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| {{#Wiki_filter:From: Wentzel, Michael Sent: Tuesday, March 9, 2021 11:34 AM To: Taylor, Andrew Charles | | {{#Wiki_filter:}} |
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| ==Subject:==
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| Sequoyah Nuclear Plant, Units 1 and 2 - Supplement 2 to Audit Plan and Setup of Online Reference Portal (EPID L-2020-LLA-0004)
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| Attachments: Sequoyah Nuclear Plant, Units 1 and 2 - Supplement 2 to Audit Plan and Setup of Online Reference Portal (EPID L-2020-LLA-0004).pdf
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| ==Dear Mr. Taylor,==
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| By {{letter dated|date=January 14, 2020|text=letter dated January 14, 2020}}, (Agencywide Documents Access and Management System (ADAMS)
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| Accession No. ML20016A396); as supplemented by letters dated February 18, May 14, August 12, and November 10, 2020 (ADAMS Accession Nos. ML20049H184, ML20135H067, ML20225A170, and ML20328A093, respectively); the Tennessee Valley Authority (TVA), submitted a license amendment request to revise the Sequoyah Nuclear Plant, Units 1 and 2, Updated Final Safety Analysis Report hydrologic analysis.
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| The U.S. Nuclear Regulatory Commissions (NRCs) External Hazards Branch (EXHB) staff is continuing to review the application and has determined that additional information is necessary to support the ongoing regulatory audit of information supporting the license amendment. The attached Supplement 2 to the NRC staffs September 14, 2020 (ADAMS Accession No. ML20261H418) audit plan identifies the additional information that the staff is requesting.
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| If you have any questions, please contact me at (301) 415 6459 or michael.wentzel@nrc.gov.
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| Sincerely, Michael J. Wentzel, Project Manager Plant Licensing Branch II-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation
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| Hearing Identifier: NRR_DRMA Email Number: 1063 Mail Envelope Properties (BLAPR09MB7267003D3AFC5F20D4B87476E7929)
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| ==Subject:==
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| Sequoyah Nuclear Plant, Units 1 and 2 - Supplement 2 to Audit Plan and Setup of Online Reference Portal (EPID L-2020-LLA-0004)
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| Sent Date: 3/9/2021 11:33:59 AM Received Date: 3/9/2021 11:33:59 AM From: Wentzel, Michael Created By: Michael.Wentzel@nrc.gov Recipients:
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| "Taylor, Andrew Charles" <actaylor@tva.gov>
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| Tracking Status: None Post Office: BLAPR09MB7267.namprd09.prod.outlook.com Files Size Date & Time MESSAGE 1259 3/9/2021 11:33:59 AM Sequoyah Nuclear Plant, Units 1 and 2 - Supplement 2 to Audit Plan and Setup of Online Reference Portal (EPID L-2020-LLA-0004).pdf 48711 Options Priority: Normal Return Notification: No Reply Requested: No Sensitivity: Normal Expiration Date:
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| SUPPLEMENT 2 TO AUDIT PLAN REGARDING LICENSE AMENDMENT REQUEST TO REVISE UPDATED FINAL SAFETY ANALYSIS REPORT HYDROLOGIC ANALYSIS TENNESSEE VALLEY AUTHORITY SEQUOYAH NUCLEAR PLANT, UNITS 1 AND 2 DOCKET NOS. 50-327 AND 50-328 I. BACKGROUND By application dated January 14, 2020 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML20014353A016); as supplemented by letters dated February 18, May 14, August 12, and November 10, 2020 (ADAMS Accession Nos. ML20049H184, ML20135H067, ML20225A170, and ML20328A093, respectively); the Tennessee Valley Authority (TVA; the licensee) submitted a license amendment request (LAR) for Sequoyah Nuclear Plant (Sequoyah), Units 1 and 2. The proposed amendments would revise the Sequoyah, Units 1 and 2, Updated Final Safety Analysis Report (UFSAR) to reflect the results from new hydrologic analysis performed by the licensee.
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| By email dated September 14, 2020 (ADAMS Accession No. ML20261H418), the U.S. Nuclear Regulatory Commission (NRC) staff issued its plan for audit of information that supports the LAR. By email dated November 10, 2020 (ADAMS Accession No. ML20316A018), the NRC staff issued a supplement to the audit plan to include the software dedication process used to support the LAR. Based on the NRC staffs review of information provided in the LAR, as well as its review of information provided as a part of the ongoing audit, the staff is issuing this second supplement to its September 14, 2020, audit plan to request additional information be provided for the NRC staff to review.
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| II. ADDITIONAL INFORMATION REQUEST The NRC staff requests the following additional information be provided on the online reference portal:
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| Information Request (IR) 2.4-3: The Nesting Method for Distributing PMP into Subbasins TVA developed a new probable maximum precipitation (PMP) nesting method, as described in Attachment B of the LAR (ADAMS Accession Number ML20016A396). The purpose of the nesting method is to distribute an estimated basin PMP depths described in the Topical Report (TVA-NPG-AWA16-A) into various sizes of sub-basins used in hydrology modeling. The NRC staff request the following supplemental information in relation to applying the nesting method to the TVAs basin:
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| a) TVA considered an allocation of PMP values across watersheds by combining different sub-basin areas as shown in Table B-2a through B-2e in Attachment B of the LAR. TVA then reviewed multiple nesting scenarios to end up with a bounding peak flood elevation
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| scenario which results in a maximum flood peak runoff at the Sequoyah site. However, detailed information on how these alternative scenarios were set up and analyzed to select a bounding (controlling) scenario for the entire basin are not provided yet.
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| Describe the rationale for establishing plausible alternative PMP nesting scenarios.
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| Provide detailed information on analyzing alternative scenarios to develop a bounding scenario. Clarify how sensitive the alternative nesting scenarios are to the resulting hydrologic analysis, especially to the peak runoff at the Sequoyah site.
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| b) TVA designed the nesting method to allocate higher PMP depths on upstream watersheds, such as the sub-basins above Douglas Dam. This nesting method results in a rainfall pattern which is different compared to the one without nesting (i.e., see data types between raw and final shown on Step 7 of Table B-6 in Attachment B of the LAR). Clarify the potential impacts of placing more PMP volumes at downstream or other parts of the watershed on the peak runoff at the basin outlet. Identify whether TVA has tested more refined nesting scenarios of primary basins (e.g., using smaller areal intervals) other than the ones reported in the LAR. Discuss the correlation between the basins PMP depths (or basins PMP volumes) and peaks of unit hydrographs for sub-basins above Chickamauga Dam and how the correlation (if significant) be used in selecting the selected critical nesting scenario.
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| c) Clarify why more refined inner nesting was not considered for sets of nesting sub-basins, such as the six sub-basins above Douglas Dam. Discuss how the nesting sets of local sub-basins were used in establishing the nesting of the overall Sequoyah basin (i.e.,
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| from the nesting of the Melton Hill Dam basin to the nesting of Chickamauga Dam basin).
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| d) TVA has previously stated that one of the objectives of the nesting method is to justify the most critical flood risk scenarios at TVA nuclear plant sites. Key flood parameters in risk analyses, if a site is inundated by flooding, include peak flood level as well as time to peak, duration of inundation, warning time, and recession time. Discuss how these flood parameters were considered in establishing alternative PMP distribution scenarios and selecting a bounding one.
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| e) Provide any tools or computer codes used in performing the nesting method applied for the LAR flood modeling, or any calculation documents that describe the process and results of applying the nesting method to the watershed area.}}
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