed the arrival time of 28.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> is supported by TVAs calculation package during the NRC staffs regulatory audit. Therefore, the NRC staff concludes that the existing flood warning time for seismic dam failure combined with a 500-year flood is valid and remains without changes.

3.3.5 Changes of the Estimated Wind-Induced Waves For the Sequoyah site, the licensee followed Regulatory Guide 1.59 and added a 2-year recurrence wind on the PMF event. The 2-year wind is defined as the wind speed that has a 50 percent chance of being exceeded in any given year. The licensee has updated wind wave effects at the Sequoyah site using more current wind data (2000 to 2014) recorded in regional airports, including Chattanooga, Knoxville, and Tr-Cities, in Tennessee, as well as Asheville, North Carolina; and Huntsville, Alabama. The licensee transferred the wind data of the airports to the Sequoyah site for wind speed analysis, using the weighted distance inverse method.

The licensee also updated critical fetch length along with new wind data and used Pearson Type III probability exceeding curve to determine a 2-year overland wind speed. The computed statistic 2-year overland wind speed at the general area around the Auxiliary, Control, and Shield Buildings is updated to 23.65 miles per hour (mph). The licensee has converted the overland wind speed to the wind speed over surface water. The conversion is based on effective fetch length and the guidance addressed in USACE Engineering Technical Letter 1110-2-8 (USACE, 1966). The results of the conversion provided that the wind speed over the water at the Diesel Generator Building is 28.4 mph, 27.2 mph for the Unit 2 Reactor Building, and 28.4 mph for the Unit 1 Reactor Building, and the Emergency Raw Cooling Water (ERCW)

Pumping Station.

The NRC staff reviewed the licensees wind speed calculations during the regulatory audit. The NRC staff confirms that the licensee provided sufficient data for the calculations, and the calculations were based on USACE Engineering Technical Letter 1110-2-8. The NRC staff confirm that TVAs calculations support the converted wind speed from land surface to water surface. The NRC staff determines that the calculated wind speeds at the Sequoyah buildings are reasonable since the calculations are updated based on additional observed wind data (2000 to 2014) and fit a Pearson Type III probability curve.

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The licensee calculated wind-induced wave height using the calculated wind speeds on water surface with the guidance of USACE Engi neering Technical Letter 1110-2-8. The NRC staff confirmed that the licensees calculations support the wind-induced wave heights at the Diesel Generator Building, ERCW Pumping Station, and both 2 Reactor Buildings indicated in the LAR.

The NRC staff accepts that the computed wind-induced wave heights are reasonable since the licensee used the updated wind speeds and followed the acceptable method, USACE Engineering Technical Letter 1110-2-8, to calculate the wind-induced wave heights.

3.3.6 Conclusions of the Site Flooding Evaluation

Based on the NRC staffs evaluation as addressed in Sections 3.3.1 and 3.3.5 of this safety evaluation, the NRC staff concludes that t he licensees revised flood hazard analyses which includes changing methodologies, considers dam failures, determines the controlling storm event, revises the target elevation of flood warning, and updates wind-induced wave heights, are technically acceptable to support the UFSAR revisions.

Based on the technical evaluation given in Se ctions 3.3.1 through 3.3.3, and 3.3.5, the NRC staff concludes that the licensees updated PMP flood event is a controlling flood hazard at the Sequoyah site and is consistent with NUREG/CR-7046, Regulatory Guide (RG) 1.59, JLD-ISG-2013-01, and NUREG-0800 Sections 2.4.1 through 2.4.4, as listed in Section 3.2.3 of this safety evaluation.

Because the change in the design basis flood PMF elevation is within the margin of error for estimating water surface elevations from floods of this magnitude, the NRC staff concludes that the updated design basis is acceptable and, therefor e, the licensees flood protection measures continue to meet the acceptance criteria of NUREG-0800 Sections 2.4.10 and 2.4.14.

Based on the evaluations addressed above, the NRC staff concludes that the licensees flood hazard analysis presented in the LAR has met NRC requirements consistent with the guidance and other documents listed in Section 3.2.3 of this safety evaluation.

4.0 STATE CONSULTATION

In accordance with the Commissions regulations, the Tennessee State official was notified of the proposed issuance of the amendments on February 2, 2024. The State official had no comments.

5.0 ENVIRONMENTAL CONSIDERATION

The amendments change a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20. The NRC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendments involve no significant hazards consideration, originally published in the Federal Register on May 5, 2020 (85 FR 26723). There has been no public comment on such finding. Accordingly, the amendments meet the eligibility criteria for catego rical exclusion set forth in 10 CFR 51.22(c)(9).

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Pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendments.

6.0 CONCLUSION

The Commission has concluded, based on the considerations discussed above, that: (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) there is reasonable assurance that such activities will be conducted in compliance with the Commissions regulations, and (3) the issuance of the amendment(s) will not be inimical to the common defense and security or to the health and safety of the public.

7.0 REFERENCES

1. Bonnin, Geoffrey M., Deborah Martin, Bingzhang Lin, Tye Parzybok, Michael Yekta, David Riley, NOAA Atlas 14, Precipitation-Frequency Atlas of the United States, Volume 2 Version 3.0: Delaware, District of Columbia, Illinois, Indiana, Kentucky, Maryland, New Jersey, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Virginia, West Virginia, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, Silver Spring, Maryland, 2004, revised 2006.

2. Froehlich, David C., 1995, Peak Outflow from Breached Embankment Dam, Journal of Water Resources Planning and Managemen t, vol. 121, no. 1, p. 90-97.

3. Perica, Sanja, Deborah Martin, Sandra Pavlovic, Ishani Roy, Michael St. Laurent, Carl Trypaluk, Dale Unruh, Michael Yekta, Geoffrey Bonnin, NOAA Atlas 14, Precipitation-Frequency Atlas of the United States, Volume 9 Version 2.0: Alabama, Arkansas, Florida, Georgia, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Weather Service, Silver Spring, Maryland, 2013.

4. USACE, 1966, Department of the Army, Office of the Chief of Engineers, Engineering Technical Letter No. 1110-2-8, August 1966.

5. Von Thun, J. Lawrence, and David R. Gillette, 1990, Guidance on Breach Parameters, unpublished internal document, U.S. Bureau of Reclamation, Denver, Colorado, March 13, 1990, 17 p.

Principal Contributors: K. See, NRR D. Zhang, NRR

Date: March 26, 2024

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Table 2: Quantitative and Qualitive Changes to the UFSAR Resulting from the New Hydrologic and Hydraulic Analyses

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