CNL-13-112, Response to NRC Request for Additional Information Related to License Amendment Request to Updated Final Safety Analysis Report Changes Associated with Hydrologic Analysis from Mechanical and Civil Engineering Branch

Response to NRC Request for Additional Information Related to License Amendment Request to Updated Final Safety Analysis Report Changes Associated with Hydrologic Analysis from Mechanical and Civil Engineering Branch

ML13296A025
Person / Time
Site:	Watts Bar
Issue date:	10/21/2013
From:	James Shea Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
CNL-13-112, TAC ME9130
Download: ML13296A025 (16)

Text

Tennessee Valley Authority, 1101 Market Street, Chattanooga, Tennessee 37402 CNL-13-112 October 21, 2013 10 CFR 50.4 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Watts Bar Nuclear Plant, Unit 1 Facility Operating License No. NPF-90 NRC Docket No. 50-390

Subject:

Response to NRC Request for Additional Information Related to License Amendment Request to Updated Final Safety Analysis Report Changes Associated With Hydrologic Analysis From Mechanical And Civil Engineering Branch (TAC ME9130)

References:

1. Letter from TVA to NRC, "Application to Revise Watts Bar Nuclear Plant Unit 1 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, TAC No. ME8200 (WBN-UFSAR-12-01)," dated July 19, 2012 (ADAMS Accession No. ML122360173)

2. Letter from TVA to NRC, "Supplement to Application to Revise Watts Bar Nuclear Plant Unit 1 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (WBN-UFSAR-1 2-01),"

dated March 1, 2013 (ADAMS Accession No. ML13067A393)

3. Letter from TVA to NRC, "Completion of Commitments Related to Updated Hydrologic Analysis Results for Sequoyah Nuclear Plants Units 1 and 2 and Watts Bar Nuclear Plant Unit 1 (TAC Nos. ME8805, ME8806, and ME8807)," dated April 29, 2013 (ADAMS Accession No. ML13126A101)

4. Electronic Mail from Andrew Hon (NRC) to Joseph W. Shea (TVA),

"Watts Bar Nuclear Station, Unit 1 - Request For Additional Information Related To License Amendment Request To Updated Final Safety Analysis Report Changes Associated With Hydrologic Analysis From Mechanical And Civil Engineering Branch (TAC No. ME9130)," dated September 20, 2013 (ADAMS Accession No. ML13263A376)

Printed on recycled paper

U.S. Nuclear Regulatory Commission Page 2 October 21, 2013 By letter dated July 19, 2012 (Reference 1), Tennessee Valley Authority (TVA) submitted a license amendment request (LAR) to revise the Watts Bar Nuclear Plant (WBN), Unit 1 Updated Final Safety Analysis Report (UFSAR) to reflect the results from new hydrologic analysis. The proposed changes in the updated hydrologic analysis include updates to input information, and updates to methodology to include the use of the U.S. Army Corps of Engineers Hydrologic Modeling System and River Analysis System software.

By letter dated March 1, 2013 (Reference 2), TVA supplemented the LAR (Reference 1) with additional information addressing the impact of the updated hydrologic analysis on the chilled water circulating pump motors for Main Control Room Chillers and the 6.9kV Shutdown Board Room Chillers.

By letter dated April 29, 2013 (Reference 3), TVA notified the NRC of the completion of commitments (Nos. 8 through 11) related to the updated hydrologic analysis results.

Commitment No. 8 addressed the installation of a permanent plant modification to provide flood protection with respect to the Design Basis Flood level for the WBN, Unit 1 Thermal Barrier Booster pumps and motors.

On September 20, 2013, the NRC transmitted a request for additional information (RAI) by electronic mail (email) (Reference 4). Enclosure 1 to this letter provides TVA's response to the NRC RAI.

There are no new regulatory commitments included in this submittal. Please address any questions regarding this submittal to Edward D. Schrull at (423) 751-3850.

I declare under penalty of perjury that the foregoing is true and correct. Executed on this 21st day of October 2013.

Respeplly, J.V~.Shea Vice President, Nuclear Licensing

Enclosure:

1. Response to NRC Mechanical and Civil Engineering Branch Request for Additional Information cc (Enclosure):

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Watts Bar Nuclear Plant, Unit 1

ENCLOSURE TENNESSEE VALLEY AUTHORITY WATTS BAR NUCLEAR PLANT, UNIT 1 RESPONSE TO NRC MECHANICAL AND CIVIL ENGINEERING BRANCH REQUEST FOR ADDITIONAL INFORMATION

Subject:

Request For Additional Information Related To License Amendment Request To Updated Final Safety Analysis Report Changes Associated With Hydrologic Analysis From Mechanical And Civil Engineering Branch (TAC NO. ME9130)

Section 2.4.2. Floods NRC RAI EMCB-RAI-2.4.2-1 As stated in section 2.4.2 of Reference 1; the historical flood data has been corrected since the time of the original license issuance and this flood data was used to calibrate the hydrologic (hydraulic) models used for the hydrologic (hydraulic) analysis.

Please provide additional information, regarding the corrections performed to the historical data that was used to calibrate the models. The response should include information on how the corrections to the updated information were incorporated into the hydraulic models and the effect that the corrections had on the analysis outputs. For those corrections that resulted in less conservative assumptions or input than used in the original licensing analysis, provide a detailed explanation of why this is acceptable.

TVA Response - EMCB-RAI-2.4.2-1 The original tabulation referred to in Section 2.4.2.1 of Reference 1 provided elevations and discharges for historic floods at the Watts Bar Nuclear Plant (WBN) site (Tennessee River Mile (TRM) 528.0) from the 1950's up to the time the plant construction permit was issued in the 1970's. The tabulation was changed in the current analysis to reflect historic flood elevations and discharges at the Watts Bar Dam tailwater gage (TRM 529.9) where data was available from the 1950's up to 2007, when the current analysis was initiated.

The elevations shown in the updated table were gaged at the Watts Bar Dam tailwater, and are not corrections to the original elevations shown for WBN. The period from the 1950's to 2007 was reviewed to identify the larger floods that had occurred on the Tennessee River system, not just at Watts Bar Dam, for use in calibration of the models. The floods selected for calibration on the Tennessee River reservoirs were the March 1973 and May 2003 events.

The March 1973 event is the flood of record for the Tennessee River main stem and was used as a calibration event in the previous and current model analysis with no changes. The more recent May 2003 flood event, one of the larger events on the Tennessee River, was also used for calibration of the models. While there may be floods that produce slightly higher Watts Bar Dam discharges or elevations, use of the largest flood for the Tennessee River system produces a more precise model calibration for the system.

No corrections or updates were made that resulted in less conservative assumptions or input than used in the original licensing analysis.

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ENCLOSURE NRC RAI EMCB-RAI-2.4.2-2 WBN Unit 1 FSAR, section 2.4.2.2 states that: "The maximum plant site flood level from any cause is Elevation 734.9." This level was revised in Reference 1 to 739.2 ft. However, Section 3.3 of Reference 1 notes that:

" the elevation to the base plate of the Thermal Barrier Booster (TBB) Pump Motors is 739.3 ft;

" the component is currently not adequately protected against flooding; and

• a temporary barrier will restore additional margin of approximately 0.8 ft above the DBF surge level of 739.7 ft Item Level (location) LAR Proposed Max Flood Elev. 734.9 ft (Plant site) 739.2 ft DBF level 740.1 ft (Aux bldg) 741 ft DBF Surge Level 739.7 ft TBB Pumps 739.3 ft (base plate) (739.7 +.8) = 740.5 ft a) Please explain the difference between the flood level of 739.2 ft (revised Probable Maximum Flood (PMF)) resulting from the maximum site flood level possible without coincident wind wave activity and the revised DBF flood level at the Auxiliary building of 741 ft, including the reason for the difference.

b) Please clarify the difference between the DBF surge level (739.7 ft) and the proposed DBF level of 741 ft. Briefly describe the relationship between these two levels, if any exists?

c) Please explain how the temporary flood barrier that results in a new flood protection elevation of 740.5 ft restores margin to the TBB pumps since the revised DBF level at the Auxiliary building is 741 ft.

TVA Response - EMCB-RAI-2.4.2-2

a. The maximum flood elevation in the table above is defined as the maximum flood level, excluding wind wave activity, produced by precipitation causing flooding of rivers and streams. TVA has referred to this level as a stillwater PMF elevation without coincident wind wave activity (wave setup and runup). The design basis flood (DBF) level at the Auxiliary building is determined by the maximum stillwater PMF elevation coincident with the calculated wind wave activity (wave setup and runup). The 741.0 ft DBF level is based on the calculated maximum flood elevation (739.2 ft) plus the wind wave activity at the auxiliary building (wave runup of 1.7 ft and wind setup of 0.1 ft).

b. The DBF surge level establishes the internal water level design basis in flooded buildings and is determined by adding 0.5 ft to the calculated maximum flood elevation, i.e.,

stillwater PMF. The additional 0.5 ft margin is included to account for water surges in the building interior that result from exterior wind wave activity. The DBF level, discussed above, and the DBF surge level are related via the maximum flood elevation. The effects of wind wave activity on the exterior building flood level are included in the DBF level while interior surges caused by exterior wind wave activity are included in the DBF surge level.

The surge value of 0.5 ft was assigned during initial plant licensing to account for the flood water inside the building and was maintained for the license amendment request.

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ENCLOSURE

c. As described above, the DBF surge level (739.7 ft) is the interior design flood level for the auxiliary building and the DBF level (741 ft) is an exterior design flood level for the auxiliary building. The TBB pumps are located within the auxiliary building; therefore, the design flood level for this equipment is 739.7 ft. The constructed barrier around the TBB pumps is built to an elevation of 740.5 ft, therefore providing approximately 0.8 ft of margin against the (interior) DBF surge level of 739.7 ft.

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ENCLOSURE NRC EMCB-RAI-2.4.2-3 With regards to the barrier installed at the thermal barrier booster pumps to protect them from flooding, please provide the following additional information:

a) A summary of any testing and/or calculations performed that demonstrate that the barrier configuration is structurally sound and adequate to withstand hydrodynamic and hydrostatic forces, earthquake forces, fires and any other relevant hazards and that these hazards will not affect the barrier and/or the TBB pumps.

b) Reference 2 notes in Attachment 1 of Enclosure 1, titled "WBN Unit 1 Thermal Barrier Booster Pump Flood Barrier" that there are "seal welds" located at the tail (note) of the fillet weld symbol. Please provide additional information that describes these seal welds.

c) Regarding Reference 2, Attachment 2, Sheet 3, please explain how the flare bevel weld located between the Tube Steel (TS) 4x4x3/8 (end) and the side of TS 4x4x3/8 was installed and completed. In addition, please provide the calculated strength of this weld based on any analyses performed during design and construction.

TVA Response - EMCB-RAI-2.4.2-3

a. TVA calculation CDQ0006042012000072, Rev. 1, "Auxiliary Building Pump Protection Measures for Revised Probable Maximum Flood (PMF)" documents the structural qualification of the barrier configuration to withstand the applicable hydrostatic and seismic forces. Regarding other relevant hazards, WBN Unit 1 UFSAR, Section 2.4.14.1.2, "Combinations of Events," and WBN design criteria WB-DC-40-29 "Flood Protection Provisions," Section 3.2 state that, "because floods above plant grade, earthquakes, tornadoes, or design basis accidents, including a LOCA, are individually very unlikely, a combination of a flood plus any of these events, or the occurrence of one of these during the flood recovery time, or the occurrence of flood during the recovery time after one of these events, is considered incredible.

As an exception to the non-combination of events, certain reduced levels of floods are considered together with a seismic event discussed in WBN Unit 1 UFSAR, Section 2.4.14.10, "Basis for Flood Protection Plan in Seismic-Caused Dam Failures." In these cases, the seismic event would occur prior to flooding and the.flooding levels for these events would never reach the barrier around the TBB pumps, therefore analysis of hydrodynamic forces for these cases are not applicable. In addition, calculated velocities in the Probable Maximum Flood are low (-2.5 feet per second at the river bank) and do not cause the hydrodynamic forces caused by high velocity floods. A fire by itself cannot affect the TBB barrier. Therefore, a fire during a flood is not considered credible. No other hazards are considered relevant.

Post installation testing was performed to demonstrate the adequacy of the barrier design.

A test containment structure was built along a portion of the exterior of the TBB pump barrier. The tested area included both a vertical wall and horizontal floor interface with the Auxiliary Building concrete structure. The tested area encompassed a vertical and horizontal grouted base plate and vertical and horizontal seal welds on the TBB pump barrier.

The test containment structure was filled and maintained to a level that imparted the designed hydrostatic pressure on the structure and was monitored hourly over a 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> period. At each hourly observation, the interior of the TBB pump barrier was inspected and any noted leakage inside the TBB pump barrier was measured. The leakage over the 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> test was 2 ounces. Extrapolating the recorded leakage across the entire exterior E1-4

ENCLOSURE perimeter of the TBB pump barrier would equate to approximately 19.7 ounces of leakage over a 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> period. This amount of leakage is inconsequential considering the entire enclosed TBB pump area is approximately 10 feet by 13 feet. TVA calculation CDQ000020080054, Rev. 3 documents the maximum level and amount of time the Probable Maximum Flood level would exceed the TBB pump floor elevation to be approximately 46 hours5.324074e-4 days <br />0.0128 hours <br />7.60582e-5 weeks <br />1.7503e-5 months <br />. This demonstrates that the test results are acceptable for the PMF event.

b. Seal welds are provided at barrier corners and transitions to ensure that the flood barrier is water tight. The seal welds do not provide any credited structural strength but are used only to provide a degree of tightness against leakage.

c. The flare bevel welds connecting the TS 4x4x3/8 (end) and TS 4x4x3/8 (side) in Reference 2, Attachment 2, Sheet 3 are not credited for strength in the qualification analysis. Only the 3/16" fillet welds are used for calculated strength (2.78 kips per inch).

The two 3/16" fillet welds and the flare bevel welds were made per approved site welding procedures. Installation of the flare bevel welds included building up to fill the gap between the straight edge of the TS side and the curved edge of the TS end. Since the submittal of Reference 2, revisions were made to this drawing during construction. A note was added to omit the weld on the near side at the field splice points. Previously a note on the drawing required the flare bevel weld to be ground flush to facilitate the installation of the cover plate. Both methods allow for proper installation of the cover plate but omission is acceptable because this weld is not credited in the analysis. The far side flare bevel was not omitted and provides some additional strength that is also not accounted for in the analysis.

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ENCLOSURE Section 2.4.3. ProbableMaximum Floods (PMF) on Streams and Rivers NRC EMCB-RAI-2.4.3-1 Reference 1 notes that in the WBN Unit 1 FSAR, Subsection 2.4.3.1, that the Probable Maximum Precipitation (PMP) is defined for TVA by Hydro-Meteorological Report (HMR)

No. 45 for watersheds above tributary dams. However, FSAR Subsection 2.4.2.3, notes that HMR No. 56 (which superseded HMR No. 45) was used for the PMP for the plant drainage systems based on local intense precipitation.

HMR No. 56 was not used in the updated hydrologic analyses described by Reference 1 which defined the PMP for determining the PMF. Additionally, Reference 1 deleted the reference to HMR No. 45 and added a new reference to HMR No. 56 in the WBN, Unit 1 FSAR, Section 2.4, references section.

Please explain why a reference to HMR No. 56 was added to the revised proposed wording in Reference 1 if it is not used in the updated hydrologic analyses, proposed by Reference 1 to define the PMP for determining the PMF.

TVA Response - EMCB-RAI-2.4.3-1 WBN Unit 1 UFSAR, Subsection 2.4.3.1 states that the "Probable Maximum Precipitation (PMP) for the watershed above Chickamauga and Watts Bar Dams has been defined for TVA by Hydrometeorological Report (HMR) No. 41. Hydrometeorological Report No. 45 defines PMP for watersheds above tributary dams." The second part of this statement regarding HMR No. 45, "Probable Maximum and TVA Precipitation for Tennessee River Basins Up to 3,000 Square Miles in Area and Durations to 72 Hours" is an incorrect statement that is corrected in the Reference 1 submittal. HMR No. 45 was not used to define the PMP for watersheds above tributary dams. The removal of the HMR No. 45 statement corrects the error in UFSAR Subsection 2.4.3.1.

HMR No. 45 was superseded by HMR No. 56, "Probable Maximum and TVA Precipitation Estimates with Areal Distribution for Tennessee River Drainages Less Than 3000 Mi2 in Area" in October 1986. HMR No. 56 is the report that defines the PMP used for local intense precipitation analysis for watersheds above tributary dams and is correctly described in UFSAR Subsection 2.4.2.3 and has been included as a reference to reflect its use in the analysis.

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ENCLOSURE NRC EMCB-RAI-2.4.3-2 With regards to the original hydrologic analysis, WBN Unit 1 FSAR, Section 2.4.3.1 states, in part, that "two basic storm situations were found to have the potential to produce a maximum flood at Watts Bar Nuclear Plant." These two basic storm situations are:

1. The 21,400-square-mile storm and

2. The 7,980-square-mile storm.

Furthermore, Reference 1 notes that the controlling PMP event has changed from the 7,980-square-mile storm (734.9 ft) to the 21,400-square-mile storm (level 739.2 ft). The revised maximum flood elevation levels are the result of various input changes to the hydrologic analysis and updates to the methodology.

Please provide additional information regarding the following:

If only the two basic storm situations (21,400-square-mile storm and 7,980-square-mile storm) were re-evaluated against the updated input changes to the hydrologic analysis and updates to the methodology, describe how these two storm configurations are still bounding in the reevaluated licensing basis and how these storms envelope any other credible storm scenarios assuming these storms were to be updated using the same criterion.

TVA Response - EMCB-RAI-2.4.3-2 As noted in Reference 1, in the previous analysis the 7,980-square-mile storm produced the maximum PMF elevation at the site of 734.9 feet while the 21,400-square-mile storm produced an elevation of 734.7 feet, i.e., 0.2 feet lower. These storms were defined for the Tennessee River watershed by the National Weather Service in HMR No. 41. HMR No. 41 specified the PMP rainfall event depths, areal distribution and temporal distribution to be used in the development of a PMF level on the Tennessee River. In the current analysis, the 21,400-square-mile storm produces a maximum elevation of 739.2 which is higher as a result of various input changes to the hydrologic analysis and updated methodology. While not reported, the calculated PMF level for the updated 7,980-square-mile storm is at an elevation of 738.7 feet, i.e., 0.5 feet lower than the 21,400-square-mile storm. For the current analysis, the 21,400-square-mile storm events were reviewed as defined in HMR No. 41 with a fixed upstream and downstream storm pattern. Based on the rainfall volume above the plant, this evaluation determined that the downstream-centered event would still be controlling. The 7,980-square-mile storm as defined in HMR No. 41 presents an elliptical pattern with the center allowed to move along the long axis of the storm pattern. To determine the critical centering, several locations were selected and evaluated which allowed the location of critical storm rainfall volume to be calculated. Based on these evaluations, the two storm configurations selected were determined to envelop the critical storm scenarios.

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ENCLOSURE NRC EMCB-RAI-2.4.3-3 Reference 1 proposes to change the inputs for defining the antecedent precipitation index (API) using 11 years of historical rainfall records (1997-2007). The WBN Unit 1 FSAR, Subsection 2.4.3.2 describes the current analysis for precipitation losses.

a) What is the range, in terms of years, of the historical data analyzed to determine the median API described in the current FSAR analysis? If the proposed range of the historical data for Reference 1 is less than the data in the FSAR, please provide a justification for using this new proposed range that demonstrates that the range selected for the proposed license amendment request (LAR) is representative and relates precipitation excess to the rainfall, week of the year, geographic location and API.

b) Please explain and justify the selection of the year 2007 as the upper range of the revised historical rainfall records, otherwise, justify why the latest rainfall records were not used.

TVA Response - EMCB-RAI-2.4.3-3 The median starting API value for this analysis was determined using the available on-line USGS stream flow data from 1997 to 2007. Variable median API values, ranging from 0.78 to 1.29, were used for this analysis as compared to a fixed median API value of 1.0 for the previous analysis. The variable median API values from 1997 to 2007 result in an area weighted value of 1.08 which is slightly more conservative than the previous fixed median starting API of 1.0.

The probable maximum flood analysis for SQN, Units 1 and 2 was originally completed in the mid-1 970s with information that had been collected for TVA's flood control purposes. The rainfall-runoff relationships for the original analysis resulted in an approximate 11%

precipitation loss during the main storm; the reevaluation of the data performed in the 2008-2009 timeframe verified that the precipitation loss still remains at approximately 11%.

Slight differences in the antecedent storm were realized but when evaluating throughout the entire storm, the losses were consistent with the original analysis results. Additional history of the use of the API method is described below.

TVA met with the United States Weather Bureau (USWB) on May 30 and June 1, 1950 to discuss the USWB method of developing rainfall-runoff relationships. This method was described in an unpublished paper entitled "Predicting the Runoff from Storm Rainfall," by M.A. Kohler and R.K. Linsley. It was later published in September 1951 as a research paper.

In 1951, a USWB natural flow river stage forecasting unit was opened in Knoxville, TN to:

1) provide predicted natural flows on uncontrolled streams to determine reservoir inflows necessary in the integrated operation of the TVA reservoir system; 2) supply TVA with predicted natural stream flows and stages for significant points on the Tennessee River and tributaries, for existing (pre-regulated) conditions, so that the controlled flood stages would be lower than the elevations obtained naturally (unregulated); and 3) prepare flood stage forecasts for those points on uncontrolled streams in the Tennessee Valley which are vulnerable to flood damage and in need of flood warning. A fundamental requirement needed to accomplish these assignments was to estimate surface runoff from storm rainfall. The USWB river stage forecasting unit developed the rainfall-surface runoff relationships in the 1950s as part of this task. The relationships were based on the antecedent precipitation index. The data used to develop the relationships was from the time period of 1937 to 1955.

Since that early development by USWB, TVA has continued to use this method to accomplish their federally mandated, integrated operation of the TVA reservoir system which includes E1-8

ENCLOSURE flood control responsibilities. The information that was retrieved from the original hydrology analysis for SQN and other TVA nuclear plants indicated that only minor adjustments have been made since TVA adopted the API method with the original relationships remaining the same.

The verification of the API was completed by comparison with two other industry standard rainfall runoff relationships; the Soil Conservation Service (SCS) curve method and the Initial/Constant (IC) loss method. When compared to both these methods, the API method produces more conservative total runoff volumes. The API method accounts for a fixed volume available in the watershed for surface and infiltration losses, and transitions to 100% runoff for any increment of rainfall occurring above that volume. Because the antecedent precipitation for the Tennessee Valley is a large event as set forth in HMR No. 41, a 1:1 runoff-to-rainfall relationship occurs at the end of the antecedent storm and early in the main storm.

The ability of the API method to fit historical stream flow data was not reviewed directly but was compared against historical data used to develop the watershed unit hydrographs. This comparison demonstrated a good fit between historical and calculated data. The unit hydrographs were originally developed using historical flood data for a period ranging from the 1940's to the 1970's. For the hydrology analysis that supports this license amendment request, the original unit hydrographs were validated with the highest of two or more floods from a new range of data between the years of 1997 to 2007. This data range was chosen because high resolution, radar-based, hourly precipitation data was available for this period.

The data was cut off in 2007 because the unit hydrograph validations were started in 2008 and completed in 2009, therefore the 2007 data was the latest available data.

In summary, although this analysis used only 11 years of data to determine the median starting API, this data compared well to previous work that was based on extensive data and was deemed appropriate for use.

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ENCLOSURE NRC EMCB-RAI-2.4.3-4 Please provide additional information that clarifies the differences between the runoff model used in the previous analysis to determine the Tennessee River flood hydrograph (45 unit areas) to the proposed analysis (40 units), as described in Reference 1. The response should also include an expanded justification for the acceptability of the differences between the models.

TVA Response - EMCB-RAI-2.4.3-4 The runoff model used in the previous analysis segmented the Clinch River watershed into six subbasins, with subbasin number 32 further broken into five smaller subbasins. This detailed refinement was completed for a proposed Clinch River Breeder Reactor in the late 1970s to early 1980s. Synthetic unit hydrographs were developed for this watershed area because detailed stream flow gage data was not available for these refined eleven unit hydrograph areas. For this analysis, the additional refinement required of the Clinch River Breeder Reactor was not necessary and use of a single unit hydrograph developed and validated using observed flood data is judged to be a better runoff model.

An original single unit hydrograph was found within historic documents. The floods used in the original single unit hydrograph development were those of March 1967, December 1969-January 1970, and March 1973. The flood inflows were obtained from reverse reservoir routing. The composite unit hydrograph was computed from the three floods using methodology proposed by Newton and Vineyard (Reference 3).

The original unit hydrograph was validated following the methodology described in ANSI/ANS-2.8-1992. With regard to verifying runoff models, ANSI/ANS-2.8, 1992 states the following:

"Deterministic simulation models including unit hydrographs should be verified or calibrated by comparing results of the simulation with the highest two or more floods for which suitable precipitation are available."

The analysis that supports this license amendment request (Reference 1) has validated the original single unit hydrograph through use of observed flood data. The two floods selected for validation were the March 1973 and May 2003 events. The TVA Simulated Open Channel Hydraulics (SOCH) program was used for this validation together with Hydrologic Engineering Center - Hydrologic Modeling System (HEC-HMS). The unit hydrograph was used with effective rainfall inputs to obtain inflow hydrographs for the two floods using HEC-HMS and used as inputs to the SOCH model. Using these inputs into the SOCH model replicated the observed discharges for the two flood events within a reasonable margin.

An additional change to the runoff model was an inclusion of a refinement at Hiwassee River.

Hiwassee River was broken up into two separate subbasins, number 44a and number 44b, instead of a single subbasin. The original subbasin number 44 area for the lower Hiwassee River was 1,082.6 square miles (sq. mi.). Review of the available gage data showed a gage at Hiwassee River mile 18.9 which could be used in the revised analysis. Splitting the basin into number 44A (686.6 sq. mi.) and number 44B (396 sq. mi.) allowed the direct validation of the unit hydrograph for the larger sub-basin number 44A against gage data. This reduced the area draining to the Chickamauga Reservoir requiring validation using reverse routing and thus was a more accurate approach.

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ENCLOSURE NRC EMCB-RAI-2.4.3-5 Reference 1 notes that the temporary flood barriers were stable for the most severe PMF headwater/tailwater conditions, using vendor recommended base friction values. If these set point values were demonstrated to be acceptable via deterministic analyses, please provide any additional details on the methodology and general approach taken to ensure that the actual base friction values, identified in the field, were comparable to the deterministic analyses.

Include any field testing, quality control and quality assurance testing, inspections, etc. in your response.

TVA Response - EMCB-RAI-2.4.3-5 The vendor who supplied the temporary flood barriers, HESCO Bastion, Inc., hired an independent company to conduct field test analyses for friction coefficients. The tests were performed on HESCO Concertainers 3' deep by 3' wide by 4' high sections filled with sand (uncompacted and lightly compacted) and were conducted on various base surfaces. The calculated friction coefficients from the test varied from 0.83 to 0.54 on earth and grass combination (muddy and muddy saturated conditions). The calculated friction coefficients from the test for concrete surfaces varied from 0.64 to 0.51.

TVA evaluated sliding of the 3-ft and 4-ft HESCO Concertainers, in different arrangements, under hydrostatic loading. These calculations assumed friction coefficients equal to 0.65 on the grass and earth embankments and 0.57 on paved sections. The calculations were completed assuming sand was used for the fill material with a unit weight of 102 pounds per cubic foot (pcf). The actual material used in the installation was a compacted crushed stone. Reference 4 provides ranges of dry densities for gravel and gravely soils from 110 to 145 pcf. The unit weight of the fill material is greater than the unit weight used in the evaluation which results in the factors of safety against sliding being larger.

An additional check is shown here using heavier fill material and back-calculating a coefficient of friction that would equal a factor of safety of 1.0 against sliding:

Hydrostatic force, FH = (yw dw 2) - 2 = 499.2 pounds per linear foot (plo where, yw = unit weight of water, 62.4 pcf, and dw = depth of water, 4 ft, conservatively assume water at top of HESCO Concertainer Uplift force, Fu = (yw x dw x w) ÷ 2 = 386.9 plf where, yw = unit weight of water, 62.4 pcf, and dw = depth of water, 4 ft, conservatively assume water at top of HESCO Concertainer w = width of HESCO Concertainer + volume increase (bulge), 3.1 ft Concertainer weight, Fc = d x w XY¥iu x fvoiume= 1452 plf where, d = depth of HESCO Concertainer, 4 ft w = width of HESCO Concertainer, 3 ft yf,1I= unit weight of the fill material, conservatively assume 110 pcf gravel and gravely soils range from 110 to 145 pcf (Reference 5), and fvoiume = volume increase factor, conservatively assume 1.1 The HESCO Concertainer will bulge when filled. The field tests conducted (Reference 5) found that a unit was typically filled 22 to 43% above a volume of 36 cubic feet (3 ft x 3 ft x 4 ft). For this conservative calculation, only a 10%

volume increase for bulging was assumed.

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ENCLOSURE Resisting Force, FR = p x (F, - Fu) = 1065.1 p where, p = coefficient of friction, F= Conceratiner weight, and F= Uplift force Factor of Safety, FS = FR - FH = 2.13p For FS = 1.0 -* p = 0.47 The back-calculated coefficient of friction of 0.47 using conservative parameters is lower than the coefficients of friction determined from the vendor tests. Therefore, no additional field testing was conducted because the values used within the calculation were judged sufficiently conservative.

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ENCLOSURE NRC EMCB-RAI-2.4.3-6 With regards to the reservoir operation guides, Reference 1 notes that in the original analysis, all of the discharge gates were considered operable, without any failure up to the point where the operating deck is flooded. However, the new analysis assumes that all gates remain operable without failure and without exceptions. Please provide additional information regarding the basis for determining that all of the discharge gates will remain operable without failure and without exceptions. Include any information related to testing, maintenance, and inspections, associated with the discharge gates that support this assumption.

TVA Response - EMCB-RAI-2.4.3-6 When describing the proposed changes to Reservoir Operating Guides in the "Evaluation of Proposed Changes" section of the license amendment request (Reference 1), the statement is made that "All gates remain operable without failure." The evaluation focuses on the inclusion or exclusion of turbine discharges in the model. In the previous model, turbine discharges were not used because the head differentials were considered too small and all gates were considered operable without failure up to the point where the operating deck is flooded. In the proposed change, turbine discharges are used until head differentials are too small or the respective powerhouse is flooded. It is intended that the statement "All gates remain operable without failure" be a continuation of the preceding statement with the operability of the gates also subject to the flooding of the powerhouse/operating deck. This is clarified in and supported by the WBN Unit 1 UFSAR Section 2.4.3.3 markup.

As noted in the Reference 1 markup for WBN Unit 1 UFSAR Section 2.4.3.3, Runoff and Stream Course Model, "All discharge outlets (spillway gates, sluice gates, and valves) for projects in the reservoir system will remain operable without failure up to the point where the operating deck is flooded for the passage of water when and as needed during the flood."

After the operating deck is flooded, the gate hoist equipment is considered inoperable with the gates in their fully open position. At that point, flood water can pass through or over the gates depending on their configuration without failure and would stay in that position until flood waters recede down to the spillway crest. A high confidence that all gates/outlets would be operable is provided by periodic inspections by TVA plant personnel, the intermediate and five-year dam safety engineering inspections consistent with Federal Guidelines for Dam Safety, and the significant capability of the emergency response teams to direct and manage resources to address issues potentially affecting gate/outlet functionality.

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ENCLOSURE

References:

1. Enclosure 1 to letter from TVA to NRC, "Application to Revise Watts Bar Nuclear Plant Unit 1 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis, TAC No. ME8200 (WBN-UFSAR-12-01)," dated July 19, 2012 (ADAMS Accession No. ML12236A164)

2. Letter from TVA to NRC, "Completion of Commitments Related to Updated Hydrologic Analysis Results for Sequoyah Nuclear Plants Units 1 and 2 and Watts Bar Nuclear Plant Unit 1(TAC Nos. ME8805, ME8806, and ME8807)," dated April 29, 2013 (ADAMS Accession No. ML13126A101)

3. Newton, D.R. and J.W. Vineyard, Computer-Determined Unit Hydrograph from Floods, Journal of the Hydraulics Division, ASCE, Vol. 93, No. HY5, September 1967

4. Holtz, Robert D and Kovacs, William D, (1981), "An Introduction to Geotechnical Engineering," Prentice-Hall, Inc., New Jersey

5. Letter from TVA to NRC, "Re-submittal of Attachments for Responses to Hydrology Action Items," dated February 4, 2011, (ADAMS Accession No. ML11307A424),

Enclosure 2, "Engineering Evaluation of Hesco Barriers Performance at Fargo, ND 2009" by Wenck Associates, Inc., dated May 2009 (ADAMS Accession No. ML110831041)

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