Tennessee Valley Authority, 1101 Market Street, Chattanooga, Tennessee 37402 CNL-16-083 June 22, 2016

10 CFR 50.4 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001

Browns Ferry Nuclear Plant, Units 1, 2, and 3 Renewed Facility Operating License Nos. DPR-33, DPR-52, and DPR-68 NRC Docket Nos. 50-259, 50-260, and 50-296 Sequoyah Nuclear Plant, Units 1 and 2 Renewed Facility Operating License Nos. DPR-77 and DPR-79 NRC Docket Nos. 50-327 and 50-328 Watts Bar Nuclear Plant, Units 1 and 2 Facility Operating License Nos. NPF-90 and NPF-96 NRC Docket Nos. 50-390 and 50-391

Subject: TENNESSEE VALLEY AUTHORITY - HYDROLOGIC ENGINEERING

CENTER RIVER ANALYSIS SYSTEM (HEC-RAS) PROJECT MILESTONES

During an April 4, 2016 public meeting to discuss technical details of the Tennessee Valley Authority (TVA) HEC-RAS external flood modelling issues, TVA agreed to provide the NRC with more definitive dates for several project deliverables, and to provide a copy of a letter sent from TVA to the United States Army Corps of Engineers (USACE) regarding some identified HEC-RAS model issues. The purpose of this letter is to provide that information. TVA’s presentation during the public meeting included a draft project milestone schedule. Several project deliverables and associated submittals to NRC were discussed in detail. The draft schedule has been further developed and refined, resulting in the project deliverable milestones listed in Table 1 on the following page.

U.S. Nuclear Regulatory Commission CNL-16-083 Page 2 June 22, 2016

TABLE 1 PROJECT DELIVERABLE SUBMITTAL MILESTONES

Project Deliverable

Submittal Milestone

1 Browns Ferry Mitigating Strategies Assessment (MSA) December 2016

2 Sequoyah MSA December 2016

3 Watts Bar MSA December 2016

4 Watts Bar Focused Evaluation (FE) * December 2017 *

5 Sequoyah FE * December 2017 *

6 Browns Ferry FE * December 2017 *

7 Sequoyah Hydrology License Amendment Request (LAR) April 2018

8 Watts Bar Hydrology LAR May 2018

* It is TVA’s plan to perform Focused Evaluations for each site in lieu of Integrated Assessments. These deliverables and dates represent revised commitments from those made in the letters dated March 12, 2015, which transmitted the Browns Ferry, Sequoyah, and Watts Bar Flood Hazard Reevaluation Reports (ML15072A130, ML15071A462, and ML15071A262), respectively. Formal revision of these commitments will be the subject of separate correspondence after the revised guidance for performing integrated assessments and more focused evaluations are issued. TVA will submit status updates on these project deliverable submittal milestones to the NRC on a six-month frequency. The first status update will be submitted no later than December 31, 2016. As discussed in the referenced public meeting, the ongoing HEC-RAS external flood modeling utilizes updated precipitation input data from that currently described in the Browns Ferry, Sequoyah, and Watts Bar Updated Final Safety Analysis Reports. The TVA calculation that documents this updated precipitation data has been completed and issued. As the precipitation input is a fundamental component of the overall external flood modeling, the associated calculation will be a key reference supporting the Sequoyah and Watts Bar LARs listed above.

U.S. Nuclear Regulatory Commission CNL-16-083 Page 3 June 22, 2016

Enclosure 1 of this letter contains the letter from TVA to USAGE regarding the HEC-RAS model issue that was requested by NRC.

Enclosure 2 provides the one new regulatory commitment contained in this letter.

If additional information is needed regarding this update, please contact Russell Thompson at (423) 751-2567.

Enclosures:

1. TVA letter to U.S. Army Corps of Engineers

2. Commitment

cc (Enclosures): NRR Director - NRC Headquarters NRR JLD Director - NRC Headquarters NRC Regional Administrator - Region II NRC Project Manager - Browns Ferry Nuclear Plant NRC Project Manager - Sequoyah Nuclear Plant NRC Project Manager - Watts Bar Nuclear Plant NRC Senior Resident Inspector - Browns Ferry Nuclear Plant NRC Senior Resident Inspector - Sequoyah Nuclear Plant NRC Senior Resident Inspector - Watts Bar Nuclear Plant

CNL-16-083 ENCLOSURE 1

TVA LETTER TO U.S. ARMY CORPS OF ENGINEERS

B41 160307 002

USACE staff confirmed that the HEC-RAS post-processor method and the internal simplified method utilize different calculations, resulting in the reach volume differences discussed previously. However, they concluded that this simplification is not a software bug but instead a misunderstanding of the internal computations on TVA's part. USACE staff indicated that a better representation may be provided in HEC-RAS through the use of more typical techniques, such as more frequent cross sections and storage nodes for large embayments.

A second call was held on January 29, 2016 between Karen Carboni, Curt Jawdy, Trevor Cropp, and Gary Brunner. During this call, the possibility of TVA recalculating the augmented cross-sections with ineffective flow areas to be more compatible I consistent with the HEC-RAS computations was discussed. Mr. Brunner indicated that this approach was "viable."

TVA appreciates the perspectives and insights provided by USACE staff and their offer of additional dialogue on these issues. TVA understands that the methodology it has used to account for storage volumes in the Tennessee River system model could be changed to better apply the HEC-RAS software. USACE staff also indicated the HEC-RAS user's manual has been revised to more clearly identify the averaging equation used in the post-processor and the internal simplified methods, and this revision will be available with the pending release of HEC-RAS Version 5.0.

TVA also understood that USACE may consider developing an additional solution scheme for a future update to HEC-RAS that might be used to assist in modeling of storage volumes.

Please let us know if your staff has a different understanding of what was discussed or the commitments that were made. And, again, thank you for your assistance.

If additional information is needed regarding this letter, please contact Karen Carboni at (423) 751-7136.

Respectfu I ly,

b.m ~ ~ >----Steve Douglas Vice President, Engineering

cc: G. W. Bruner, USACE

CNL-16-083 ENCLOSURE 2

COMMITMENT

1. TVA will submit status updates on the HEC-RAS project deliverable submittal milestones

to the NRC on a six-month frequency. The first status update will be submitted no later than December 31, 2016.