Text

Enclosure 1 Evaluation of Proposed Changes Tennessee Valley Authority Watts Bar Nuclear Plant, Unit 1
	ML12236A164
Person / Time
Site:	Watts Bar
Issue date:	07/19/2012
From:	; Tennessee Valley Authority
To:	; Office of Nuclear Reactor Regulation
References
	WBN-UFSAR-12-01
	Download: ML12236A164 (142)
	v • d • e

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES TENNESSEE VALLEY AUTHORITY WATTS BAR NUCLEAR PLANT, UNIT 1

Subject:

Application to Revise Watts Bar Nuclear Plant (WBN) Unit 1 Updated Final Safety Analysis Report Regarding Changes to Hydrologic Analysis (WBN-UFSAR-12-01) 1.0

SUMMARY

DESCRIPTION 2.0 DETAILED DESCRIPTION 2.1 Proposed Changes 2.2 Need for Proposed Changes

3.0 TECHNICAL EVALUATION

3.1 Evaluation 3.2 Uncertainties 3.3 Margins 3.4 Conclusions

4.0 REGULATORY EVALUATION

4.1 Applicable Regulatory Requirements and Criteria 4.2 Precedent 4.3 Significant Hazards Consideration 4.4 Conclusions

5.0 ENVIRONMENTAL CONSIDERATION

ATTACHMENTS

1.

Proposed WBN Unit 1 UFSAR Text Changes (Markups)

2.

Proposed WBN Unit 1 UFSAR Tables

3.

Proposed WBN Unit 1 UFSAR Figures (Public)

4.

Proposed WBN Unit 1 UFSAR Figures (Non-Public)

Page 1 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES 1.0

SUMMARY

DESCRIPTION The probable maximum flood (PMF) for WBN Unit 1 at the time of Operating License issuance was elevation 738.1 ft, and included assumptions based on the existing understanding of dam structural stability and capability during seismic and extreme flood events in the 1970's. In the 1980's and 1990's, TVA implemented a Dam Safety Program (DSP) that resulted in dam safety modifications that increased dam structural stability and capability Between 1995 and 1998, TVA completed a hydrologic reanalysis to credit the results of the dam safety modifications that had been completed. This reanalysis resulted in lowering the WBN Unit 1 calculated probable maximum flood (PMF) to elevation 734.9 ft, although no physical changes to WBN Unit 1 site flooding protection features were implemented as a result of the decreased design basis flood (DBF) elevations.

On October 30, 2007, TVA submitted an application for a combined operating license (COLA) for the proposed Bellefonte Nuclear Plant (BLN) Units 3 and 4, in accordance with 10 CFR 52.

During review of the BLN Units 3 and 4 Final Safety Analysis Report (FSAR), the NRC performed an audit of the hydrologic analysis which resulted in the issuance of three Notice of Violations (NOVs) on March 19, 2008 (

Reference:

NRC Letter to TVA, Bellefonte Combined License Application - Nuclear Regulatory Commission Inspection of the Implementation of the Quality Assurance Program Governing the Simulated Open Channel Hydraulics Model -

Inspection Report Numbers 05200014/2008--01 and 05200015/2008-001 and Notice of Violation, Accession No. ML080640487). In response to these NOVs, TVA completed a revised hydrologic analysis to support the BLN Units 3 and 4 COLA.

As a result of the revised BLN Units 3 and 4 hydrologic analysis, a hydrologic analysis for the proposed WBN Unit 2 was performed to support the NRC review of the application for an Operating License in accordance with 10 CFR 50. The WBN Unit 2 hydrologic analysis includes updated input information, and updates to methodology which includes use of the U.S. Army Corps of Engineers (USACE) Hydrologic Modeling System (HEC-HMS) and River Analysis System (HEC-RAS) software. As the differences between the existing WBN Unit 1 licensing basis and the proposed WBN Unit 2 licensing basis were identified, corrective action documents were written and evaluations performed to assess the estimated impact of the issues on the WBN Unit 1 design bases. After the differences had been reviewed for impact to WBN Unit 1 operability and design bases, the corrective action documents were closed to a single corrective action document, WBN Problem Evaluation Report (PER) 154477, tracking resolution of the final hydrologic analysis and update of the WBN Unit 1 Updated Final Safety Analysis Report (UFSAR) licensing basis.

The update of the hydrologic analysis for WBN Units 1 and 2 includes, but is not limited to, changes to the description of the current hydrosphere, use of more recent flood history information, changes to the inputs used for determining probable maximum precipitation (PMP) and resulting PMF and DBF elevations at the plant site including changes to the runoff and stream course model, changes to the determination of seismically induced dam failure flood impacts at the plant site, changes to the analysis for determining that adequate water is available for operation of WBN Unit 1, and updates to flooding protection requirements. The update to the runoff and stream course model includes updated discharge rating curves to address recently identified rim leaks for Fort Loudoun Reservoir and Watts Bar Reservoir which result in bypass flow around the respective dams.

As a result of the issues and updates associated with the WBN Unit 1 hydrologic analysis, the PMF elevation at the WBN site is increased from elevation 734.9 ft to 739.2 ft, and the resulting DBF elevations affecting the Page 2 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES safety-related WBN Unit 1 systems, structures, and components (SSCs) are increased. Most of the SSCs that are required to be protected from a flood are not impacted by the increased DBF elevations, because either margin remains between the DBF elevations and the elevation of the SSCs, or the existing flood protection measures are still effective.

However, there are exceptions that require temporary modifications to ensure adequate flood protection in the interim, with permanent plant modifications planned to restore or gain additional margin between the revised DBF elevations and limiting safety-related systems, structures, and components.

TVA is requesting NRC review and approval of all of the technical changes to the WBN Unit 1 UFSAR described in this enclosure to address the cumulative effects that have occurred in the WBN Unit 1 hydrologic analysis since issuance of the Operating License.

Specific technical changes are proposed in WBN Unit 1 UFSAR Sections 2.4, 2.4.1, 2.4.2, 2.4.3, 2.4.4, 2.4.11, and 2.4.14. Additional editorial changes are shown for WBN Unit 1 UFSAR Section 2.4, and do not require NRC review and approval. to this enclosure provides the existing WBN Unit 1 UFSAR text pages marked up to show the proposed changes.

provides the proposed replacement WBN Unit 1 UFSAR Section 2.4 tables. provides the proposed replacement WBN Unit 1 UFSAR Section 2.4 figures (public version). Attachment 4 provides the proposed replacement WBN Unit 1 UFSAR Section 2.4 figures (non-public version).

2.0 DETAILED DESCRIPTION 2.1 Proposed Changes Section 2.4, Hydrolo-gical En-gineering Several technical changes are proposed for WBN Unit 1 UFSAR Section 2.4 to reflect the changes described further in the remaining sections. These include the following:

Revising the maximum flood elevation that would result from an occurrence of the probable maximum storm from elevation 734.9 ft to elevation 739.2 ft.

Adding information regarding coincident wind wave activity that results in up to an additional 2.5 ft for determining the DBF elevations.

Adding information regarding run up on the 4:1 slopes approaching the Diesel Generator Building reaching elevation 741.6 ft, wind wave run up on the critical wall of the Intake Pumping Station reaching elevation 741.7 ft, and wind wave run up on the walls of the Auxiliary, Control and Shield Buildings reaching elevation 741.0 ft.

Revising the estimated probable minimum flow past the site from 2,000 cfs to 3,200 cfs.

Additional editorial changes are shown in the mark-ups provided in Attachment 1.

Page 3 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES Section 2.4.1, Hydrological Description The following technical changes are proposed for WBN Unit 1 UFSAR Section 2.4.1 to update the hydrological description to reflect the most current information available regarding operations of the TVA system of reservoirs and dams that affect the hydrologic analysis for WBN Unit 1. Additional editorial changes are shown in the mark-ups provided in Attachment 1.

Hydrosphere As described in WBN Unit 1 UFSAR Subsection 2.4.1.2, there are 12 major dams in the TVA system upstream of the WBN site. This change proposes to update the descriptions of these dams and their associated reservoirs with updated detention areas and capacities. The flood detention capacity reserved in the TVA system varies seasonally, with the greatest amounts during the January through March flood season. WBN Unit 1 UFSAR Figures 2.4-15 through 2.4-24 show the original reservoir seasonal operating guides for reservoirs above the plant site.

This change provides updated figures showing the current reservoir seasonal operating guides.

Section 2.4.2, Floods The following technical changes are proposed for WBN Unit 1 UFSAR Section 2.4.2 to update the discussion of floods to reflect the most current information available regarding historical floods that affect the hydrologic analysis for WBN Unit 1. Additional editorial changes are shown in the mark-ups provided in Attachment 1.

Flood History As described in WBN Unit 1 UFSAR Subsection 2.4.2.1, flood records for the period 1952 to date can be considered representative of prevailing conditions for the Tennessee River Valley watershed under existing TVA river operations procedures. This change proposes to update the highest flow at Watts Bar Dam tailwater located upstream of the WBN Unit 1 site to add additional historical flood events that have occurred since issuance of the original WBN Unit 1 Operating License and corrects historical flood data.

This historical information is used to calibrate the hydrologic models used for the hydrologic analysis.

Flood Design Considerations As described in WBN Unit 1 UFSAR Subsection 2.4.2.2, the maximum PMF plant site flood level was elevation 734.9 ft, with wind waves of 2.0 ft high (trough to crest) predicted.

This change proposes to increase the PMF level to elevation 739.2 ft and to update coincident wind wave activity that results in up to an additional 2.5 ft for determining the DBF elevations.

As described in WBN Unit 1 UFSAR Subsection 2.4.2.2, wind wave run up during the PMF at the Diesel Generator Building was postulated to reach elevation 736.9 ft which is 5.1 ft below the operating floor elevation of 742.0 ft. The change proposed increases the DBF including wind wave run up during the PMF to elevation 741.6 ft, which is 0.4 ft below the Diesel Generator Building operating floor elevation of 742.0 ft.

As described in WBN Unit 1 UFSAR Subsection 2.4.2.2, the design basis flood level for the Intake Pumping Station (except for the electrical equipment room) was elevation 740.1 ft. The Page 4 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES change proposed increases the DBF including wind wave run up during the PMF to elevation 741.7 ft.

As described in WBN Unit 1 UFSAR Subsection 2.4.2.2, the design basis level for the Auxiliary, Control and Shield Buildings was 740.1 ft. The change proposed increases the design basis level for the Auxiliary, Control and Shield Buildings to elevation 741.0 ft.

Section 2.4.3, Probable Maximum Flood (PMF) on Streams and Rivers The following technical changes are proposed for WBN Unit 1 UFSAR Section 2.4.3 to update the discussion of PMF on streams and rivers to reflect the most current information available as inputs, and to use updated methodologies such as the USACE HEC-HMS and USACE HEC-RAS software for elements of the hydrologic analysis for determining the PMF for streams and rivers for WBN Unit 1. Additional editorial changes are shown in the mark-ups provided in.

PMF on Streams and Rivers As described in WBN Unit 1 UFSAR Subsection 2.4.3, the PMF was determined from PMP for the watershed above the plant with consideration given to seasonal and areal variations in rainfall.

This change proposes revisions to inputs and use of different methodologies for determining PMF as further described below. As a result of these changes, the PMF elevation at the plant is increased from elevation 734.9 ft to 739.2 ft, excluding wind wave effects.

Probable Maximum Precipitation (PMP)

As described in WBN Unit 1 UFSAR Subsection 2.4.3.1, PMP is defined for TVA by Hydrometeorological Report No. 45 for watersheds above tributary dams. However, as already described in WBN Unit 1 UFSAR Subsection 2.4.2.3, Hydrometeorological Report No. 56 which supersedes Hydrometeorological Report No. 45 is used for PMP for the plant drainage systems based on local intense precipitation.

Hydrometeorological Report No. 56 is not used in the updated hydrologic analyses to define PMP for determining PMF.

Therefore, this change proposes deleting the reference to Hydrometeorological Report No. 45 and adding a new reference to Hydrometeorological Report No. 56 in WBN Unit 1 UFSAR Section 2.4, References.

Also as described in WBN Unit 1 UFSAR Subsection 2.4.3.1, the PMF was determined from PMP for the watershed above the plant with consideration given to seasonal and areal variations. Two basic storm situations were found to have the potential to produce a maximum flood at Watts Bar Nuclear Plant.

These are (1) a sequence of March storms producing maximum rainfall on the 21,400-square-mile watershed above Chattanooga, hereafter called the 21,400-square-mile storm, and (2) a sequence of March storms centered and producing maximum rains in the basin to the west of the Appalachian Divide and above Chattanooga, hereafter called the 7,980-square-mile storm. In the most recent analysis prior to this proposed change, a PMF of elevation 734.9 ft was produced by the 7,980 square-mile storm, with the 21,400 square-mile storm producing a PMF of elevation 734.7 ft. In the proposed change, a PMF elevation 739.2 ft is produced by the 21,400 square-mile storm, which has become the controlling PMP event. This change is due to the various input changes to the hydrologic analysis, and updates to methodology.

Page 5 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES Precipitation Losses As described in WBN Unit 1 UFSAR Subsection 2.4.3.2, a multi-variable relationship, used in the day-to-day river operations of the TVA system, has been applied to determine precipitation excess directly.

The relationships were developed from observed data.

They relate precipitation excess to the rainfall, week of the year, geographic location, and antecedent precipitation index (API).

For the original study, a median API, as determined from past records, was used at the start of the antecedent storm.

This change proposes revising the inputs for defining API using an 11-year period of historical rainfall records (1997-2007) at the start of the antecedent storm.

Runoff and Stream Course Model As described in WBN Unit 1 UFSAR Subsection 2.4.3.3, the original runoff model used to determine Tennessee River flood hydrographs at WBN was divided into 45 unit areas and included the total watershed above Chickamauga Dam downstream.

Unit hydrographs are used to compute flows from the unit areas. The unit area flows are combined with appropriate time sequencing or channel routing procedures to compute inflows into the most upstream reservoirs which in turn are routed through the reservoirs using standard techniques. Resulting outflows are combined with additional local inflows and carried downstream using appropriate time sequencing or routing procedures including unsteady flow routing. This change proposes revising the runoff model to use 40 unit areas as a result of removing additional detail in modeling the subbasins on the Clinch and Holston Rivers that was not needed. The additional unit areas are not required, and validation of the runoff model is more efficient by eliminating these unnecessary modeling details.

The results remain essentially the same due to the calibration of the model to the large flood events.

TVA developed the Simulated Open Channel Hydraulics (SOCH) model for flood routing calculations for the Tennessee River and selected tributaries. The SOCH computer model is the hydraulic model used to determine flood elevations at each TVA operating nuclear plant site.

The SOCH model has been calibrated for main stem reservoirs, and Melton Hill and Tellico tributary reservoirs, to reasonably replicate observed river discharges and elevations for known historic events.

Once calibrated, the SOCH model can be used to reliably predict flood elevations and discharges for events of other magnitudes. Additional details including specific changes to the SOCH model analysis are described below.

Tributary reservoir routings: In the original routing model, the Goodrich semigraphical method and flat pool storage conditions (except Tellico) were used. In the proposed change, the Melton Hill routing is revised to adopt unsteady flow for better refinement for dam seismic failure cases as further described in the next discussion. The Goodrich semigraphical method described in the WBN Unit 1 UFSAR is the same as standard reservoir routing described in the proposed WBN Unit 1 UFSAR text.

Discharge rating curves: In the original hydrologic analysis, initial dam rating curves were developed based on the existing geographical information available. In the proposed change, temporary flood barriers have been installed on the earthen embankments of four dams (Cherokee, Fort Loudoun, Tellico, and Watts Bar Reservoirs) to increase the height of embankments and are included in the discharge rating curves for these four dams. Increasing the height of embankments at these four dams prevents embankment overflow and failure of the embankment. The vendor supplied temporary flood barriers were shown to be stable for the Page 6 of 41

ENCLOSUREI EVALUATION OF PROPOSED CHANGES most severe PMF headwater/tailwater conditions using vendor recommended base friction values. These temporary flood barriers are discussed in greater detail in the discussions for proposed changes to WBN Unit 1 UFSAR Subsection 2.4.3.4 below. Also, there are additional rim leaks for Fort Loudoun Reservoir and Watts Bar Reservoir identified through use of the latest available geographical information system (GIS) information. These rim leaks result in bypass flow around the respective dams that are addressed in updated discharge rating curves.

A single postulated Fort Loudoun Reservoir rim leak north of the Marina Saddle Dam was added as an additional discharge component for the Fort Loudoun Dam which discharges into the Tennessee River at Tennessee River Mile (TRM) 602.3.

For Watts Bar Dam, flow is considered through seven Watts Bar Reservoir rim leaks.

Three of the rim leak locations discharge to Yellow Creek, entering the Tennessee River three miles downstream of Watts Bar Dam.

The remaining four rim leak locations discharge to Watts Creek, which enters Chickamauga Reservoir just below Watts Bar Dam. The changes are made to update and refine the model.

Unsteady flow model: In the original routing model, the main river and Tellico were modeled with unsteady flow techniques, with calibration of the unsteady flow model performed using the steady flow profiles from the USACE HEC-2 backwater computer code.

In the proposed change, the main river reservoirs, Tellico, and Melton Hill are modeled with unsteady flow techniques and calibrated using profiles computed from the more recent USACE HEC-RAS computer code. The change is made to adopt the most recent computer code.

Unsteady flow model - Fort Loudoun Reservoir specific discussion: In the original routing model for Fort Loudoun Reservoir, there were 24 reaches, verified at three gauged points using 1963 and 1973 flood data, and five cross-sections used for the Fort Loudoun and Tellico canal physically connecting the reservoirs. In the proposed change, there are 29 cross-sections with additional sections interpolated between each for a total of 59 cross-sections.

The Fort Loudoun and Tellico canal was modeled using nine cross-sections with an average cross-section spacing of about 0.18 mile. The Fort Loudoun unsteady flow model was verified using the March 1973 flood data. Tellico Dam was not closed until 1979, and thus was not in place during the March 1973 flood for verification.

The unsteady flow model for the Fort Loudoun-Tellico complex, which includes both reservoirs and the Fort Loudoun and Tellico canal, is verified using the May 2003 flood data. The Tellico reservoir SOCH model is also used to replicate the Federal Emergency Management Agency (FEMA) published 100-year and 500-year flood profiles.

Unsteady flow model - Cherokee and Douglas Dams specific discussion: In the original routing model, the model extended up to Douglas Dam and Cherokee Dam on the French Broad River and Holston River, respectively, with the models verified at one gauged point each using the 1963 and 1973 flood data. In the proposed change, French Broad River is modeled using 25 cross-sections with additional sections interpolated between the original cross-sections for a total of 49 cross-sections, and the Holston River is modeled using 29 cross-sections with additional sections interpolated between the original cross-sections for a total of 53 cross-sections. The French Broad River and Holston River models are verified at two gauged points each using the March 1973 flood and at one point each using the May 2003 flood. The models are also verified by replicating the FEMA published 100-year and 500-year flood profiles.

Unsteady flow model - Little Tennessee River specific discussion: In the original routing model, the Little Tennessee River was modeled from Tellico Dam, mile 0.3, through Tellico Reservoir to Page 7 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES Chilhowee Dam at mile 33.6 and upstream to Fontana Dam at mile 61.0. The model for Tellico Reservoir to Chilhowee Dam was tested for adequacy by comparing its results with steady-state profiles at 1,000,000 and 2,000,000 cfs computed by the standard-step method.

Minor decreases in conveyance in the unsteady flow model yielded good agreement. The average conveyance correction found necessary in the reach below Chilhowee Dam to make the unsteady flow model agree with the standard-step method was also used in the river reach from Chilhowee to Fontana Dam. In the proposed change, the Little Tennessee River was modeled from Tellico Dam, Little Tennessee River mile (LTRM) 0.3 to Chilhowee Dam at LTRM 33.6.

The Little Tennessee River from Tellico Dam to Chilhowee Dam at LTRM 33.6 is described by 23 cross-sections with additional sections being interpolated between the original sections for a total of 49 cross-sections in the SOCH model, with a variable cross-section spacing of up to about 1.8 miles.

Unsteady flow model - Watts Bar reservoir specific discussion: In the original routing model, 34 reaches were used.

In the proposed change, 39 cross-sections with two additional cross-sections in the upper reach (a total of 41 cross-sections) are used with a variable cross-section spacing of up to about 2.8 miles. The model also includes a junction with the Clinch River up to Melton Hill Dam with one additional cross-section being interpolated between each of the original 13 cross-sections.

Unsteady flow model - Junction at Tennessee River mile 601.1 to Tellico Dam at Little Tennessee River mile 0.3: This short segment of stream was not considered in the original analysis. In the proposed change, five cross-sections with spacing of 0.08 miles are added.

The change is made to refine the model.

Unsteady flow model - Chickamauga reservoir specific discussion: In the original routing model, 28 reaches were used and verified at four gauged points using 1973 flood data.

In the proposed change, 29 cross-sections with additional cross-sections interpolated between the original cross-sections for a total of 53 are used. The model includes a junction with the Dallas Bay arm and the Hiwassee River arm. The model is verified using both the March 1973 and May 2003 flood data. The change is made to refine the model and add unsteady flow modeling for Dallas Bay and Hiwassee River.

Model verification: In the original analysis, the TVA standard-step backwater program or USACE HEC-2 software for river hydraulics (both solve the same equations, although not specified in the WBN Unit 1 UFSAR) was verified using the March 1963 and March 1973 flood data. This model was then used to compute steady state profiles for flows up to 1,500,000 cfs.

In the proposed change, steady-state profiles are computed using the USACE HEC-RAS software, using March 1973 and May 2003 flood data for verification. This level of detail is not included in the WBN Unit 1 UFSAR.

Reservoir Operating Guides: In the original routing model, normal operating procedures at the time were used during the antecedent storm, including turbine and sluice discharge in tributaries. Turbine discharges were not used in main river reservoirs after large flood flows develop, because the head differentials were considered to be too small.

Normal operating procedures were also used during the main storm except for turbine discharge for tributaries and main river dams. All gates were considered operable without failure up to the point where the operating deck is flooded. In the proposed change, turbine discharges are included in the analysis (main river and tributaries) until head differentials are too small or the respective powerhouse is flooded. All gates remain operable without failure. Previously, the point where Page 8 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES turbine discharge was eliminated was an assumption and was not a calculated value.

In the updated analysis, these points are determined using actual elevation data.

Median initial reservoir elevations:

In the original routing model, median initial reservoir elevations were used at the start of the storm sequence. While not specifically stated in the original analysis, the initial median reservoir levels for the appropriate season were used. In the proposed change, the updated analysis uses the same method to determine the initial median reservoir levels.

However, these median levels are different as a result of changes to the reservoir operating guidelines.

Temporary flood barriers: In the routing model described in the WBN Unit 1 UFSAR, temporary flood barriers were not used and the earth embankments at the main river dams were not overtopped as a result of dam safety modifications that have been implemented since original licensing of WBN Unit 1.

In the proposed change, the height of embankments are physically increased using temporary flood barriers to prevent earth embankment overtopping at Cherokee, Fort Loudoun, Tellico and Watts Bar Reservoirs.

While the flood barriers are "temporary structures," there is a structural analysis for the headwater loading behind the temporary flood barriers that verifies that failure of the barriers themselves would not occur.

Experience data on the use of the selected temporary flood barriers during historic floods and the vendor documentation on barrier testing were evaluated prior to selection and use.

Additionally, although not credited in the seismically induced dam failure analyses, a seismic evaluation completed on the flood barriers (without headwater behind the barriers) verifies that failure of the flood barriers themselves would not occur. A potential exists for runaway barges to float downstream and impact the temporary flood barriers at two of the four dams where the barriers are in place. Barges along these reservoirs are typically tied off at barge terminals or mooring cells during high flow events, such as a PMF event. The mooring facilities, however, are not designed for PMF elevations and velocities, so barges could break loose. There is no barge traffic on Cherokee Reservoir, so no potential for impact exists.

The Fort Loudoun Reservoir has limited to moderate barge traffic. Using typical barge dimensions, a barge would have to weigh less than 70-80% of full load capacity in order to strike the barriers. However, the earthen embankments of the dam where the temporary flood barriers are placed are located at a distance from the main channel. The stream flow during a high flow event is directed toward the concrete overflow portion of the dam, and the barges would likely be carried by the current away from the temporary flood barriers. At the Tellico Reservoir, there is very infrequent barge traffic. Conservatively assuming there will be a barge on the reservoir, and using typical barge dimensions, a barge would have to weigh less than 40-50% of full load capacity in order to strike the barriers. However, the earthen embankments of the dam where the temporary flood barriers are placed are located at a distance from the main channel. The stream flow during a high flow event is directed toward the concrete overflow portion of the dam, and the barges would likely be carried by the current away from the temporary flood barriers. There is limited to moderate barge traffic at the Watts Bar Reservoir.

An evaluation using typical barge dimensions for the Tennessee River, and conservatively assuming barges are empty (less draft allows for the barge to run closer to the top of the dam), demonstrates that barges are not likely to impact the temporary flood barriers. A spatial analysis shows that the closest edge of the temporary flood barrier would have to be at least 9.0 ft away from the upstream edge of the earthen embankment in order to prevent impact. The temporary flood barriers are located at least this distance from the edge of the earthen embankment, ensuring that there is no potential for barge impact.

In summary, this qualitative evaluation of the possibility of barge impacts affecting the temporary flood barriers during a PMF event concludes that due to the physical features of the earthen embankments including width and slope, the expected direction of flows Page 9 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES towards the dam spillway gates during the PMF event, as well as the placement of the temporary flood barriers at a sufficient distance from the main channel and from the shoreline, it is unlikely that a barge would impact any of the barriers. The temporary flood barriers are not credited in the analysis of seismically induced dam failure combinations, but are credited in the hydrologic analysis for determining the PMF.

Probable Maximum Flood Flow As described in WBN Unit 1 UFSAR Subsection 2.4.3.4, the analysis to determine the PMF flow included evaluation of PMP over the total watershed with consideration of critical seasonal and areal variations. In the most recent analysis prior to this proposed change, the controlling PMF discharge was 1,288,000 cfs from the 7,980 square-mile storm centered at Bulls Gap. In the proposed change, PMF discharge is 1,003,363 cfs from Watts Bar Dam with an additional 97,990 cfs from Watts Creek and 243,782 cfs from Yellow Creek for the 21,400 square-mile storm in March with a downstream storm pattern. This is a change in the controlling storm and is a change as a result of other input changes.

However, the 21,400 square-mile storm is currently discussed in the WBN Unit 1 UFSAR as the second most-controlling storm, so a new storm is not introduced. Discharge differences are due to input changes and a change in how discharge from the Watts Bar West Saddle Dike, and rim leakage from Watts Bar Reservoir, are routed through Watts Creek and Yellow Creek in the hydrologic analysis. Additional details including specific changes to the PMF flow analysis are described below.

Watts Bar and Chickamauga Dams: In the original analysis, the West Saddle Dike at Watts Bar Dam was considered to be overtopped and breached with the discharge input at Watts Bar Dam, and the Chickamauga Dam was considered to be overtopped but not postulated to fail. In the proposed change, the West Saddle Dike at Watts Bar Dam is overtopped and breached with the discharge input at the mouth of Yellow Creek, and Chickamauga Dam is overtopped but not postulated to fail.

Therefore, the proposed change also revises the location where the discharge from Watts Bar West Saddle Dike breach is added back to the river. Also, additional rim leakage for Watts Bar Reservoir has been addressed in the updated hydrologic analysis. As a result, rim leakage is routed through either Watts Creek, whose confluence with the Tennessee River is at TRM 528.0, or Yellow Creek, whose confluence with the Tennessee River is at TRM 526.82. The update is to provide a more realistic modeling configuration.

Concrete section analysis: In the original analysis, comparisons were included between design headwater and tailwater levels and those that prevail during the PMF for all dams. If overturning and horizontal forces were not increased by more than 20%, then the structures were considered safe against failure and were then excluded from further consideration.

Because overturning and horizontal forces were increased greater than 20% for Douglas, Fort Loudoun and Watts Bar Dams, they were further examined and judged stable. In the proposed change, factors of safety in sliding are determined by comparison of design headwater and tailwater levels to PMF headwater and tailwater levels for all dams including those previously considered safe against failure. If the factor of safety is greater than 1.0, then the structures are considered safe against failure. Therefore, the proposed change evaluates the possibility of failure for all of the upstream dams instead of just those whose headwater/tailwater comparison were greater than 20%. This is a more comprehensive evaluation of the dams for the updated PMF levels.

Spillway gates: In the original analysis, there was limited discussion involving the radial spillway gates at Fort Loudoun and Watts Bar Dams. In the proposed change, the Watts Bar Dam spillway gates are described in general, with the yield stress and stress in trunnion pins noted to Page 10 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES be less than allowable design stress.

This is a change to add details to the WBN Unit 1 UFSAR.

Waterborne Objects: In the original analysis, discussion was included for barge (end on) impacts on spillway gates and bents and broadside impacts. This information is revised in the WBN Unit 1 UFSAR to more accurately reflect the engineering judgment used in evaluating potential impacts. A new subsection discussing the potential for barge impacts to the temporary flood barriers at Cherokee, Fort Loudoun, Tellico, and Watts Bar Reservoirs is also added in UFSAR Section 2.4.3.4 as previously discussed.

Lock Gates: In the original analysis, Fort Loudoun, Watts Bar and Chickamauga lock gates were examined for possible failure. This information is revised in the WBN Unit 1 UFSAR to more accurately reflect the evaluation of the lock gate structural elements.

Embankment Breaching: In the original analysis, detailed discussion on the methodology used for earth embankments breach was included. The first part of the paragraph was since revised to discuss that the potential for embankment breaching was examined and no breaching would occur except Watts Bar West Saddle Dike which is completely failed. In the proposed change, this discussion is deleted since there is no other embankment breach in the current hydrologic analysis. This is a change to delete obsolete information.

Wave from Watts Bar Breaching: In the original analysis, analysis of the wave front from postulated breach of earth embankment at Watts Bar was shown. It was retained for historic purposes since the current hydrologic analysis determined there would be no breach of earth embankment. In the proposed change, this discussion is deleted since there is no embankment breach except Watts Bar West Saddle Dike (temporary flood barriers prevent overtopping) in the current hydrologic analysis.

Water Level Determination As described in WBN Unit 1 UFSAR Subsection 2.4.3.5, elevations from the potential controlling PMF events were evaluated to determine the limiting PMF for the WBN site. In the most recent analysis prior to this proposed change, a PMF of elevation 734.9 ft was produced by the 7,980 square-mile storm, with the 21,400 square-mile storm producing a PMF of elevation 734.7 ft.

In the proposed change, a PMF elevation 739.2 ft is produced by the 21,400 square-mile storm, which has become the controlling PMP event. This change is due to the various input changes to the hydrologic analysis, and updates to methodology.

Coincident Wind Wave Activity As described in WBN Unit 1 UFSAR Subsection 2.4.3.6, wind waves are likely when the PMF crests. In the original analysis, wind wave and runup elevations (as well as fetch lengths to determine those levels) were given for safety-related structures.

In the proposed change, updated wind wave and runup elevations (as well as fetch lengths to determine those levels) are given. Wind speed is kept the same, but fetch lengths are changed due to the updated PMF level which therefore increases the wind wave height and runup level. Methodology used is the same as the original analysis and inputs are the same except for the updated PMF stillwater elevation.

Page 11 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES Section 2.4.4, Potential Dam Failures, Seismically Induced The following technical changes are proposed for WBN Unit 1 UFSAR Section 2.4.4 to update the discussion of potential flood levels from seismically induced dam failures to reflect the most current information available as inputs, and to use updated methodologies such as the USACE HEC-HMS and USACE HEC-RAS software for elements of the hydrologic analysis for determining dam failure outflows from tributary dams for WBN Unit 1.

Additional editorial changes are shown in the mark-ups provided in Attachment 1.

As described in WBN Unit 1 UFSAR Subsection 2.4.4, procedures described in Appendix A of Regulatory Guide (RG) 1.59 were followed when evaluating potential flood levels from seismically induced dam failures. Site flooding levels at WBN from potential seismically induced dam failures are determined using the SOCH model, with changes previously discussed regarding proposed change to the runoff and stream course model.

Also as described in WBN Unit 1 UFSAR Subsection 2.4.4, the original discussion included general information concerning the TVA inspection and maintenance program. In the proposed change, the TVA DSP, which is consistent with the Federal Guidelines for Dam Safety, is described in detail. As part of the TVA DSP, inspection and maintenance activities are carried out on a regular schedule to confirm the dams are maintained in a safe condition. This is a change to add details to the WBN Unit 1 UFSAR.

Dam Failure Permutations As described in WBN Unit 1 UFSAR Subsection 2.4.4.1, analyses to determine dam integrity during seismic events were performed to determine site flooding levels at WBN from potential seismically induced dam failures.

In the analyses, two basic conditions are used, including:

1. Determination of the water level at the plant during one-half the PMF during an operating basis earthquake (OBE).

2. Determination of the water level at the plant during a 25-year flood during a safe shutdown earthquake (SSE).

The OE and SSE are defined in WBN Unit 1 UFSAR Subsections 2.5.2.4 and 2.5.2.7 as having maximum horizontal rock acceleration levels of 0.09 g and 0.18 g respectively.

The one-half PMF as used in the analyses is developed by taking half of the rainfall-induced PMF inflows calculated from the controlling 21,400 square mile March downstream centered design storm, which consists of a three-day antecedent storm, three-day dry period, and a three-day main storm. At the start of the antecedent storm, the reservoirs are at the initial median levels used as inputs to the rainfall-induced PMF analysis. This is consistent with the guidance of RG 1.59, Revision 2.

In the most recent analysis prior to this proposed change, only one combination of potential seismically induced dam failures was determined to cause a flood elevation above plant grade elevation.

This combination was failure of Norris, Cherokee, and Douglas Dams during a 25-year flood during a SSE. In the proposed change, the analyses are updated to include Page 12 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES Tellico Dam failure in all the originally controlling combinations as a conservative assumption, and a reduced partial failure of Fontana Dam.

In the updated analysis, there are four controlling combinations of potential seismically induced dam failures during one-half the PMF during an OBE, including:

" Norris and Tellico Dams; Fontana and Tellico Dams; Fontana, Tellico, Hiwassee, Apalachia, and Blue Ridge Dams; and Cherokee, Douglas, and Tellico Dams.

In the updated analysis, there is one controlling combination of potential seismically induced dam failures during a 25-year flood during a SSE, which is failure of the Norris, Cherokee, Douglas, and Tellico Dams. Based on the updated hydrologic analysis, the peak water surface elevation at WBN is produced by this controlling combination at elevation 731.2 ft.

Combinations of seismic induced dam failures have changed due to all the input updates and the inclusion of Tellico into the failure scenario. Specific changes include the following:

Fontana Dam failure: In the original analysis, partial failure was postulated to occur using engineering judgment. In the proposed change, partial failure to a higher elevation is postulated due to modifications of the dam, and additional analysis utilizing finite element analysis.

Tellico Dam failure: In the original analysis, Tellico Dam was judged to be stable for OBE seismic events. In the proposed change, Tellico Dam is postulated to fail for all seismic events due to lack of supporting analysis. Tellico Dam failure is combined with all seismic failure cases resulting in a bounding case.

Seismic Outflow Hydrograph for Norris, Cherokee, Douglas and Fontana Dams: In the original analysis, outflow from Norris, Cherokee, Douglas and Fontana Dams was based on a SOCH unsteady flow model developed in sufficient detail to define outflow from postulated dam failure.

In the proposed change, outflow from Norris, Cherokee, Douglas, and Fontana Dams is based on USACE HEC-HMS software model using the same dam failure rating curves for Norris, Cherokee, and Douglas Dams, and a revised dam failure rating curve for Fontana Dam for partial failure at a higher elevation, with results validated by comparing results with TRBROUTE.

Use of USACE HEC-HMS storage routing versus unsteady flow is a different method, but provides essentially the same result.

Unsteady Flow Analysis of Potential Dam Failures As described in WBN Unit 1 UFSAR Subsection 2.4.4.2, unsteady flow routing techniques were used to evaluate plant site flood levels from postulated seismically induced dam failures wherever their inherent accuracy was needed. In the proposed change, the HEC-HMS storage routing is used to compute the outflow hydrograph from the postulated failure of each dam except main river dams. In the case of dams which are postulated to fail completely (Hiwassee, Apalachia and Blue Ridge), SOCH is used to develop the outflow hydrograph. For Tellico Dam, Page 13 of 41

ENCLOSUREI EVALUATION OF PROPOSED CHANGES the complete failure is analyzed with the SOCH model. The failure time and initial reservoir elevations for each dam are determined from a pre-failure TRBROUTE analysis. HEC-HMS is used to develop the post failure outflow hydrographs based on the previously determined dam failure rating curves.

The outflow hydrographs are validated by comparing the HEC-HMS results with those generated by simulations using TRBROUTE.

This additional detail for the unsteady flow routing techniques is provided for completeness.

Water Level at Plant Site As described in WBN Unit 1 UFSAR Subsection 2.4.4.3, the unsteady flow analyses of the postulated combinations of seismic dam failures coincident with floods analyzed yielded a maximum elevation of 727.5 ft excluding wind wave effects.

In the proposed change, the maximum elevation excluding wind wave effects is increased to elevation 731.2 ft from the one controlling combination of potential seismically induced dam failures during a 25-year flood during a SSE, which is failure of the Norris, Cherokee, Douglas, and Tellico Dams. Coincident wind wave activity is required by the guidance in RG 1.59 to be addressed in determining whether rainfall induced PMF events or seismically induced dam failure flood events are the bounding event for design of flooding protection features. However, wind wave activity on the order calculated for rainfall induced PMF events if added on top of the limiting elevation of 731.2 ft for seismically induced dam failure flood events would still result in water surface elevations several feet below the rainfall induced PMF elevation of 739.2 ft described in UFSAR Section 2.4.3. Therefore, based on this qualitative analysis, the rainfall induced PMF elevation of 739.2 ft is bounding for WBN Unit 1.

Sections 2.4.5 through 2.4.10 Editorial changes for WBN Unit 1 UFSAR Sections 2.4.5 through 2.4.10 are shown in the mark-ups provided in Attachment 1. In addition, the name of the TVA Water Management Organization is changed to TVA River Operations (RO) to reflect the current organization name.

There are no technical changes proposed for these sections.

Section 2.4.11, Low Water Considerations The following technical changes are proposed for WBN Unit 1 UFSAR Section 2.4.11 to update the discussion of low water considerations to reflect the most current information for the hydrologic analysis for WBN Unit 1. Additional editorial changes are shown in the mark-ups provided in Attachment 1.

Low Flow in Rivers and Streams As described in WBN Unit 1 UFSAR Subsection 2.4.11.1, analyses are performed to determine probable minimum water level at WBN Unit 1 and the minimum flow requirement at the essential raw cooling water (ERCW) intake. In the original analysis, water level at the WBN site upon loss of downstream dam from headwater (HW) at elevation 682.5 ft (normal summer level) began to drop in 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months and reached elevation 666.0 ft (minimum river elevation) in 22 hours2.546296e-4 days 0.00611 hours 3.637566e-5 weeks 8.371e-6 months .

In the proposed change, water level at the WBN site upon loss of downstream dam from HW at elevation 682.5 ft begins to drop in 3 hours3.472222e-5 days 8.333333e-4 hours 4.960317e-6 weeks 1.1415e-6 months and reaches elevation 666.0 ft in 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months . This change includes updates to the routing model cross-sectional data using new bathymetry and recalibration of the models. The timing changes are due to these updates to the model. This value is determined by postulating loss of Chickamauga Dam and no flow from Watts Bar Dam.

Page 14 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES The result from the update is that the required minimum elevation of 666.0 ft is actually reached at a later time (27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months instead of 22 hours2.546296e-4 days 0.00611 hours 3.637566e-5 weeks 8.371e-6 months ). Therefore, there would be additional time to allow discharge from Watts Bar Dam to maintain the minimum required elevation of 666.0 ft. In the original analysis, a minimum flow of 2,000 cfs is required to maintain an elevation of 665.9 ft producing a 5.9 ft depth at the ERCW intake channel. In the proposed change, flow required to maintain an elevation of 665.9 ft producing a 5.9 ft depth at the ERCW intake channel is 3,200 cfs.

Historical Low Water As described in WBN Unit 1 UFSAR Subsection 2.4.11.3, historical low water records for WBN Unit 1 at the ERCW intake are provided. In the original analysis, average daily flows less than 5,000 cfs were determined to occur 0.9% of the time, and average daily flows less than 10,000 cfs were determined to occur 4.8% of the time. In the proposed change, average daily flows less than 5,000 cfs are determined to occur 2.2% of time, and average daily flows less than 10,000 cfs are determined to occur 10.4% of the time. This update is based on analysis of additional years of record.

Plant Requirements As described in WBN Unit 1 UFSAR Subsection 2.4.11.5, plant requirements for WBN Unit 1 at the ERCW intake are provided. In the original analysis, records of the minimum natural flow at the plant site before construction of dams on the Tennessee River was estimated to be 2,700 cfs. In the proposed change, estimated low flow for the period (1903 - 2010) on the basin above WBN show the 15 day, 30 day, 50 day and 100 day sustained low flow as 2,907 cfs, 3,158 cfs, 3,473 cfs, and 4,012 cfs, respectively. This update is based on analysis of additional years of record.

Sections 2.4.12 and 2.4.13 Editorial changes for WBN Unit 1 UFSAR Sections 2.4.12 and 2.4.13 are shown in the mark-ups provided in Attachment 1. There are no technical changes proposed for these sections.

Section 2.4.14, Flooding Protection Requirements The following technical changes are proposed for WBN Unit 1 UFSAR Section 2.4.14 to update the discussion of flooding protection requirements to reflect the most current information for the hydrologic analysis for WBN Unit 1. Additional editorial changes are shown in the mark-ups provided in Attachment 1.

Floodinq Protection Requirements As described in WBN Unit 1 UFSAR Subsection 2.4.14, an evaluation is performed to determine the methods by which the WBN is capable of tolerating floods above plant grade without jeopardizing public safety. The design basis flood (DBF) is the calculated upper-limit flood that includes the PMF plus the wave runup caused by a 21 mph overland wind as discussed in WBN Unit 1 UFSAR Subsection 2.4.3.6. As a result of the changes to inputs and methodology discussed previously, the DBF elevations at various plant locations that would result for the controlling PMF are increased from those currently provided in the WBN Unit 1 UFSAR as follows:

Page 15 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES Plant Location Current DBF Level (ft.)

New DBF Level (ft.)

Probable Maximum Flood (still reservoir) 734.9 739.2 DBF Runup on 4:1 sloped surfaces 736.9 741.6 DBF Runup on critical vertical wall of the Intake Pumping Station 736.9 741.7 DBF Surge level within flooded structures 735.4 739.7 The DBF level on Watts Bar Lake is deleted in the proposed change.

Warninq Plan As described in WBN Unit 1 UFSAR Subsection 2.4.14.8, plant grade elevation 728.0 ft can be exceeded by rainfall floods and closely approached by seismically induced dam failure floods.

A warning plan is needed to assure plant safety from these floods. In the proposed change, the warning time for WBN has been reevaluated because the initial median reservoir levels and flood operational guides have been revised, dam rating curves have changed at some dams, and the SOCH model of the Tennessee River has been updated to meet current quality assurance standards. Based on the current hydrologic analysis, the following specific changes are required to the warning plan analysis and results:

Rainfall Floods: As described in WBN Unit 1 UFSAR Subsection 2.4.14.8.1, protection of WBN from rainfall floods that might exceed plant grade utilizes a flood warning issued by TVA's Water Management. TVA's climatic monitoring and flood predicting systems and flood control facilities permit early identification of potentially critical flood producing conditions and reliable prediction of floods which may exceed plant grade well in advance of the event. In the proposed change, the organizational title is changed to TVA River Operations (RO), and the forecasted levels for issuing Stage I and Stage II warnings are changed to reflect the updated hydrological basis for the warning plan as described further in WBN Unit 1 UFSAR Subsection 2.4.14.9.4.

Seismically-Induced Dam Failure Floods: As described in WBN Unit 1 UFSAR Subsection 2.4.14.8.2, only one postulated combination of seismically induced dam failures and coincident storm conditions was shown to result in a flood which could exceed elevation 727.0 ft at the plant.

In the proposed change, three postulated combinations of seismically induced dam failures are considered, to reflect the updated hydrological basis for the warning plan as described further in WBN Unit 1 UFSAR Subsection 2.4.14.9.4.

Basis for Flood Protection Plan in Rainfall Floods As described in WBN Unit 1 UFSAR Subsection 2.4.14.9, large Tennessee River floods can exceed plant grade elevation 728.0 ft at WBN, and plant safety in such an event requires shutdown procedures which may take 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to implement. TVA flood forecast procedures are used to provide at least 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months of warning before river levels reach elevation 727.0 ft.

Use of elevation 727.0 ft, 1.0 ft below plant grade, provides enough margin to prevent wind generated waves from endangering plant safety during the final hours of shutdown activity.

Based on the current hydrologic analysis, the following specific changes are required to the warning plan analysis and results for rainfall floods:

Overview: As described in WBN Unit 1 UFSAR Subsection 2.4.14.9.1, the estimated probability is less than 0.0032 that a Stage I warning could possibly be issued during the 40-year life of the Page 16 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES plant. In the proposed change, no probability is quoted since there is no regulatory requirement or regulatory guidance to determine the probability.

TVA Forecast System: As described in WBN Unit 1 UFSAR Subsection 2.4.14.9.2, TVA has in constant use an extensive, effective system to forecast flow and elevation as needed in the Tennessee River basin monitored by TVA RO. This permits efficient operation of the reservoir system and provides warning of when water levels could possibly exceed critical elevations at selected, sensitive locations. In the original WBN Unit 1 UFSAR text, there was an extensive description of process, gage network, and forecast procedures.

In the proposed change, updates are included to reflect current processes, gage network and forecast procedures.

These changes reflect refinements which have been implemented to the process over time.

Basic Analysis:

As described in WBN Unit 1 UFSAR Subsection 2.4.14.9.3, the forecast procedure to assure safe shutdown of WBN for flooding is based upon an analysis of 17 hypothetical PMP storms, including their antecedent storms.

In the proposed change, the procedure is based upon an analysis of nine of the 17 hypothetical storms up to PMP magnitude judged to be controlling. This change reflects the updated hydrological basis for the warning plan as described further in WBN Unit 1 UFSAR Subsection 2.4.14.9.4.

Hydrologic Basis for Warning System:

As described in WBN Unit 1 UFSAR Subsection 2.4.14.9.4, a minimum of 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months has been allowed for preparation of the plant for operation in the flood mode.

An additional 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months for communication and forecasting computations is provided to translate rain on the ground to river elevations at the plant. Hence, the warning plan provides 31 hours3.587963e-4 days 0.00861 hours 5.125661e-5 weeks 1.17955e-5 months from arrival of rain on the ground until elevation 727.0 ft could be reached.

The 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months allowed for shutdown at the plant consists of a minimum of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months of Stage I preparation and an additional 17 hours1.967593e-4 days 0.00472 hours 2.810847e-5 weeks 6.4685e-6 months for Stage II preparation that is not concurrent with the Stage I activity.

The threshold river levels at WBN for initiating Stage I and Stage II preparations differ with the season. In the original analysis, during the October 1 - April 15 "winter" season, Stage I threshold elevation was 714.5 ft. Corresponding Stage I threshold river elevation of the April 16 - September 30 "summer" season was elevation 726.5 ft.

In the proposed change, during the "winter" season, Stage I threshold elevation is 715.5 ft.

Corresponding Stage I threshold river elevation for the "summer" season at WBN is elevation 720.6 ft. The specific periods (October 1 - April 15 and April 16 - September 30) were judged to be too rigid and elevations changes are a result of updated analysis which change the curve and subsequently change the threshold elevations. This change is due to the various input changes to the hydrologic analysis, and as a result from the updates to methodology.

Hydrologic Basis for Threshold Flood Warning Levels: As described in WBN Unit 1 UFSAR Subsection 2.4.14.9.5, predicted threshold flood warning levels which assure adequate shutdown times are evaluated.

In the original analysis, the procedure for establishing the threshold levels for WBN shutdown was described with a threshold level in winter at elevation 714.5 (Stage I) and 727.0 (Stage II), and summer at elevation 726.5 (Stage I) and 727.0 (Stage II).

In the proposed change, the procedure for establishing the threshold levels for WBN shutdown is described with a threshold level in winter at elevation 715.5 (Stage I) and 727.0 (Stage II), and in summer at elevation 720.6 (Stage I) and 727.0 (Stage II). This change is due to the various input changes to the hydrologic analysis, and as a result from the updates to methodology.

Communications Reliability:

As described in WBN Unit 1 UFSAR Subsection 2.4.14.9.6, Communication between projects in the TVA power system is via: (a) TVA-owned microwave Page 17 of 41

ENCLOSUREI EVALUATION OF PROPOSED CHANGES network, (b) Fiber-Optics System, and (c) by commercial telephone. In emergencies, additional communication links are provided by Transmission Power Supply radio networks.

The four networks provide a high level of dependability against emergencies. In the original description of the communications systems, the original technology was described.

In the proposed change, the description is updated to reflect current technology.

Basis for Flood Protection Plan in Seismic-Caused Dam Failures:

As described in WBN Unit 1 UFSAR Subsection 2.4.14.10, floods resulting from combined seismic and flood events can closely approach plant grade, thus requiring emergency measures. In the most recent analysis previous to this update, only one seismic dam failure combination coincident with a flood, i.e., the SSE failure of Norris, Cherokee and Douglas Dams concurrent with the 25-year flood, would result in a flood approaching plant grade. In the proposed change, plant grade would be exceeded by three of five candidate combinations. This change is due to the various input changes to the hydrologic analysis, and as a result from the updates to methodology.

Section 2.4 Tables In support of the technical changes proposed for the WBN Unit 1 UFSAR, and to reflect the most current information for the hydrologic analysis for WBN Unit 1, the tables associated with WBN Unit 1 UFSAR are proposed to be revised as follows:

" WBN Unit 1 UFSAR Table 2.4-1, Facts About Major TVA Dams and Reservoirs, is revised to incorporate the latest information for the TVA system dams and reservoirs. The revised table is also renamed to delete the word "Major," reformatted using a more recent word processing program, and renumbered as Table 2.4-2 to be in the correct sequence for this section.

WBN Unit 1 UFSAR Table 2.4-2, Facts About Non-TVA Dams and Reservoirs - Historical Information, is renamed to delete the "Historical Information" reference, reformatted using a more recent word processing program, and renumbered as Table 2.4-5 to be in the correct sequence for this section. There are no technical changes to the table.

WBN Unit 1 UFSAR Table 2.4-3, Flood Detention Capacity - TVA Projects Above Watts Bar Nuclear Plant, is revised to incorporate the latest information for the TVA system projects above the WBN site.

The revised table is also reformatted using a more recent word processing program, and renumbered as Table 2.4-6 to be in the correct sequence for this section.

A new WBN Unit I UFSAR Table 2.4-3, TVA Dams - River Mile Distances to WBNP, is added.

" WBN Unit 1 UFSAR Table 2.4-4, Location of Surface Water Supplies in the 58.9 Mile Reach of the Mainstream of the Tennessee River Between Watts Bar Dam (TRM 529.9) and Chickamauga Dam (TRM 471.0), is revised to incorporate the latest information for these surface water supplies. The revised table is also reformatted using a more recent word processing program, and renumbered as Table 2.4-1 to be in the correct sequence for this section.

Page 18 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES

" WBN Unit 1 UFSAR Table 2.4-5, Probable Maximum Storm Precipitation and Precipitation Excess, is revised to incorporate the latest information for the determination of these inputs to the hydrologic analysis. The revised table is also reformatted using a more recent word processing program, and renumbered as Table 2.4-11 to be in the correct sequence for this section.

WBN Unit 1 UFSAR Table 2.4-6, Unit Hydrograph Data, is revised to incorporate the latest information for the unit hydrograph data used in the hydrologic analysis. The revised table is also reformatted using a more recent word processing program, and renumbered as Table 2.4-13 to be in the correct sequence for this section.

" WBN Unit 1 UFSAR Table 2.4-7, Flood Flow and Elevation Summary, is deleted because the information provided is redundant to that included in the WBN Unit I UFSAR Section 2.4 text.

A new WBN Unit 1 UFSAR Table 2.4-7, Peak Streamflow of the Tennessee River at Chattanooga, TN (USGS Station 03568000) 1867 - 2007, is added.

" WBN Unit 1 UFSAR Table 2.4-8 Floods from Postulated Seismic Failure of Upstream Dams (Plant Grade is Elevation 728), is revised to incorporate the latest information for the results of the flood analysis.

The revised table is also reformatted using a more recent word processing program, and renumbered as Table 2.4-14 to be in the correct sequence for this section.

" WBN Unit 1 UFSAR Table 2.4-9 was previously deleted from the WBN Unit 1 UFSAR. The number is reused as discussed for renumbered WBN Unit 1 UFSAR Table 2.4-15.

" WBN Unit 1 UFSAR Table 2.4-10, Well and Spring Inventory Within 2-mile Radius of Watts Bar Nuclear Plant Site (1972 Survey Only) - Historical Information, is renamed to delete the "Historical Information" reference, reformatted using a more recent word processing program, and renumbered as Table 2.4-15 to be in the correct sequence for this section.

There are no technical changes to the table.

" A new WBN Unit 1 UFSAR Table 2.4-10, Seasonal Variations of Rainfall (PMP), is added.

" WBN Unit 1 UFSAR Table 2.4-11 was previously deleted from the WBN Unit 1 UFSAR. The number is reused as discussed for revised and renumbered WBN Unit 1 UFSAR Table 2.4-5.

WBN Unit 1 UFSAR Table 2.4-12 was previously deleted from the WBN Unit 1 UFSAR. The number is reused for a new WBN Unit 1 UFSAR Table 2.4-12, Historical Flood Events, which incorporates the latest information for historical floods.

WBN Unit 1 UFSAR Table 2.4-13 was previously deleted from the WBN Unit 1 UFSAR. The number is reused as discussed for revised and renumbered WBN Unit 1 UFSAR Table 2.4-6.

WBN Unit 1 UFSAR Table 2.4-14, Weir Length Description and Coefficients of Discharge for Areas 3 and 4, is reformatted using a more recent word processing program and Page 19 of 41

ENCLOSUREI EVALUATION OF PROPOSED CHANGES renumbered as Table 2.4-8 to be in the correct sequence for this section. There are no technical changes to the table.

" WBN Unit 1 UFSAR Table 2.4-15, Drainage Area Peak Discharge, is reformatted using a more recent word processing program and renumbered as Table 2.4-9 to be in the correct sequence for this section. There are no technical changes to the table.

" WBN Unit 1 UFSAR Table 2.4-16, Dam Safety Modification Status (Hydrologic), is deleted because the information provided is not appropriate information for the WBN Unit 1 UFSAR.

Section 2.4 Figures In support of the technical changes proposed for the WBN Unit 1 UFSAR, and to reflect the most current information for the hydrologic analysis for WBN Unit 1, the figures associated with WBN Unit 1 UFSAR are proposed to be revised as follows:

WBN Unit 1 UFSAR Figures 2.4-1 through 2.4-14 are deleted and have been replaced by new figures with different information that support the updated hydrologic analysis in WBN Unit 1 UFSAR Section 2.4.

The information provided by the WBN Unit 1 UFSAR figures is not necessary to support the updated hydrologic analysis.

WBN Unit 1 UFSAR Figures 2.4-15 through 2.4-24 and 2.4-26 are replaced by a new Figure 2.4-3 with similar but updated information.

The information provided by the WBN Unit 1 UFSAR figures is updated in the replacement figure to reflect current TVA RO operating guidelines that are used as inputs to the updated hydrologic analysis.

" WBN Unit 1 UFSAR Figures 2.4-25 and 2.4-27 through 2.4-38 are replaced by a new Figure 2.4-4 with similar but updated information.

The information provided by the WBN Unit 1 UFSAR figures is updated in the replacement figure to reflect current reservoir areas and volumes based on the current TVA RO operating guidelines that are used as inputs to the updated hydrologic analysis.

WBN Unit 1 UFSAR Figures 2.4-39, 2.4-54, 2.4-56, 2.4-58, 2.4-60, 2.4-64, 2.4-70, 2.4-84, 2.4-85, 2.4-92, 2.4-100, and 2.4-101 are not used. These figures were previously deleted in the WBN Unit 1 UFSAR.

These figures are not referenced in the proposed WBN Unit 1 UFSAR text.

" WBN Unit 1 UFSAR Figure 2.4-40 is replaced by new Figure 2.4-5 with similar but updated information. The information provided by the WBN Unit 1 UFSAR figure is updated in the replacement figure to reflect information from additional historical flood events that are used as inputs to the updated hydrologic analysis.

WBN Unit 1 UFSAR Figures 2.4-40a through 2.4-40g are retained but updated, with a new WBN Unit 1 UFSAR Figure 2.4-40c added, to reflect the current site grading and drainage plans.

WBN Unit 1 UFSAR Figures 2.4-40h, 2.4-68, 2.4-71, 2.4-76, 2.4-77, 2.4-79 through 2.4-83, 2.4-86, 2.4-88 through 2.4-91, 2.4-93, 2.4-94, 2.4-99, 2.4-102 through 2.4-105, 2.4-108, and 2.4-109 are retained without any technical changes.

Page 20 of 41

ENCLOSUREI EVALUATION OF PROPOSED CHANGES WBN Unit 1 UFSAR Figures 2.4-41 through 2.4-53, 2.4-55, 2.4-59, 2.4-61 through 2.4-63, 2.4-65, 2.4-67, 2.3-69, 2.4-73 through 2.4-75, 2.4-95 through 2.4-97, and 2.4-111 are deleted. The information provided by these figures is not necessary to support the updated hydrologic analysis. These figures are not referenced in the proposed WBN Unit 1 UFSAR text.

WBN Unit 1 UFSAR Figure 2.4-57 is replaced by new Figure 2.4-25 with similar but updated information. The information provided by the WBN Unit 1 UFSAR figure is updated in the replacement figure to reflect the new PMF elevation at WBN in the updated hydrologic analysis.

" WBN Unit 1 UFSAR Figure 2.4-66 is renumbered as Figure 2.4-29 with the same information.

WBN Unit 1 UFSAR Figures 2.4-72, 2.4-78, and 2.4-87 are retained but updated to reflect improvements to the embankments effectively raising their heights.

WBN Unit 1 UFSAR Figure 2.4-98 is renumbered as Figure 2.4-27 with updated information.

WBN Unit 1 UFSAR Figures 2.4-106 and 2.4-107 are retained without any technical changes, and are older revisions of the drawing.

WBN Unit 1 UFSAR Figure 2.4-110 is retained but updated to reflect the changes to the warning time basis in the updated hydrologic analysis.

" New WBN Unit 1 UFSAR Figures 2.4-15 through 2.4-26 and 2.4-28 are added to reflect the changes to the analysis of PMF on streams and rivers in the updated hydrologic analysis.

" New WBN Unit 1 UFSAR Figures 2.4-112 and 2.4-114 through 2.4-116 are added to reflect the changes to the analysis of seismically induced dam failures in the updated hydrologic analysis.

2.2 Need for Proposed Changes TVA submitted an application for a COLA for the proposed BLN Units 3 and 4 on October 30, 2007, in accordance with 10 CFR 52. During review of the BLN Units 3 and 4 FSAR, the NRC performed an audit of the hydrologic analysis which resulted in the issuance of three NOVs on March 19, 2008 (

Reference:

NRC Letter to TVA, Bellefonte Combined License Application -

Nuclear Regulatory Commission Inspection of the Implementation of the Quality Assurance Program Governing the Simulated Open Channel Hydraulics Model - Inspection Report Numbers 05200014/2008--01 and 05200015/2008-001 and Notice of Violation, Accession No. ML080640487). In response to these NOVs, TVA completed a revised hydrologic analysis to support the BLN Units 3 and 4 COLA.

The revised TVA hydrologic analysis included reconstitution of the inputs and updates to calculations and software, and is documented in TVA quality-related calculations prepared, reviewed, and approved in accordance with an approved 10 CFR 50, Appendix B Quality Assurance program.

Page 21 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES As a result of the revised BLN Units 3 and 4 hydrologic analysis, an update of the hydrologic analysis for the proposed WBN Unit 2 was performed to support the NRC review of the application for an Operating License in accordance with 10 CFR 50, and updates to the WBN Unit 2 FSAR were submitted to the NRC.

During development of the WBN Unit 2 hydrologic analysis, conforming changes were identified in the existing design and licensing basis hydrologic analysis for WBN Unit 1. In addition, the hydrologic analysis for BLN Units 3 and 4, and subsequently WBN Unit 2, incorporates updated input information, application of specific project model data to correct and update dam failure rating curves, and updated methodology in the hydrologic analysis which uses the most recent hydrologic data and which includes use of USACE HEC-HMS and HEC-RAS software for elements of the methodology. As these differences between the existing WBN Unit 1 licensing basis and the proposed WBN Unit 2 licensing basis were identified, corrective action documents were written and evaluations performed to assess the estimated impact of the differences on the WBN Unit 1 design bases. After the differences had been reviewed for impact to WBN Unit 1 operability and design bases, the corrective action documents were closed to a single corrective action document, WBN PER 154477, tracking resolution of the final hydrologic analysis and update of the WBN Unit 1 UFSAR licensing basis.

The proposed changes are the result of the conforming changes that were identified in the existing design basis hydrologic analysis for WBN Unit 1 in the WBN PERs that have been closed to WBN PER 154477.

The proposed changes are also necessary to address the cumulative effects that have occurred in the WBN Unit 1 hydrologic analysis since issuance of the Operating License. TVA is requesting NRC review and approval of all of the technical changes to the WBN Unit 1 UFSAR described in this enclosure, specifically the technical changes proposed in WBN Unit 1 UFSAR Sections 2.4, 2.4.1, 2.4.2, 2.4.3, 2.4.4, 2.4.11, and 2.4.14, to incorporate the cumulative effects that have occurred in the WBN Unit 1 hydrologic analysis since issuance of the Operating License.

The proposed technical changes to the WBN Unit I UFSAR described in this enclosure include changes that incorporate updates previously submitted in support of the initial licensing of WBN Unit 2 as well as more recently discovered input information.

Given that the WBN Unit 1 UFSAR and Unit 2 Final Safety Analysis Report (FSAR) are separate documents at this time, TVA will submit an update to the affected sections of the WBN Unit 2 FSAR on or before August 30, 2012 to assure that the hydrology licensing bases are consistent.

3.0 TECHNICAL EVALUATION

3.1 Evaluation Section 2.4, Hydrologic Engineering This section provides a summary of information that is more explicitly discussed in the applicable subsections. The justification for the changes in these sections is provided in the applicable subsections below.

Section 2.4.1, Hydrological Description The proposed changes to WBN Unit 1 UFSAR Section 2.4.1 update the hydrological description of the TVA system upstream of the WBN site, and update figures showing the current TVA Page 22 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES reservoir seasonal operating guides. These changes are updates to the inputs used for the hydrologic analysis to reflect the most current information available regarding operations of the TVA system of reservoirs and dams, and do not represent any changes to the methodologies used in the updated hydrologic analysis. These changes represent the most current, complete, and substantiated information relative to the hydrologic description in the vicinity of the site and site regions important to the design and siting of WBN Unit 1 as expected for review by the NRC in NUREG-0800, Standard Review Plan (SRP), Section 2.4.1.

Section 2.4.2, Floods The proposed changes to WBN Unit 1 UFSAR Section 2.4.2 update the discussion of floods to reflect the most current information available regarding historical floods that affect the hydrologic analysis for WBN Unit 1. These changes are updates to the inputs used for the hydrologic analysis to reflect the most current information available regarding historical floods, and do not represent any changes to the methodologies used in the updated hydrologic analysis. These changes represent the most current, complete, and substantiated information relative to the local intense precipitation, flooding causal mechanisms, and the controlling flooding mechanism important to the design and siting of WBN Unit 1 as expected for review by the NRC in SRP Section 2.4.2.

Section 2.4.3, Probable Maximum Flood (PMF) on Streams and Rivers The proposed changes to WBN Unit 1 UFSAR Section 2.4.3 update the discussion of PMF on streams and rivers to reflect the most current information available as inputs, and to use updated methodologies such as the USACE HEC-HMS and USACE HEC-RAS software for elements of the hydrologic analysis for determining the PMF for streams and rivers for WBN Unit 1. As a result of these changes, PMF elevation at the plant is increased from elevation 734.9 ft to 739.2 ft, excluding wind wave effects.

The design basis PMF event for WBN is based on storms with the heavy rainfall occurring in the middle of the three-day main storm (adopted distribution). This time distribution is supported by evaluation of actual storm events which have occurred in the region and is consistent with regulatory guidance and accepted practice.

Inputs to the simulations include calibrated SOCH models (geometry files and Manning's n values) of each reservoir, operational guides and initial median reservoir levels, initial dam rating curves, as well as inflow hydrographs. The model is divided into three segments for ease of computation. Segment I is the model comprising Fort Loudoun-Tellico, Melton Hill and Watts Bar reservoirs.

Segment la is the model comprising the Melton Hill, Watts Bar and Chickamauga reservoirs. Segment 2 is the model comprising the Chickamauga, Nickajack, and Guntersville reservoirs. Watts Bar Dam is an appropriate location to divide Segments 1 and 2 of the model, because there is not a significant submergence effect by the tailwater on the discharge from Watts Bar Dam that would require modeling simultaneously with the downstream dam unless the concrete wall at Watts Bar Dam fails due to overtopping. If the concrete wall failure occurs, Segment Ia is required to model the submergence effects at Watts Bar Dam.

Initial dam rating curves have been developed for six main stem dams and one tributary dam (Melton Hill Dam) to be used as inputs to the SOCH models. The initial dam rating curves were developed using an average tailwater to determine outflow from the dam based on data from Page 23 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES steady-state profiles at varying flows. In modeling the design storms it is necessary to adjust some of the initial dam rating curves to more accurately account for the submergence effect of the tailwater on the discharge that may occur at the dams during a large flood event.

A SOCH model analysis of hypothetical storms on the Tennessee River Watershed above the Guntersville Dam was conducted using the methodology described above. The current lock configuration with 18 spillway bays was used for modeling Chickamauga Dam.

Additional details including specific changes to the SOCH model analysis are described in Section 2.1 of this enclosure.

These changes represent the most current, complete, and substantiated information relative to the probable maximum flooding on streams and rivers important to the design and siting of WBN Unit 1 as expected for review by the NRC in SRP Section 2.4.3.

Section 2.4.4, Potential Dam Failures, Seismically Induced The proposed changes to WBN Unit 1 UFSAR Section 2.4.4 update the discussion of potential flood levels from seismically induced dam failures to reflect the most current information available as inputs, and to use updated methodologies such as the USACE HEC-HMS and USACE HEC-RAS software for elements of the hydrologic analysis for determining dam failure outflows from tributary dams for WBN Unit 1.

As described in WBN Unit 1 UFSAR Subsection 2.4.4, procedures described in Appendix A of RG 1.59 were followed when evaluating potential flood levels from seismically induced dam failures.

Site flooding levels at WBN from potential seismically induced dam failures are determined using the SOCH computer hydraulic model, with changes previously discussed regarding proposed change to the runoff and stream course model in WBN Unit 1 UFSAR Subsection 2.4.3.

In the updated hydrologic analysis, the following bounding combinations are used for evaluating potential flood levels from seismically induced dam failures:

A Seismic failures of Tellico Dam and Norris Dam during one-half the PMF during an OBE.

B Seismic failures of Tellico Dam, Norris Dam, Cherokee Dam, and Douglas Dam during a 25-year flood during a SSE.

E Seismic failure of Tellico Dam and partial seismic failure of Fontana Dam during one-half the PMF during an OBE.

F Partial seismic failure of Fontana Dam and seismic failures of Tellico Dam, Hiwassee Dam, Apalachia Dam, and Blue Ridge Dam during one-half the PMF during an OBE.

H Seismic failures of Cherokee Dam, Douglas Dam, and Tellico Dam during one-half the PMF during an OBE.

A SOCH model analysis of these bounding dam failure combinations was conducted using the methodology described above for WBN Unit 1 UFSAR Section 2.4.3.

Additional details including specific changes to the SOCH model analysis are described in Section 2.1 of this enclosure. Table 1 provides a summary of the peak elevations and discharges at TVA dams Page 24 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES and WBN produced by the five potential seismically induced dam failure combinations.

As shown in Table 1, the peak water surface elevation at WBN from seismically induced dam failures is produced by Seismic Dam Failure Combination B, elevation 731.2 ft.

Table 1 - Summary of Maximum Elevations and Discharges in the Tennessee River Watershed Seismic Seismic Seismic Seismic Seismic Dam Dam Dam Dam Dam Failures Failures Failures Failures Failures Combination Combination Combination Combination Combination Location Parameter A

B E

F H

Fort Headwater 817.2 833.3 817.2 817.2 836.2 Loudoun (feet)

Dam Discharge 443,594 2,573,677 383,877 383,877 2,101,716 Dam (cfs)

Headwater 818.5 813.2 818.4 818.4 818.6 Tellico (feet)

Dam Discharge 1,829,012 1,855,019 3,549,639 3,549,639 1,731,678 (cfs)

Headwater 817.4 817.0 795.0 795.0 795.0 Melton (feet)

Hill Dam Discharge 1,053,348 876,224 88,269 88,269 88,275 (cfs)

Headwater 763.0 765.6 756.2 756.2 763.1 Watts Bar feet)

Dam Discharge 1,074,582 1,195,727 744,786 744,786 1,059,008 (cfs)

Headwater 728.7 731.2 720.7 722.1 729.1 Watts Bar (feet)

Nuclear Discharge 917,284 979,385 743,668 742,572 902,687 (cfs)

These changes represent the most current, complete, and substantiated information relative to the effects of dam failures important to the design and siting of WBN Unit 1 as expected for review by the NRC in SRP Section 2.4.4.

Sections 2.4.5 throuqh 2.4.10 There are no technical changes proposed for these sections requiring NRC review and approval.

Section 2.4.11, Low Water Considerations The proposed changes to WBN Unit 1 UFSAR Section 2.4.11 update the discussion of low water considerations to reflect the most current information for the hydrologic analysis for WBN Unit 1.

As described in WBN Unit 1 UFSAR Subsection 2.4.11.1, analyses are performed to determine probable minimum water level at WBN Unit 1 and the minimum flow requirement at the ERCW intake. In the proposed change, water level at the WBN site upon loss of downstream dam from Page 25 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES HW 682.5 water surface at site begins to drop in 3 hours3.472222e-5 days 8.333333e-4 hours 4.960317e-6 weeks 1.1415e-6 months and reaches elevation 666.0 ft in 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months , and the flow required to maintain an elevation of 665.9 ft producing a 5.9 ft depth at the ERCW intake channel is 3,200 cfs. These changes are the result of updates to the routing model cross-sectional data using new bathymetry and recalibration of the models, as determined by postulating loss of Chickamauga Dam and no flow from Watts Bar Dam. The result from the update is that the required minimum elevation of 666.0 ft is actually reached at a later time (27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months instead of 22 hours2.546296e-4 days 0.00611 hours 3.637566e-5 weeks 8.371e-6 months ). Therefore, there would be additional time to allow discharge from Watts Bar Dam to maintain the minimum required elevation of 666.0 ft. The higher minimum flow is well within the capability of operations of Watts Bar Dam to supply to WBN Unit 1.

These changes represent the most current, complete, and substantiated information relative to the low water effects important to the design and siting of WBN Unit 1 as expected for review by the NRC in SRP Section 2.4.11.

Sections 2.4.12 and 2.4.13 There are no technical changes proposed for these sections requiring NRC review and approval.

Section 2.4.14, Flooding Protection Requirements The proposed changes to WBN Unit 1 UFSAR Section 2.4.14 update the discussion of flooding protection requirements to reflect the most current information for the hydrologic analysis for WBN Unit 1.

As described in WBN Unit 1 UFSAR Subsection 2.4.14, an evaluation is performed to determine the methods by which the WBN is capable of tolerating floods above plant grade without jeopardizing public safety. The DBF is the calculated upper-limit flood that includes the PMF plus the wave runup caused by a 21 mph overland wind as discussed in WBN Unit 1 UFSAR Subsection 2.4.3.6. As a result of the changes to inputs and methodology discussed previously, the DBF elevations at various plant locations that would occur by the limiting large rainfall and seismically induced dam failure floods are increased to the following:

Plant Location DBF Level (ft.)

Probable Maximum Flood (still reservoir) 739.2 DBF Runup on 4:1 sloped surfaces 741.6 DBF Runup on critical vertical wall of the Intake Pumping Station 741.7 DBF Surge level within flooded structures 739.7 WBN is designed in accordance with the Regulatory Position 2 of RG 1.59, Revision 2, August 1977, which specifies that at least those structures, systems, and components necessary for cold shutdown and maintenance thereof are designed with hardened protective features to remain functional while withstanding the entire range of flood conditions up to and including the worst site-related flood probable (e.g., PMF, seismically induced flood, hurricane, surge, seiche, heavy local precipitation) with coincident wind-generated wave action as discussed in Regulatory Position 1 of the RG.

Although the DBF elevations at various plant locations that would occur by the limiting large rainfall and seismically induced dam failure floods are increased from those currently provided in the WBN Unit I UFSAR, there are only two distinct changes to the physical flooding Page 26 of 41

ENCLOSUREI EVALUATION OF PROPOSED CHANGES protection features of WBN Unit 1 required.

All other safety-related systems, structures, and components identified in Regulatory Guide 1.29 are designed to withstand the flood conditions associated with the updated DBF elevations, and would remain functional during external floods.

The UFSAR currently requires the Reactor Building and Diesel Generator Buildings to remain dry during flood mode. No barriers for these structures would be breached due to the revised flood elevations.

The Intake Pumping Station (IPS) is designed to have the Essential Raw Cooling Water (ERCW) System and the High Pressure Fire Protection (HPFP) System remain fully function for the DBF. The revised DBF elevation for the critical face of the IPS results in the possibility of flooding of the IPS impacting ERCW equipment required for flood mode operation located on elevation 722 ft. The IPS structure contains various equipment required to support the ERCW and HPFP systems. The IPS contains the ERCW and HPFP pumps, travelling water screens and support equipment including screen wash pumps, ERCW strainers and support equipment including backwash valves and pressure indicators, and HPFP strainers and support equipment including backwash valves and pressure indicators. During a DBF event, surge is accounted for by considering the sum of the wind wave and runup on the critical face of the IPS combined with the PMF stillwater elevation, which conservatively results in an internal flood elevation of 741.7 ft for the IPS. While this does not wet any flood-sensitive equipment on elevation 741.0 ft, the ERCW strainers and support equipment are located on elevation 722.0 ft of the IPS, connected to elevation 741.0 ft via stairwells and doors W001 and W002 at elevation 741.0 ft.

The critical elevation of flood-sensitive equipment located on elevation 722.0 ft is approximately 18 inches above the floor elevation. Doors W001 and W002 both have 0.5 ft concrete berms at the opening to elevation 741.0 ft, which raises the critical elevation for floodwaters to be capable of wetting elevation 722.0 ft to elevation 741.5 ft. As a result of this increase, a compensatory measure of staged sandbags to be constructed into a berm at any time prior to or during the event of a Stage I flood warning has been implemented. These sandbags will be constructed into a berm at least 12 inches in height to prevent water intrusion to elevation 722.0 ft.

Additionally, two non-safety related sump pumps in each of the ERCW Train A and B strainer rooms, connected to safety-related power sources, are available to expel water leakage to this elevation outside the structure. TVA's established corrective action program requirements are being implemented to address the need for additional compensatory measures necessary to provide flood protection for the IPS internal systems and components, including the need for permanent plant modifications.

The Service, Turbine, Auxiliary, and Control Buildings are permitted to flood as the water exceeds the plant level entrances. No permanent barriers to specifically protect flood sensitive plant equipment exist in any of these structures although, as discussed further in Section 3.2 of this enclosure, temporary compensatory measures are in place to ensure adequate flood protection if a PMF event were to occur, and permanent plant modifications are planned to restore or gain additional margin between the revised DBF elevations and limiting safety-related systems, structures, and components in the Auxiliary Building.

Specifically, a limiting safety-related component required to be available during a plant flood affected by the increase in DBF elevations is the Thermal Barrier Booster (TBB) Pump Motors. The DBF surge level within flooded structures is elevation 739.7 ft.

Field measurements taken from the floor and calibrated with benchmark locations near the TBB Pump Motors indicate that the actual elevation to the baseplate of the TBB Pump Motors is elevation 739.3 ft. Therefore, there is not adequate flood protection for these motors. To restore margin for the TBB Pump Motors, a Page 27 of 41

ENCLOSUREI EVALUATION OF PROPOSED CHANGES temporary flood protection barrier has been designed to be installed around the TBB Pump Motors prior to the event of a Stage I flood warning.

Installation of the temporary flood protection barrier is in progress at WBN Unit 1. The barrier encompasses the TBB Pump Motors providing approximately 0.8 ft of margin above the DBF surge level. There are seven major components that are part of the barrier (three end attachment units and four panels), with two end attachment units that attach the L-shaped barrier to the West and South walls that are permanently attached to the surrounding structure walls.

Regulatory Position 2 of RG 1.59, Revision 2, August 1977, also specifies that sufficient warning time is shown to be available to shut the plant down and implement adequate emergency procedures.

TVA RO is responsible for operations of the TVA dams and reservoirs for the Tennessee River watershed, including controls and scheduling the releases from all flood storage dams above the nuclear sites as an integrated system. The USACE is responsible for operation of the locks on the Tennessee River. TVA RO operates the TVA dams and reservoirs for purposes that include:

Flood control and storm water management of the Tennessee River and major tributaries, Irrigation of land along the Tennessee River,

" Generation of hydroelectric power,

° Cooling of fossil and nuclear power plants,

" Navigation of recreational and commercial vessels, Public and industrial water supplies, and General recreation of the public.

As a part of TVA RO's flood control responsibilities, forecast and warning procedures have been established that reflect the updated hydrologic analyses. The safety of the plant in extreme events requires shutdown procedures which may take 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months to implement. TVA's calculation demonstrates that the time is available for TVA RO's forecast and warning procedures to provide at least 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months before river levels reach elevation 727.0 ft. Use of elevation 727.0 ft, 1.0 ft below plant grade, provides enough margin to prevent wind generated waves from endangering plant safety during the final hours of shutdown activity.

Flood warning is based upon rainfall already reported to be on the ground on the watershed above WBN. Although the warning time for WBN has been reevaluated, the only significant change in the results of the analysis of warning time is the use of a revised forecasted plant site water levels where Stage I actions are required to begin. Use of these revised Stage I action levels does not reduce the effectiveness of the warning plan, as there is still a minimum of 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months to prepare for operation in the flood mode.

The warning time for WBN has been reevaluated because the initial median reservoir levels and flood operational guides have been revised, dam rating curves have changed at some dams, and the SOCH model of the Tennessee River has been updated to meet current quality assurance standards. Details of the warning plan analysis follows:

Page 28 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES The design basis PMF event for WBN is based on storms with the heavy rainfall occurring in the middle of the three-day main storm (adopted distribution). This time distribution is supported by evaluation of actual storm events which have occurred in the region and is consistent with regulatory guidance and accepted practice. In order to address the warning time issue, different time distributions have been evaluated.

The original analysis tested the effects of varied time distribution of rainfall by alternatively placing the maximum daily rainfall on the first, middle, and the last day of the three-day main storm to ensure that the shortest warning times were captured for the hypothetical storms. This analysis showed that the fastest rising floods occur when the heavy rainfall is applied at the end of the storm. The current analysis consists of nine hypothetical storms ranging from slightly more than five inches (equivalent to the largest flood event since regulation) up to PMP and enveloped potentially critical areal, seasonal variations and time distribution of rainfall.

The warning time is based on those storm situations which resulted in the shortest time interval between watershed rainfall and elevation 727.0 ft.

The warning time is divided into two stages: Stage I, a minimum of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months long and Stage II, a minimum of 17 hours1.967593e-4 days 0.00472 hours 2.810847e-5 weeks 6.4685e-6 months long so that unnecessary economic consequences can be avoided, while adequate time is allowed for preparing for operation in the flood mode.

Stage I allows preparation steps causing minimal economic consequences to be sustained but postpones major economic damage to the plant until a Stage II warning predicts a likely forthcoming flood above plant grade. If the flood does not develop beyond a Stage I warning, major economic damage is avoided.

To be certain of 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months for pre-flood preparation, flood warnings with the prospect of reaching elevation 727.0 ft must be issued early when lower threshold flood warning levels are forecast.

Consequently, some of the Stage I warnings may not progress into a Stage II warning. For this reason pre-flood preparations are divided into two stages. Stage I steps requiring 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months are easily revocable and cause minimum economic consequences to the plant.

Additional rain and stream-flow information obtained during Stage I activity determines if the more serious steps of Stage II need to be taken with the assurance that at least 17 hours1.967593e-4 days 0.00472 hours 2.810847e-5 weeks 6.4685e-6 months is available before elevation 727.0 ft is reached.

Considering results of the original analysis, six storms were identified in the current analysis to envelop the potentially critical variations of rainfall. Separate controlling winter and summer storm events were selected for use with the critical time distribution.

The controlling winter event selected was the 21,400 square-mile March downstream centered storm. The controlling summer event selected was the 7,980 square-mile June Bulls Gap centered event. The six selected storms represent average basin rainfall from 5.2 inches up to the PMP. The selected events are described below.

1. 21,400 square-mile March downstream centered PMP storm with heavy rainfall on the last day (HLD)

2. 7,980 square-mile June Bulls Gap centered PMP storm (HLD)

3. 21,400 square-mile March downstream centered PMP storm scaled to 10 inches above Chickamauga Dam, (HLD)

4. 7,980 square-mile June Bulls Gap centered PMP storm scaled to 10 inches above Chickamauga Dam, (HLD)

Page 29 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES

5. 21,400 square-mile March downstream centered PMP storm scaled to 5.2 inches above Chickamauga Dam, (HLD)

6. 7,980 square-mile June Bulls Gap centered PMP storm scaled to 5.2 inches above Chickamauga Dam, (HLD)

In addition to the six storms above, the three PMF candidate events listed below are analyzed to determine the design basis flood at WBN.

7. 21,400 square-mile, March, downstream centered PMP storm with the adopted distribution

8. 7,980 square-mile, March, Bulls Gap centered PMP storm with the adopted distribution

9. 7,980 square-mile, June, Bulls Gap centered PMP storm with the adopted distribution For events 7 to 9 listed above, the heavy rainfall is in the middle of the three-day main storm and is referred to as the adopted distribution.

The results of the six simulations performed as part of this analysis, together with the results of the three additional simulations, were used to develop the relationship between basin average rainfall and peak river elevation at WBN.

The maximum calculated water surface elevation at WBN for each storm was plotted against average basin rainfall depth.

Summer and winter relationships were developed using a polynomial curve. This approach did not result in curves which passed through all calculated points. To ensure that the curve would envelop the calculated points, a systematic adjustment of the polynomial coefficients was applied until the curve passed thru the calculated points or was within 0.1 ft above.

The adopted warning time curves envelop all routing simulation results for their respective seasons using inflows from the selected worst case storm events. Therefore, these curves are a bounding condition for determining the warning times at WBN.

The warning time to assure safe shutdown of WBN for flooding resulting from seismic dam failures coincident with flood events is based upon analysis of potentially critical combinations of dam failures.

Flood warnings are issued in real-time by TVA RO. Flood control operations for a major storm that spans the majority of the Tennessee Valley would necessitate the integrated operation of all the reservoirs in the system. The flood storage available to TVA for minimizing flood damages is finite, and does not allow TVA to eliminate flooding at all areas along the regulated rivers.

Thus, TVA efforts are directed toward using the available flood storage to minimize downstream flooding, rather than eliminating downstream flooding. During extreme flood events, TVA would focus on minimizing downstream flood damage to the extent possible, operating the projects to ensure the safety and integrity of the dams and appurtenant structures, and providing frequent flood warning time and elevation forecasts.

The one-half PMF, developed as part of the seismic flood analysis, addresses item 2 of Regulatory Position 2, Regulatory Guide 1.59, Revision 2. This storm was developed by taking one-half the runoff ordinates of the design basis flood including the antecedent flood (21,400 square-mile PMP storm) plus base flow and routing them through the reservoir system.

Page 30 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES Based on use of the warning time methodology described above, a minimum of 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months has been allowed for preparation of the plant for operation in flood mode. An additional 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months is allowed for communication and forecasting computations by the TVA RO organization to translate rain on the ground to river elevations at the plant. Hence, the warning time provides a minimum of 31 hours3.587963e-4 days 0.00861 hours 5.125661e-5 weeks 1.17955e-5 months from arrival of rain on the ground until elevation 727.0 ft could be reached.

A minimum of 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months is allowed for shutdown at the plant which consists of a minimum of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months of Stage I preparation and a minimum of 17 hours1.967593e-4 days 0.00472 hours 2.810847e-5 weeks 6.4685e-6 months for Stage II preparation that is not concurrent with the Stage I activity.

Although reservoir elevation 727.0 ft, 1.0 ft below plant grade to allow for wind waves, is the controlling elevation for determining the need for plant shutdown, lower forecast threshold warning flood elevations are used in some situations to assure that the 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months pre-flood transition interval is always available. The threshold warning flood elevations differ with season of the year.

Determination of the warning time and flood elevations at WBN requires the following:

1. The elevation hydrograph at the plant for each simulated rainfall event;

2. A plot of the cumulative storm rainfall for each rainfall event; and

3. The relationship between average basin rainfall and plant elevation.

The shortest warning time scenario for the winter was determined to be produced by the March 21,400 square mile downstream centered PMP with the heavy rainfall on the last day and postulated Fontana Dam failure. The shortest warning time for the summer flood was produced by the 7,980 square mile June Bulls Gap centered PMP with the heavy rainfall on the last day.

Inasmuch as the hydrologic procedures and threshold flood warning levels have been established to provide adequate shutdown time in the flood producing the shortest warning time, longer times are available in other floods. In such cases there is a waiting period, after the Stage 1, 10-hour shutdown activity during which activities shall be in abeyance until TVA RO determines, based upon weather conditions, that plant operation can be resumed, or if Stage II shutdown should be implemented.

For rainfall induced floods, the available warning times are adopted as results and have been evaluated for the flood conditions producing the shortest warning time. Therefore, more time would be available in all other flood situations. Table 2 includes the predicted threshold flood warning levels for the shortest warning time flood conditions which assure adequate warning time for plant shutdown.

Table 2 - Warning Threshold Flood Warning Levels Stage I Shutdown Stage Wt Shutdown Season Elevation (ft)

Rainfall* (inches)

Elevation (ft)

Rainfall* (inches)

Winter 715.5 8.6 727.0 11.2 Summer 720.6 9.3 727.0 11.3

Rainfall in table refers to "inches of rain on the ground above Wafts Bar Dam."

Page 31 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES For seismically induced dam failure floods, Table 3 provides the maximum elevations and warning times at WBN for the five seismic combinations evaluated. The following three seismic dam failure combinations would result in flood levels above plant grade elevation (728.0 ft).

1. SSE failure of Norris, Cherokee, Douglas and Tellico Dams coincident with the 25-year flood;

2. OBE failure of Norris and Tellico Dams coincident with one-half PMF; and

3. OBE failure of Cherokee, Douglas, and Tellico Dams coincident with one-half PMF.

As shown in Table 3, the other two candidate combinations of events would create flood levels below plant grade elevation (728.0 ft). The times from seismic occurrence to arrival of failure surge at the WBN are shown in Table 3. The failure of Norris and Tellico Dams in an OBE event coincident with one-half PMF produces the shortest arrival time at 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months and is adequate to permit safe plant shutdown in readiness for flooding.

Dam failure during non-flood periods was not reevaluated, but would be bounded by the three critical failure combinations described above.

The warning time for safe plant shutdown in seismically induced dam failure flood events is based on the fact that a failure combination of critically centered large earthquake conditions must exist before the flood waves from seismically induced dam failures could exceed plant grade.

Table 3 - Floods from Postulated Seismic Failure of Upstream Dams (Plant Grade is Elevation 728.0 ft)

WBN Plant Flood Wave Travel Elevation (ft)

Timec (hr)

OBE Failure with One-Half Probable Maximum Flood Norris - Tellico 728.7 27 Cherokee - Douglas - Tellico 729.1 44 "

Fontana - Tellico - Hiwassee - Apalachia - Blue Ridgea 722.1 a NA Fontana - Tellicoa 720.7 d NA SSE Failure with 25-Year Flood Norris - Cherokee - Douglas - Tellico) 731.2 d 35

a.

Includes failure of four ALCOA dams and one Duke Energy dam - Nantahala (Duke Energy, formerly ALCOA), upstream; Santeetlah, on a downstream tributary; and Cheoa-i, Calderwood, and Chilhowee, downstream. Fort Loudoun gates are inoperable in open position.

b. Gate opening at Fort Loudoun prevented by bridge failure.

c.

Time from seismic dam failure to arrival of failure wave at WBN elevation 727.0 ft (1.0 ft below plant grade).

d.

Does not include most recent modeling changes to address additional rim leakage for Fort Loudoun Reservoir, Tellico Reservoir, and Watts Bar Reservoir, since these are not the limiting case for the warning time assessment.

For flood conditions resulting from one-half PMF, RG 1.59 specifies that all safety-related facilities designed in accordance with Regulatory Position 2 must be designed to withstand the flood conditions resulting from a Standard Project event (defined as flow rates generally 40 percent to 60 percent of the PMF) with attendant wind-generated wave activity that may be produced by the worst winds of record and remain functional.

Page 32 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES The one-half PMF (magnitude in range of Regulatory Guide specifications) would produce a maximum elevation at WBN of 717.2 ft.

This is 10.8 ft below plant grade elevation 728.0 ft.

RG 1.59 specified attendant wind-generated wave activity that may be produced by the worst winds of record would not present a problem due to the short wind fetch lengths (0.8, 1.1, and 1.3 miles) and the elevation margin of 10.8 ft available to plant grade elevation. The modeling conventions used in this simulation produce conservative results at WBN.

These changes represent the most current, complete, and substantiated information relative to the emergency operations important to the design and siting of WBN Unit 1 as expected for review by the NRC in SRP Section 2.4.14.

As previously stated, the warning time is divided into two stages: Stage I, a minimum of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months long and Stage II, a minimum of 17 hours1.967593e-4 days 0.00472 hours 2.810847e-5 weeks 6.4685e-6 months long so that unnecessary economic consequences can be avoided, while adequate time is allowed for preparing for operation in the flood mode.

The plant procedure governing preparations for operation in the flood mode includes the following initial assumptions:

1. WBN Unit 1 is at 100% power.

2. Stage I Flood Warning is issued by TVA RO at the threshold flood warning levels during the

'winter" season of elevation 715.5 ft, or during the "summer' season of elevation 720.6 ft.

Upon issuance of the Stage I Flood Warning, the following actions are taken in accordance with the appropriate procedures:

1. A "Notification of Unusual Event" would be declared per the Emergency Plan, and support staffing of the Technical Support Center would be initiated.

2. Rapid shutdown of the unit using an approved "Rapid Load Reduction" procedure would be initiated.

3. Cooldown of the Reactor Coolant System (RCS) to Mode 4 conditions of approximately 340-345OF would be performed.

4. Operation of the Main Steam System would be terminated following cooldown of the RCS to Mode 4.

5. RCS temperature would be stabilized to maintain Mode 4 conditions using Steam Generator Power Operated Relief Valves, and natural circulation conditions would be monitored.

6. Other preparations and contingency actions would be implemented as necessary; for example, ensuring Emergency Diesel Generators are in service to shutdown boards if offsite power is lost.

Upon issuance of the Stage II Flood Warning or if RO confirms that elevation 727.0 ft is likely to be exceeded, the following actions are taken in accordance with the appropriate procedures:

1. An "Alert" would be declared per the Emergency Plan.

2. The steam supply to the Turbine Driven Auxiliary Feedwater pump would be isolated.

Page 33 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES

3. Tanks that will be submerged would be vented and filled to prevent collapsing or floating of the tanks or radioactive release.

4. The auxiliary charging pumps which are located in the Auxiliary Building at elevation 757.0 ft would be aligned for flood mode boration supply.

5. High Pressure Fire Protection water would be aligned to the Steam Generators.

6. Auxiliary air on elevation 757.0 ft would be placed in service, and control and station service air compressors would be shutdown.

7. The Component Cooling Water System would be shutdown and realigned to Emergency Raw Cooling Water via spool pieces to essential components such as spent fuel pool, Residual Heat Removal System, and thermal barrier and sample heat exchangers. Once cooling via Emergency Raw Cooling Water is established, the normal charging pump would be shutdown and Chemical and Volume Control System isolated.

8. Heat sink would be maintained using Steam Generators supplied by High Pressure Fire Protection water, and Reactor Coolant System inventory would be maintained by starting and stopping the Auxiliary Boration System Charging Pumps locally.

9. Phase A containment isolation would be manually initiated.

Flood mode operation would continue until conditions as described in the appropriate plant procedures allow either further plant cooldown or restart.

3.2 Uncertainties As stated in RG 1.59, Revision 2, Probable Maximum Water Level is defined by the Corps of Engineers as "the maximum still water level (i.e., exclusive of local coincident wave runup) which can be produced by the most severe combination of hydrometeorological and/or seismic parameters reasonably possible for a particular location.

Such phenomena are hurricanes, moving squall lines, other cyclonic meteorological events, tsunami, etc., which, when combined with the physical response of a body of water and severe ambient hydrological conditions, would produce a still water level that has virtually no risk of being exceeded." The PMF for streams and rivers for sites like WBN Unit 1 is the hypothetical flood (peak discharge, volume, and hydrograph shape) that is considered to be the most severe, reasonably possible, based on comprehensive hydrometeorological application of PMP and other hydrologic factors favorable for maximum flood runoff such as sequential storms and snowmelt. The primary standards followed for development of the PMF are the American National Standard ANSI/ANS 2.8 and RG 1.59, Revision 2. These guidance documents state that the PMF be derived from the combination of circumstances that collectively represent a risk probability that is acceptable for nuclear plant accidents.

Each element in the development of the PMF is based on best available data including PMP estimates from the National Weather Service, rain-runoff relationships developed from historical storms, time distribution of PMP consistent with storms in the region, seasonal and areal considerations of rainfall, current reservoir operations, and verification of runoff and stream course models against large historic floods. Per ANSI/ANS 2.8 and RG 1.59, the techniques applicable to PMF and seismically induced floods for nuclear power plants are estimates. The calculations which support the PMF analysis document all Page 34 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES assumptions and approaches which are consistent with ANSI/ANS 2.8 and RG 1.59. The PMF analysis is a best estimate and is consistent with these standards and guidelines. However, it is realized that various elements of the analysis when modified result in different elevations, some higher and some lower, and those elements discussed in further detail below are consistent with these standards and guidelines demonstrating that the PMF analysis is a reasonable best estimate.

As discussed in NUREG/CR-7046, "Design-Basis Flood Estimation for Site Characterization at Nuclear Power Plants in the United States of America," the appropriate method to address the uncertainty in the hydrologic analysis is through calibration of the model to historic flood events or sensitivity analyses. TVA calibrated the model to historic flood events using the two highest recent flood events where data exists. The floods used for calibration are March 1973 and May 2003 with elevations at WBN of approximately 697 ft and 694 ft for those two storms.

In addition to the calibration using historic data, sensitivity analyses for the Bellefonte model have been completed. While the input assumptions regarding failure of Chickamauga dam and the Chickamauga dam spillway gate configurations differ between the two models, insights may be applied to WBN based on these sensitivity analyses. The rainfall-to-runoff transformation (unit hydrographs) is completed. The unit hydrographs are peaked by 25% and the results show that the model is not sensitive to this parameter.

The rainfall loss rate is another parameter that has been evaluated through sensitivity analysis.

TVA uses the Antecedent Precipitation Index rain-runoff relationship.

This is the same methodology used by TVA for the daily reservoir operations.

This parameter does show sensitivity to the model resulting in several feet added to the PMF elevation when increasing the runoff from 89% to 100%. However, the rainfall loss rate used in the TVA model is a realistic value for TVA based on historic data over more than 60 years, and there is high confidence in this value as the appropriate value for hydrology modeling. In addition, this parameter was tested by comparison to other acceptable methods for determining rain-runoff relationships discussed in NUREG/CR-7046. The methodology used by TVA is conservative when compared to the other acceptable methods.

Other parameters in the stream course model such as the Manning's n value or resistance to flow could be increased or decreased during extreme flood events such as the PMF. The adopted values in the model are based on calibration against two of the largest floods of record.

If it is postulated that debris in the overbanks would result in an increase in resistance to flow and thus an increase in the Manning's n values, then the elevation at WBN would increase. If it is postulated that Manning's n values would be decreased, then the elevations at WBN would be decreased.

Such decreases have been documented for large flood events on the Mississippi River and could have been considered in the updated hydrologic analysis, but Manning's n values were conservatively not decreased. Based on this documented experience, there is conservatism in the applied Manning's n values but it is difficult to quantify since the flood has never been out of channel to the extreme that it would be in the PMF.

Therefore, TVA uses the best estimate approach with calibration to the two largest recorded floods with data.

Dam rating curves are developed assuming that all gates will be open, and TVA River Operations has committed to making this occur during the PMF event. When it is certain that a PMF event is occurring, the gates will be lifted and left open during the flood. The model has not been tested for loss of gate capacity although it may be assumed that loss of gate capacity Page 35 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES could result in an increase in PMF elevation. However, quantification of the change is not easily determined.

While not tested during sensitivity analyses, it is known that a conservative assumption is made regarding the downstream dam, Chickamauga dam. This dam is overtopped during the PMF but is assumed to not fail.

Finally, the TVA hydrology model has been reviewed by an expert panel and the panel agreed that due to the complexity of the system, it would be very difficult to highlight conservatisms in the model. The panel concluded after review that the analysis was rigorous and technically sound. Based on this discussion, the hydrologic analysis is considered to be a reasonable best estimate that has accounted for uncertainties based on regulatory guidance using the best data available.

3.3 Margins As previously discussed in Tennessee Valley Authority (TVA) Submittal to NRC Document Control Desk, "Commitments Related to Updated Hydrologic Analysis Results for Sequoyah Nuclear Plant, Units 1 and 2, and Watts Bar Nuclear Plant, Unit 1," dated June 13, 2012 (ADAMS Accession No. ML12171A053), a limiting safety-related component required to be available during a plant flood affected by the increase in DBF elevations is the Thermal Barrier Booster (TBB) Pump Motors.

The DBF surge level within flooded structures is elevation 739.7 ft. Field measurements taken from the floor and calibrated with benchmark locations near the TBB Pump Motors indicate that the actual elevation to the baseplate of the TBB Pump Motors is elevation 739.3 ft. Therefore, there is not adequate flood protection for these motors.

To restore margin for the TBB Pump Motors, a temporary flood protection barrier has been designed to be installed around the TBB Pump Motors prior to the event of a Stage I flood warning. Installation of the temporary flood protection barrier is in progress at WBN Unit 1. The barrier encompasses the TBB Pump Motors providing approximately 0.8 ft of margin above the DBF surge level. There are seven major components that are part of the barrier (three end attachment units and four panels), with two end attachment units that attach the L-shaped barrier to the West and South walls that are permanently attached to the surrounding structure walls. As committed to in the June 13, 2012 submittal, TVA will install a permanent plant modification to provide flood protection with respect to the DBF level for the WBN, Unit 1 TBB Pump Motors by March 31, 2013.

Other limiting safety-related components required to be available during a plant flood affected by the increase in DBF elevations and previously discussed in the June 13, 2012 TVA submittal are the ERCW equipment required for flood mode operation located on elevation 722.0 ft of the IPS. The IPS structure contains various equipment required to support the ERCW and HPFP systems. The IPS contains the ERCW and HPFP pumps, travelling water screens and support equipment including screen wash pumps, ERCW strainers and support equipment including backwash valves and pressure indicators, and HPFP strainers and support equipment including backwash valves and pressure indicators.

During a DBF event, surge is accounted for by considering the sum of the wind wave and runup on the critical face of the IPS combined with the PMF stillwater elevation, which conservatively results in an internal flood elevation of 741.7 ft for the IPS. While this does not wet any flood-sensitive equipment on elevation 741.0 ft, the ERCW strainers and support equipment are located on elevation 722.0 ft of the IPS, connected to elevation 741.0 ft via stairwells and doors W001 and W002 at elevation 741.0 ft.

The critical elevation of flood-sensitive equipment located on elevation 722.0 ft is approximately Page 36 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES seven feet above the floor elevation. Doors W001 and W002 both have 0.5 ft concrete berms at the opening to elevation 741.0 ft, which raises the critical elevation for floodwaters to be capable of wetting elevation 722.0 ft to elevation 741.5 ft. As a result of this increase, a compensatory measure of staged sandbags to be constructed into a berm at any time prior to or during the event of a Stage I flood warning has been implemented. These sandbags will be constructed into a berm at least 12 inches in height to prevent water intrusion to elevation 722.0 ft.

Additionally, two non-safety related sump pumps in each of the ERCW Train A and B strainer rooms, connected to safety-related power sources, are available to expel water leakage to this elevation outside the structure. TVA's established corrective action program requirements are being implemented to address the need for additional compensatory measures necessary to provide flood protection for the IPS internal systems and components, including the need for permanent plant modifications.

In addition to these limiting safety-related components required to be available during a plant flood, the Spent Fuel Pit Cooling Pump Motors have a reduced new margin of approximately 0.7 ft between the DBF surge level and the base of the motors. As committed to in the TVA Submittal to NRC Document Control Desk, "Commitments Related to Updated Hydrologic Analysis Results for Sequoyah Nuclear Plant, Units 1 and 2, and Watts Bar Nuclear Plant, Unit 1," dated June 13, 2012 (ADAMS Accession No. ML12171A053), TVA will install a permanent plant modification to provide additional flood protection margin with respect to the DBF level for the WBN, Unit 1 Spent Fuel Pit Cooling Pump Motors by March 31, 2013.

3.4 Conclusions The revised DBF elevations at the WBN Unit I site are determined to impact some of the safety-related systems, structures, or components required to be available during a plant flood.

However, temporary compensatory measures are in place to ensure adequate flood protection if a PMF event were to occur. Except for the limited cases of the TBB Pump Motors and ERCW equipment required for flood mode operation located on elevation 722 ft of the IPS, no physical change to the systems, structures, or components is necessary to ensure that they remain adequately protected from the effects of external floods. Permanent plant modifications are planned to restore or gain additional margin between the revised DBF elevations and limiting safety-related systems, structures, and components. Also, the warning time for WBN shows that there is sufficient time available in both rainfall and seismically induced dam failure floods for safe plant shutdown.

In addition, the updated low water level analysis demonstrates that there is sufficient flow to support operations of WBN Unit 1. Although there are numerous changes to inputs for the hydrological analysis, the cumulative effects of these changes do not impact the original conclusions of the WBN Unit 1 UFSAR that adequate flooding protection features and procedures are in place. The hydrologic analysis is considered to be a reasonable best estimate that has accounted for uncertainties based on regulatory guidance using the best data available. The updated hydrologic analysis shows that the design and siting of WBN Unit 1 is adequate to meet the regulatory requirements and criteria specified to be addressed for WBN Unit 1 UFSAR Section 2.4 and that WBN Unit 1 is capable of tolerating floods above plant grade in a manner that does not jeopardize public health and safety.

4.0 REGULATORY EVALUATION

4.1 Applicable Regulatory Requirements and Criteria 10 CFR Part 100, requires identifying and evaluating hydrologic features of the site.

Page 37 of 41

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES 10 CFR 100.23(d) sets forth the criteria to determine the siting factors for plant design bases with respect to seismically induced floods and water waves at the site.

10 CFR 50, Appendix A, General Design Criteria (GDC) 2, requires 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 A, GDC 44, requires providing an ultimate heat sink for normal operating and accident conditions.

In addition to regulatory requirements, acceptable guidance for hydrologic analysis of the site is included in the following:

" Regulatory Guide 1.27 describes the applicable ultimate heat sink capabilities.

" Regulatory Guide 1.29 identifies seismic design bases for safety-related SSC.

Regulatory Guide 1.59, as supplemented by best current practices, provides guidance for developing the hydrometeorological design bases.

Regulatory Guide 1.102 describes acceptable flood protection to prevent the safety-related facilities from being adversely affected.

The WBN Unit 1 hydrologic analysis conforms to the above regulatory requirements and guidance, using the most recent data and updated methodology which includes use of USACE HEC-HMS and HEC-RAS software.

The WBN Unit 1 hydrologic analysis as described in this License Amendment Request, and as presented in the proposed revision of the WBN Unit 1 UFSAR, contains substantiated sufficient information pertaining to the hydrologic description at the proposed site.

The hydrologic analysis meets the requirements of 10 CFR 100 as it relates to:

1. Identifying and evaluating the hydrology in the vicinity of the site and site regions, including interface of the plant with the hydrosphere,

2. Hydrological causing mechanisms,

3. Surface and ground water uses,

4. Spatial and temporal data sets,

5. Alternate conceptual models of site hydrology,

6. Identification and consideration of local intense precipitation and flooding at the site,

7.

Identification and consideration of the probable maximum flooding on streams and rivers at the site and in the surrounding area, Page 38 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES

8. Identification and consideration of the effects of dam failures at the site and in the surrounding area,

9. Low water effects important to the design and siting of this plant, and

10. The appropriate site phenomena in establishing emergency operations for SSCs important to safety.

Further, the hydrologic analysis considers the most severe natural phenomena that have been historically reported for the site and surrounding area while describing the hydrologic interface of the plant with the site, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated.

The NRC staff has generally accepted the methodologies used to determine the severity of the phenomena, the local intense precipitation, flooding causal mechanisms, controlling flooding mechanism reflected in these site characteristics, the probable maximum flooding on streams and rivers, the effects of dam failures, the potential for low water conditions, and consideration of the appropriate site phenomena in establishing emergency operations for SSCs important to safety, as documented in safety evaluation reports for previous licensing actions. Accordingly, the use of these methodologies results in site characteristics and procedures containing sufficient margin for the limited accuracy, quantity, and period of time in which the data have been accumulated.

In view of the above, the site characteristics previously identified as described in the proposed changes to the WBN Unit 1 UFSAR are acceptable for use in establishing the design bases for SSCs important to safety and site procedures.

4.2 Precedent TVA evaluated license amendment requests and requests for issuance of Combined Operating Licenses in which the NRC had reviewed and approved changes to or initial hydrologic analysis for existing and proposed new nuclear power plants. Although there are similar requests for various changes to the hydrologic analysis or for a new hydrologic analysis for these other nuclear power plants, no precedent was identified for a rebaselining of an existing hydrologic analysis similar to this request.

4.3 Significant Hazards Consideration The proposed changes modify WBN Unit 1 UFSAR hydrologic analysis and results, including the DBF elevations required to be considered in the flood protection of safety-related systems, structures, or components during external flooding events, and verify the adequacy of the warning time for WBN for both rainfall and seismically induced dam failure floods.

The proposed changes do not alter the conclusions presented in the WBN Unit 1 UFSAR that equipment required for operation in the flood mode is either above the DBF or suitable for submerged operation considering the temporary compensatory measures in place and upon completion of planned permanent plant modifications, and that there is sufficient time available in both rainfall and seismically induced dam failure floods for safe plant shutdown. No physical changes to safety-related systems, structures, or components, or any credited flooding protection feature, are required to ensure that they remain adequately protected from the effects of external floods except for the limited cases of the TBB Pump Motors and ERCW equipment required for flood mode operation located on elevation 722 ft of the IPS. However, temporary Page 39 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES compensatory measures are in place to ensure adequate flood protection if a PMF event were to occur, and permanent plant modifications are planned to restore or gain additional margin between the revised DBF elevations and limiting safety-related systems, structures, and components.

The TVA has concluded that the changes to WBN Unit 1 UFSAR do not involve a significant hazards consideration.

TVA's conclusion is based on its evaluation in accordance with 10 CFR 50.91 (a)(1) of the three standards set forth in 10 CFR 50.92, "Issuance of Amendment,"

as discussed below:

1.

Does the proposed amendment involve a significant increase in the probability or consequence of an. accident previously evaluated?

Response: No Although the proposed changes require some physical changes to plant systems, structures, or components to add flood protection features to restore or gain additional margin between the revised DBF elevations and limiting safety-related systems, structures, and components; they do not 1) prevent the safety function of any safety-related system, structure, or component during an external flood; 2) alter, degrade, or prevent action described or assumed in any accident described in the WBN Unit 1 UFSAR from being performed since the safety-related

systems, structures, or components remain adequately protected from the effects of external floods; 3) alter any assumptions previously made in evaluating radiological consequences; or 4) affect the integrity of any fission product barrier.

Therefore, this proposed amendment does not involve a significant increase in the probability or consequences of an accident previously evaluated.

2.

Does the proposed amendment create the possibility of a new or different kind of accident from any accident previously evaluated?

Response: No.

The proposed changes do not introduce any new accident causal mechanisms, nor do they impact any plant systems that are potential accident initiators.

Therefore, the proposed amendment does not create the possibility of a new or different kind of accident from any accident previously evaluated.

3.

Does the proposed amendment involve a significant reduction in a margin of safety?

Response: No.

The proposed changes do not alter the permanent plant design, including instrument set points, that is the basis of the assumptions contained in the safety analyses.

However, permanent plant modifications are planned to restore or gain additional margin between the revised DBF elevations and limiting safety-related systems, structures, and components. Although the results of the updated hydrologic analysis increase the DBF elevations required to be considered in the flood protection of Page 40 of 41

ENCLOSUREI EVALUATION OF PROPOSED CHANGES safety-related systems, structures, or components during external flooding events, the proposed changes do not prevent any safety-related systems, structures, or components from performing their required functions during an external flood considering the temporary compensatory measures in place and upon completion of planned permanent plant modifications. Consistent with existing regulatory guidance including regulatory recommendations and discussions regarding calibration of hydrology models using historical flood data and consideration of sensitivity analyses, the hydrologic analysis is considered to be a reasonable best estimate that has accounted for uncertainties using the best data available.

Therefore, the proposed changes do not involve a significant reduction in a margin of safety.

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

5.0 ENVIRONMENTAL CONSIDERATION

A review has determined that the proposed amendment would not change a requirement with respect to installation or use of a facility component located within the restricted area, as defined in 10 CFR 20, or would change an inspection or surveillance requirement. Also, the proposed amendment does not involve (i) a significant hazards consideration, (ii) a significant change in the types or significant increase in the amounts of any effluents that may be released offsite, or (iii) a significant increase in individual or cumulative occupational radiation exposure.

Accordingly, the proposed amendment meets the eligibility criterion for categorical exclusion set forth in 10 CFR 51.22(c)(9). Therefore, pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the proposed amendment.

Page 41 of 41

ENCLOSURE1 EVALUATION OF PROPOSED CHANGES ATTACHMENT 1 Proposed WBN Unit I UFSAR Text Changes (Markups)

WBNP-2.4 HYDROLOGIC ENGINEERING Watts Bar Nuclear Plant is located on the right bank of Chickamauga Lake at Tennessee River Mile (TRM) 528 with plant grade at Elevation levation 728.0 ft MSL. The plant has been designed to have the capability for safe shutdown in floods up to the computed maximum water level, in accordance with regulatory position 2 of Regulatory Guide 1.59, Revision 2, August 1977.

Determination of the maximum flood level included consideration of postulated dam failures from seismic and hydrologic causes. The maximum flood Elevation 731elevation 739.2 ft would result from an occurrence of the probable maximum storm. Allowances for ccneurrent wind waves ould raise lake levels to Elevati. n 736.2 with runCoincident wind wave activity results in wind waves of up to 2.2 ft (crest to trough). Run up on the 4:1 slopes approaching the plant reaching about Elevation 736.1Diesel Generator Building reaches elevation 741.6 ft. Wind wave run up on the critical wall of the Intake Pumping Station reaches elevation 741.7 ft and wind wave run up on the walls of the Auxiliary, Control and Shield Buildings reaches elevation 741.0 ft.

The nearest surface water user located downstream from Watts Bar Nuclear Plant is Dayton, Tennessee, at TRM 503.8, 24.2 miles downstream. All surface water supplies withdrawn from the 58.9 mile reach of the mainstream of the Tennessee River between Watts Bar Dam (TRM 529.9) and Chickamauga Dam (TRM 471.0) are listed in Table 2"-42.4-1.

The probable minimum flow past the site is estimated to be 2-03.Q,200 cfs, which is more than adequate for plant water requirements.

2.4.1 Hydrological Description 2.4.1.1 Sites and Facilities The location of key plant structures and their relationship to the original site topography is shown on Figure 2.1-5. The structures which have safety-related equipment and systems are indicated on this figure and are tabulated below along with the elevation of exterior accesses.

Structure Access Accesses Elev Intake Pumping (1)

Access Hatches 3

728.0 SFuettreStation (2)

Stairwell Entrances 2

741.0 (3)

Access Hatches 6

741.0 Auxiliary and (1)

Door to Turbine Bldg.

1 708.0 Control Bldgs.

(2)

Door to Service Bldg.

2 713.0 (3)

Railroad Access Opening 1

729.0 (4)

Door to Turbine Bldg 2

729.0 (5)

Emergency Exit 1

730.0 (6)

Door to Turbine Bldg.

2 755.0 2.4-1

WBNP-Shield Building (1)

Personnel Lock 1

714.0 (2)

Equipment Hatch 1

753.0 (3)

Personnel Lock 1

755.0 Diesel Generator (1)

Equipment Access Doors 4

742.0 Building (2)

Emergency Exits 4

742.0 (3)

Personnel Access Door 1

742.0 (4)

Emergency Exit 1

760.5 Additional (1)

Equipment Access Door 2

742.0 Diesel (2)

Personnel Access Door 1

742.0 Generator (3)

Emergency Exit 1

742.0 Building (4)

Emergency Exit 1

760.5 Exterior accesses are also provided to each of the Class 1E electrical systems manholes and handholes at elevations varying from 714.5 feet MSL to 728.5 feet MSL, depending upon the location of each structure.

The relationship of the plant site to the surrounding area can be seen in Figures 2.1-4a and 2.1-5.

It can be seen from these figures that significant natural drainage features of the site have not been altered. Local surface runoff drains into the Tennessee River.

2.4.1.2 Hydrosphere The Watts Bar Nuclear Plant site, along with the Watts Bar Dam Reservation, comprises approximately 1770 acres on the west bank of Chickamauga Lake at TRM 528. As shown by Figure 2.1-4a, the site is on high ground with the Tennessee River being the major potential source of flooding. The Watts Bar Nuclear Plant is located in the Middle Tennessee Chickamauga watershed, U.S. Geological Survey (USGS) hydrologic unit code 06020001, one of 32 watersheds in the Region 06 - Tennessee River Watershed (Figure 2.4-1).

The Tennessee River above the Watts Bar plant site drains 17,319 square miles. Watts Bar Dam, 1.9 miles upstream, has a drainage area of 17,310 square miles. Chickamauga Dam, the next dam downstream, has a drainage area of 20,790 square miles. Two major tributaries, Little Tennessee and French Broad Rivers, rise to the east in the rugged Southern Appalachian Highlands. They flow northwestward through the Appalachian Divide which is essentially defined by the North Carolina-Tennessee border to join the Tennessee River which flows southwestward. The Tennessee River and its Clinch and Holston River tributaries flow southwest through the Valley and Ridge physiographic province which, while not as rugged as the Southern Highlands, features a number of mountains including the Clinch and Powell Mountain chains. The drainage pattern is shown on Figure 2.1-1. About 20% of the watershed rises above Elevatienelevation 3,000 ft with a maximum elevation of 6.684 ft at Mt. Mitchell, North Carolina. The watershed is about 70% forested with much of the mountainous area being 100% forested.

2.4-2

WBNP-The climate of the watershed is humid temperate. Mean annual pr.cipitatin for-the Tenne....

Valley is shown in. Figre 2.1 1 (histeor.ial info.mation). Above Watts Bar Dam annual rainfall averages 50 inches and varies from a low of 40 inches at sheltered locations within the mountains to high spots of 90 inches on the southern and eastern divide. Rainfall occurs fairly evenly throughout the year. The lowest monthly average is 2.8 inches in October. The highest monthly average is 5.4 inches in July, with March a close second with an average of 5.1 inches.

Major flood-producing storms are of two general types: the cool-season, winter type, and the warm-season, hurricane type. Most floods at Watts Bar Nuclear Plant, however, have been produced by winter-type storms in the main flood-season months of January through early April.

Watershed snowfall is relatively light, averaging about 14 inches annually above the plant.

Snowfall above the 3,000-feet elevation averages 22 inches annually. The highest average annual snowfall in the basin is 63 inches at Mt. Mitchell, the highest point east of the Mississippi River. Individual snowfalls are normally light, with an average of 13 snowfalls per year.

Snowmelt is not a factor in maximum flood determinations.

The Tennessee River, particularly above Chattanooga, Tennessee, is one of the most highly regulated rivers in the United States. The TVA reservoir system is operated for flood control, navigation, and power generation with flood control a prime purpose with particular emphasis on protection for Chattanooga, 64 miles downstream from Watts Bar Nuclear Plant.

Chickamauga Dam, 57 miles downstream, affects water surface elevations at Watts Bar Nuclear Plant. Normal full pool elevation is 682.5 feet. At this elevation the reservoir is 58.9 miles long on the Tennessee River and 32 miles long on the Hiwassee River, covering an area of 35,40036,050 acres, with a volume of 628, 0622 500 acre-feet. The reservoir has an average width of nearly 1 mile, ranging from 700 feet to 1.7 miles. At the Watts Bar site the reservoir is about 1100 feet wide with depths ranging between 18 feet and 26 feet at normal pool elevation.

There are 12 major reer.v.irs in the TVA system upstr-ea Wats Bar-Nu.lear Plant, ten o which have substantial r-esef-ved flcod detention capacity during the flood season. Table 2.4 1 (histor-ical infcRmnation) lists pertinent data fcr-TAIA's major dams prior-to medifications made by the Dam Safety Program (See Table 2.4 16). Figures 2.1 2 through 2.4 114 shw general plans, elevations, and sections for t dams and Chickam1auga Dam downstream.. dams (South Holston, Boone, Fort Patrick Henry, Watauga, Fontana, Norris, Cherokee, Douglas, Tellico, Fort Loudoun, Melton Hill, and Watts Bar) in the TVA system upstream from Watts Bar Nuclear Plant, ten of which (those previously identified excluding Fort Partick Henry and Melton Hill) provide about 4.4 million acre-ft of reserved flood-detention (March 15) capacity during the main flood season. Table 2.4-2 lists pertinent data for TVA's dams and reservoirs. Figure 2.4-2 presents a simplified flow diagram for the Tennessee River system. Table 2.4-3 provides the relative distances in river miles of upstream dams to the Watts Bar Nuclear Plant site. Details for TVA dam outlet works are provided in Table 2.4-4. In addition, there are six major dams owned by the Aluminum Company of America (ALCOA). The ALCOA reservoirs often contribute to flood reduction, but they do not have dependable reserved flood detention capacity. Table 2.42 (hist*ori*c infeo-mati,)2.4-5 lists pertinent data for the ALCOA dams and Walters Dam (Waterville Lake). The locations of these dams are shown on Figure 2. 1-1.

2.4-3

WBNP-Flood control above the plant is provided largely by eight tributary reservoirs. Tellico Dam is counted as a tributary reservoir because it is located on the Little Tennessee River although, because of canal connection with Fort Loudoun Dam, it also functions as a main river dam. On March 15, near the end of the flood season, these provide a minimum of 4,069,2003,937,.400 acre-feet of detention capacity equivalent to 6-45.5 inches on the 12,4 5 l 3508-square-mile area they control. This is 89% of the total available above the plant. The two main river reservoirs, Fort Loudoun and Watts Bar, provide 490,000 acre-feet equivalent to -.92.4 inches on the remaining 481-93 802-square-mile area above Watts Bar Dam.

The flood detention capacity reserved in the TVA system varies seasonally, with the greatest amounts during the January through March flood season. Figures 2.1 15 through 2.1 21 (all historical information) showFigure 2.4-3 (12 sheets) shows the reservoir seasonal operating guides for reservoirs above the plant site. Table 2.--32.4-6 shows the flood control reservations at the multiple-purpose projects above Watts Bar Nuclear Plant at the beginning and end of the winter flood season and in the summer. Total assured system detention capacity above Watts Bar Dam varies from 6-.24.9 inches on January 1 to 494.8 inches on March 15 and decreasing to

-41.5 inches during the summer and fall. Actual detention capacity may exceed these amounts, depending upon inflows and power demands.

Chickamauga Dam, the headwater elevation of which affects flood elevations at the plant, has a drainage area of 20,790 square miles, 3,480 square miles more than Watts Bar Dam. There are eight majer-tributa*y damsseven major tributary dams (Chatuge, Nottely, Hiwassee, Apalachia, Blue Ridge, Ocoee No. 1 and Ocoee No. 3) in the 3,480-square-mile intervening watershed, of which thfreefour have substantial reserved capacity. On March 15, near the end of the flood season, these provide a minimum of 3 "66,7)0o379 30* acre-feet equivalent to 7-45.9 inches on the 961.,200-square-mile controlled area. Chickamauga Dam contains 34-7,00345,300 acre-feet of detention capacity on March 15 equivalent to 2-2.8 inches on the remaining 2-,_5422,280 square miles. Figure 2.4 26 (histeri-cal infinafion) 2.4-3 (Sheet 1) shows the seasonal operating guide for Chickamauga.

Elevation-storage relationships for the 4-2-reservoirs above the site and Chickamauga, downstream, are shown in Figures 2.1 25 and 2.4 27 through 2.4 3. (all histor ial info*r.....at.in)

Figure 2.4-4 (13 sheets). Cur~cs determined at seicctcd ycarS as part of TVA's progr-am of monitoring changes du osdmn atioae shown. These show that sedimentation is not significant in these reservoir-s.

The or.iginal hydr**olgic design of the dams w'A' s;q based upon a combin.ationi Of design &fod a*d*

freeboar-d v.hich in somfe Ceases did-not mneet probable maximum flood (PN4F) cr-iter-ia imposed by Regulatory Guide 1.70.17, January 1975. The potential consequences of ovropn)f these dams noet mneeting those criter~ia were evaluated wherce failur-ewould signifieantly influence plant site flood levels and wer-e dcscr-ibed in Section 2.4.3.4 per-the or-iginal analysis.

2.4-4

WBNP-in 1982, T.VA offiially began a safe.' review of all its dams. The TVA Dam Safev' Program was desiged to be consistent with the Federal Emffer-gency Management Agency's (FEMA "FeLderail Guidelines For4-Da;m-Safe-4y'" and similar-effon~s by other-Federal agencies. Techaical swi-dics Rand engineer-ing analyses 'wA.erecoducted and physical modifications implemented to ensur-e the hydroelogic and sesi ner'of the TVA dams and demonstrate that TVA's dams can be oper-ated in accraefd-ance with FEMA guidelines. Table -2.1 16 proevides the status of TVNA Dam Safcet' hydrologic modifications as of early 1998. These moedifications enable thes proejects to safely pass the proebable maximum flood. The r-emaining hydr-ologic moidificationis planned for-Bear-Cr-eek Damf and Chiekamnauga Dam will net affect Walls Bar-in any mnaffer which might invalidate the reanalysis described belo'w.

in 1997 98, TXVA reanalyzed the nucelear-plant design basis flood ev~ents. The pufpose of th-e r-eanalysis 'was to Revaluate the effects of the hydroelogic dam sa",fe~'mdifications on the flood-eleatinsand r-esponse. tim-es in the UFSAR and to confin; the adequacy of the plantflo plans. The following mnetho-d-s an;d asupios'er-e applied to the reanalyslis

i.

The computer-programs and modeling methods 'were the same as pr-eviously used and docuimented in the plant UFSAR.

2.

-Probable maximumn pr-ecipitationi, timfe distr-ibution Of preiiain precipitation losses and rceryoi oeaing proedurcsq 'wve-r-e uncrehanged from the or-iginal analysis.

3.

All of the or-iginal stability analyses and postulated seismic dam failur~e assumptions wer-e conser-,atively assumed to occur-in the same manner-and in combination with the same pr-eviously postul-atead rainfall events. No cr-edit was taken for the 1988 post tensioning ot Fonanaand Melton Hill Dams to pr-event seismic failure. Nor-was any credit taken for-Dam Safety seismic evaluations o~f NoFfis, Cher-okee, Douglas, Fodt Loudon, Tellico, Hi'wassee, Apalachia, and Blue Ridge Dams which demonstr-ated their-stmuetr-al integrit' forf a; sesi Yeet'ith -A rebm per-iod of approeximately 10,000 years.

4.

The planned moedification Of Chiekamfauga Dam (afmor-ing the embankmnent to pennit ovedoppinig) 'was reonsen'vatively assumned to have been implemented for-the purpose ot.

calculating flood effects. However-, the condition of Chickamfaudga Dam, 57 miles downstream has negligible impact flood levels at the plant.

5.

Bea;r-Cr-eek am1 is d'nremof the nuclear-plant and its plapmed moldification has no reevne t this reassessment.

Daily flow volumes at the plant, for all practical purposes, are represented by discharges from Watts Bar Dam with a drainage area of 17,3 10 square miles, only 9 square miles less than at the plant. Momentary flows at the nuclear plant site may vary considerably from daily averages, depending upon turbine operations at Walls Bar and Chickamauga Dams. There may be periods of several hours when no releases from either or both Walls Bar and Chickamauga Dams occur.

Rapid turbine shutdown at Chickamauga may sometimes cause periods of upstr-eamtreverse flow in Chickamauga Reservoir.

2.4-5

WBNP-Based upon Watts Bar Dam discharge records since dam closure in 1942, the average daily streamflow at the plant is 2-7,80)27,000 cfs. The maximum daily discharge was !87,000 efs-en December. 30, 1911 prr t pr..t regulatien208,400 cfs on May 8. 1984. Daily average releases of zero have been recorded on seven occasions during the past 2-951 years. Flow data for water years 1960-4-982010 with regulation essentially equivalent to present conditions indicate an average rate of about 237,7-023,000 cfs during the summer months (May-October) and about 3-,90031,500 cfs during the winter months (November-April). Flow durations based upon Watts Bar Dam discharge records for the period 1960-1-9972010 are tabulated below:

Average Daily Percent of Time Discharge, cfs Equaled or Exceeded 5,000 9",-97.4 10,000 9-3-87.9 15,000 4.-477.5 20,000 69464.2 25,000 50.648.5 30,000 32-933.4 35,000 2-0421.4 Channel velocities at the Watts Bar site average about 2.3 fps under normal winter conditions.

Because of lower flows and higher reservoir elevations in the summer months, channel velocities average about 1.0 fps.

The Watts Bar plant site is underlain by geologic formations belonging to the lower Conasauga Formation of Middle Cambrian age. The formation consists of interbedded shales and limestones overlain by alluvial material averaging 40 feet in thickness. Ground water yields from this formation are low.

All surface water supplies withdrawn from the 58.9 mile reach of the mainstream of the Tennessee River between Watts Bar Dam (TRM 529.9) and Chickamauga Dam (TRM 471.0) are listed in Table 2-A-42.4-1. See Section 2.4.13.2 (histi*,--al informatin)* for description of the ground water users in the vicinity of the Watts Bar site.

2.4.2 Floods 2.4.2.1 Flood History Histeica! Infr;natin The nearest location with extensive formal flood records is 64 miles downstream at Chattanooga, Tennessee, where continuous records are available since 1874. Knowledge about significant floods extends back to 1826 based upon newspaper and historical reports. Flood flows and stages at Chattanooga have been altered by TVA's reservoir system beginning with closure of Norris Dam in 1936 and reaching essentially the present level of control in 1952 with closure of Boone Dam, the last major dam with reserved flood detention capacity constructed above Chattanooga prior to construction of Tellico Dam. Tellico Dam provides additional reserved flood detention capacity; however, the percentage increase in the total Fetiefdetention capacity above the Watts Bar site is small. Therefore, flood records for the period 1952 to date can be considered representative of prevailing conditions. Figufe-2.440Table 2.4-7 provides 2.4-6

WBNP-annual peak flow data at Chattanooga. Figure 2.4-5 shows the known flood experience at Chattanooga in diagram form. The maximum known flood under natural conditions occurred in 1867. This flood reaehed Ele;'ationwas estimated to reach elevation 716.3 ft at Watts Bar Nuclear Plant site with a discharge of about 440,000 cfs. The maximum flood elevation at the site under present-day regulation reached Elevation 696. 18 at the sitc en March 17, 1 973would be approximately elevation 698 ft based on a maximum tailwater elevation of 698.23 ft at Watts Bar Dam located just upstream.

The following tabulation lists the highest floods at theWatts Bar Dam (TRM 529.9) tailwater located upstream of Watts Bar Nuclear Plant site under present-day regulation:

Elevation, Discharge, Date Feet cfs February 2, 1957 69*_No Record 157,600 November 19, 1957 69-3.-1No Record 151,600 March 13, 1963 693.85694.75 167,700 j.nua.y 1, 1979December 31, 1969 694-2.2693.28 167,300 March 17, 1973 696t-8696.95 184,800 May 28, 1973 695.24 175,200 April 5, 1977 694.79 181,600 May 8, 1984 698.23 208,400 April 20, 1998 694.67 167,500 May 7, 2003 694.17 153,100 There are no records of flooding from seiches, dam failures, or ice jams. Historic information about icing is provided in Section 2.4.7.

2.4.2.2 Flood Design Considerations TVA has planned the Watts Bar project to conform with regulat.. y p.siti. n 2 of Regulatory Guide 1.59 including position 2.

The types of events evaluated to determine the worst potential flood included (1) prfebable maximum precipitationProbable Maximum Precipitation (PMP) on the total watershed and critical suýb 'water sh sub-watersheds including seasonal variations and potential consequent dam failures and (2) dam failures in a postulated SSE or OBE with guide specified concurrent flood conditions.

Specific analysis of Tennessee River flood levels resulting from ocean front surges and tsunamis is not required because of the inland location of the plant. Snow melt and ice jam considerations are also unnecessary because of the temperate zone location of the plant. Flood waves from landslides into upstream reservoirs required no specific analysis, in part because of the absence of major elevation relief in nearby upstream reservoirs and because the prevailing thin soils offer small slide volume potential compared to the available detention space in reservoirs. Seiches pose no flood threats because of the size and configuration of the lake and the elevation difference between normal lake level and plant grade.

2.4-7

WBNP-The maximum PMF plant site flood level frem any eause is Elevation 734.9elevation 739.2 ft.

This elevation would result from the PMP critically centered on the watershed as described in Section 2.4.3. The maximum flood level is 3.2 feet lower-than or.iginally determined in the FSAR as a result of dam safety moedifications.

Wind waves from a March wind with velocitybased on an overland wind speed of 21 miles per hour waswere assumed to occur coincident with the flood peak. This would create,aves 3-.maximum wind waves up to 2.2 feet high (trough to crest) and pr-oduce m.. imu. m la.ke levels to Elevation 736.2.

All safety-related facilities, systems, and equipment are housed in structures which provide protection from flooding for all flood conditions up to plant grade at Elevatienelevation 728.0 ft.

See Section 2.4.10 (histor-ial information) for more specific information.

Other rainfall floods will also exceed plant grade gev-ationelevation 728.0 ft and require plant shutdown. Fledwarning eriteria and fereeaSting techniques have be*n dcvclped to assur.e tha there Will always be adequate time to shut the plant down and be ready for-floodwater.s above i÷a, e.,-. Section 2.4.14 describes emergency protective measures to be taken in flood events exceeding plant grade.

Seismic and flood events could cause dam failure surges appreaehingexceeding plant grade Elevatienelevation 728.0 ft. in all such events there is t'Im f Ir 4afe pln shutd n 1 after-thee seismi, event before plant grade w..uld be crosed. Section 2.4.14 describes emergency protective measures to be taken in seismic events exceeding plant grade.

For the condition where flooding exceeds plant grade, as described in Sections 2.4.3 and 2.4.4, those safety-related facilities, systems, and equipment located in the containment structure are protected from flooding by the Shield Building structure with those accesses and penetrations below the maximum flood level designed and constructed as watertight elements. The Diesel Generator Building and Essential Raw Cooling Water (ERCW) pumps are located above this flood level, thereby providing protection from flooding.

Wind wave run up during the PMF at the Diesel Generator Building reaches Elevation 73 6.9 whieh is 54-.feetwould reach elevation 741.6 ft which is 0.4 ft below the operating floor.

Consequently, wind wave run up will not impair the safety functions of the Diesel Generator Building.

Those Class 1 E electrical system conduit banks located below the PMF plus wind wave run up flood level are designed to function submerged with either continuous cable runs or qualified, type tested splices. The ERCW pumps are structurally protected from wind waves. Therefore, the safety function of the ERCW pumps will not be affected by floods or flood-related conditions.

The Turbine, Control, and Auxiliary Buildings will be allowed to flood. All equipment required to maintain the plant safely during the flood, and for 100 days after the beginning of the flood, is either designed to operate submerged, is located above the maximum flood level, or is otherwise protected.

2.4-8

WBNP-The electrical equipment room of the intake pumping,stfcutreIntake Pumping Station will flood at Elevatie elevation 728.0 ft. However, the design basis watefflood level for the remaining structure is Elevation 710.1 elevation 741.0 ft. The Auxiliary and Control Buildings will flood with the water level at Eleationelevation 729.0 ft. The design basis watefexternal flood level for the Auxiliary, Control and Shield Building is Ellevation 71.0.14741.0 ft. The Diesel Generator Building is located above the design basis.,,,',ater leel (E vatin 74"..)flood level of elevation 741.6 ft.

2.4.2.3 Effects of Local Intense Precipitation All streams in the vicinity of the plant shown on Figure 2.1-4a were investigated, including Yellow Creek, with probable maximum flows from a local storm and from breaching of the Watts Bar Dam west saddle dike West Saddle Dike and were found not to create potential flood problems at the plant. Local drainage which required detailed design is from the plant area itself and from a 150-acre area north of the plant.

The underground storm drainage system is designed for a maximum lone-hour rainfall of 4four inches. The 4one-hour rainfall with 1% exceedance frequency is 3.3 inches. Structures housing safety-- related facilities, systems, and equipment are protected from flooding during a local PMF by the slope of the plant yard. The yard is graded so that the surface runoff will be carried to Chickamauga Reservoir without exceeding the elevation of the accesses given in Section 2.4.1 1. The exterior accesses that are below the grade elevation for that specific structure exit from that structure into another structure and are not exterior in the sense that they exit or are exposed to the environment. For any access exposed to the environment and located at grade elevation, sufficient drainage is provided to prevent water from entering the opening. This is accomplished by sloping away from the opening.

PMP for the plant drainage systems has been defined for TVA by the Hydrometeorological Branch of the National Weather Service and is described in Hydrometeorological Report No. 56.[351 Ice accumulation would occur only at infrequent intervals because of the temperate climate.

Maximum winter precipitation concurrent with ice accumulation would impose less severe conditions on the drainage system than would the PMF.

Figure 2.4-40a (Sheetsheet 1) shows the Watts Bar site grading and drainage system and building outlines for the main plant area. Direction of flow for runoff has been indicated by arrows.

Figure 2.4-40b shows the Watts Bar general plan-a-: Figure 2.4-40c shows the yard site grading and drainage system for flood studies for the area north and northwest of the plant along with the outline of the low-level radwaste storage facility. The 150-acre drainage area north of the site has been outlined on Figure 2.4-40b with direction of flow for runoff indicated by arrows.

Figure 2.4-40d (three sheets) shows the plans and profiles for the perimeter roads; Figure 2.4-40e (two sheets) shows the plan and profile for the access highway. Figure 2.4-40f (three sheets) shows the plan, sections, and profiles for the main plant railroad tracks. Figure 2.4-40g (three sheets) shows the yard grading, drainage, and surfacing for the switchyard.

2.4-9

WBNP-In testing the adequacy of the site drainage system, all underground drains were assumed clogged. Peak discharges were evaluated using storm intensities for the maximum 4-one-hour rainfall obtained from the PMP mass curve shown on Figure 2.4-40h (histo-ic*al inifoation).

Runoff was assumed equal to rainfall. Each watershed was analyzed using the more appropriate of two methods: (1) when flow conditions controlled, standard-step backwater from the control section using peak discharges estimated from rainfall intensities corresponding to the time of concentration of the area above the control or (2) when ponding or reservoir-type conditions controlled, storage routing the inflow hydrograph equivalent to the PMP hydrograph using 2two-minute time intervals.

Computed maximum water surface elevations are below critical floor E1ek'atieielevation 729.0 ft. The separate watershed areas are numbered for identification on Figure 2.4-40a.

Runoff from the employee parking lot and the areas south of the office building and west of the Turbine Building (area 1) will flow along the perimeter road west of the switchyard and drain into the area surrounding the chemical holdup ponds. The control is the drainage ditch and road which acts as a channel between the west end of the switchyard and the embankment to the west.

To be conservative it was assumed water would not flow into the switchyard. Maximum water surface elevations at the office and Turbine Buildings computed using method (1) were less than elevation 729.0 ft.

Flow from the area west of the Service, Auxiliary, Reactor, and Diesel Generator Buildings and north of the office building and gatehouse (area 2) will drain along and then across the perimeter road, flow west through a swale and across the low point in the access road. The swale and the roads have sufficient capacity to keep water surface elevations below 729.0 ft at all buildings.

Method (1) was used in this analysis.

The area east of the Turbine, Reactor, and Diesel Generator Buildings (area 3) forms a pool bounded by the main and transformer yard railroad tracks with top of rail elevations at 728.00 ft and 728.25 ft respectively. Method (2) was used to route the inflow hydrograph through this pool from an initial elevation of 728.00 ft with outflow over the railroads. Maximum water surface elevations at the mtu-bineTurbine and Reactor Buildings were less than Elevationelevation 729.0 ft. Use of method (1) starting just downstream of the railroad confirmed this result.

The flow from area 3 over the railroad north of the east-west baseline drains north along a channel between the main railroad and the ERCW maintenance road and east between the ERCW maintenance road and the north cooling tower. Flow from area 3 over the railroad south of the east-west baseline drains south along a channel between the storage yard road and the switchyard past the storage yard to the river. Analysis using method (1) shows that flow over the Diesel Generator Building road controls the elevations at the Turbine and Reactor Buildings.

Maximum water surface elevations were computed to be less than E!e-atinelevation 729.0 ft.

Flow from the switchyard and transformer yard (area 4) will drain to the east, west, and south.

Maximum water surface elevations at the Turbine Building obtained using method (2) were less than Elevatienelevation 729.0 ft.

Table 2. -142.4-8 provides the weir length description and coefficient of discharge used in the analysis for areas 3 and 4.

2.4-10

WBNP-Flow from the 150-acre drainage area north of the site drains two ways: (1) 50 acres drain east through the double 96-inch culvert under the access railroad shown on Figure 2.4 402.4-40c and (2) drainage from the remaining 100 acres is diverted to the west through an 81 -inch by 59-inch pipe arch and, when flows exceed the pipe capacity, south over a swale in the construction access road. The flow over the construction access road drains to the west across the access highway. The following information provides details of our analysis.

The discharge hydrograph for the 100-acre area north of the plant and upstream from the construction access road was determined using a dimensionless unit graph based upon SCS procedures and PMP defined by the National Weather Service.E*'-35 The PMP mass curve used in the determination is shown on Figure 2.4-40h. Runoff was assumed equal to rainfall. The construction access road will act as a dam with the 81-inch by 59-inch pipe arch acting as a low-level outlet. Flow is prevented from draining to the east above the construction access road by a dike with top elevation at 736.5 ft (dike location and cross-section shown on Figure 2.4-40c). The profile of the construction access road and the location of the pipe arch are shown on Figure 2.4-40c. The discharge hydrograph was routed using 2two-minute time intervals through the pipe arch and over the construction access road using standard storage routing techniques. The rating curve for flow over the construction access road was developed from critical flow relationships with losses assumed equal to 0.5 V2/2g.

The maximum elevation reached at the construction access road was 735.28_ft. The pipe arch is designed for AASHTO H-20 loading which we judge is adequate for the loading expected. In the unlikely event of pipe arch failure and flow blockage, the maximum flood level at the construction access road would increase only 0.12 feet, from Elevatienelevation 735.28 ft to 735.4 ft. The peak flow over the construction road was used in computations.

Flow over the construction access road discharges into the 67-acre area west of the Service, Auxiliary, Reactor, and Diesel Generator Buildings and north of the office building and gatehouse (area 2 of Figure 2.4-40a) before flowing west across the access highway (Figure 2.4-40e). Flow from 60 additional acres to the northwest of the site is also added to this area just upstream of the main access road. Elevations for area 2 were examined to include these additional flows. Backwater was computed from downstream of the access highway, crossing the perimeter road, to the Reactor, Diesel Generator, and Waste Evaporation System Buildings.

The elevation at the access highway control was computed conservatively assuming that the peak flows from area 2 and over the construction road added directly. The maximum flood elevation reached in the main plant area was less than Ele-atienelevation 729.0 ft.

The discharge hydrograph for the 50-acre area north of the plant was conservatively assumed equivalent to the PMP hydrograph using 2 minute time intervals. This hydrograph was routed using 2two-minute time intervals through the double 96-inch culvert using standard storage routing techniques.

The maximum elevation reached at the culvert was 725.67_ft. Flow is prevented from entering the main plant area by site grading as shown on Figure 24-402.4-40c.

2.4-11

WBNP-The double 96-inch culvert is designed to carry a Cooper E-80 loading as recommended by the American Railway Engineering Association (AREA). The culvert has already been exposed to the maximum loading (the generator stator with a total load of 792 tons on 22 axles) with no damage to the pipes or tracks. This maximum loading is less than the design load. Loading conditions will not be a problem.

The site will be well maintained and any debris generated from it will be minimal; therefore, debris blockage of the double 96-inch culvert or the 81-inch by 59-inch pipe arch will not be a problem.

Table 2.4--1-52.4-9 provides a description of drainage area, estimated peak discharge, and computed maximum water surface elevation for each subwatershed investigated in the site drainage analysis.

A local PMF on the holding pond does not pose a threat with respect to flooding of safety-related structures. The top of the holding pond dikes is set at Elevatienelevation 714.0 ft, whereas water level must exceed the plant grade at E4evationelevation 728.0 ft before safety-related structures can be flooded. A wide emergency spillway is cut in original ground at an elevation 2 feet below the top of the dikes. During a local PMF the water trapped by the pond rise will be considerably less than the 14-feet difference between the top of the dikes and plant grade.

2.4.3 Probable Maximum Flood (PMF) on Streams and Rivers The guidance of Appendix A of Regulatory Guide 1.59 was followed in determining the PMF.

The PMF was determined from PMP for the letal-watershed above the plant with consideration given to seasonal and areal variations in rainfall. The,original PM.nF dete..:i.nn:tin.- nl 0*~

givenl to seasonl an areal11 varations in XX rainall The

X orgial 1*Iv~l~g PMF detefminati also considered a PM4F at upstreamn tributar-y dams whose failure has the potential to cause aiu plant site flood levels. Dam sa f.emdifications have eliminated the potential Aof.a PFat upstreamn tributary dlams to eause maximumn plant site flood leVels.

Two basic storm situations were found to have the potential to produce a-maximum flood levels at Watts Bar Nuclear Plant. These are (1) a sequence of Mar-c-storms producing maximm iain&UPMP depths on the 21,400-square-mile watershed above Chattanooga and (2) a sequence of Mar-eh-storms center-ed and proeducing maximum r-ains in the basin to the west of the Appalachian Divide

,-andabo*A.ve.

Chattaneega, producing PMP depths in the basin above Chattanooga and below the five major tributary dams (Norris, Cherokee, Douglas, Fontana, and Hiwassee), hereafter called the 7,980-square-mile storm. The maximum flood level at the plant would be caused by the March PMP en the 7,9890 square mile 21,400-square-mile storm. The flood level for the 21,100 square mile7,980-square-mile storm would be slightly less.

In both storms, the west saddle iWest Saddle Dike at Watts Bar Dam would be overtopped and breached. No other failure would occur. Maximum discharge at the plant woeei-be 1,288,000 cfs for the 7,980 square mile is 1 003,363 cfs from Watts Bar Dam with an additional 97,990 cfs from Watts Creek and 243.782 cfs from Yellow Creek for the 21,400-square-mile storm. The resulting probable maximurPnMF elevation at the plant would be 734.9739.2 ft excluding wind wave effects.

2.4-12

WBNP-2.4.3.1 Probable Maximum Precipitation (PMP)

Probable maximum precipitation (PMP) for the watershed above Chickamauga and Watts Bar Dams for determining PMF has been defined for TVA by the Hydrometeorological Report No. 41.[4] Hydrometeorological Repoe No. 4 54 defines PMP for watersheds above tributari dams. hese-This reports defines depth-area-duration characteristics. seasonal variations, and antecedent storm potentials*-8iSew and incorporates orographic effects of the Tennessee River Valley. Due to the temperate climate of the watershed and relatively light snowfall, snowmelt is not a factor in generating maximum floods for the Tennessee River at the site.

Two basic total 'watershed storm positions have the potential to produce a maximum flood at the Watts Bar plant site. storms with three possible isohyetal patterns and seasonal variations described in Hydrometeorological Report No. 41 [4] were examined to determine which would produce maximum flood levels at the Watts Bar plant site. One would produce maximum rainfall ;erfPMP depths on the 21,400-square-mile watershed above Chattanooga. The-ethe would preduce m.

.imum r.ains en the part of the basin d.wnstr.eam fromn major TVA tributary ree.'ir, her-eafter-r-efeffed to as the :7,980 square maile sterm. These sterms would eceur-in March. Depths for other months would be less.Two isohyetal patterns are presented in Hydrometeorological Report No. 41141 for this storm. The isohyetal pattern with downstream center would produce maximum rainfall on the middle portion of the watershed and is shown in Figure 2.4-6.

Repert-No. 4 0`4 The pattern critical to this study is the dontempattern' shown in Figur-e

2.

4 A1 (hister-ieal infe*oafn along with. the m i

ho....

ur-,

depths. The second storm described in Hydrometeorological Report No. 41 [41 would produce PMP depths on the 7,980-square-mile watershed above Chattanooga and below the five major tributary dams. The isohyetal pattern for the 7,980-square-mile storm is shown in Figure 2.1 42 (histoerical information) along with the maximum 6 hour6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months Storm depths. The pattem is not e--egf-aphie*..ygeographically fixed and can be moved parallel to the long axis, northeast and southwest. along the va44eyTennessee Valley. The isohyetal pattern centered at Bulls Gap, Tennessee, would produce maximum rainfall on the upper part of the watershed and is shown in Figure 2.4-7.

Potential storm amounts differ-ing by seasons were analyzed in suffffic~ienqt number-to make cer-tain th-at the MacWtom

.ould be controlling. Enough centering wrinestigated to assur-e that a most ocritical position was used.Seasonal variations were also considered. Table 2.4-10 provides the seasonal variations of PMP. The two seasons evaluated were March and June. The March storm was evaluated because the PMP was maximum and surface runoff was also maximum. The June storm was evaluated because the June PMP was maximum for the summer season and reservoir elevations were at their highest levels. Although September PMP is somewhat higher than that in June, less runoff and lower reservoir levels more than compensate for the higher rainfall.

2.4-13

WBNP-A 3 day storm, 3 days antecedent to the 3 day main storm, was assumed to occur-in all total basin PMP situations. DepthsAll PMP storms are nine-day events. A three-day antecedent storm was postulated to occur three days prior to the three-day PMP storm in all PMF determinations. Rainfall depths equivalent to 40% of the main storm were used for the antecedent storms with uniform areal distribution as recommended in Report No. 41.[41 in the or-iginal analysis, storms pr-oduceing PMP aboec uipstr-eam tr-ibutarzy dams were also, evaluated. Storm dcpths and isohyetal patterns fcr tr-ibutary watcr-shcds with. driagearas less than 3000 squarce miles arc defined in Hydr.o......r..gieal Rcpo. No. 4

.F drainage area 1511 square m.iles, a 72 hour8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months PM.P storm depth of 22.3 inc.h.e.s w

.as dete.mine using Hydr*om*eteor.ologic.al Rpo+rt No. A5H data. Nonorographic PMP was determined from Figure 3 11 from Report N

o.1 reaslly for-5ye mean annual pr-ecipitation above Douglas oA mean annfual nooor phipecipitation for the area (Figurde 3 16 from Report No Areal distribution of PMaPed rinf as paffemed ean annual prlecipitation isohyets and isohyets of Hydrometeorological Report No. 150hilaces estimates for subbasins above Dcuglas Dam. Residual rainfall en the area below Douglas and above Watts Bar-was such that the total above Watts Bar-was 80-% of the PM4P. These stormfs

'would occurf in the June to October 'warmf season months. Fcr-conscr-,'atism, a Jutly date was postulated because r-eservýoir-s would-be, -atmaximum summer-levels.

A day storm, 3 days antecedent to the 3 day mistr, was applied in these small area PM4P situations. Depths equal te 30% of the main storm were used fer the anteedenet st formswith unifom areal distrhibution as reofmmended in Reprtl Nc.

from A standard time distribution pattern was adopted for alithe storms based upon major observed storms transposable to the Tennessee Valley and in conformance with the usual practice of Federal agencies. The adopted distribution is shown on Figure 2.1 13 (historical informfationl) within the limits stipulated in Chapter VII of Hydrometeorological Report No. 41 [41. This places the heaviest precipitation in the middle of the storm. The adopted sequence closely conforms to that used by the U.S. Army Corps of Engineers. A typical distribution mass curve resulting from this approach is shown in Figure 2.4-8.

Both the 21,100 square mile total basin stor. m with dcwnstr.eam. or*ogr-aphi-ally fixed pattern (Figure 2.1 41, histoerial infeFmation) and the 7,980 square mile storm centered at Bulls Gap, Tennessee (50 miles northeast of Knoxville) pr-oduce essentially the same peak stage. These stermnswould follow an antecedent storm commfencing on March 15. For-the pur-pose of this report all subsequent rfainfal statistics- -arefo the 21,100 square mile storm because ot equivalent flood levels, larger-total somvol-u-mes involved and the fact that it pr-oduceste controlling storm for the Sequoyah Nuclear Plant downstrteam. T eanslation of the P dP foum Repn e

to the basin above Watts Bar-ruesults in an antecedent stohrm produeing an aver-age pr-ecipitation of 6.114 inches in 3 days, follc'wed by a 3 day dry period, and then by the main storm preoducing an aver-age pr-ecipitatien of 16.314 inches in 3 days. F-igur~e 2.1 411 (histor-ical information) is an isohyetal map of the maximum 3 day PMP. Basin rainfall depths af-invf ilTable 2.4*-5The PMF discharge at the Watts Bar Nuclear Plant was determined to result from the 21.400-square-mile storm producing PMP on the watershed with the downstream storm pattern, as defin ed in Hydrometeorological Report No. 4 [] The PMP storm would occur in the month of March and would produce 16.25 inches of rainfall in three days on the watershed 2.4-14

WBNP-above Chickamauga Dam. The storm producing the PMP would be preceded by a three-day antecedent storm producing 6.18 inches of rainfall, which would end three days prior to the start of the PMP storm. Precipitation temporal distribution is determined by applying the mass curve (Figure 2.4-8) to the basin rainfall depths in Table 2.4-11.

2.4.3.2 Precipitation Losses A multi-variable relationship, used in the day-to-day operation of the TVA reservoir system, has been applied to determine precipitation excess Pe-directly. The relationships were developed from observed storm and flood data. They relate precipitation excess to the rainfall, week of the year, geographic location, and antecedent precipitation index (API). In their application, Peprecipitation excess becomes an increasing fraction of rainfall as the storm progresses in time and becomes equal to rainfall when from 6 to 16 inches have fallenin the later part of extreme storms. An API determined from an 11-year period of historical rainfall records (1997-2007) was used at the start of the antecedent storm. The precipitation excess computed for the main storm is not sensitive to variations in adopted initial moisture conditions because of the large antecedent storm.

For. this study, a median API as determind flm Past r.eord.s ;v.'as Used at the sta.

oft ante**edet Stor.

The antecedent storm is So lar.ge, ho.weVer., that the main StOff is not sensitive to.ariations in adopted API.

For-r-eview pur-pses, pr-ecipitation losses have been determined by subtr-acting Pfem efainfall.

in the critical probable maximum storm losses are 2.24 inehes, amounting to 35% of rainfall, fr the 3 day anteedent st.om, and 1.78 inches, 11P% of rainfall, fcr-the 3 day main storm above Watts Bar.m..

Tab.le 2.4 5 displays the API, rain, and precipitation excess for each of the 45 subwater.sheds of the hydrologic m, del for-the Watts Bar-probable maximum flood.Basin rainfall, precipitation excess, and API are provided in Table 2.4-11. The average precipitation loss for the watershed above Chickamauga Dam is 2.33 inches for the three-day antecedent storm and 1.86 inches for the three-day main storm. The losses are approximately 38% of antecedent rainfall and 11% of the PMP, respectively. The precipitation loss of 2.33 inches in the antecedent storm compares favorably with that of historical flood events shown in Table 2.4-12.

2.4.3.3 Runoff and Stream Course Model The runoff model used to determine Tennessee River flood hydrographs at Watts Bar Nuclear Plant is divided into 4-540 unit areas and includes the total watershed above Chickamauga Dam dOWHetre

. Unit hydrographs are used to compute flows from the unit areas. The watershed unit areas are shown in Figure 2.4-9. The unit area flows are combined with appropriate time sequencing or channel routing procedures to compute inflows into the most upstream tributary reservoirs which in turn are routed through the reservoirs using standard routing techniques.

Resulting outflows are combined with additional local inflows and carried downstream using appropriate time sequencing or routing procedures including unsteady flow routing.--Figure 2.1 415 (histor-ical information) shows unit areas of the water-shed uipstreamn fromF Chickaffaulga 2.4-15

WBNP-The runoff model dif-fer-s from that used in the PSAR beeausc of r-efinements made ini somfe elements of the model during PN4F studies for-other-nuelear Plants an;d tho4se made from..

information gained fromn the 1973 flood, the large.,t that has occurrFed for-pr-esenit r-esernoi cOnlditiOns.

ChangeS are ientified When appr-opr-i*aIteI in the text. They ilude both

adioa-l and r-evised unit hydroegaphs and additional and r-evised unsteady flow stream course mfodels.

Unit hydrographs were developed for each unit area for which discharge records were available from maximum flood hydrographs either recorded at stream gaging stations or estimated from reservoir headwater elevation, inflow, and discharge data using the procedures described by Newton and Vineyard-Reference E5 3 For non-gaged unit areas synthetic unit graphs were developed from relationships of unit hydrographs from similar watersheds relating the unit hydrograph peak flow to the drainage area size, time to peak in terms of watershed slope and length, and the shape to the unit hydrograph peak discharge in cfs per square mile. Unit hydrograph plots are provided in Figure 2.4-10 (11 Sheets). Table 2.4-13 contains essential dimension data for each unit hydrograph.The number of unit areas has been increased from 31 to

45. The differ-enees in.lude:

1.

Combining the two unit areas fcr-Watauga River- (Sugar-Groeve and Watauga loeal) into one unit area and dividing the Chero-kee to Gate Cit,' area into two unit areas (Sir-geinsville local and Cher-okee local below Sur-gainsville);

2.

Ireasing the unit areas on the Clinch Riv.er. from 1 to 11 and the Wa.ts Bar leal from 1

3.

Changes to add an unsteady flow model for the Fort Loudoun Tellico Dam complexi whic-h included dividing the lower-Little Tennessee River-into two unit areas (Fontana to Chilhcwee and Chillhowee to Tellico), and the Fort Loudoun local unit area into thr-ee unit areas (French Broad River local, H.lst.n River-l.cal and F.ora Ludoun local);.

4.

Combining the two unit areas above Fontana (Tuek~asegee River-at BI=)sonf Cit'yn Oeeonaluftee River-at Bir-dtewn) into one unit area (Tuekasegee River-at Br-ysea City)

5.

Combining the two unit areas above Ooeec No. 1 (Ococe No. 1 and Ococe No. 3) into one unit area (Ocoee No. 1 to Blue Ridge) and dividing the Chickamnauga local unit area into two unit areas (Chiekamauga local and-lo-v.er-Hiwassee).

In

additioni, 12 of the unit gr~aphs have been r-evised. Figure 2.1 16 (histor-cal iniforatfion),

which contains 11 sheets, shows the unit hydroegr-aphs. Table 2.1 6 contains essential dimeso data for-each unit hydroegr-aph and identification of those hydroegr-aphs which are new or-r-evised.

2.4-16

WBNP-Tributary reservoir routings, except for Tellico and Melton Hill, were made using the Geedieh semi*graphical methodstandard reservoir routing procedures and flat pool storage conditions.

M-ainThe main river reservoirs a-n4-Tellico, and Melton Hill routings were made using unsteady flow techniques. This differ-from the PSAR in that:

1.

An unsteady flow model has been added &Fo Fert oudoun Tellico

.emplex.,

and

2.

The Chickamnauga unsteady flow moedel has been revised using the 1973 flood data and the HEC 2 backwater-comlputer-proegramf.

Unsteady flow routings were computer solved with athe Simulated Open Channel Hydraulics (SOCH) mathematical model based on the equations of unsteady flow-Reference 61. Ba..daoy conditions pr-escr-ibed wer-e inflow hydr-egr-aphs at the upstr-eam boundary, loca inflows-,*and hea4dwater-discahar-ge relationships -At the dowsteamboudar-y based upon nonnal opertn

,ules, cr-based upo n rated cu

.aes when geometfy contrclled.The SOCH model inputs include the reservoir geometry, upstream boundary inflow hydrograph, local inflows, and the downstream boundary headwater discharge relationships based upon operating guides or rating curves when the structure geometry controls. Seasonal operating curves are provided in Figure 2.4-3 (12 Sheets).

Discharge rating curves are provided in Figure 2.4-11 (13 Sheets) for the reservoirs in the watershed at and above Chickamauga. The discharge rating curve for Chickamauga Dam is for the current lock configuration with all 18 spillway bays available. Above Watts Bar Nuclear Plant, temporary flood barriers have been installed at four reservoirs (Watts Bar, Fort Loudoun, Tellico and Cherokee Reservoirs) to increase the height of embankments and are included in the discharge rating curves for these four dams. Increasing the height of embankments at these four dams prevents embankment overflow and failure of the embankment. The vendor supplied temporary flood barriers were shown to be stable for the most severe PMF headwater/tailwater conditions using vendor recommended base friction values. A single postulated Fort Loudoun Reservoir rim leak north of the Marina Saddle Dam which discharges into the Tennessee River at Tennessee River Mile (TRM) 602.3 was added as an additional discharge component to the Fort Loudoun Dam discharge rating curve. Seven Watts Bar Reservoir rim leaks were added as additional discharge components to the Watts Bar Dam discharge rating curve. Three of the rim leak locations discharge to Yellow Creek, entering the Tennessee River three miles downstream of Watts Bar Dam. The remaining four rim leak locations discharge to Watts Creek, which enters Chickamauga Reservoir iust below Watts Bar Dam.

The unsteady flow mathematical model for the 19.9 mile leng Fot L'oudoen Reser.'oir was divided into 21, 2.08 mile r-eaehes. The moedel was ver-ified at 3 gaged points in Fort Loudoun Reser.voir. using 1963 and 1973 flood data configuration for the Fort Loudoun-Tellico complex is shown by the schematic in Figure 2.4-12. The Fort Loudoun Reservoir portion of the model from TRM 602.3 to TRM 652.22 is described by 29 cross-sections with additional sections being interpolated between the original sections for a total of 59 cross-sections in the SOCH model, with a variable cross-section spacing of about 1 mile. The unsteady flow model was extended upstream on the French Broad and Holston Rivers to Douglas and Cherokee Dams, respectively.

2.4-17

WBNP-The French Broad and Holiston River unsteady flow models were ver.ified at one gaged point each (mile 7.1 and 5.5, r.espectively) using 1963 and 1973 floodda4a River from the mouth to Douglas Dam at French Broad River mile (FBRM) 32.3 was described by 25 cross-sections with additional sections being interpolated between the original sections for a total of 49 cross-sections in the SOCH model, with a variable cross-section spacing of about 1 mile. The Holston River from the mouth to Cherokee Dam at Holston River mile (HRM) 52.3 was described by 29 cross-sections with one additional cross-section being interpolated between each of the original sections for a total of 57 cross-sections in the SOCH model, with a variable cross-section snacing of about 1 mile.

The Little Tennessee River was modeled from Tellico Dam, m ile 0.3, through Tellico Rese..oii to Chilhowee Dam at mile 33.6 and upstream to Fontana Dam at mile 6 1.0. The model tLr Tellieo Reservoir to Chilhowee Dam was tested for adequacy by ecomparing its r eesults with steady state proefiles at 1,000,000 and 2,000,000 cfs cmputed by the standard step meod Minor-decr-eases in el i

the unsteady flow model yielded good agreement. The aver-age conveyanee co fcionfund necessary in the r-each below Chiffiowee Dam to make the unsteady flow moedel agree with the standard step mnethod was also used in the river-r-each from Chilhowec to Fontana Dam Little Tennessee River mile (LTRM) 0.3 to Chilhowee Dam at Little Tennessee River mile (LTRM) 33.6. The Little Tennessee River from Tellico Dam to Chilhowee Dam at LTRM 33.6 was described by 23 cross-sections with additional sections being interpolated between the original sections for a total of 49 cross-sections in the SOCH model, with a variable cross-section spacing of up to about 1.8 miles.

The Fort Loudoun and Tellico unsteady flow models wer-ear joined by a canal unsteady flow model. The canal was modeled with five equally spaced croess sections at 525 feet inter-,'als fo~r the 2100 foet leng canl an interconnecting canal. The canal was modeled using nine cross-sections withan average cross-section spacing of about 0.18 miles.

The Fort Loudoun-Tellico complex was verified by two different methods as follows:

(1)

Using the available data for the March 1973 flood on Fort Loudoun Reservoir and for the French Broad and Holston rivers. The verification of the 1973 flood is shown in Figure 2.4-13 (2 Sheets). Because there were limited data to verify against on the French Broad and Holston rivers, the steady-state HEC-RAS model was used to replicate the Federal Emergency Management Agency (FEMA) published 100- and 500-year profiles. Tellico Dam was not closed until 1979, thus was not in place during the 1973 flood for verification.

(2)

Using available data for the May 2003 flood for the Fort Loudoun-Tellico complex. The verification of the May 2003 flood is shown in Figure 2.4-14 (3 Sheets). The Tellico Reservoir steady-state HEC-RAS model was also used to replicate the FEMA published 100- and 500-year profiles.

A schematic of the steady-state SOCH model for Watts Bar Reservoir is shown in Figure 2.4-15.

The unsteady flow routing model for the 72.4-mile-long Watts Bar Reservoir was-4v.4ded-4i-te 34, 2.13 mile re.. ache.s. The Watts. R md

.as ver.ified at two gauged points within the reser..oir using 1973 flood data described by 39 cross-sections with two additional sections being 2.4-18

WBNP-added in the upper reach for a total of 41 sections in the SOCH steady state model with a variable cross-section spacing of up to about 2.8 miles. The model also includes a junction with the Clinch River at Tennessee River mile (TRM) 567.7. The Clinch River arm of the model goes from Clinch River mile (CRM) 0.0 to CRM 23.1 at Melton Hill Dam with one additional section being interpolated between each of the original 13 sections and cross-section spaces of up to about 1 mile. Another junction at TRM 601.1 connects the Little Tennessee River arm of the model from the mouth to Tellico Dam at LTRM 0.3 with cross-section spaces of about 0.08 miles. The time step was tested between 5 and 60 seconds which produced stable and comparable results over the full range. A time step of 5 seconds was used for the analysis to allow multiple reservoirs and/or river segments to be coupled together with different cross-section spacing. The verification of Watts Bar Reservoir for the March 1973 and the May 2003 floods are shown in Figure 2.4-16 and Figure 2.4-17, respectively.

TheA schematic of the unsteady flow r model for Chickamauga Reservoir is shown in Figure 2.4-18. The model for the tea4 58.9-mile-long Chickamauga Reservoir wasE--

-fit 28, 2.1 mailc reaches.ThChcaugRcc.orutadflwmdlaseiidator gauged peints within the r-esefrvoir-using 1973 flood data. This differ-s fromi the PSAR in that the 19:73 floodwas added for Verifitation replaeaig the 1963 fle"d. The 19.73 flod is the largest which has oefued since closurce of South Holston Dam in 1950. Comparisons between obser.ved and ompu.ted stages in Chi.kaf.auga Rese.ir. are shown in Figure 2.1 50 (histor.ical i..fe.ma.ief. described by 29 cross-sections with one additional section being interpolated between each of the original 29 sections for a total of 53 sections in the SOCH model with a variable cross-section spacing of up to about 1 mile. The model also includes a junction with the Dallas Bay embayment at TRM 480.5. The Dallas Bay arm of the model goes from Dallas Bay mile (DB) 5.23 to DB 2.86, the control point for flow out of Chickamauga Reservoir. Another iunction at TRM 499.4 connects the Hiwassee River arm of the model from the mouth to the Charleston gage at HRM 18.9. The time step was tested between 5 and 50 seconds producing stable and comparable results over the full range. A time step of 5 seconds was used for the analysis to allow multiple reservoirs and/or river segments to be coupled together with different cross-section spacing. The verification of Chickamauga Reservoir for the March 1973 and the May 2003 floods are shown in Figure 2.4-19 and Figure 2.4-20, respectively.

it is impossible to verify the modelVerifying the reservoir models with actual data approaching the magnitude of the PMF. The best remaining alte.ativewas to co.mpar.e the model elevations in a state o~f steady flow with elevat.ion coMpue by the standard step method. This was done for-steady flows ranging up to 1,500,000 cfs. An example shown by the rat.in cu-oF Figure 2.4 51 (histervi*al information) shows the good agreement is not possible, because no such events have been observed. Therefore, using flows in the magnitude of the PMF (1,200,000 -

1,300,000 cfs), steady-state profiles were computed using the HEC-RASE' 41 steady state model and compared to computed elevations from the SOCH model. An example of the comparison between HEC-RAS and SOCH profiles is shown for Chickamauga Reservoir in Figure 2.4-21.

This approach was applied for each of the SOCH reservoir models. Similarly, the tailwater rating curve was compared at each project as shown for Watts Bar Dam in Figure 2.4-22. In this figure, the initial tailwater curve is compared to results from the HEC-RAS or SOCH models.

The rnnoieff model wffas ver-ified by using it to r-eproducee the MaUrch 1963 and Marcoh 1973 floods.

This di-ffiers_ froem the PSAR in that the 1973 flooed-wfas -ad-ded for-ver-ification reOplacing the 1957 2.4-19

WBNP-flood. The 1973 flood is the largest whih has occuffed since elesure of SOUth Hoston DamA in 1950. Obsern'ed volumnes of pr-ecipitation exeess wer-e used in the ver-ification. Compar-isons between otbsenvedi ana computed eutfnows from watts Har-ana Gnicikamauga 44ams ior-tne i ý00 and; 1973 floodA aF.igures..

2.1 4 48 And 2.4 19 (oth histor.ical info.A ation),

From a study ef the basie units of the praediting system and the systems rSponel tialterations in various basie elements, it is oencluded that the anoff madel w

Securent reserveirao efnsexiatively to detein a ximum flood levels. This eonclusion is based in pawr upon watudes byromthetribut]and reservoirshe Turbin ofischare weireaso usdedate the asmption ofrineart in unrvit huydgaphs, an iMpoftant element of the ste is valid when they are developed frwo large, out of bank floeds priodued by majer, basin gaide stoms oer will duplicate such floeds.

Resemrvilr routings sta ble at median oeai ed elrevations for the appropriate season, mid Marcth for-the lage area P anP storms. Median levels ng ee rfeevaluated using optrat experiencefor:

i.

The toetal projiect period, o

2.

The 5 year-period, 1972 1976, fOr those prtojects whose operating guides were changed in Because of the wet years of 1972 1975 and the operaeng guide hanges, median elevatonas were higher-for 7 of the 13 tributary resenriry whiepre arouting is involved.

Normal reserr oeraoting precedurves were used in the antecedent storm. These used turfbine and sluiee discharge in the trPibuta r

oirs. Turbine discharges are not used in the i i r-eservoir-s after-lar-ge flooed flow;.s ddevelop because head differentials are too small. Normal oratinc epredries ern e used in the principal steom except that turbine disehargewas not used-in either the tributary or-main river dams. All gateswere determined to be operable without failures dur-ing the flood. gates ona main r-iver-dams would be ffialy raised, thus r-equiringn additional oper-ations, by the last day of the stormf which is before the stf~utures and acc-ess roads would be inundated.The reservoir operating guides applied during the SOCH model simulations mimie, to the extent possible, operating policies and are within the current reservoir operating flexibility. In addition to spillway discharge, turbine and sluice discharges were used to release water from the tributary reservoirs. Turbine discharges were also used at the main river reservoirs Up to the point where the head differentials are too small and/or the powerhouse would flood. All discharge outlets (spillway gates, sluice gates, and valves) for projects in the reservoir system will remain operable without failure up to the point the operating deck is flooded for the passage of water when and as needed during the flood. A higzh confidence that all gates/outlets will be operable is provided by periodic inspections by TVA plant personnel, the intermediate and five-year dam safely engineering inspections consistent with Federal Guidelines for Dam Safety, and the significant cap~ability of the emergency response teams to direct and manage resourees to address issues potentially impacting gate/outlet functionality.

Median initial reservoir elevations for the appropriate season were used at the start of the PMF storm sequence used to define the PM4F to be. Use of median elevations is consistent with statistical experience and to-aveidavoids unreasonable combinations of extreme events. As a 2.4-20

WBNP-result 550% of the total r-eservedl system flood dletention capacity above the plant was occupied at the start of the main flood. This is consider-ed to he amply conservative. Neither: the initial reservir le;lsAno the Operating rules would have significant efferat on maximum flood diseharges and elevations at the plant site because spillway capacities, -And-henOe uncoentr-olled conditions, were r-eached ear*,ly in the flood.

The flood from the antecedent storm occupies about 70% of the reserved system detention capacity above Watts Bar Dam at the beginning of the main storm (day 7 of the event).

Reservoir levels are at or above guide levels at the beginning of the main storm in all but Apalachia and Fort Patrick Henry Reservoirs, which have no reserved flood detention capacity.

2.4.3.4 Probable Maximum Flood Flow The analy1Asis to determinle the PN-F, fl-ow. inc-lude-d-evalumatin of PN4P o)ver-the total water-shed With conHsiderationf Of critical seasonal -And-aea var-ations. A compar-ison of candidate events is pro.vided by Table 2.1 -7.

The PMF discharge at the Watts Bar Nuclear Plant was determined to be 1,288,000,fs resulting from the 7,980 square mil-e stormi centered at Bulls Gap. The peak dischar-ge of 1,230,000 cfs r-esulting from the total basin, dewnstr-eamn center-ed, oroegr-aphically fixed stormf produceda equivalent peak stage 1,088,625 cfs. This flood would result from the 21,400-square-mile storm in March with a downstream orographically-fixed storm pattern (Figure 2.4-6).

safely pass the PMF. The west saddle dikeThe PMF discharge hydrograph is shown in Figure 2.4-23. The West Saddle Dike at Watts Bar Dam (Figure 2.4-24) would be overtopped and the earth embankment breached. The discharge from the failed West Saddle Dike flows into Yellow Creek which joins the Tennessee River at mile 526.82, 1.18 miles below Watts Bar Nuclear Plant. Chiekamauga Dam would be overtopped but was assumed not to fail (failure would r-educe the flood level at the site). in the or-iginal analysis, Fort Loudoun T-elfico and Watts Bar Chickamauga Dam downstream would be overtopped and the eafrh embankments breached. The PMF hydroegr-aph is shown in Figure 2.4 52. Rainfall and pr-ecipitation shown in Figure 2.1 52 are for the total basin storm. The dam was postulated to remain in place, and any potential lowering of the flood levels at Watts Bar Nuclear Plant due to dam failure at Chickamauga Dam was not considered in the resulting water surface elevation.

Following is a more complete descrption of candidate sitvations examined.

The storm.. pr1oducing the P. F d

.ischar.ge is the 7,980 squar.e mile storm centerAed at Bulls gap, Tensse

=90 F esAR fRieAtt J

ea t

e Knoxv eI, sut evV, al guAIL 1

\\Afzf e

HfflI~AI~A~~

aIA.t O.filA.

The flood fromn this storm would over-top and br-each the west saddle dike at Watts Bar-Dam (Figufe24A-53)

March storm producing the PP on the -21,100 square mile watershed above Chattanooga with 2.4-21

WBNP-downstr-eam center-ed and or-ogr-aphically fixed isohytal patterin sho*w Il i Figurfe 2.1 11 (historical information), and is more completely described in Section 2.1.3.1. The flood would overtop and breach the e...h wVest Saddle ike at Watts Bar-Dam upstream. Chie.amauga Dam, downstream, would be overtpped but was conservatively assumed not to fail.

The previeus PMF evaluations als onsid--red-eandidate Situations involving upstr.eam tributar dams, Douglas an Watauga. These two dams 'werae overtpped and brleaced in their t

PFws. These two situations weroe shown at that time to be no n go eri. Dam safety mudifications have sinee elimginated the patential failures of these dams, therefore, these two candidate situiations have been elimninated.

Flleowing st aoc plete description of dam stability and earth embanuaent breaching analysis madte tor.d..efine PMF conditions.

Concrete Section Analysis For concrete dam sections, cmaioswere made betwveenl the. or-iginal design headwater-an tailwater-levels and those.ta ol pr-evail in the PM4F. if the ove~rning moments and hor-izontal forcees wer-e not incr-eased by more than about 20-1,, thefactors of safety in sliding were determined by comparison of the existing design headwater/tailwater levels to the headwater/tailwater levels that would occur in the PMF as described in Section 2.4.3. The structures were considered safe against failure if a factor of safety greater than 1.0 for sliding was demonstrated. All upstr-eam d-amsn--The dams uipstream of Wafts Bar Nuclear Plant passed this test except Douglas, Fort Loudoun, and Watts Bar-. Original designs showed the spillway detail and judged to be stable.

Spillway Gates During peak PMF conditions, the radial spillway gates of Fort Loudoun and Watts Bar Dams wil-beare wide open with flow over the gates and under the gates. For this condition1 both the static and dynamic load stresses in the main structural members of the Watts Bar Dam spillway gate will-beare determined to be less than the yield stress by a factor of 3. The stress in the tnunnion pin is less than the allowable design stress by a fa*cr" Of greater than 10. The trunnion pin is prevented from dislodgement by a key int the gate anchor.age assembly and fitting into a 4eti4he-pia and the stress in the trunnion pin is less than the allowable design stress. The open radial spillway gates at other dams upstream of Watts Bar Dam were determined to not fail by comparison to the Watts Bar Dam spillway gate analysis.

The gates were also investigated for-the condition when rising headwater-level first begins to exceed the bottom of the gates in the 'wide open position. This condition proeduces the largest forcees, tending to roetate the radial gates up'ward. in the wideopen position the gates are dogged against steel gate stops anchor-ed to the concr-ete pier-s. The stresses in the gate stop mnembers are less than the yield stress of the mater-ial by a factor of 2.

it is concluded that the above listed

.ar.gins are suffic.ient to pro÷vide assurance also that the gates will not fail as a result of additional stresses which may result from possible vibrations o2 the gates acating as or-ificesa.

2.4-22

WBNP-Waterborne Objects Consideration has been given to the effect of waterborne objects striking the spillway gates and bents supporting the bridge across Watts Bar Dam at peak water level at the dam. The most severe potential for damage weou-lis postulated to be by a barge which has been torn loose from its moorings and floats into the dam.

Should the barge approach the spillway portion of the dam end on, one bridge bent could be failed by the barge and two spillway gates could be damaged and possibly swept away. The loss of one bridge bent will likely not collapse the bridge because the bridge girders are continuous members and the stress in the girders willis postulated to be less than the ultimate stress for this condition of one support being lost. Should two gates be swept away, the nape of the water surface over the spillway weir would be such that the barge would likely-be grounded on the tops of the concrete spillway piers and provide a partial obstruction to flow comparable to un-failed spillway gates. Hence the loss of two gates from this cause will have little effect on the peak flow and elevation.

Should the barge approach the spillway portion broadside, two and possibly three bridge bents eould be failedmay fail. For this condition the bridge would likely collapse on the barge and the barge would be grounded on the tops of the spillway piers. This would be probable because the appro.ah vel, ity of the barge would be frm.4 to 7 miles per-hour and the bottom of the barge would be about 6 in.hes above the tops of the piers. For this condition the barge would be grieundedlikely ground before striking the spillway gates because the gates are about 20 feet downstream from the leg of the upstream bridge bents.

Lock Gates The lock gates at Fort Loudoun, Watts Bar, and Chickamauga were examined for possible failure with the conclusion that no potential for failure exists beeause the gates a. e designed for a

.....er.e.iai.ye...stati n...

gr.eater ta..

tnat w.

. ex.sts Atir..g tne pr-ea. e maximum 1looA.

The lock gate structural elements may experience localized yielding and may not function normally following the most severe headwater/tailwater conditions.

The potential for-embankment br-eachingwas examined for-All PF ci i the 1998 r-eanalysis the only embankment fa;ilure wovuld-be the w~est s-addle dik-e at Watts Bar. Dam.

C-hiekaffmauga Dam, 57 m~iles; dov.'n-;;stream of the plant, would be oveitopped but was assumfed noAt to -fail. This is coenser.'-ative -AS failurfe woveuld slightly lower-flood-elevationis at the plant-.

Figure 2.1 47b, shows the headwater-and tailwater-dischar-ge r-elatienships for-Fort Louden Dam as modified. Figur~es 2.4 47e and 2.4 55 show these r-elationiships for Tellico and Watts Bar Dams, r-espectively.

2.4-23

WBNP-The adopted relationship to compute the rate of erosion in an eaflh damn failure is that developed and used by the Bureau of Reclamation in connection with its safei'*

f dams program.---Thee onI+*i below r-e*late th I

vo+lum at-er-oafl aea otevlm fwt i

throeugh the br-each. The equation is:

Q~se4 where Q soil-Voelumeof soil eroded in each timie pe.r-iod Q water -Volume of water-discharged each timne period K

Constant Of pop*'ionalit, 1 for the soil and discharge relationships in this study 0 -

Base of namr~al logarithm sy'stemf

4 wheN-b -

EBase

,length Of Overflow chael at any given timne H4-Hydraulic head at any given time Developed angle of friction of soil material. A consen'ative value of 13 degrees was adopted for-materials in the dams investigated.

Solv'ing the Oequationl, 'which was comfputer-ized, involves a trial and eaFor procedure ov'er shod-depth and time incr-ements. in the proegr-am depth changes of 0. 1 foot or-less are used to keep time increments to less than on.e seeona Atur.fing rapd ansuare and utip to a rtout U secods pno t

-e.ehint-2 The solution of an earth embanlfaent breacsh begins by solving the eroesion eqjuationl usina headwater~~~

~~~

elvto yrgap-asmn ofilure. Eroesion is postulated to occur-across the enitireo eau~h section and to stafi at the downstr-eam edge whnhe-adfawatr-elevations r-eacheda selected depth above the dam top elvto.Subsequently, w~hen erosion rAcPVhes teupstr-eamn edge of the embanlrnment, breaching and rapid lowe~ng of the embaffAnment begins. Thereafter, ceomputations include headwater-adjustmnents for-increased r-eser-voir-outflow r-esulting from the Watts Bar-West Saddle. Dikep Failure would be a complete washout and would adld to the dischar-ge from Watts Bar-Dam.

Given the -hour-I Of fat-ilure, the peak discshar-ge 'was dete~mined based upon the headwater-and tailwater-depths at that time. Unsteady flow roeuting techniques wer-e used to define the rest ot the outflow hydr-ogr-aph. This was accomnplished using headwater-dischar-ge relationships at the 2.4-24 8

WBNP-Somnever-ification for-the br-eaching copuaioa proceedur-es illustrated above was obtained by coprsn and acatual failures r~eported ini the lieameanid in informal discussion with hyd.These repots show that overtopped earth embankments do not necessarily fail. Earh emban--rents have sustained evertopping of several feet fer several hoeu-rs before failure eccurred. An extreme example is Oros earh dam in Brazilm. which was overoepped to a depth of approeximately 2.6 feet along a 2,000 foot length for-12 hours befoebreacin began.

Once an ear4h embankment is breached, failrte tends to progress r.apidly., however. How rapidly depends upon the material and headwater depths durng failure. Complete failures computed in this and other-studies have varied Worne about one halt to 6 hour~s after-initial br-eaching. This is conisistent with aeffial failures.

In the original analysis, the failureo earthem ba at Chiekamauga Dam, 57 miles downstr-eam fromn Wafts Bar-Nuclear-Plant r-educed-flooAd-levels at the plant by 0.2 foot.Fufr embank~ment improvements arc planned for-Chickamnauga Dam which might evcntually invaidat asumpton f failur~e. Ther-efor-e, the dam was assum-ed noit to fail in determnn flood elevantions ifor the plant. This asup ioni eonscrativ~e.

This subsection descr-ibes analysis of the 'wave front r-esulting from the embafflunent failure of Watts Bar Dam. Embankment impro.vements made by the Dam Saf' Progrm esrtha failure in the PM* F will not occur-. This subsection is retained for histor1ical purposes.

Because W-atts Bar -Nucear Plant is located only 1.9 miles downstream from Waifs Bar-Dam, thc magnitude and shape of the wave front resulting from the sudden failure of-the WattsBa embankment duringe the PM4F was examined in detail. The analysis consisted-of determininae the magnitude of the wave r~esuilting from the embankment failur (1) immediaeyatrfiue 2 after-traveling downistr-eamf 8500 feet and striking a r-idge on the left bank, and (3) r-eflected froem the r-idge dir-ected toward the plant on the right bank.

The water-levels uipstr-eam and downstr~eam fromn the earth embankment for-the conditionis which exs utprior to and immediately, following emnbank~ment failure are shown sehematieally on Figure 2.1 61 (historical information). immediately after-failuire of the 750 foot wide portion of the e~omba__nlnent, a borrevwill develop. The relative height Of the borFe has been defined by Stakefýý by the following ffinetional r-elationiship, which is shown gr-aphically On Figur-e 2.1 62 (histor-ical inforimation).

0 ha-,hu) inhwhiehh-,hý--k-and-4i, are defined in Figure 2.1 61 (histor-ical informfation)-.

For-the-initial conditions shown scemenatically on Figure 2.1 61 (histor-ical informfation).

2.4-25

WBNP-hu 0.435 4-7 62-00462-Entering Figure 2.1 62 (histodeal inf....ation) wi.t h...,

hi

....3

- 0.250 Solving this equation for-the bor-e height above talaer h-h0 Solving for-the depth behind the bore, It, giive&

h2- 0.250 b4=ho - 0.250 -x 62 += 27 - 12.5 feet The initial bor-e velocity per-Refer-ence [3 1], can be computed fromw 44 ghU (hg:442 12.0 + (32.2) L 12.0 +4 2.0 5-15.0 fps In. which,

- 12 fps 9

PiAdw'e*ea RF.

000 x 24 Ther-efer-e, immlfediately after: the failure of the embankment, the height of the bor-e, will be 15.5 feet above the initial tailwater depth, its widthwill be 750 feet at the damn and it'will mov toward Blalock Ridge with an initial veloc%'y of 51 feet per-second.

The bor-e is conscn'atively estimated to spread later-ally, at 100 as it moves o)ut fromF the dlam, as sho'wn on Figure 2.1 63 (histerical info~mation). The height ef the bor-e'would be r-educed by the l0' expansion fromn 15.5 feet at the dam to 3.1 f&eet at BaokRidge. The r-idge is sloped and heavily wooedand would asr a large perceentage of the incident bor-e cnler-g.

Sincee wave ener-gy is proepodional to the sqluar-e of the wave ampliwide Atýý the followin exrsio can be written:

A.i a

a-4-k-in whichi subscripts i, r-, and a, refer to) incidenlt, reflected and~all.plefileA Gemponcnts, respectively.

2.4-26

WBNP-This equation may be wr-itten as:

(A+-l (Au)-

1-t ccffcint the exrsso becmes

/-!

J1

/'*?"

J1 1

-~ -

Inct r-efecion coemeient and &15 incthe ahso ntlon The -f-llwing table was computed using this expr o.

V 0

04 072 04 074 0-.7 04-

+/-4 04 079 04 04 C

--A

/A; 0

0-.--

042 0,63 0-74 0-.77 0784 4.

4-0g95 0484 0-.7 0,774 0-75 044 0

Whef-e!

t Ct is the fraction of-wave energ' absorbed A

is the fraction of wave energy reflecte*

Values ottG r-angrn tromn about 0. 16 up to 0.72 have been repoed in the literaturre for 4r-ee like materials, w.it mo:

w.u...,ld he nb'Arýhe

t values being near-er-the 0. 16ý

-a-umfini by the ridge and 500% reflected, gives from the pF I-n I-~

eeedingtable A -0.71 InA I-T With Aq 3.1 feet, the reflected bore height would be:

A

-fn)nC-..

11 A.Iý ik%9!

i i

2.4-27

WBNP-

.g nn of hors enenr: the fnI1nwin~r I

~~0 In -i+/-iiti-rn to ths~ con~n'nti'.ze n~nmntmn regnr~1inp' the nhwrnti, additional faetor-s which would fuither-r-educe the r-eflected wave height have been neglected.

(1)

The shape of the ridge is such that it does not parallel the wave Ofrnt, ther.efore oly a petiffl Of the waVe fronHt wou1-ld-bediete owr the plant site. Cnieal disperSiOn of the way e front would result from the eured shape of Wlalek *ridge-.

(2)

The r-idge is sloped, ther-eferc fun off would f*..her-r 1edue the reflected wave height.

(3)

After-striking Blalock Ridge noe fufither-avexnson has been consider-ed in Thus, a wave height of about 2.2 feet is eonser.'afively estim-ated-to-be reflected from Bialock Ridge and dir-eetcd towar-d the plant site. This Meprswt the computed wave height in the PSAR of 2.0 feet. This wave would tr-avel at about 30 fcet* pe-r second on the flood plain as it approeaches the plant site-The rarrive at the plant site in about 5 to 10 minutes following e.. ba.A....ent failuire (embankment failuire was postulated to occur-at 11:ý30 on Marceh 21). At the time h wave affives at the plant site, the water-level is about Elevation 728 as shown in FSAR Figurfe 2.1 64 proeducing a water-level at the plant site of Elevation :730.2. The maximumn PM4F level occur-s some one and one half days later.

Forces resulting frem the 2.2 foot bore were evaluated for each of the safe"y related str.etures required to r-emain functionial Mand maintain their-structurval integr-ity during the proebabl maximum flood.' The design cr-iteria for-the structur~es described in Section 3.8 wereno e-eeed d.

2.4.3.5 Water Level Determinations The maximumn plant site flood Elevation 731.9 is produced by the 7980 sqluar-e mile stormf. The less critia 21,10 squar-e m~ile storm would produce Elevation 731.7 at the plant site. The fleodThe controlling PMF elevation at the Watts Bar Nuclear Plant was determined to be 739.2 ft, produced by the 21,400-square-mile storm in March and coincident with overtopping failure of the West Saddle Dike at Watts Bar Dam. The PMF elevation hydrograph is shown in Figure 2A.4572.4-25. Elevations were computed concurrently with discharges for he-site using the SOCH unsteady flow reservoir model. Figure 2.1 65 shows the described in Section 2.4.3.3.

The PMF profile together with the regulated maximum known flood_ median summer elevation and thlweg bottom profiles along a 6-four-mile reach of the Chickamauga Reservoir which encompasses the plant location, is shown in Figure 2.4-26.

2.4-28

WBNP-The third candidate situation considered, failure of Douglas Dam during its PN4F was shown in 2.4.3.6 Coincident Wind Wave Activity Some wind waves are likely when the probable maximum flood crests at Watts Bar Nuclear Plant. The flood would be near its crest for a day beginning about 2 days after cessation of the probable maximum storm (Figure 24--572.4-25). The day of occurrence would be in the month of March or possibly the first week in April.

Figure 2.4-982.4-27 shows the main plant general grading plan. The Diesel Generator Buildings to the north and the pumping station to the eastsoutheast of the main building complex must be protected from flooding to assure plant safety. The Diesel Generator Buildings operating floors are at 4eevationelevation 742.0 ft which are wel4-above the maximum computed elevation including wind wave runup. The pumping station is shielded from direct wave action on all sides except to the south by either buildings, ea..h embankments, or-the cooling towers. The aimm effective feth of 1.3 miles ELeur-s from both the southwest and northeast dir-c.tions tyigtire 14 16-1 This al Iowýs ior-the snel trlng ceietiet-sevcr-alfif I S n mce scumh fnvebanik which become islands at maximum flood le-velsThe electrical equipment room of the Intake Pumping Station will flood at elevation 728.0 ft. The Auxiliary and Control Buildings are allowed to flood. All equipment required to maintain the plant safely during the flood is either designed to operate submerged, is located above the maximum flood level, or is otherwise protected. Those safety-related facilities, systems, and equipment located in the containment structure are protected from flooding by the Shield Building structure with those accesses and penetrations below the maximum flood level designed and constructed as watertight elements.

The maximum effective fetches for the structures are shown on Figure 2.4-28. Effective fetch accounts for the sheltering effect of several hills on the south riverbank which become islands at maximum flood levels. The maximum effective fetch in all cases, except for the west face of the Intake Pumping Station occurs from the northeast or east northeast direction. The maximum effective fetch for the west face of the Intake Pumping Station occurs from the west direction.

The Diesel Generator Building maximum effective fetch is 1.1 miles, and the critical west face of the Intake Pumping Station maximum effective fetch is 1.3 miles. The maximum effective fetch for the Auxiliary, Control, and Shield Buildings is 0.8 miles.

For the Watts Bar-UFSgAPplant site, the two-year extreme wind for the season in which the PMF could occur was adopted to associate with the PMF crest as specified in Regulatory Guide 1.59.

The storm studies on which the PMF determination is based 4 j show that the season of maximum rain depth is the month of March. Wind velocity was determined from a statistical analysis of maximum March winds observed at Chattanooga, Tennessee.

2.4-29

WBNP-Records of daily maximum average hourly winds for each direction are available at the Watts Bar site for the period May 23, 1973, through April 30, 1978. This record, however, is too short to use in a statistical analysis to determine the -two-year extreme wind, as specified in ANSI Standard N 170-1976, an appendix to Regulatory Guide 1.59. Further, the necessary 30-minute wind data are not available. To determine applicability of Chattanooga winds at the Watts Bar plant, a Kolmogorov-Smirnov (K-S) statistical test was applied to cumulative frequency distributions of daily maximum hourly winds for each direction at Chattanooga and Watts Bar.

The winds compared were those recorded at Chattanooga during the period 1948-74 (the period when the necessary triple-register records were available for analysis) and the Watts Bar record.

A concurrent record is not available; however, the K-S test showed that (except for the noncritical east direction) the record of daily maximum hourly velocities at Chattanooga were equal to or greater than that at Watts Bar. From this analysis it was concluded that use of the Chattanooga wind records to define seasonal maximum winds at the Watts Bar site is conservative.

The available data at Chattanooga included 30-minute and hourly winds by seasons and direction for the 27-year period 1948 through 1974. The March 30 minute wind data w;,'hich was used dir-ectly in subsequent wind wave calculations wer-e adjusted to 30 feet by the equatiefn-V'T* -

VIZ (3 0 '/ Z)14 where.

-r wind speed at 30 feet

-V

, wind speed at height Z above the ground The adj~usted 30-minute wind data were analyzed for both the southwest and northeast directions. I The winds from the northeast are considerably less than those from the southwest; hence, the southwest direction is controlling. Figure 2.4 66 (historical information)2.4-29 shows the plot of I the Chattanooga March maximum 30-minute winds from the critical southwest direction. The 2two-year, 30-minute wind speed is 21 miles per hour determined from a mathematical fit to the Gumbel distribution. This compares with 15 miles per hour determined for the March season from the noncontrolling northeast direction.

Wind wave calculations call for-a 28 mninute sustained wind. There is, hoever no igificant diff~er-ence between a 2 year-, 30 minute wind velocity and a 2 year-, 28 Mint' id eoiy Thus, the 2 'oar, 30 m.inute, 21 mile per h..r wind veloty.wasu compute wind waves.

Computation of wind waves used the procedures of the Corps Of Engineers.[' 41 The -ritieal dir-ection for-the. _PMAF elvtosi-sfrom the southwest with an effective fetch of 1.3 milesa shown in Figure 2.4 67. For-a 28 minute sustained 21 mile per-hour wind, 99.61% of the waves approaching the plant would be less than 2.0 feet high, cr-est to troeugh, resulting in m~aximumfil

'ater elevation of 736.2.Wind speed was adiusted based on the effective fetch length for over water conditions. For the Diesel Generator Building, the adiusted wind speed is 23.8 miles per hour. The Intake Pumping Station maximum adjusted wind speed is 24.2 miles per hour for the critical west face. For the Auxiliary. Control, and Shield Buildings the adjusted wind speed is 23.4 miles per hour.

2.4-30

WBNP-For waves approaching the Diesel Generator Building, the maximum wave height (average height of the maximum 1 percent of waves) would be 1.7 ft high, crest to trough, and the significant wave height (average height of the maximum 33-1/3 percent of waves) would be 1.0 ft high, crest to trough. The corresponding wave period is 2.0 seconds. For the Intake Pumping Station, the maximum wave height would be 2.2 ft and the significant wave height would be 1.3 ft, with a corresponding wave period of 2.3 seconds. For the critical west face, the maximum wave height would be 1.9 ft high, and the significant wave height would be 1.1 ft high. The corresponding wave period is 2.1 seconds. The maximum wave height approaching the Auxiliary, Control, and Shield Buildings would be 1.5 ft high, and the significant wave height would be 0.9 ft high. The corresponding wave period is 1.9 seconds.

Computation of wind setup used the procedures of the Corps Of Engineers[143. The maximum wind setup is 0.1 ft for all structures. Computation of runup used the procedures of the Corps Of Engineers[14]. At the Diesel Generator Building, corresponding runup on the earth embankment with a 4:1 slope wold-be-b-2.is 2.3 fet, *eahing Elevation 736.9 and reaches elevation 741.6 ft, including wind setup. The runup on the sethcritical west face wall of the puing-statien weuld be to Elevation 736.9.ntake Pumping Station is 2.1 ft and reaches elevation 741.7 ft, including wind setup. The configuration of the north face of the Intake Pumping Station, opposite of the intake channel, allows higher runup of 3.4 ft. The remaining south and east faces allow runup of 2.4 ft. However, there are no credible entry points to the structure on the north, south, or east faces. Therefore, the runup on these faces is discounted. The runup on the walls of the Auxiliary, Control, and Shield Buildings is 1.7 ft and reaches elevation 741.0 ft, including wind setup.

Runup does not exceed the design basis flood level for any of the structures. Additionally, runup at the Diesel Generator Building is maintained on the slopes approaching the structure and is below all access points to the building. Runup has no consequence at the Shield Building because all accesses and penetrations below runup are designed and constructed as watertight elements.

Wind wav i~nt a pro.blem. sin.e the wind diretion i eppesite to the flow of the rIver.

The static effect of wind waves was accounted for by taking the static water pressure from the maximum height of the runup. The dynamic effects of wind waves were accounted for as follows:

The dynamic effect of nonbreaking waves on the walls of safety-related structures was investigated using the RainflewSainflou methodtI51. Concrete and reinforcing stresses were found to be within allowable limits.

The dynamic effect of breaking waves on the walls of safety-related structures was investigated using a method developed by D. D. Gaillard and D. A. MolitarE1 6]. The concrete and reinforcing stresses were found to be less than the allowable stresses.

The dynamic effect of broken waves on the walls of safety-related structures was investigated using the method proposed by the U.S. Army Coastal Engineering Research Center.1 153 Concrete and reinforcing stresses were found to be within allowable limits.

2.4-31

WBNP-2.4.4 Potential Dam Failures, Seismically Induced The procedures described in Appendix A of Regulatory Guide 1.59 were followed when evaluating potential flood levels from seismically induced dam failures.

The plant site and upstream reservoirs are located in the Southern Appalachian Tectonic Province and, therefore, subject to moderate earthquake forces with possible attendant failure.

All rupstieamUpstream dams whose failure has the potential to cause flood problems at the plant were investigated to determine if failure from seismic events would endanger plant safety.

It should be clearly understood that these studies have been made solely to ensure the safety of Watts Bar Nuclear Plant against failure by floods caused by the assumed failure of dams due to seismic forces. To assure that safe shutdown of the Watts Bar Nuclear Plant is not impaired by flood waters, TVA has in these studies added conservative assumptions to be able to show that the plant can be safety controlled even in the event that all these unlikely events occur in just the proper sequence. TVA is of the strong epinien that the chances ef the assumed events oeeurrng approeach zero proebabilit'ý'

By furnishing this information TVA does not infer or concede that its dams are inadequate to withstand earthquakes that may be reasonably expected to occur in the TVA region under consideration. TVA has a program of inspe.tion and maintenane

.arried out on a regular, schedule to keep its dams safe.The TVA Dam Safety Program (DSP), which is consistent with the Federal Guidelines for Dam Safety 37, conducts technical studies and engineering analyses to assess the hydrologic and seismic integrity of agency dams and verifies that they can be operated in accordance with Federal Emergency Management Agency (FEMA) guidelines. These guidelines were developed to enhance national dam safety such that the potential for loss of life and property damage is minimized. As part of the TVA DSP, inspection and maintenance activities are carried out on a regular schedule to confirm the dams are maintained in a safe condition. Instrumentation of the dams to help keep cheek an their-beha-viorto monitor the dams' behavior was installed in many of the dams during original construction.--Othef and other instrumentation has been added since and is still being added as the need may appear or as new techniques beeome available. I,-shieBased on the implementation of the DSP, TVA has confidence that its dams are safe against catastrophic destruction by any natural forces that could be expected to occur.

2.4.4.1 Dam Failure Permutations There are 12 major dams above Watts Bar Nuclear Plant whose failure could influence plant site flood levels. Dam locations with respect to the Watts Bar Nuclear Plant site are shown in Figure 2.4-2. These are Watts Bar and Fort Loudoun Dams on the Tennessee River; Watauga, South Holston, Boone, Fort Patrick Henry, Cherokee, and Douglas Dams above Fort Loudoun; and Norris, Melton Hill, Fontana, and Tellico Dams between Fort Loudoun and Watts Bar. These were examined individually, and in combinations, to determine if failure might result from a seismic event and, if so, would failure concurrent with storm runoff create maximum flood levels at the plant.Dam locations with respect to the plant site are shown in Figure 2.1 1.

2.4-32

WBNP-dam integrit i

ents wee made fOr two basic A

I nalyses to dEtewmine eanatiRS

1. Deermintionof the water level at the plant during one hafteP-MF with full Fec.or if itS creSt wer-e augmented by flood waveas 4-from the postulated failure of upstr-eam damas during an oper'ating basis ea.thquake (09-E+.

2.

Determiniatien of the water-level at the plant dur-ing a 25 year fleed with full r-esen'cir-s it its er-est wer-e augmented by flood waves from the postulated failure of ufpstr-eam dams dutr-ing a safe shutdown earthquake (SS-E).

The procedures referred to in Regulatory Guide (RG) 1.59, Appendix A, were followed for evaluating potential flood levels from seismically induced dam failures. In accordance with this guidance, seismic dam failure is examined using the two specified alternatives:

(1) the Safe Shutdown Earthquake (SSE) coincident with the peak of the 25-year flood and a two-year wind speed applied in the critical direction.

(2) the Operating Basis Earthquake (OBE) coincident with the peak of the one-half PMF and a two-year wind speed applied in the critical direction.

The OBE and SSE are defined in Sections 2.5.2.4 and 2.5.2.7 (both hister*ieal infoer.atfio as having maximum horizontal rock acceleration levels of 0.09 g and 0.18 g respectively. As described in Section 2.5.2.4.(hi.se*;

-eal 4ei*_atiaa), TVA agreed to use 0.18 g as the maximum bedrock acceleration level for the SSE.

Prior to the 1998 reanalysis, the flod eveals from postulated seismic failure of tr.ibutary dams ge.

.i this repo.t.were higher-than those in the PSAR. The.se higher levels resulted from

1.

Use o~f unsteadyý flo)w models for the Clinch and Little Tennessee Rivers for the routinig Of Norris and Fontana seismic dam failure sur.ges which r-eplaced appro.ximate

.ou4ting pro.edur.es used in the PSAR analysis.

2.

Use of improeved Watts Bar-ReseR~cir-unsteady flow models which extended up the Clinc-h River-embayment to Melon Hill Dam.

-3.

Usee of-;- adischarge rating for-Norris failed dlam section developed by P/IA Enginern Laboratory model studies.

in the 1998 r-eanalysis at potentially crFitic.al seismnic events involving dam failures abovth plant were re evaluated. These events include the postulated OBE failure of Fontana, the pestulated OBE failure of Ncn2s, the postulated OBE failure of Cherkee and Douglas,th postulated SSE failure of Norris, Cheroekee and Douglas, and the postulated SSE failur-e Ot Norris, Douglas, Fort Loudon and Tellico.

The highest flood level at Wa~s Bar-froem diff-erFent seismic damfailurfle Aand flood comabinations wovuld-be El-Ievation 727.5 frmfailur-e of Norris, Cherokee and Douglas Dams dur-ing the SSE earthquake coincident with the twenty five year-flood. Wind wave could raise the levelt Elevation 728.2. Runup could r-each Elevation 729.0 on a 3.1 slope.

2.4-33

WBNP-Plant saf.ty.would be assured by shutdown prior-to this flood

.r.ssing p

lant

grFade, levation 728, using the warning system described in Section 2.4.14.

This is the only c

embination of seismic dam d

f ilures iath coincieant flods which culd rsesult inet a flooAd-at Watts Bar-emceeding plant gr-ade. All other-combinationsli wopuld proeduce flood levels well below plant gr-ade-.

The effcet o~f postulated seismic bridge failur-e and rcesulting failuire of spillway gate anchor-s at Watts Bar-and Fort Loudoun Dams would not create a safety hazar~d at the Watts Bar-plant-.From the seismic dam failure analyses made for TVA's operating nuclear plants, it was determined that five separate, combined events have the potential to create flood levels above plant grade at Watts Bar Nuclear Plant. These events are as follows:

(1)

The simultaneous failure of Fontana and Tellico Dams in the OBE coincident with one-half PMF.

(2)

The simultaneous failure of Fontana, Tellico, Hiwassee, Apalachia, and Blue Ridge Dams in the OBE coincident with one-half PMF.

(3)

The simultaneous failure of Norris and Tellico Dams in the OBE coincident with one-half PMF.

(4)

The simultaneous failure of Cherokee, Douglas, and Tellico Dams in the OBE coincident with one-half PMF.

(5)

The simultaneous failure of Norris, Cherokee, Douglas, and Tellico Dams in the SSE coincident with a 25-year flood.

Tellico has been added to all five combinations which was not included in the original analyses for TVA's operating nuclear plants. It was included because the seismic stability analysis of Tellico is not conclusive. Therefore, Tellico was postulated to fail.

Concrete Structures The standard method of computing stability is used. The maximum base compressive stress, average base shear stress, the factor of safety against overturning, and the shear strength required for a shear-friction factor of safety of 1 are determined. To find the shear strength required to provide a safety factor of 1, a coefficient of friction of 0.65 is assigned at the elevation of the base under consideration.

The analyses for earthquake are based on the pseudo-static analysis method as given by Hinds 171 with increased hydrodynamic pressures determined by the method developed by Bustamante and Flores[1'8. These analyses include applying masonry inertia forces and increased water pressure to the structure resulting from the acceleration of the structure horizontally in the upstream direction and simultaneously in a downward direction. The masonry inertia forces are determined by a dynamic analysis of the structure which takes into account amplification of the accelerations above the foundation rock.

2.4-34

WBNP-No reduction of hydrostatic or hydrodynamic forces due to the decrease of the unit weight of water from the downward acceleration of the reservoir bottom is included in the analysis.

Waves created at the free surface of the reservoir by an earthquake are considered of no importance. Based upon studies by Chopra[] 9] and Zienkiewicz 1 2 °1 it is TVA's judgment that before waves of any significant height have time to develop, the earthquake will be over. The duration of earthquake used in this analysis is in the range of 20 to 30 seconds.

Although accumulated silt on the reservoir bottom would dampen vertically traveling waves, the effect of silt on structures is not considered. There is only a sm...all am un.t Of silt.w present, and-theThe accumulation rate is slow, as measured by TVA for many years.[211 Embankment Embankment analysis was made using the standard slip circle method. The effect of the earthquake is taken into account by applying the appropriate static inertia force to the dam mass within the assumed slip circle (pseudo-static method).

In the analysis the embankment design constants used, including the shear strength of the materials in the dam and the foundation, are the same as those used in the original stability analysis.

Although detailed dynamic soil properties are not available, a value for seismic amplification through the soil has been assumed based on previous studies pertaining to TVA nuclear plants.

These studies have indicated maximum amplification values slightly in excess of two for a rather wide range of shear wave velocity to soil height ratios. For these analyses, a straight-line variation is used with an acceleration at the top of the embankment being two times the top of rock acceleration.

As discussed in Section 2.4.3, temporary flood barriers are installed on embankments at Cherokee, Watts Bar, Fort Loudoun and Tellico Reservoirs. However, the temporary flood barriers are not required to be stable following an OBE or SSE and are not assumed to increase the height of the embankments for these loading conditions.

Flood Routing The runoff model of Section 2.4.3.3 was used to reevaluate potentially five critical seismic events involving dam failures above the plant. The-ai.-nin-Other events addressed in earlier studies (the postulated OBE single failures of Watts Bar, the p.stulatedd Q OB failure of and Fort Loudoun,_

the p..v, lated SSE failurc ofifont.ianao and Douglas, the postulated SSE combination failure of Nefr4s Fontana and Douglas, and the postulated SSE combination failure of Fontana, Fort Loudoun, and Tellico, the SSE combination failure of Norris, Douglas, Fort Loudoun and Tellico, and the single SSE failure of Norris) produced plant site flood levels sufficiently lower than the controlling events and therefore were not re-evaluated.

2.4-35

WBNP-The procedures prescribed by Regulatory Guide 1.59 require seismic dam failure to be examined using the SSE coincident with the peak of the 25-year flood, and the OBE coincident with the peak of one-half the PMF.

Reservoir operating procedures used were those applicable to the season and flood inflows.

OBE Concurrent With One-Half the Probable Maximum Flood Watts Bar Dam Stability analyses of Watts Bar Dam powerhouse and spillway sections result in the judgment that these structures will not fail. The analyses show low stresses with about 381% of allin the spillway base, and abo-t 42% of the powerhouse basc cm es

. Resui4sOriginal results are given in Figure 2.4-68 and were not updated in the current analysis (historical inforation-).

Dynamic analysis of the concrete structures resulted in the determination that the base acceleration is amplified at levels above the base. This differ-s fromfl the previou.s analysis where amplificatien was no.t cnsidered.

glipThe original slip circle analysis of the earth embankment section results in a factor of safety of-l--.- greater than 1, and the embankment is judged not to fail. Results arc given in Figure 2.1 69 (histor-ical infcrmnation).

For the condition of peak discharge at the dam for one-half the probable maximum flood the spillway gates are in the wide-open position with the bottom of the gates above the water. This condition was not analyzed because the condition with bridge failure described in the following paragraphs produces the controlling condition.

Analysis of the bridge structure for forces resulting from the OBE, including amplification of acceleration results in the determination that the bridge could fail as a result of shearing the anchor bolts. The downstream bridge girders are assumed to strike the spillway gates. The impact of the girders striking the gates is assumed to fail the bolts which anchor the gate trunnions to the pier anchorages allowing the gates to fall on the spillway crest and be washed into the channel below the dam. The flow over the spillway crest would be the same as that prior to bridge and gate failure, i.e., peak discharge for one-half the probable maximum flood with gates in the wide-open position. Hence, bridge failure will cause no adverse effect on the flood.

A potentially severe -ond1ition is the OBEat the onset o-fhe main portion of one half the probable m-aximumfifl flooed floAwA into-Watts Bar. Re-servoir4 wlhen most spillway gates would be closed during bridge failure, as descrvibed above,.

The gate hoisting machiner-y inoper-able from being struck by the br~idge with the rcsuilt that the flood 'would wouldd er-estwth the gates closed and the bridge deck and girdcrs lying on top of the spill'way pier-s. Analysis of the concrete portionis of the dam for-the headwater-for-this condition sho'ws that the dam'will not fail.

For-the condition described above with the most preobable embankment br-eaching from overflow, the outtlo'w of Waifs Bar-Dam 'would increase rapidly, from about 200,000 cfs prior-to the br.each to about 660.000 cfs when br-eachine is complete. Br-each time 'would be aot 5 hou rs.

2.4-36

WBNP-The 660,000 cfs br-each flow is the cr-est. The flood level at Watts Bar Dam reached Elevation 717.5. Elevation at the plant site will be somewhat less, 'which is safely below plant grade E Ieavatioen 72 8. T h is f lood level was not rne evalu11ated using the-mo._del-de0-scr6ibed in Sectio 2.1.3.3, as amended, nor-as part of the 10998-reanalysis, because it is clearly5 not controlling.

For-flow coenditionls between the 25 year flood and one half the probable maximuma flood, 'when striking of the gates by the downistr-eam bridge gir-der-s will result in failuire of the gate liPftig chains. The gates will roetate to the closed position. This condition i lsssver-e than that descr~ibed above for-gates remaining closed dur-ing one half the probable maximum flood,-

consequently, the resulting flood levels were no.t dete.mined-.Previous evaluations determined that if the dam was postulated to fail from embankment overtopping in the most severe case (gate opening prevented by bridge failure) that the resulting elevations at Watts Bar Nuclear Plant would be several ft below plant grade elevation 728.0 ft. Therefore, this event was not reevaluated.

Fort Loudoun Dam Stability analyses of Fort Loudoun Dam powerhouse and spillway sections result in the judgment that these structures will not fail. The analyses show low base stresses, with near two-thirds of the base in compression. Resu4sThe original results are given in Figure 2.4-71 -(his.reeal i......na.tien) were not updated for the current analysis.

Slip circle analysis of the earth embankment results in a factor of safety of 1.26, and the embankment is judged not to fail. Results-aThe original results given in Figure 2.4-72 (histori-cal information) were not updated in the current analysis.

The spillway gates and bridge are of the same design as those at Watts Bar Dam. Conditions of failure during the OBE are the same, and no problems are likely. Coincident failure at Fort Loudoun and Watts Bar does not occur.

For the potentially critical case of Fort Loudoun bridge failure at the onset of the main portion of one-half the probable maximum flood flow into Fort Loudoun Reservoir, in an earlier analysis it was found that the Watts Bar inflows are much less than the condition resulting from simultaneous failure of Cherokee. and Douglas, and Tellico Dams as described later.

Tellico Dam "To nail Ot I'lie I6 m r~afi; 4k112;e1 tE) fil Re'mltS 6f the Stahilitx anlalVw' for-a tx'nicnl

erfio'w blck and a ty.pial spi.. way blok are shown in Fig.ure 2.1 73 (histoerial infoMatio The result o~f the stability, analysis of the earth emb-tankment is shown in Fiprfe 2.1 71 (histofr-info.mation) and indicates a factor of safety of 1 28.Although, not included in the original analyses for TVA's operating nuclear plants, Tellico is judged to fail completely because the seismic stability analysis of Tellico is not conclusive. No hydrologic results are given for the single failure of Tellico because the simultaneous failure of Tellico Dams with other dams discussed under multiple failures, is more critical.

n).

2.4-37

WBNP-Norris Dam Results of the Norris Dam stability analyses for-a typical spillway block and a typical nonn overflow section of maximum height are shown on Figure 2.4 75 (histor-ical informfation).

Because only a small per""entage of the spillway base is in cu mpr-ession, this structure is judged to fail. The high nonover-fiow sectionfl with -a small pcrceentage of the basei opeso and With high ecmprfessive an haIn stresIes iseas judged to fail.

Although an evaluation made in 1975 by Agbabian Associates concluded that Norris Dam would not fail in an OBE (with one-half PMF) or SSE (with 25-year flood), the original study postulated failure in both seismic events. To be consistent with prior studies, Norris was conservatively postulated to fail. Figure 2.4-76 (historical information) shows the 4kelypostulated condition of the dam after OBE failure. Based en stability analyses the nonERE oVerfo blck rIEmann inF place ar-ejudge'd to-withst-and the OBE. B-locks 3311 a4 judged to fail by evemi.

The location of the debris is not based on any calculated procedure of failure because it is believed that this is not possible. It is TVA's judgment, however, that the failure mode shown is one logical assumption; and, although there may be many other logical assumptions, the amount of channel obstruction would probably be about the same.

The discharge rating for this controlling, debris section was developed from a 1: 150 scale hydraulic model at the TVA Engineering Laboratory and was verified closely by mathematical analysis.

in the hydr-ologic r-outing fer-this failure, Melton Hill Dam was postulate-d to-fahilwh~en the flood wave r-eached headwater-Elevation 801, based on strcurle41mal -analysis. The headwater-at Watts Bar-Dam woAeuild reach Elevation 758.1, 8-9 feet be-low the top of the eai~h embanklnent of the main dam. Hova'ever-, the West Saddle Dike would be ovet~opped and br-eached. A complete washout of the dike was assumaed. The r-esuilting water-level at the nucllear-plant site is 721.5, 6.5 feet bele,,' 728 plant grade.No hydrologic results are given for the single failure of Norris Dam because the simultaneous failure of Norris and Tellico Dams, discussed under multiple failures, is more critical.

Cherokee Dam Results of the original Cherokee Dam stability analysis for a typical spillway block are shown in Figure 2.4-77 (histerical infermation). Based on this analysis teTh__e spillway is judged stable at the foundation base Wevatieelevation 900.0 ft. Analyses made for other elevations above f1eyationelevation 900.0 ft, but not shown in Figure 2.4-77 (histerical information), indicate the resultant of forces falls outside the base at E4evaienelevation 1010.0 ft. The spillway is assumed to fail at this elevation.

The non-overflow dam is embedded in fill to Elevationelevation 981.5 ft and is considered stable below that elevation. However, original stability analysis indicates failure will occur above the fill line.

2.4-38

WBNP-The powerhouse intake is massive and backed up by the powerhouse. Therefore, it is judged able to withstand the OBE without failure.

Results of the original analysis for the highest portion of the south embankment are shown on Figure 2.4-78 (historical informaticn). The analysis was made using the same shear strengths of material as were used in the original analysis and shows a factor of safety of 0.85. Therefore, the south embankment is assumed to fail during the OBE. Because the north embankment and saddle damnSaddle Dams 1, 2, and 3 are generally about one-half, or less, as high as the south embankment, they are judged to be stable for the OBE.

Figure 2.4-79 (historic*al information) shows the assumed condition of the dam after failure. All debris from failure of the concrete portion is assumed to be located downstream in the channel at elevations lower than the remaining portions of the dam and, therefore, will not obstruct flow.

No hydrologic results are given for the single failure of Cherokee Dam because the simultaneous failure of Cherokee. and-Douglas, and Tellico Dams discussed under multiple failures, is more critical.

Douglas Dam Results of the original Douglas Dam stability analysis for a typical spillway block are shown in Figure 2.4-80 (histor'ial inf.-matin). The upper part of the Douglas spillway is approximately 12 feet higher than Cherokee, but the amplification of the rock surface acceleration is the same.

Therefore, based on the Cherokee analysis, it is judged that the Douglas spillway will fail at Ele-atienelevation 937.0 ft, which corresponds to the assumed failure elevation of the Cherokee spillway.

The Douglas non-overflow dam is similar to that at Cherokee and is embedded in fill to Ele-atienelevation 927.5 ft. It is considered stable below that elevation. However, based on the Cherokee analysis, it is assumed to fail above the fill line. The abutment non-overflow blocks 1-5 and 29-35, being short blocks, are considered able to resist the OBE without failure.

The powerhouse intake is massive and backed up downstream by the powerhouse. Therefore, it is considered able to withstand the OBE without failure.

Results of the original analysis of the saddle-dafnSaddle Dam shown on Figure 2.4-81 -(hi*i*-iea infenatie;)

indicate a factor of safety of 1. Therefore, the saddle damSaddle Dam is considered to be stable for the OBE.

Figure 2.4-82 (hist.ri.al infe.matie.)

shows the portions of the dam judged to fail and the portions judged to remain. All debris from the failed portions is assumed to be located downstream in the channel at elevations lower than the remaining portions of the dam and, therefore, will not obstruct flow.

No hydrologic results are given for the single failure of Douglas Dam because the simultaneous failure of Cherokee. and-Douglas, and Tellico Dams as discussed later under multiple failures, is more critical.

2.4-39

WBNP-Fontana Dam Fontana Dam was assumf-ed-to fail in the OBE although no stability analysis wa'-s ma-de. FontanaB is a high dam eonstfctietd-with thrfe-e lengitudinal contr-action joints in the higher-blocks.

Although the joints are keyed and gr-outed, it is possible that the grouting was not ffilly effective.

Coensequently, there is some question as to how this stfmeur-e will r-espond to the mootion Of -a severe earhquake. To be e..se,.ative, ther`efore, it is assumed that Fontana Dam will not resist the OBE without failure.

The original hydrological analysis used a conservative seismic failure condition for Fontana Dam. A subsequent review which takes advantage of later earthquake stability analysis and dam safery modifications performed for the TVA DSP has defined a conservative but less restrictive seismic failure condition at Fontana. This subsequent review used a finite element model for the analysis and considered the maximum credible earthquake expected at the Fontana Dam site.

Figure 2.4-83 (histoer*i*al infomation) shows the part of Fontana Dam judged to remain in its original position after postulated failure. and the assum.ed location on the debris of the failed poi

n. The l

,eation f the debris after failu.r. e is one logieal assumptin based on a failu.r.e a the dam at the lengitudinal contr-aetien joints. There may be other-logical assumptions, but the amount of channel obstmcation would probably be about the same.

The higher blocks 9 to 27 e.ntaining either-two or-three longitudinal joint t ÷o fail.

Right abutment blocks 1 to 8 and left abutment blocks 28 and beyond wer-e judged to be stable for-the following r-easons:

Their-heights are less than one h~alf the maximumn height of the dam.

None o-f these blocks have moere than one logidialcntr-action joint, -and so-mle hlave no longitudinal joints.

The baek slope of Fontana Dam is 1 on 0.76 which the or-iginal stability analysis shows is flatter than that required for stability for the normal statie loading.

Although not investigated, it was assumned that Nantahala Dam upstr-eam from Fontana n Santeetlah on a dOwnstream tr-ibutary and the three ALCOA dams downstream on the Little Tennessee River would fail along with Fontana in the OBE. instant vanishment was assumfed.

T-ellio and Watts B3ar Dam spillway gates would be operable durring and after-the OBE. FailurIe ofthe bridge at Fo.t Loudoun Dam woul rendr. the spilla. gso able in the wide open The Fontana failure wave euld overtop and fa.+rI Tellieeo em-.1..bankments. Tr.ansfer of water. into Fort Loudoun would occur but would net be sufficient to ertop the dam or-to pr event failure ot Tellico. Tellico was postalated to completely fail. Watts Bar-headwater-wouild r-each Eilevationl 761.3, 5.7 feet below the top of the embanlun~ent. No embank~ment failure would occur However, the West Saddle Dike would be over1opped and breached. The elevation at the plant site..wo.d be 725.2, 2.8 feet bel.' 728 plant grade. No hydrologic results are given for the single failure of Fontana Dam because the simultaneous failure of Fontana and Tellico Dams, as discussed later under multiple failures, is more critical.

2.4-40

WBNP-Multiple Failures Attenuation studies of the 0OBEF -show that above Watts Bar-Dam onl), the sim-ult-AnmL2eou11s fai lur-es of Cherokee and Douglas Damis need be con. isider-ed with respect to Watts Bar-Nuclear, Plant safeg. These tvo dams are @nly 15 miles apart, and an OBE located Midway between them is a

tu e t cause thei o

failure. The degree of f.a.iluean likely position ofdebris abe judged to be ampareable ti that shown for single failure of these dams in Figures 2.4 79 and 2.4 n2 (both hiSteqieal infonuation).

The postulated simultaneous failures of CherFokee and Douglas Dam wtOld rCeach a mimu headwater elevation of 833.3 feet at Fort Loudon Dam, 0.55 foot above the top of the embankment. Forft Loudon would be overtopped for-only' about six hour-s to a maximum depth of 0.55 fseot. Brieaing analysis inditates that this srvertopping time and shallow overfflow depth wiuld not fail the. d

.fr Atough ans o

Tellico iwoue tulate simuld oeour, the maxmum headwater would only reach elevatien 826, whirah is four feet below top of dam. At Watts Bar Dam the headwater would reah Elevatin 758.2, 8.8 feet beleow the tep of the eanth embankW ent fdthe main dam. However-, the West Saddle Dike woutld be overopped and breached. A dcmplete washout of the dike was assumed. The elevation at the plant site woulA be 723.1, 1.9 feet below plant grade Elevation 728.Previous attenuation studies of the OBE above Watts Bar Dam result in the judgment that the following simultaneous failure combinations require reevaluation:

(1)

The Simultaneous Failure of Fontana and Tellico Dams in the OBE Coincident with One-Half PMF Figure 2.4-83 shows the postulated condition of Fontana for the OBE event. Tellico was conservatively postulated to completely fail.

The seismic failure scenario for Fontana and Tellico include postulated simultaneous and complete failure of non-TVA dams on the Little Tennessee River. Cheoah. Calderwood. and Chilhowee Dams and on its tributaries. Nantahala and Santeetlah Dams. Failure of the bridge at Fort Loudoun Dam would render the spillway gates inoperable in the wide open position. Watts Bar Dam spillway gates would be operable during and after the OBE.

Watts Bar Dam headwater would reach 756.13 ft. 13.87 ft below the top of the embankment.

The West Saddle Dike at Watts Bar Dam with top elevation of 757.0 ft would not be overtopped.

The peak discharge at Watts Bar Nuclear Plant would be 743,668 cfs. The elevation at Watts Bar Nuclear Plant would be 720.65 ft. 7.35 ft below plant grade elevation 728.0 ft.

(2)

The Simultaneous Failure of Fontana. Tellico. Hiwassee, Apalachia. and Blue Ridge Dams in the OBE Coincident with One-Half PMF Fontana. Tellico. Hiwassee. Apalachia and Blue Ridge Dams could fail when the OBE is located within a flattened oval-shap~ed area located between Fontana and Hiwassee Dams (Figure 2.4-112). Failure scenarios for Fontana. Tellico. Hiwassee. Ap~alachia, and Blue Ridge Dams include postulated simultaneous failure of non-TVA dams on the Little Tennessee River.

Cheoah. Calderwood and Chilhowee Dams and on its tributaries. Nantahala and Santeetlah Dams.

2.4-4 1

WBNP-Based on previous attenuation studies, the OBE event produces maximum ground accelerations of 0.09 g at Fontana, 0.09 a at Hiwassee, 0.07 g at Apalachia, 0.08 g at Chatuge, 0.05 g at Nottely, 0.03 g at Ocoee No. 1, 0.04 g at Blue Ridge, 0.04 g at Fort Loudoun and Tellico, and 0.03 g at Watts Bar. Figure 2.4-83 shows the postulated condition of Fontana Dam after failure.

Hiwassee, Apalachia, Blue Ridge, and Tellico Dams are postulated to completely fail. Chatuge Dam is judged not to fail in this defined OBE event.

Nottely Dam is a rock-fill dam with large central impervious rolled fill core. The maximum attenuated ground acceleration at Nottely in this event is only 0.054 g. A field exploration boring program and laboratory testing program of samples obtained in a field exploration was conducted. During the field exploration program, standard penetration test blow counts were obtained on both the embankment and its foundation materials. Both static and dynamic (cyclic) triaxial shear tests were made. The Newmark Method of Analysis utilizing the information obtained from the testing program was used to determine the structural stability of Nottely Dam.

It is concluded that Nottely Dam can resist the attenuated ground acceleration of 0.054 g with no detrimental damage.

Ocoee No. 1 Dam is a concrete gravity structure. The maximum attenuated ground acceleration is 0.03 g. Based on past experience of concrete dam structures under significantly higher seismic ground accelerations, the Ocoee No. 1 Dam is judged to remain stable following exposure to a 0.03 g base acceleration with amplification.

Ocoee No. 1 and Ocoee No. 3 Dams, downstream of Blue Ridge Dam, would be overtopped and were postulated to completely fail at their respective maximum headwater elevations. Ocoee No. 2 Dam has no reservoir storage and was not considered.

Fort Loudoun and Watts Bar spillways would remain operable. The Fontana failure wave would transfer water through the canal from Tellico into Fort Loudoun, but it would not be sufficient to overtop Fort Loudoun Dam. The maximum headwater at Fort Loudoun would reach elevation 817.13 ft, 19.87 ft below the top of the dam. Watts Bar headwater would reach elevation 756.13 ft, 13.87 ft below the top of dam. The West Saddle Dike at Watts Bar with a top elevation of 757.00 ft would not be overtopped.

The peak discharge at the Watts Bar Nuclear Plant site produced by the OBE failure of Fontana, Tellico, Hiwassee, Apalachia, and Blue Ridge coincident with the one-half PMF is 742,572 cfs.

The peak elevation is 722.01 ft, 5.99 ft below 728.0 ft plant grade.

(3)

The Simultaneous Failure of Norris and Tellico Dams in the OBE Coincident with One-Half PMF Figure 2.4-76 shows the postulated condition of Norris Dam for the OBE event. Tellico was conservatively postulated to completely fail in this event.

In the hydrologic routing for this failure, Melton Hill Dam would be overtopped and was postulated to fail when the flood wave reached headwater elevation 817.0 ft, based on the structural analysis and subsequent structural modifications performed at the dam as a result of the Dam Safety Program.

2.4-42

WBNP-The headwater at Watts Bar Dam would reach elevation 762.96 ft, 6.54 ft below top of dam. The West Saddle Dike at Watts Bar with top at elevation 757.0 ft would be overtopped and breached.

A complete washout of the dike was assumed. Chickamauga headwater would reach 701.05 ft, 4.95 ft below top of dam. The embankments at Nickajack Dam would be overtopped but was postulated not to breach which is conservative.

The peak discharge at the Watts Bar Nuclear Plant site produced by the OBE failure of Norris and Tellico Dams coincident with the one-half PMF is 917,284 cfs. The peak elevation is 728.67 ft, 0.67 ft above 728.0 ft plant grade.

(4)

The Simultaneous Failure of Cherokee, Douglas, and Tellico Dams in the OBE Coincident with One-Half PMF Figures 2.4-79 and 2.4-82 show the postulated condition after failure of Cherokee and Douglas Dams, respectively. Tellico was conservatively postulated to completely fail.

In the hydrological routing for these postulated failures, the headwater at Watts Bar Dam would reach elevation 763.1 ft, 6.9 ft below the top of the dam. The West Saddle Dike at Watts Bar with a top elevation of 757.0 ft would be overtopped and breached. A complete washout of the dike is assumed. Chickamauga Dam headwater would reach 702.95 ft, 3.05 ft below the top of the dam. The embankments at Nickajack Dam would be overtopped but were conservatively postulated not to breach.

The peak discharge at the Watts Bar Nuclear Plant site produced by the OBE failure of Cherokee, Douglas, and Tellico with the one-half PMF is 902,687 cfs. The peak elevation is 729.07 ft, 1.07 ft above 728.0 ft plant grade.

SSE Concurrent With 25-Year Flood The SSE will produce the same postulated failure of the Fort Loudoun and Watts Bar bridges as described for the OBE described earlier. The resulting flood level at the Watts Bar plant was not determined because the larger flood during the OBE makes that situation controlling.

Watts Bar Dam A reevaluation using the revised amplification factors was not made for Watts Bar Dam for SSE conditions. However, even if the dam is arbitrarily removed instantaneously, the level at the nuclear-plant site would be Elevation 723, 5 feet belew~ plant grade. This flood level was not r-eevaluated using the ;;]o;A model deScribed in Sectien 2.4.3.3 as amended because it is clearly

let eentrit4*g Watts Bar Nuclear plant based on previous analyses would be below elevation 728.0 ft plant grade.

2.4-43

WBNP-Fort Loudoun Dam Results of the original stability analysis for Fort Loudoun Dam are shown on Figure 2.4-86 (histoerical infomatin4). Because the resultant of forces falls outside the base, a portion of the spillway is judged to fail. Based on previous modes of failure for Cherokee and Douglas, the spillway is judged to fail above E4evationelevation 750.0 ft as well as the bridge supported by the spillway piers.

The results of the original slip circle analysis for the highest portion of the embankment are shown on Figure 2.4-87 (historic*al information). Because the factor of safety is less than one, the embankment is assumed to fail.

No analysis was made for the powerhouse under SSE. However, an analysis was made for the OBE with no water in the units, a condition believed to be an extremely remote occurrence during the OBE. Because the stresses were low and a large percentage of the base was in compression, it is considered that the addition of water in the units would be a stabilizing factor, and the powerhouse is judged not to fail.

Figure 2.4-88 (historical information) shows the condition of the dam after assumed failure. All debris from the failure of the concrete portions is assumed to be located in the channel below the failure elevations.

No hydrologic routing for the single failure of Fort Loudoun, including the bridge structure, is made because its simultaneous failure with other dams is considered as discussed later in this subparagraph.

Tellico Dam No Structural analysis was maade for-Tellico Dam failure in the SSE. Because of the similar-ity to Fert Leudoun, the spillway and entir-e embankment are judged te fail in a mane iilar-to Fer Louidoun. Figure 2.1 29 (historical inforemation) shows after-failure conditiefns with all debris assumed lecated in the chamicl below the failure elevation.

No hydrologic routing for the single failure of Tellico is made for the reasens given above fer F-e*t Lou because its simultaneous failure with other dams is more critical as discussed later in this sub-paragraph.

Norris Dam Under-SSE conditions blocks 31 to 45 (833 feet of length) are judged tofal Although an evaluation made in 1975 by Agbabian Associates concluded that Norris Dam would not fail in the SSE (with 25 year flood), Norris Dam was postulated to fail. The resulting debris downstream would occupy a greater span of the valley cross section than would the debris from the OBE but with the same top level, Elevationelevation 970.0 ft. Figure 2.4-90-(histeriea4 in-femation) shows the part of the dam judged to fail and the location and height of the resulting debris.

2.4-44

WBNP-This postulated single failurfe would r.esult in peak headwate, at Wa..s Bar-of

71*7.9, 9.1 feet.

below the top of the eai~h poffiens of the dami. Routing was net carried ffi~her-because it was evident that flood levels at the plant site would b-e cnieal lower-than for: the Norris failure The discharge rating for this controlling, debris section was developed from a 1: 150 scale hydraulic model at the TVA Engineering Laboratory and was verified closely by mathematical analysis. The somewhat more extensive debris in SSE failure restricts discharge slightly compared to OBE failure conditions.

No hvdrologic routing for the sinale failure of Norris was made because the simultaneous failure with Cherokee, Douglas and Tellico Dams, discussed under multiple failures, is more critical.

e, DORxs anx Co.nsidered separ-ately, the SSE will produce the same postulated failures of Cherokee, Douglas, and Fontana Dams as were descr-ibed for-the OBE. None of these single ffailures need tb carried downstr-eam, however-, becauise elevations would be lew~er-than the same failures in

.. ne.

half the probable maximum flood.

Cherokee The SSE is judged to produce the same postulated failure of Cherokee as was described for the OBE. The single failure does not need to be carried downstream because elevations would be lower than the same OBE failure in one-half the probable maximum flood.

Douglas The SSE is judged to produce the same postulated failure of Douglas as was described for the OBE. The single failure does not need to be carried downstream because elevations would be lower than the same OBE failure in one-half the probable maximum flood.

Multiple Failures Although consider-ed, as discussed in the following par-agraphs, TN'A believes that multiple damf.

failures are an extr-emely unlikely event. TVA's searceh of the literature r-eveals no recor-d ot failure of concr-ete dams from ead~hguake. The postulation of an SSE of 0. 18 g a~eeler-ation is a ver.y ense..ative upper im.it in itself in addition, theTVA considered the following multiple SSE dam failure combinations.

(5)

The Simultaneous Failure of Norris, Cherokee, Douglas and Tellico Dams in the SSE Coincident with 25-year Flood The SSE must be located in a very precise region to have the potential for multiple dam failures.

In order to fail Norris, Cherokee, and-Douglas, and Tellico-dams Dams, the epicenter of SSE must be confined to a relatively small area the shape of a football, about 10 miles wide and 20 miles long.

2.4-45

WBNP-Figure 2.4-91 shows the location of an SSE, and its attenuation, which produces 0.15 g at Norris, 0.09 g at Cherokee and Douglas, 0.08 g at Fort Loudoun and Tellico, 0.05 a at Fontana, and 0.03 g at Watts Bar. Fort Loudoun and Watts Bar have previously been judged not to fail for the OBE (0.09 M). The bridge at Fort Loudoun Dam, however, might fail under 0.08 4 forces, falling on any open gates and on gate hoisting machinery. Trunnion anchor bolts of open gates would fail and the gates would be washed downstream, leaving an open spillway. Closed gates could not be opened. By the time of the seismic event at upstream tributary dams the crest of the 25 year flood would likely have passed Fort Loudoun and flows would have been reduced to turbine capacity. Hence, spillway gates would be closed. As stated before, it is believed that multiple dam failure is extremely remote, and it seems reasonable to exclude Fontana on the basis of being the most distant in the cluster of dams under consideration. For the postulated failures of Norris, Cherokee, and Douglas the portions judged to remain and debris arrangements are as given in Figures 2.4-90, 2.4-79, and 2.4-82, respectively. Tellico is conservatively postulated to completely fail.

As discussed in Section 2.4.3, temporary flood barriers are installed on embankments at Fort Loudoun and Tellico Reservoirs. The temporary flood barriers are assumed to fail in the SSE and are thus not credited for increasing the height of the Fort Loudoun or Tellico Reservoirs embankments. The flood for this postulated failure combination would overtop and breach the south embankment and Marina Saddle Dam at Fort Loudoun. At Watts Bar Dam, the headwater would reach elevation 765.54 ft, 4.46 ft below the top of the earth embankment of the main dam.

However, the West Saddle Dike with top at elevation 757.0 ft would be overtopped and breached. The headwater at Chickamauga Dam would reach elevation 701.14 ft, 4.86 ft below top of dam. The embankments at Nickajack Dam would be overtopped but was conservatively postulated not to breach.

The maximum discharge at Watts Bar Nuclear Plant would be 979,385 cfs. The elevation at the plant site would be 731.17 ft, 3.17 ft above 728.0 ft plant grade. This is the highest flood elevation resulting from any combination of seismic events.

The flood elevation hydrograph at the plant site is shown on Figure 2.4-114.

In addition to the SSE failure combination of Norris, Cherokee, Douglas, and Tellico identified as the critical case, three other combinations were evaluated in earlier studies. These three originally analyzed combinations produced significantly lower elevations and were therefore not reevaluated.

In order to fail Norris, Douglas, Fort Loudoun, and Tellico d4ansDams, the epicenter of an SSE must be confined to a triangular area with sides of approximately one mile in length. However, as an extreme upper limit the above twe-combinations of dams a-r-eis postulated to fail as well as the combination of (1) Fontana, Fort Loudoun, and Tellico; and (2) Fontana and Douglas.

An SSE centered between Fontana and the Fort Loudoun-Tellico complex was postulated to fail these three dams. The four ALCOA dams downstream from Fontana and Nantahala, aria Duke Energy dam (formerly ALCOA-dainl upstream were also postulated to fail completely in this event. Watts Bar Dam and spillway gates would remain intact, but failure of the ro.adway bridge 2.4-46

WBNP-was postulated which would reender; the spillway ga inoperable. At the time of seismic failure, dischar.ges would be small in the 25 year-flood. For eenseatism, Watts Bar gates were assumed inoperable in the closed position after-the SSE event. Using the

falr mde show on Figures 2.1 83, 2.1 88, and 2.1 89 (all histsoreal inforamnation) for Fontana, Fr Loudoun, and Tellieso r-espectively, unsteady r-outing showed the failur-e wave overtoppling Watts Bar-Dam with r-esulting e-mb-anlumenHt failure. Initial Waifs Bar RembbanIenPt failure begins at headwater-level 763.0. Headwater-levels will continue to rise to Elevation 761.7 because of noe spill'way dischar-ge. This event would result in a flood level at the nuclear-plant site of 720.7, 7.3 feet below 728 plant gr-ade. This flood level was not reevaluated using the update Mnoff model descr-ibed in Section 2.4.3.3 as amended nor-as part of the 1998 r-eanalysis because it is clear-ly noteot-eeltflhng because previous analysis showed it was not control~ling.

(histo~efial infor-miationf) shows the location of an SSE, and its ateutowhich produces 0.15 g at Norris, 0.09 g at Cher-okee and Douglas, 0.08 g at Fort Loudoun and Telc,0.05 g-at Fontana, and 0.03 g at Watts Bar-. Fort Lo.doun, Telli.o, and Waifs Bar v previously been judged noet to f~ail for the OBE (0.09 g). The bridge at Fort Loudoun Damn, hoeemght fail under- 0.08 g forcees, faling on any, open gates and on gate hoisting mnachinerpy Taumnion anchor bolts of open gates would fail and the gates 'would be washed do'wnstream, leaving an open sp ill'way. Closed gates co ulId-no-t bhe opened. By thRe ti MiRe Of thie 'se0ism-ic Pe vent at upstream tributary dams the cr.est of the 25 year-floodwould likely have passed Fort LoudouIn and flows wouild have been r-educed to turbineaai.

Hence, spillway gates would be closed. At least this most e.nsen'.ativ. assmpio 'was used. As stated before, it is believed that multiple damm failure is extr-emely remote, and it seems r-easonable to exclude Fontana on the basis of being the most distant in the cluster-of dams under-consider-ation. For the postulated failures of Norris, C~heroklcee, and Douglas, the portions judged to remain and debris arranigemfents are as given in Figures 2.1 76, 2.1 "79, and 2.4 82 (all histor.ical info.mation) for. single dam failure.

The food for: the postu latted-f e ombination w vertop and breach Fo rtA Loud11oun Dam.

Although tr-ansfer-of 'water-into Tellico 'would occur-, the maximumn headwater-would Only r-each Elevation 820, which is 10 feet belo'w top of dam. At WaitftsBa Dam the headwater would reach Elevation 764.9, 2.1 feet belo.w the top of the earth embaffunent f.the main dam. However-, th. e West Saddle Dike 'would be oertoepped and br-eached. The elevation at the plant site would be 727.5, 0.5 feet below plant gr-ade Elevation 728.0. This is the highest flood r-esulting from any combination of seismic and flood events.

The flood elevation hydroegr-aph at the plant site is shown on Figure 2.1 11-1.

Norris, Douglas, Fort Loudoun, and Tellico Dams were postulated to fail simultaneously. Figure 2.4-93 (historical information) shows the location of an SSE, and its attenuation, which produces 0.12 g at Norris, 0.08 g at Douglas, 0.12 g at Fort Loudoun and Tellico, 0.07 g at Cherokee, 0.06 g at Fontana, and 0.04 g at Watts Bar. Cherokee is judged not to fail at 0.07 g; Watts Bar has previously been judged not to fail at 0.09 g; and, for the same reasons as given above, it seems reasonable to exclude Fontana in this failure combination. For the postulated failures of Norris, Douglas, Fort Loudoun, and Tellico, the portions judged to remain and the debris arrangements are as given in Figures-2--4762.4-90, 2.4-82, 2.4-88 and 2.4-89 (all histor.ical informf;ation) for 2.4-47

WBNP-single dam failure. For-this r. e evaluation, Fort Loudoun and Tellico were assumedpostulated to fail completely as the portions judged to remain are relatively small. This is consern.atiecombination was not reevaluated.

T-his no-4Stulated failure combination results in Watts Bar-head water-Elevation 75.9. 8.1 f eet

below the top of the earth e-mbhankIcment of the mnain dam. Hvowever, the West Saddle Dike would be evertopped and br-eaehed. A eomplete washout of the dike was assumed. The elevation at the plant site would be 722.8, 5.2 feet below plant gr-ade Elevation :728.0.

Douglas and Fontana Dams were postulated to fail simultaneously. Figure 2.4-94 (histerieal infeimatiei) shows the location of an SSE and its attenuation, which produces 0.14 g at Douglas, 0.09 g at Fontana, 0.07 g at Cherokee, 0.05 g at Norris, 0.06 g at Fort Loudoun and Tellico, and 0.03 g at Watts Bar. For the postulated failures of Douglas and Fontana Dams, the portions judged to remain and the debris arrangements for Douglas Dam are as given in Figures 2.4-82 and 2.4-83 (both histeo4ral informatie)*

for single dam failure. Fort Loudoun,-Te-li4e, and Watts Bar Dams have previously been judged not to fail for the OBE (0.09 g). The*brid.gea**,F4e Loui-do-un Dam, however-, mnight fail under- 0.06 g forces, falling on gates and on gate hoist ing bcaehinef.

Foirt Leudoun gates were assumed inoperable in the closed position following the SSE event. The Fontana failure flodwave would verokep and bre gac andh T

DAM ;di saUdle dikes. The flood fRom the Douglas failu wountild rach Fort Leudoun after-Tellic been ovetowpped and brreaced. Altheugh Fert L eudeun gates ev iuble in the closd position, the Fort Loudoun Tellico canal would proevide enouigh relief to keep Fort Loudoun Dam from being overtopped. The i

c mbined Douglas Fentana failure surigewould creach Elewation 751.7 at Watts Bar-Dam, 5.3 feet below dam top. Resulting water-sur-face at the Watts Bar-plant would r-each Elevation 721.2, 6.8 feet below plant gr-ade. This flood level was not r~eevaluated using the mo wmddell described i n

Se ction 2.

4.33.3.3 as amended nOr as pa Of the 1998 r-eanaly because it is not oantroinopuation ouflow failrenhi ombtion has not been evaluated but is bounded by the SSE failure of Norris. Cherokee. Douglas and Tellico.

2.4.4.2 Unsteady Flow Analysis of Potential Dam Failures Unsteady flow routing techniquest (Reference 23) were used to evaluate plant site flood levels from postulated seismically induced dam failures wherever their inherent accuracy was needed.

In addition to the flow models described in Section 2.4.3.3, the unsteady flow, models descr-ibed below were used as adjuncts to roeute floods from postulated dam failuresmodels described below were used to develop the outflow hydrographs from the postulated dam failures. The HEC-HMS storage routing was used to compute the outflow hydro graph from the postulated failure of each dam except main river dams. In the case of dams which were postulated to fail completely (Hiwassee, Apalachia and Blue Ridge), HEC-RAS or SOCH was used to develop the outflow hydrograph. For Tellico Dam, the complete failure was analyzed with the SOCH model.

Unsteady flow techniquies wer-e applied in NorrZis Resenvoir. The Norris Reservoir-moedel was developed in sufficie*..nt detail to define the manner in which the reservoir would supplya*,nd sustain outflowA following postulated dam failure. The model was ver-ified by compar-ing t roeuted headwater level in the one half PM4F with those using storaige routinig techniques-.

H4eadwfxfater-level agreed within a foot, and the moi-del] wIas,-

cosdrdadeqluate fOr the purpose.

2.4-48

WBNP-Unsteady flow techniques were also applied in Cherokee, Douglas, and Fontana Reservoir-s. The r-eservoir-models were developed in sufficient detail to define the manner-in which the r-esris would supply and sustain

.utflow following postulated dam failure.The failure time and initial reservoir elevations for each dam were determined from a pre-failure TRBROUTE analysis.

HEC-HMS was used to develop the post failure outflow hydrographs based on the previously determined dam failure rating curves. The outflow hydrographs were validated by comparing the HEC-HMS results with those generated by simulations using TRBROUTE.

2.4.4.3 Water Level at Plant Site The unsteady flow analyses of differ-en the five postulated combinations of seismic dam failures coincident with floods described in Section 2.1.1.! analyzed yields a maximum elevation of

7g-7-5731.17 ft; at Watts Bar Nuclear Plant excluding wind wave effects. As shown in Table 2.4- "The maximum elevation would result from the SSE failure of Norris, Cherokee, and Douglas, and Tellico Dams coincident with the twenty-five-25-year flood postulated to occur in June when reservoir levels are high. A June wind With 50-% eOceedan. e pr.. babi..,o*ver-the-1 1.3 mile effe.tive fetch is 12 miles per-hour-over.land. Flood waves, crest to tgh, are abot 1.0 foot high resuilting inmniu water-elevation of 728.2. Runup could r-each Elevation

729.0 on -A 3:.1 ea-Ah slope. The static and dyanaic effects of wind waves on stfuetur~es are described in Section 24,.3.6Table 2.4-14 provides a summary of flood elevations determined for the five failure combinations analyzed.

Coincident wind wave activity for the PMF is described in Section 2.4.3.6. Wind waves were not computed for the seismic events, but superimposed wind wave activity from guide specified two-year wind speed would result in water surface elevations several ft below the PMF elevation 739.2 ft described in section 2.4.3.

2.4.5 Probable Maximum Surge and Seiche Flooding Chickamauga Lake level during non-flood conditions would not exceed Elevatiofnelevation 682.5 ft, normal maximum pool level, for any significant time. No conceivable meteorological conditions could produce a seiche nor reservoir operations a surge which would reach plant grade Elevationelevation 728.0 ft, some 45 feet above normal maximum pool levels.

2.4.6 Probable Maximum Tsunami Flooding Because of its inland location the Watts Bar plant is not endangered by tsunami flooding.

2.4.7 Ice Effects Histe'ical hif*or'atie Because of its location in a temperate climate significant amounts of ice do not form on lakes and rivers in the plant vicinity and ice jams are not a source of major flooding.

The present potential for generator of significant surface ice at the site is less today than prior to closure of Chickamauga and Watts Bar Lakes in 1940 and 1942, respectively. This condition exists because of (1) daily water level fluctuations from operating Chickamauga Reservoir downstream and Watts Bar Reservoir upstream would break up surface icing before significant 2.4-49

WBNP-thickness could be formed, (2) flows are warmed by releases from near the bottom of Watts Bar Reservoir, and (3) increased water depths due to Chickamauga Reservoir result in a greater mass needing to be cooled by radiation compared to pre-reservoir conditions.

After closure of Watts Bar in January 1942, there have been no extended periods of cold weather and no serious icing conditions in the Watts Bar Nuclear Plant site region. On several occasions, ice has formed near the shore and across protected inlets but has not constituted a problem on the main reservoirs.

The lowest water temperature observed in Watts Bar Lake at the dam during the periods 1942-1953, and June 1967 to November 1973 for which records were kept, was 39 degrees on January 30, 1970, the coldest January since 1940 in the eastern part of the Basin. This lake temperature is indicative of the lowest water temperature released from Watts Bar Lake during winter months.

The most severe period of cold weather recorded in the Valley was January and early February 1940 prior to present lake conditions at the plant site. A maximum ice depth of five inches was recorded on the Tennessee River at Chattanooga. There were no ice jams except one small one on the lower French Broad River.

Records of icing are limited and none are available at the site prior to 1942. From newspaper records, the earliest known freeze in the vicinity was at Knoxville in 1796. More recently, newspaper accounts and U.S. Weather Bureau records for Knoxville provide a fairly complete ice history from 1840 to 1940. At Knoxville; the Tennessee River was frozen over 16 times, and floating ice was observed six other times.

The most severe event in this period prior to 1940 was in December-January 1917-18 when ice jammed the Tennessee River at Knoxville for 1 to 2 weeks, reaching 10 feet high at some places.

In late January rain and temperature rise produced flooding on the Clinch River referred to by local people as the "ice tide." There is no record of ice jamming, however.

There are no safety-related facilities at the Watts Bar site which could be affected by an ice jam flood, wind-drive ice ridges, or ice-produced forces other than a flooding of the plant itself. An ice jam sufficient to cause plant flooding is inconceivable. There are no valley restrictions in the 1.9-mile reach below Watts Bar Dam to initiate a jam, and an ice dam would need to reach at least 68 feet above streambed to endanger the plant.

Intake pump suctions which will be used for the intake of river water will be located a minimum of 7.6 feet below minimum reservoir water level; hence, no thin surface ice which may form will effect the pipe intake. In the assumed event of complete failure of Chickamauga Dam downstream, the minimum release from Watts Bar Dam will ensure a 5.9 feet depth of water in the intake channel.

2.4.8 Cooling Water Canals and Reservoirs Hi..torial Infoma*ti, The intake channel, as shown in Figure 2.1-5, extends approximately 800 feet from the edge of the reservoir through the flood plain to the intake pumping statinlntake Pumping Station.

2.4-50

WBNP-The channel, as shown in Figure 2.4-99, has an average depth of 36 feet and is 50 feet wide at the bottom. The side slopes are 4 on 1 and are designed for sudden drawdown, due to assumed loss of downstream dam, coincident with a safe shutdown earthquake.

In response to multipurpose operations, the level of Chickamauga Reservoir fluctuates between a normal minimum of 675.0 feet and a normal maximum of 682.5 feet. The minimum average elevation of the reservoir bottom at the intake channel is 656 feet and the elevation of the intake channel bottom is 660 feet. The 15 feet normal minimum depth of water provided in the intake channel is more than ample to guarantee flow requirements. The intake provides cooling water makeup to the closed-cycle cooling system and the ERCW Systemessential raw cooling water systems. The maximum flow requirement for the plant for all purposes is 178 cfs based on four ERCW pumps and six RCW pumps inservice.

The protection of the intake channel slopes from wind-wave activity is afforded by the placement of riprap, shown in Figures 2.4-99 in accordance with TVA design standards, from Ele;'atienelevation 660.0 ft to Elevation elevation 690.0 ft. The riprap is designed for waves resulting from a wind velocity of 50 mph.

2.4.9 Channel Diversions Histori-al Information Channel diversion is not a potential problem for the plant. Currently, no channel diversions upstream of the Watts Bar plant would cause diverting or rerouting of the source of plant cooling water, and none are anticipated in the future. The feed plainfloodplain is such that large floods do not produce major channel meanders or cutoffs. The topography is such that only an unimaginable catastrophic event could result in flow diversion above the plant.

2.4.10 Flooding Protection Requirements Historical nfeormatin Assurance that safety-related facilities are capable of surviving all possible flood conditions is provided by the discussions given in Sections 2.4.14, 3.4, 3.8.1, 3.8.2 and 3.8.4.

The plant is designed to shut down and remain in a safe shutdown condition for any rainfall flood exceeding plant grade, up to the "design basis flood" discussed in Section 2.4.3 and for lower, seismic-caused floods discussed in Section 2.4.4. Any rainfall flood exceeding plant grade will be predicted at least 28 hours3.240741e-4 days 0.00778 hours 4.62963e-5 weeks 1.0654e-5 months in advance by TVA's Water ManagementRiver Operations (RO) organization.

Notification of seismic failure of key upstream dams will be available at the plant approximately 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months before a resulting flood surge would reach plant grade. Hence, there is adequate time to prepare the plant for any flood.

See Section 2.4.14 for a detailed presentation of the flood protection plan.

2.4.11 Low Water Considerations Histori.al Infoerm aio Because of its location on Chickamauga Reservoir, maintaining minimum water levels at the Watts Bar plant is not a problem. The high rainfall and runoff of the watershed and the regulation afforded by upstream dams assure minimum flows for plant cooling.

2.4-51

WBNP-2.4.11.1 Low Flow in Rivers and Streams The probable minimum water level at the Watts Bar plant is Elevation 6-3levation 675.0 ft and would occur in the winter flood season as a result of speeial-Chickamauga Reservoir pFefeed drawdown, at which time flows would* he-* ubstantialoperation. The most severe drought in the history of the Tennessee Valley region occurred in 1925. Frequency studies for the 1874-1935 period prior to regulation show that there is less than one percent change that the 1925 observed minimum tone-day flow of 3300 cfs downstream at Chattanooga might occur in a given year.

At the plant site the corresponding minimum tone-day flow is estimated to be 2700 cfs compar-ed to 2600 efs in the PSAR.

Although dependable flo'w uinder-extr-eme dr-ought conditions is sufficient to meet all plan rqieents, there is the added assur-ance of large quantities of 'water-in TVIA's Multiple purpose tributar r.eseroirs upstr.eam. Stored water at p-r e sc.ribed-m

.inimum.

pool levels in the-*se r-eservoir-s (Tellico ReservoAi-r eaxcluded) could proevid-e me-Are than 1,000 cfs at the Watts Bar-site for-2 years with noe r-ainfall. These minimum levels 'will not be violated withouit specific TVA Board-ofDir-ectors' action in which the safeat' of Watts Bar woud e -A confitroelling consider-ation.

This ýxar-antees that adeqiuate 'water-would be available if needed at the Waifs Bar-site.

In the assumed event of complete failure of Chickamauga Dam and with the headwater before failure assumed to be the normal summer level, Elevatienelevation 682.5 ft, the water surface at the-siteWatts Bar Nuclear Plant will begin to drop 43 hours4.976852e-4 days 0.0119 hours 7.109788e-5 weeks 1.63615e-5 months after failure of the dam and will fall at a fairly uniform rate to Elevationelevation 666.0 ft in approximately -22_7 hours from failure.

This time period is more than ample for initiating the release of water from Watts Bar Dam.

The estimated minimum flow requirement for the ERCW System is 50 cfs; however, in order to guarantee both ample depth and supply of water, a minimum flow of 20003,200 cfs will be released from Watts Bar Dam. With flow of 2*O3,200 cfs water surface elevation would be 665.9 ft producing 5.9-feet depth in the intake channel.

2.4.11.2 Low Water Resulting From Surges, Seiches, or Tsunami Because of Watts Bar's inland location on a relatively small, narrow lake, low water levels resulting from surges, seiches, or tsunamis are not a potential problem.

2.4.11.3 Historical Low Water From the beginning of stream gage records at Chattanooga in 1874 until the closure of Chickamauga Dam in January 1940, the estimated minimum daily flow at Watts Bar Nuclear Plant site was 2700 cfs on September 7 and 13, 1925. The next lowest estimated flow of 3900 cfs occurred in 1881 and also in 1883.

Since January 1942 low flows at the site have been regulated by TVA reservoirs, particularly by Watts Bar and Chickamauga Dams. Under normal operating conditions, there may be periods of several hours daily when there are no releases from either or both dams, but average daily flows at the site have been less than 5,000 cfs only 49about 2.2% of the time and have been less than 10,000 cfs eony 4 %about 10.4% of the time.

2.4-52

WBNP-On March 30 and 31, 1968, during special operations for the control of waten-'rnilfbilwater milfoil, there were no releases from either Watts Bar or Chickamauga Dams during the -two-day period. Daily average releases cfzero have been recorded on four other occasions during the past-25-y5,efsOver the last 25 years (1986 - 2010) the number of zero flow days at Watts Bar and Chickamauga Dams have been 0 and 2, respectively.

Since January 1940, water levels at the plant have been controlled by Chickamauga Reseweir.

Sinee-thenDam. For the period (1940 - 2010), the minimum level at the dam was 673.3 ft on January 21, 1942.

2.4.11.4 Future Control Future added controls which could alter low flow conditions at the plant are not anticipated because no sites that would have a significant influence remain to be developed. However, any control that might be considered would be evaluated before implementation.

2.4.11.5 Plant Requirements The Engineering Safety Feature System water supply requiring river water is the ERGWEssential Raw Cooling Water (ERCW). Also, the high pressure fire pumps perform an essential safety function during flood conditions by providing a feedwater supply to steam generators, makeup to the spent fuel pool, and auxiliary boration makeup tank. For interface of the fire protection system with the Auxiliary Feedwater System, see Section 10.4.9. The ERCW pumps are located on the intake pumping stationlntake Pumping Station deck at Elevatienelevation 741.0 ft and the ERCW pump intake is at Elevatienelevation 653.33 feet. The ERCW intake will require 5 feet of submergence. Based on a minimum river surface elevation of 665.9 feet, a minimum of 12.0712.57 feet of pump suction submergence will be provided.

In the assumed event of complete failure of Chickamauga Dam and with the headwater before failure assumed to be the normal summer level, Elevatienelevation 682.5 ft, the water surface at the site will begin to drop 43 hours4.976852e-4 days 0.0119 hours 7.109788e-5 weeks 1.63615e-5 months after failure of the dam and will fall at a fairly uniform rate to Elevationelevation 666.0 ft in approximately 22-27 hours from failure. This time period is more than ample for initiating the release of water from Watts Bar Dam.

The estimated minimum flow requirement for the ERCW System is 50 cfs. However, in order to guarantee both ample depth and supply of water, a minimum flow of 2,000 efs wi143,200 cfs can be released from Watts Bar Dam. This flow will give a river surface elevation of 665.9 ft, which ensures a 5.9-feet depth of water in the intake channel and approximately 10 feet in the river.

The river-sur-facze elevation is controalled by the wveir-efffect-o-f H4unterfl Shoals, Elevation 661.2, located appr-eximnately 7.5 miles downstream from the site. The stage dischar-gc ratinig cutn'e at the entr.ance to the intake channel is shown by Figre 2.1 95. Coss sections of Hunter-Shoals

.a.re.sh.

in Figure 2.1 96. Fi...

2.1 97 shows the channel profile of the Tenessee River fho the r-1neh frm i*mie 9200 tn !21 32"7 2.4-53

WBNP-A flow of at least 2,WO03.200 cfs can be released at the upstream dam, Watts Bar Dam, through the spillway gates, the turbines or the lock. The spillway gates offer the largest flow of water.

There are twenty 40-feet-wide radial gates operated by two traveling gate hoists on the deck and one of the hoists is always located over a gate. At minimum headwater Elevation levation 735.0 ft, one gate opened 2 fet will provide a flow of 2,000 cfs;, full) open, 15,000 will be p..evded there are several gate arrangements that could be used to supply the minimum 3,200 cfs flow.

There are five turbines, each with a maximum flow of 9,400 cfs and an estimated speed/non-load flow of 900 to 1100 cfs. The lock culvert emptying and filling valves are electrically operated segmental type with a bypass switch located in each of the four valve control stations. These can be used at any time to open or close both filling and emptying valves.

In the improbable event of loss of station service power at the dam, a 300-kVA gasoline-engine-driven generator located in the powerhouse will supply emergency power. The generator feeds into the main board when used and the emergency power is adequate to operate each of the three sources of water supply discussed.

For concurrent loss oflhe upstream and downstream dams, assurance that sufficient flow will be available is provided by record of the miiu n... ra flow at the plantsite befiere eonst tion of dams on the Tennessee P4iver-. This flow is estimated to be 2,700 efs. Since this flo, exceeds the 2,000 efs specified above to be released throeugh Watts Bar-Dam, it is net neeessapf to reser*e a sterage voelume in Wads Bar Reer-evir review of the estimated low flows for the period 1903 - 2010 on the basin above Watts Bar Dam which shows that the 15 day, 30 day, 50 day, and 100 day sustained low flow would be 2907 cfs, 3158 cfs, 3473 cfs, and 4012 cfs, respectively. If additional flow is needed to supply the minimum 3,200 cfs it could be supplemented by use of upstream reservoir storage.

2.4.12 Dispersion, Dilution, and Travel Times of Accidental Releases of Liquid Effluents 2.4.12.1 Radioactive Liquid Wastes A discussion of the routine handling and release of liquid radioactive wastes is found in Section 11.2, "Liquid Waste Management Systems." The routine and nonroutine nonradiologial liquid discharges are addressed in the Watts Bar Nuclear Plant's NPDES permit (Permit No.

TNO020168) and the Spill, Prevention, Control, and Countermeasure Plan (SPCC plan),

respectively. The nonradiological liquid discharges are under the regulatory jurisdiction of the State of Tennessee.

2.4.12.2 Accidental Slug Releases to Surface Water Hi. teria!e

,,fo..atn An accidental release of radioactive or nonradioactive liquid from the plant site would be subject to naturally induced mixing in the Tennessee River. The worst case for a given volume, V, (cubic feet), of liquid is a release which takes place over a short period of time. Calculations have been made to determine the reduction in concentration of such a release as it progresses downstream; particular emphasis has been placed on the concentrations at the surface water 2.4-54

WBNP-intakes downstream of the plant. The model used here is based on the convective diffusion equation as applied to the dispersion in natural streams[24'251. The major assumptions used in this analysis are:

I.

The release is assumed to occur at the right bank with no diffuser induced mixing whether the release occurs at the bank or through the diffuser.

2.

The effluent becomes well mixed vertically (but not horizontally) relatively rapidly (well before reaching first downstream water intake). This assumption is usually justified in riverine situations. [26,271

3.

The river flow is uniform and one-dimensional over a rectangular cross-section.

Other less restrictive assumptions are described in Reference [27].

Under assumption 2, the two-dimensional form of the convective diffusion equation is sufficient and may be written as ac ac a2c a~c

&c + u-* a EXX2c + E(1) a Xux2 Y

5 y2 in which C is the concentration of radioactive effluent in the river; u is cross-sectionally averaged river velocity; x and y are coordinates in the downstream and lateral directions, respectively; and Ex and Ey are the dispersion coefficients in the x and y directions. Following Reference [25], it is assumed that the formal dependence of Ex and Ey on river parameters is EX= axU*H (2a) and Ey= ayU*H (2b) in which ax and ay are empirical coefficients, U* is the river shear velocity, and H is the river depth. Relationships between U* and bulk river parameters may be found in any open channel hydraulics text.[281 Equation (1) was solved for the slug release by applying the method of images[2 7,29] to the instantaneous infinite flow field solution of equation (1) which is given in Reference [29]

[(x-ut) 2 (Y-YO) 2 C

V0 exp

[ 4Ext 4Eyt ]

(3)

Co 4it Ht Ex Ey in which Co is the initial concentration of radioactive material in the liquid effluent, t is the time elapsed since the release of the slug and y is the distance of the release from the right bank.

Equation (3) was used in the method of images solutions.

2.4-55

WBNP-2.4.12.2.1 Calculations gitoial iformanio The above model was applied to predict the maximum concentrations which would be observed on the right bank of the Tennessee River at two downstream locations; the right bank concentrations will always be higher than those on the left bank. The release is assumed to occur on the right bank at Tennessee River Mile (TRM) 528; the river width is assumed constant at 1,100 feet and the river depth is assumed constant at 30 feet. The Watts Bar Dam discharge equaled or exceeded 50% of the time is 28,200 cfs.

The coefficients ax and ay in Equation (2) were chosen to be 100 and 0.6, respectively; these values are based on the results in Reference [25]. The shear velocity, U* was computed assuming a Manning's n of 0.030 to describe the bed roughness of the river. Because the actual release volume, VO, is not known a priori, results are presented in terms of a relative concentration defined as C/(Co,Vo). Thus, to obtain the concentration reduction factor C/C0, this relative concentration must be multiplied by the release volume V0 (in cubic feet).

Calculations show that the concentrations along the right bank at the downstream water intakes will be as follows:

Relative Tennessee Concentration Water Intake River Mile (l/cu, bic ft-)

Dayton 503.8 2.8 x 10-9 East Side Utility 473.0 1.3 x 109 (formerly Volunteer Army Ammunition Plant) 2.4.12.3 Effects on Ground Water Historical nfbrmatio:n The plant site is underlain by terrace deposits of gravel, sand, and clay, having an average thickness of 40 feet. The deposit is variable in grain-size composition from place to place.

Locally, very permeable gravel is present. Essentially all of the ground water under the site is in this deposit.

Bedrock of the Conasauga Shale underlies the terrace deposit. Foundation exploration drilling and foundation excavation revealed that very little water occurs in the bedrock. The average saturated thickness of the terrace deposit is about 25 feet. Discharge from this material is mostly small springs and seeps to drainways along the margin of the site. Directions of ground water flow are discussed in Section 2.4.13.

The nearest point of probable ground water discharge is along a small tributary to Yellow Creek, which at its nearest point is 2,600 feet from the center of the plant. In this direction, the hydraulic gradient (dh/dl) is 26 feet (maximum) in 2,600 feet, or 0.01. The hydraulic conductivity (K) of the terrace materials is estimated to be 48 feet/day. (The basis for this estimate is described in Section 2.4.13.3.) Porosity (0) is estimated to be 0.15.

Average ground water velocity = (K dh/dl)/O = 3.2 ft/day or 812 days average travel time through the terrace deposit to the nearest point of ground water discharge.

2.4-56

WBNP-Estimating the density of the water-bearing material to be 2.0 and the distribution coefficient for strontium to be 20, the computed average travel time for strontium indicates a period of over 200 times longer than that for water, or 1.8 x 105 days (almost 500 years) travel time from the plant site to the nearest point of ground water discharge. This time of travel would be further increased by accounting for the delay resulting from movement through and absorption by unsaturated materials above the water table.

Water available for dilution, based on-the estimated porosity of 0.15 and a saturated thickness of 25 feet, is estimated to be 3.75 cubic feet per square feet of surface area. In a 1000-feet wide strip extending from the plant site to the nearest point of ground water discharge, the volume of stored water would be 9.8 x 106 cubic feet.

There are no data on which to base a computation of dispersion in the ground water system. For a conservative analysis, it would be necessary to assume that no dispersion occurs.

2.4.13 Groundwater 2.4.13.1 Description and On-Site Use Histeorial Inf-.....on Only the Knox Dolomite is regionally significant as an aquifer. This formation is the principal source of base flow to streams of the region. Large springs, such as Ward Spring 2.7 miles west of the site, are fairly common, especially at or near the contact between the Knox Dolomite and the overlying Chickamauga Limestone. Water occurs in the Knox Dolomite in solution openings formed along bedding planes and joints and in the moderately thick to thick cherty clay overburden. The formation underlies a 1-one-mile to 2-two-mile wide belt 2.5 miles west of the site at its nearest point; a narrow slice, the tip of which is about one mile north of the site; and a 1-one-mile to 2two-mile wide belt, one mile east of the site and across Chickamauga Lake.

Within a two-mile radius of the site, there is no use of the Knox Dolomite as a source of water to wells for other than small supplies.

Other formations within the site region, described in detail in Section 2.5.1.1, include the Rome Formation, a poor water-bearing formation; the Conasauga Shale, a poor water-bearing formation; and the Chickamauga Limestone, a poor to moderate water-bearing formation that normally yields no more than 25 gallons per minute (gpm) to wells.

The plant site is underlain by the Conasauga Shale, which is made up of about 84% shale and 16% limestone and occurs as thin discontinuous beds (Section 2.5.1.2). SurfaeialSurficial materials are older terrace deposits and recent alluvial deposits, fine-grained, poorly sorted, and poorly waterbearing.

The pattern of groundwater movement shown on Figure 2.4-105 indicates that recharge of the shallow water-bearing formations occurs from infiltration of local precipitation and from lateral under-flewunderflow from the area north of the plant site. All ground-water discharge from the site is to Chickamauga Lake, either directly or via Yellow Creek.

Potable water for plant use is obtained from the Watts Bar Utility District. Their water is obtained from 3 wells located 2.5 miles northwest of the plant.

2.4-57

WBNP-2.4.13.2 Sources Histý,i-cal Informati.n Ground water sources within a two-mile radius of the site are listed in Table.a4-4-42.4-15 and their locations are shown on Figure 2.4-102. Of the 89 wells listed, only 58 are equipped with pumps. Two of the thirteen spring sources listed are equipped with pumps. Seventy-nine residences are supplied by ground water, with one well supplying five houses. Assuming three persons per residence and a per capita use rate of 75 gpd, total ground-water use is less than 10,000 gpd.

Draw downDrawdown data are available only for the Watts Bar Reservation wells, as listed in the previous section.

Water-level fluctuations have been observed monthly in six observation wells since January 1973. Data collection for wells 7, 8, & 9 began in December 1981. The locations of these wells are shown on Figure 2.4-104. Data for the period January 1973 through December 1975 is shown on Figure 2.4-103.

As elsewhere in the region, water levels normally reach maximum elevations in February or March and are at minimum elevations in late summer and early fall. Depth to the water table is generally less than 20 feet throughout the plant site.

Figure 2.4-105 is a water-table contour map of the area within a two-mile radius of the plant site, based on 48 water-level measurements made in January 1972. The water table conforms fairly closely to surface topography, so that directions of ground-water movement are generally the same as those of surface-water movement. The water-table gradient between plant site and Chickamauga Lake at maximum water-table elevation and minimum river stage is about 44 feet in 3200 feet, or 0.014.

Water occurs in the Consauga Shale in very small openings along fractures and bedding planes.

Examination of records of 5500 feet of foundation exploration drilling showed only one cavity, 0.6-feet thick, penetrated.

Water occurs in the terrace deposit material in pore spaces between particles. The deposit is composed mostly of poorly-sorted clay-to gravel-sized particles and is poorly water bearing, although an approximately six-feet-thick permeable gravel zone is locally present at the base of the terrace deposit. The foundation excavation required only intermittent dewatering after initial drainage. The excavation was taken below the base of the terrace deposit into fresh shale. No weathered shale was found to be present; the contact between the terrace deposit and fresh shale is sharp.

The average depth to the water table in the plant area, based on data collected during August through December 1970, is 17 feet; the average overburden thickness is 40 feet; the saturated overburden thickness is therefore, 24 feet. No weathered zones or cavities were penetrated in the Conasauga Shale below a depth of 85 feet, so that the average saturated thickness of bedrock is assumed to be less than 50 feet.

2.4-58

WBNP-The plant site is hydraulically isolated by Yellow Creek and Chickamauga lake to the west, south, and east; it is hydraulically isolated to the north by the relatively impermeable Rome Formation underlying the site. Therefore, it is believed that any off-site groundwater withdrawals could not result in altered groundwater movement at the site.

No attempt was made to measure hydraulic properties of overburden or of bedrock at this site because of the very limited occurrence of ground water and the heterogeneity and anisotropy of the materials underlying the site.

2.4.13.3 Accident Effects Hit.o-ria! I;nforation Assuming a maximum annual range in saturated thickness of overburden of between 23 feet and 33 feet, and a porosity of 0.15, total water stored in this material, and the maximum volume available for dilution, ranges seasonally between 4.6 and 6.6 cubic feet per square feet of surface area. Water available for dilution in bedrock is very small and may be less than 0.01 cubic feet per square feet of surface area.

Since dispersion and exchange characteristics are not known, it must be assumed that these are not factors in a release of liquid radioactive material which would then travel to discharge points at the same rate as water movement. There are no direct pathways to ground-water users since all groundwater discharge from the site is to adjacent surface-water bodies.

Groundwater travel time has been estimated for water in the terrace deposit, in which essentially all ground water at the site occurs.

The nearest point of possible groundwater discharge is 2600 feet west of the plant site, along a tributary to Yellow Creek. In this direction the maximum hydraulic gradient is 26 feet in 2600 feet, or 0.01. The maximum hydraulic conductivity of the terrace materials is estimated to be 48 ft/day, based on particle-size analyses of terrace-deposit materials as related to permeability.[30 1 Kdh/dl 17 0

where v

=

mean velocity, ft/ day; K = hydraulic conductivity = 48 ft/ day; dh/dl

=

hydraulicgradient =.01 O = porosity

=

0.15 (estimated average effective)

(.0 1) v = 48 3.2 ft/day

(.15) or 812 days travel time from plant to nearest point of groundwater discharge.

Packer tests on the Conasauga Shale in foundation holes, using water at 50 psi, showed no acceptance, although one 0.6 feet cavity was penetrated in one hole in a total of more than 5,000 feet of drilling. Therefore, no estimate of time of water travel was made for water in bedrock.

2.4-59

WBNP-2.4.13.4 Monitoring and Safeguard Requirements Historia,i.format,-in The potential for the plant to affect groundwater users is very low because of its physical location, however, any provisions for radiological groundwater monitoring will be as described in the Watts Bar OQD4MMonitoring Plan. A network of observation wells will be maintained as needed and ground water will be analyzed for radioactivity as required by the Technical Specifications.

In the event of accidental release of radioactivity to the groundwater system, nearby groundwater users will be advised not to use their wells for drinking water until an investigation can be made of the extent, rate, and direction of movement of the contaminant.

Monitoring and notification for both the routine and any accidental nonradioactive liquid discharges to either surface or groundwaters would be implemented as required by the facilities NPDES permit (Permit No. TNO020168) and the Spill, Prevention, Control, and Countermeasure Plan (SPCC plan), respectively. These requirements for the nonradiological liquid discharges are under the regulatory jurisdiction of the State of Tennessee.

2.4.13.5 Design Basis for Subsurface Hydrostatic Loading Histori*al Information The ground water levels used for structural design are discussed in Section 2.5.4.6.

Dewatering of the construction excavation is discussed in Section 2.5.4.6.

2.4.14 Flooding Protection Requirements ASSUFMnGe that safeaty related facilities are capable of sunviving all possible flood condition~sThe plant grade elevation at WBN can be exceeded by large rainfall and seismically-induced dam failure floods. Assurance that WBN can be safely shut down and maintained in these extreme flood conditions (Section 2.4.2.2 and this Section 2.4.14) is provided by the discussions given in Section 2.4.2.2, Se.tion 3.1, Secti.ns 3.8.1 and 3.8.4 and this section, 2.44.lSections 3.4. 3.8.1.

and 3.8.4.

2.4.14.1 Introduction This subsection describes the methods by which the Watts Bar Nuclear P!antWBN is capable of tolerating floods above plant grade without jeopardizing public safety. Since flooding of this magnitude, as illustrated in Seetien-42Sections 2.4.2 and 2.4.4 is most unlikely, extreme steps are considered acceptable, including actions that create or allow extensive economic consequence to the plant. The actions described herein will be implemented for floods ranging from slightly below plant grade, to allow for wave runup to the design basis flood. The plant Flood Protection Plan (Technical Requirement 3.7.2) specifies the flood warning conditions and subsequent actions.

2.4-60

WBNP-2.4.14.1.1 Design Basis Flood The design basis flood (DBF) is the calculated upper-limit flood that includes the PMF plus the wave runup caused by a 21 mph overland wind; this is discussed in Section 2.4.3.6. The table below gives representative levels of the DBF at different plant locations.

Design Basis Flood (DBF) Levels Probable Maximum Flood (still reservoir) 734.9739.2 ft DrF An lake 736.2 DBF Runup on 4:1 sloped surfaces

746.9741.6 ft DBF Runup on critical vertical walls with base Elevation 728.0 of the 736.9741.7 ft Intake Pumping Station DBF Surge level within flooded structures 357.4739.7 ft In addition to flood level considerations, plant flood preparations cope with the "fastest rising" flood which is the calculated flood, including seismically induced floods, that can exceed plant grade with the shortest warning time. Reservoir levels for large rainfall floods in the Tennessee Valley can be predicted well in advance. By dividing the pre-flood preparation steps into two stages, a minimum of a 27 hour3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months , pre-flood transition interval is available between the time a flood warning is received and the time the flood waters exceed plant grade. The first stage, a minimum of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months long, commences upon receipt of a flood warning. The second stage, a minimum of 17 hours1.967593e-4 days 0.00472 hours 2.810847e-5 weeks 6.4685e-6 months long, is based on a confirmed estimate that conditions will produce a flood above plant grade. This two-stage scheme is designed to prevent excessive economic loss in case a potential flood does not fully develop. Refer to Section 2.4.14.4.

2.4.14.1.2 Combinations of Events 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 of the flood during the recovery time after one of these events, is considered incredible. However, as an exception, certain reduced levels of floods are considered together with a-seismic events. Refer to Section 2.4.14.10 and 2.4.4.

2.4.14.1.3 Post Flood Period Because of the improbability of a flood above plant grade, no detailed procedures are established for return of the plant to normal operation unless and until a flood actually occurs. If flood mode operation (Section 2.4.14.2) should ever become necessary, it is possible to maintain this mode of operation for a sufficient period of time (100 days) so that appropriate recovery steps can be formulated and taken. The actual flood waters are expected to recede below plant grade within 1 to 45 days.

2.4-61

WBNP-2.4.14.1.4 Localized Floods Localized plant site flooding due to the probable maximum storm (Section 2.4.2.3) will not enter vital structures or endanger the plant. Any offsite power loss resulting from water ponding on the switchyard or water entry into the Turbine Building will be similar to a loss of offsite power situation as described in Chapter 15. The other steps described in this subsection are not applicable to this case. Refer to Section 2.4.2.3.

2.4.14.2 Plant Operation During Floods Above Grade "Flood mode" operation is defined as the set of conditions described below by means of which the plant is safely maintained during the time when flood waters exceed plant grade (Elevatien elevation 728.0 ft) and during the subsequent period until recovery (Section 2.4.14.7) is accomplished.

2.4.14.2.1 Flooding of Structures Any l-eensed unitThe Reactor Building will be maintained dry during the flood mode. Walls and penetrations are designed to withstand all static and dynamic forces imposed by the DBF; minor seepage through the concrete walls and through the leading penetrations into the annulus will be allowed to flow to the Reactor Building floor and equipment drain sump by removing the blind flange on penetration X-I 18. The Reactor Building floor and equipment drain sumps are more than capable of pumping this flow.

The Diesel Generator Buildings also will remain dry during the flood mode since its lowest floor is at Elevatienelevation 742.0 ft. Other structures, including the Service, Turbine, Auxiliary, and Control Buildings, would be allowed to flood as the water exceeds their grade level entrances.

Equipment that is located in these structures and required for operation in the flood mode is either above the DBF or suitable for submerged operation.

2.4.14.2.2 Fuel Cooling Spent Fuel Pool Fuel in the spent fuel pool is cooled by the Spent Fuel Pool Cooling and Cleanup System (SFPCCS), the active components of which are located above flood waters. During the flood mode of operation, heat is removed from the heat exchangers by essential raw cooling water instead of component cooling water. The SFPCCS cooling circuit is assured of two operable SFPCCS pumps (a third pump is available as a backup) as well as two SFPCCS heat exchangers.

High spent fuel pool temperature causes an annunciation in the Main Control Room indicating equipment malfunction. Additionally, that portion of the cooling system above flood water is inspected approximately every 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months to confirm continued proper operation. As a backup to spent fuel cooling, water from the High Pressure Fire Protection (HPFP) System can be added to the spent fuel pool.

2.4-62

WBNP-Reactors Residual core heat is be removed from the fuel in the reactors by natural circulation in the reactor coolant system. Heat removal from the steam generators is accomplished by adding river water from the HPFP System and relieving steam to the atmosphere through the power operated relief valves. This transition from auxiliary feedwater to river water is accomplished during Stage II of the flood preparation procedures. Refer to Section 2.4.14.4.1. Reactor coolant system pressure is maintained at less than 350 psig by operation of the pressurizer relief valves and heaters.

Secondary side pressure is maintained below 125 psig by operation of the power operated relief valves. At times beyond approximately 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months following shutdown of the plant two relief valves have sufficient capacity to remove the steam generated by decay heat. Since 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months is less than the minimum flood warning time available, the plant can be safely shut down and decay heat removed by operation of two power operated relief valves per unit.

The earliest that the HPFP pumps would be utilized to supply auxiliary feedwater would be about 20 hours2.314815e-4 days 0.00556 hours 3.306878e-5 weeks 7.61e-6 months after reactor shutdown. At this time, in order to remove the decay heat from the reactor unit, the water requirement to the steam generators would be approximately 300 gpm.

Later times following reactor shutdown would have gradually decreasing HPFP Systemsvstem makeup water flow rate requirements. With the steam generator secondary side pressure less than 125 psig, a single HPFP pump can supply makeup water well in excess of the requirement of 300 gpm. Additional surplus flow is available since there are four HPFP pumps, two powered from each emergency power train.

The main steam power operated relief valves are adjusted by controls in the auxiliary control room as required to maintain the steam pressure within the desired pressure range. The controls in the main control room also can be utilized to operate the valves in an open-closed manner.

Also, a manual loading station and the relief valve handwheel provide additional backup control for each relief valve.

The power operated relief valves would be used to depressurize the steam generators as discussed above to maintain steam generator pressure sufficiently below the developed head of the fire pumps. Note that even in the event of a total loss of makeup water flow at the time of maximum decay heat load, approximately 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months are available to restore makeup water flow before the steam generators would boil dry.

If the reactor is open to the containment atmosphere during the refueling operations, then the decay heat of the fuel in the reactor and spent fuel pool heat is removed in the following manner.

The refueling cavity is filled with borated water (nominal ppm boron concentration) from the refueling water storage tank. The SFPCCS pump takes suction from the spent fuel pool and discharges to the SFPCCS heat exchangers. The SFPCCS heat exchanger output flow is directed by a temporary piping connection to the Residual Heat Removal (RHR) System upstream to the RHR heat exchangers. This piping (spool piece) connection is prefabricated and is installed only during preparation for flood mode operation. (The tie-in locations in the SFPCCS and RHRS are shown in Figures 2.4-106 and 2.4-107 respectively.)

2.4-63

WBNP-After passing through the RHR heat exchangers, the water enters the reactor vessel through the normal cold leg RHR injection paths, flows downward through the annulus, upward through the core (thus cooling the fuel), then exits the vessel directly into the refueling cavity. This results in a water level differential between the spent fuel pool and the refueling cavity with sufficient water head to assure the required return flow through the twenty-inch diameter fuel transfer tube thereby completing the path to the spent fuel pool.

Any leakage from the reactor coolant system will be collected to the extent possible in the reactor coolant drain tank; nonrecoverable leakage is made up from supplies of clean water stored in the four cold leg accumulators, the pressurizer relief tank, and the demineralized water tank. Even if these sources are unavailable, the fire protection system can be connected to the auxiliary charging system (Section 9.3.6) as a backup. Whatever the source, makeup water is filtered, demineralized, tested, and borated, as necessary, to the normal refueling concentration, and pumped by the auxiliary charging system into the reactor (see Figures 2.4-108 and 2.4-109).

2.4.14.2.3 Cooling of Plant Loads Plant cooling requirements with the exception of the fire protection system which must supply makeup water to the steam generators, are met by the ERCW System. The inake pumpi s4aiefoIntake Pumping Station is designed to retain full functional capability of the ERCW system and HPFP system water intakes for all floods up to and including the DBF. The ERCW System and HPFP System water intakes also remain fully functional in the remote possibility of a flood induced failure of Chickamauga Dam. (Refer to Sections 9.2.1 and 9.5.1.)

2.4.14.3 Warning Scheme See Section 2.4.14.8 (Warning Plan).

2.4.14.4 Preparation for Flood Mode An abnormal operating instruction is available to support operation of U4t-4-the plant.

At the time the initial flood warning is issued, the plant could be operating in any normal mode.

This means that the-uniteither or both units may be at power or in any stage of refueling.

2.4.14.4.1 Reactor Initially Operating at Power If the reactor is operating at power, Stage I and then, if necessary, Stage II procedures are initiated. Stage I procedures consist of a controlled reactor shutdown and other easily r-eekablerevocable steps, such as moving flood mode supplies above the maxii*imum pessibleprobable maximum flood elevation and making load adjustments on the onsite power supply. After scram, the reactor coolant system is cooled by the auxiliary feedwater (Section 10.4.9) and the pressure is reduced to less than 350 psig.

2.4-64

WBNP-Stage II procedures are the Leastless easily fevekablerevocable and more damaging steps necessary to have the plant in the flood mode when the flood exceeds plant grade. HPFP System water (Section 9.5.1) will replace auxiliary feedwater for steam generator makeup water. Other essential plant cooling loads are transferred from the Component Cooling Water System to the ERCW System and the ERCW replaces raw cooling water to the ice condensers (Section 9.2.1).

The radioactive waste (Chapter 11) system will be secured by filling tanks below DBF level with enough water to prevent flotation. One exception is the waste gas decay tanks, which are sealed and anchored against flotation. Power and communication cables below the DBF level that are not required for submerged operation are disconnected, and batteries beneath the DBF level are disconnected.

2.4.14.4.2 Reactor Initially Refueling If time permits, fuel is removed ifthe uitiifrom the unit undergoing refueling and placed in the spent fuel pool; otherwise fuel cooling is accomplished as described in Section 2.4.14.2.2. If the refueling canal is not already flooded, the mode of cooling described in Section 2.4.14.2.2 requires that the canal be flooded with borated water from the refueling water storage tank. If the flood warning occurs after the reactor vessel head has been removed or at a time when it could be removed before the flood exceeds plant grade, the flood mode reactor cooling water flows directly from the vessel into the refueling cavity.

Flood mode operation requires that the prefabricated piping be installed to connect the RHR and SFPC Systems, that the proper flow to the spent fuel pit diffuser and the RHRS be established and that essential raw cooling water be directed to the secondary side of the RHRS and SFPCCS heat exchangers. The connection of the RHR and SFPC Systems is made using prefabricated in-position piping which is normally disconnected. During flood mode preparations, the piping is connected using prefabricated spool pieces.

2.4.14.4.3 Plant Preparation Time The steps needed to prepare the plant for flood mode operation can be accomplished within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months of notification that a flood above plant grade is expected. An additional 3 hours3.472222e-5 days 8.333333e-4 hours 4.960317e-6 weeks 1.1415e-6 months are available for contingency margin. Site gr.ading and building design pr.event any fl..ding befr. e the end of the 27 hour-preflood per-iod.

2.4.14.5 Equipment Both normal plant components and specialized flood-oriented supplements are utilized in coping with floods. Equipment required in the flood mode is either located above the DBF, within a nonflooded structure, or is suitable for submerged operation. Systems and components needed only in the preflood period are protected only during that period.

2.4-65

WBNP-2.4.14.5.1 Equipment Qualification To ensure capable performance in this highly unlikely, limiting design case, only high quality components are utilized. Active components are redundant or their functions diversely supplied.

Since no rapidly changing events are associated with the flood, repairability is an available option for both active and passive components during the long period of flood mode operation.

Equipment potentially requiring maintenance is accessible throughout its use, including components in the Diesel Generator Building.

2.4.14.5.2 Temporary Modification and Setup Normal plant systems used in flood mode operation and in preparation for flood mode operation may require modification from their normal plant operating configuration. Such modification, since it is for a limiting design condition and since extensive economic consequences are acceptable, is permitted to allow operation of systems outside of their normal plant configuration. However, most alterations will be only temporary and inconsequential in nature.

For example, the switchover of plant cooling loads from the component cooling water to ERCW is done through valves and prefabricated spool pieces, causing little system disturbance or damage.

2.4.14.5.3 Electric Power Because there is a possibility that high winds could destroy power lines and disconnect the plant from offsite power at any time during the preflood transition period, the preparation procedure and flood mode operation are accomplished assuming only onsite power circuits available.

While most equipment requiring ac electric power is a part of the permanent emergency onsite power distribution system other components, if required, could be temporarily connected, when the time comes, by prefabricated jumper cables.

The loads that are normally supplied by onsite power but are not required for the flood are disconnected early in the preflood period. Those loads used only during the preflood period are disconnected from the onsite power system during flood mode operations. DC electric power is similarly disconnected from unused loads and potentially flooded cables.

Charging is maintained for each battery by the onsite ac power system as long as it is required.

Batteries that are beneath the DBF level are disconnected during the preflood period when they are no longer needed.

2.4.14.5.4 Instrument, Control, Communication and Ventilation Systems The instrument, control, and communication wiring or cables required for operation in the flood mode are either above the DBF or within a nonflooded structure, or are suitable for submerged operation. Unneeded wiring or cables that run below the DBF level will be disconnected to prevent short circuits.

2.4-66

WBNP-Instrumentation is provided to monitor vital plant parameters such as the reactor coolant temperature and pressure and steam generator pressure and level. Important plant functions are either monitored and controlled from the main control area, or, in some cases where time margins permit, from other points in the plant that are in close communication with the main control area.

Communications are provided between the central control area (the Main and Auxiliary Control Rooms) and other vital areas that might require operator attention, such as the Diesel Generator Building.

Ventilation, when necessary, and limited heating or air conditioning is maintained for locations throughout the plant where operators might be required to go or where required by equipment heat loads.

2.4.14.6 Supplies The equipment and most supplies required for the flood are on hand in the plant at all times.

Some supplies may require replenishment before the end of the period in which the plant is in the flood mode. In such cases supplies on hand are sufficient to last through the short time (Section 2.4.14.1.3) that flood waters will be above plant grade and until replenishment can be supplied.

2.4.14.7 Plant Recovery The plant is designed to continue safely in the flood mode for 100 days even though the water is not expected to remain above plant grade for more than 1 to 4 days. After recession of the flood, damage will be assessed and detailed recovery plans developed. Arrangements will then be made for reestablishment of off-site power and removal of spent fuel. A decision based on economics would be made on whether or not to regain the plant for power production. In either case, detailed plans would be formulated after the flood, when damage can be accurately assessed. The 100-day period provides a more than adequate time for the development of procedures for any maintenance, inspection, or installation of replacements for the recovery of the plant or for a continuation of flood mode operations in excess of 100 days.

2.4.14.8 Warning Plan Plant grade Etevationelevation 728.0 ft can be exceeded by rainfall floods and elesely app..eaehe4-by-seismic-caused dam failure floods. A warning plan is needed to assure plant safety from these floods.

The warning plan is divided into two stages: Stage I, a minimum of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months long and Stage II, a minimum of 17 hours1.967593e-4 days 0.00472 hours 2.810847e-5 weeks 6.4685e-6 months so that unnecessary economic consequences can be avoided, while adequate time is allowed for preparing for operation in the flood mode. Stage I allows preparation steps causing minimal economic consequences to be sustained but will postpone major economic damage until the Stage I warning predi.ts a likely fo-thecming flocd above g-adeforecasts a likely forthcoming flood above elevation 727.0 ft.

2.4-67

WBNP-2.4.14.8.1 Rainfall Floods Protection of the Watts Bar Plant from rainfall floods that might exceed plant grade utilizes a flood warning issued by TVA's Water-Management. TVA's climatic mo.niter*ing and flood prediefingRO. TVA's climatic monitoring and flood forecasting systems and flood control facilities permit early identification of potentially critical flood producing conditions and reliable prediction of floods which may exceed plant grade well in advance of the event.

The Watts Bar Nuclear PlantWBN flood warning plan provides a minimum of 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months to prepare for operation in the flood mode, 3 hours3.472222e-5 days 8.333333e-4 hours 4.960317e-6 weeks 1.1415e-6 months more than the 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months needed. Four additional preceding hours would be available to gather and analyze rainfall data and produce the warning.

The first stage, Stage I, of shutdown begins when there is sufficient rainfall on the ground in the upstream watershed to yield a prejected plantsite water-level of Elevation 711.5 in the winter months (Oetober 1 through April 15) and Elevation 72 6.5 in the summer- (April 16 t ou* g Septefnber3.

forecasted plant site water level of elevation 715.5 ft in the winter months and elevation 720.6 ft in the summer. This assures that additional rain will not produce water levels to E-evatienelevation 727.0 ft in less than 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months from the time shutdown is initiated. The water level of Elevatienelevation 727.0 ft (one feet below plant grade) allows margin so that waves due to winds cannot disrupt the flood mode preparation.

The plant preparation status is held at Stage I until either Stage II begins or TVA's Water ManageaeffntRO determines that floodwaters will not exceed E.e

.atien levation 727.0 ft at the plant. The Stage II warning is issued only when enough additional rain has fallen to predietforecast that Elevatenelevation 727.0 fl (winter or summer) is likely to be reached.

2.4.14.8.2 Seismically-Induced Dam Failure Floods Only one postulated cembinatioinThree postulated combinations of seismically induced dam failures and coincident storm conditions was shown to result in a fleedwere shown to result in floods which could exceed Ele;atienelevation 727.0 ft at the plant. Watts Bar plant preteetien fro. this flood

.tilizes T.A's Water Management foreeastWBN's notification of these floods utilizes TVA's RO forecast system to identify when a critical combination exists. Stage I shutdown is initiated upon notification that a critical dam failure combination has occurred or loss of communication prevents determining a critical case has not occurred. Stage I shutdown continues until it has been determined positively that critical combinations do not exist. If communications do not document this certainty, shutdown procedures continue into Stage II activity. Stage II shutdown continues to completion or until lack of critical combinations is verified.

2.4-68

WBNP-2.4.14.9 Basis For Flood Protection Plan In Rainfall Floods 2.4.14.9.1 Overview Large Tennessee River floods can exceed plant grade E4e-vationelevation 728.0 ft at Watts Bar Nuelear-P antWBN. Plant safety in such an event requires shutdown procedures which may take 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to implement. TVA flood forecast procedures are used to provide at least 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months of warning before river levels reach iElevatienlevation 727.0 ft. Use of Elevatienelevation 727.0 ft, 4-one feet below plant grade, provides enough margin to prevent wind generated waves from endangering plant safety during the final hours of shutdown activity. Forecast will be based upon rainfall already reported to be on the ground.

To be certain of 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months for preflood preparation, flood warnings with the prospect of reaching E4evationelevation 727.0 ft must be issued early when lower target elevations are forecast.

Consequently, some of the warnings may later prove to have been unnecessary. For this reason preflood preparations are divided into two stages. Stage I steps requiring 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months are easily r-evekablerevocable and cause minimum economic consequences. The estimated probability is lcss than 0..

32small that a Stage I warning will be issued during the 40-year life of the plant.

Added rain and stream-flow information obtained during Stage I activity will determine if the more serious steps of Stage II need to be taken with the assurance that at least 17 hours1.967593e-4 days 0.00472 hours 2.810847e-5 weeks 6.4685e-6 months will be available before Elevaoienelevation 727.0 ft is reached. The probability of a DRF ee gStage II warning during the 40-year-life of the plant is very small.

Flood forecasting and warnings, to assure adequate warning time for safe plant shutdown during floods, will be conducted by TVA's Water ManagementRO.

2.4.14.9.2 TVA Forecast System TVA has in constant use an extensive, effective system to forecast flow and elevation as needed in the Tennessee River basin. This permits efficient operation of the reservoir system and provides warning of when water levels will exceed critical elevations at selected, sensitive locations which includes WBN.

TVA's RO normal operation produces daily forecasts by 12 noon made from data collected at 6 a.m. Central time. During major flood events, RO may issue forecasts as frequent as 4 to 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months at specific site locations.

Elements of the present (4-99820 10) forecast system above Watts Bar Nuclear PlantWBN include the following.

1.

Nine*y-*eight-(9. *More than 90 rain gages measure rainfall, with an average density of 165 squa. miles per-rain gage. Of these, 54 are GOESabout 190 square miles per rain gage. All are Geostationary Operational Environmental Satellites (GOES) Data Collection Platform (DCP) satellite telemetered gages, and 27 are Data Logger-teephefie telemetered gages which depend upen connnercial telephone lines, and 17 are observet gages located at TVA hydr-e and fossil plants and non TV1A hydr~o plants. in ease ot 2.4-69

WBNP-commercial telephone line failur.,

field pe.s...el can be notified by radio to in.te..ogate and provide data fr.m the 27 Data Logger gagestelemetered gages.

The telephone gae ar initerrogated on a two hour-inter,'al on the even hour- (Centr~al preVailinig timfe) to obtain hourly5 ranall readings. During flood per-iods, the gages can ben*.terrogated mo.re frequently if desired. TheSome of the satellite gages transmit hourly rainfall data every 3 hours3.472222e-5 days 8.333333e-4 hours 4.960317e-6 weeks 1.1415e-6 months during normal oper.atins. in addition, the satellite gages event r

.epo.

when 0..1 inch or greater-r-ainfall a..umulates. The normal and event transmissiens are conveyed on separate satellite channels. Perseomfel at theTY installations r-er-d six hour rainfall data. Information from these sites is available for-the normal forecast run at 6 a.m. Central timewhile others transmit hourly during normal operations.

Streamfiow data are received for aaiIiable fromm 23 gages in the system. Of these, 12 are GOES Data Collection Platfeiofl satellite telemetered gages, and 11 are Data Logge~r telephone telemeter-ed gages which depend upon commerceial telephone lines.

The telephoe gae r interrogated on a two hour-intern'al on the even hour- (Central pr-evailing time) to obtain 15 mninute stage r-eadings. During flood per-iods, the gages can be interrogated moere frequently if desir-ed.Streamflow data are received from 23 gages in the system. All are GOES Data Collection Platform satellite telemetered Rages. The satellite gages transmit 15-minute stage data every -3three hours during normal operations. Random str.ea gage tr.ansm-issieon; is cu.,ently being tested. Information from these sites is available for-the normal for-ecast run at 6 a.m. Central time.

2.

3.

I4eHIi-Real-time headwater elevation, tailwater elevation, and discharge data are received from 14 TAOA oAn I non T,, A hydr.o plants. More frequent dat-a c.an be obtained du.ing f*l8d operations. 21 TVA hydro projects (Watts Bar, Melton Hill, Fort Loudoun, Tellico, Norris, Douglas, Cherokee, Fort Patrick Henry, Boone, Watauga, Wilbur, South Holston, Chickamauga, Ocoee No. 1, Ocoee No. 2, Ocoee No. 3, Blue Ridge, Apalachia, Hiwassee, Chatuge and Nottely) and hourly data are received from non-TVA hydro plants (Chilhowee, Cheoah, Calderwood and Santeelah).

4.

Weather forecasts including quantitative precipitation forecasts received at least twice daily and at other times when changes are expected.

5.

Computer programs which translate rainfall into streamflow based on current runoff conditions and which permit a forecast of flows and elevations based upon both observed and predicted rainfall. A network of UNIX wer-kstatienservers and personal computers are utilized and are designed to provide backup for each other. One computer is used primarily for data collection, with the others used for executing forecasting programs for reservoir operations. The time interval between receiving input data and producing a forecast is less than 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months . Forecasts normally cover at least a -3three-day period.

2.4-70

WBNP-As effective as the forecast system already is, it is constantly being improved as new technology provides better methods to interrogate the watershed during floods and as the watershed mathematical model and computer system are improved. Also, in the future, improved quantitative precipitation forecasts may provide a more reliable early alert of impending major storm conditions and pkasthus provide greater flood warning time.

TVA's normal eperatien proeduees daily forcceasts by 12 noon made from data collected at 6 a.m.

Central time. Wh~en ser-ious flood situations demand, per-some!l of Resenvoir-Operaitions werk ar-ound the clock With for-eeasts as frequtent as at 6 hour6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months intenvals.

2.4.14.9.3 Basic Analysis The forecast procedure to assure safe shutdown of Watts Bar Nuclear PlantWBN for flooding is based upon an analysis of 17 hypothetical P-M-P orms, in.luding-thei. anteedenft* tor T-heynine hypothetical storms up to PMP magnitude. The storms enveloped potentially critical areal and seasonal variations and time distributions of rainfall. To be certain that fastest rising flood conditions were included, the effects of varied time distribution of rainfall were tested by alternatively placing the maximum daily PMP enin the first, the middle, and the last day of the 3three-day main storm. In each day the. m..aximum 6 hour-depth was placed during the sec interval except when the maximum daily raini was placed on the last da~y. Then the maximumn 6 hour-amount was pla.d i. the.ast 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months . The warning System is based en these PMPEarlier analysis of 17 hypothetical storms demonstrated that the shortest warning times resulted from storms in which the heavy rainfall occurred on the last day and that warning times were significantly longer when heavy rainfall occurred on the first day. Therefore, heavy rainfall on the first day was not reevaluated. The warning system is based on those storm situations which resulted in the shortest time interval between watershed rainfall and Elevationelevation 727.0 ft at WBN, thus assuring that this elevation could be predicted at least 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months in advance.

The procedures used to compute flood flows and elevations for those flood conditions which establish controlling elements of the forecast system are described in Section 2.4.3 as amended.

The analyses o~f the rcmfaining fleods 'which identified the cr-itical flood conditions wer-e made using e-lier ver.sios of the pr.ecedr.es des.r.ibed in Secion 2.4.3.

2.4.14.9.4 Hydrologic Basis for Warning System A minimum of 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months has been allowed for preparation of the plant for operation in the flood mode, three hours more than the 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months needed. An additional 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months for communication and forecasting computations is provided to allow TVA's RO to translate rain on the ground to river elevations at the plant. Hence, the warning plan provides 31 hours3.587963e-4 days 0.00861 hours 5.125661e-5 weeks 1.17955e-5 months from arrival of rain on the ground until El'vati.enelevation 727.0 ft could be reached. The 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months allowed for shutdown at the plant consists of a minimum of 10 hours1.157407e-4 days 0.00278 hours 1.653439e-5 weeks 3.805e-6 months of Stage I preparation and an additional 17 hours1.967593e-4 days 0.00472 hours 2.810847e-5 weeks 6.4685e-6 months for Stage II preparation that is not concurrent with the Stage I activity.

Although river E4evationelevation 727.0 ft, 4-one feet below plant grade to allow for wind waves, is the controlling elevation for determining the need for plant shutdown, lower forecast target levels are used in some situations to assure that the 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months pre-flood transition interval will always be available. The target river levels differ with season.

2.4-71

WBNP-During the Octeber 1 th*rough April 15 "winter" season, Stage I shutdown procedures will be started as soon as target river Elevation 71 elevation 715.5 ft has been forecast. Stage II shutdown will be initiated and carried to completion if and when target river Elevation elevation 727.0 ft at WBN has been forecast. Corresponding target river elevations for the April !56 through September-30 "summer" season at WBN are Elevatin: 726.5 anid Ele-vati-nl 727elevation 720.6 ft and elevation 727.0 ft.

Inasmuch as the hydrologic procedures and target river elevations have been designed to provide adequate shutdown time in the fastest rising flood, longer times will be available in other floods.

In such cases there wil4may be a waiting period after the Stage I, 10-hour shutdown activity during which activities shall be in abeyance until weather conditions determine if plant operation can be resumed, or if Stage II shutdown should be implemented.

Resumption of plant operation following just Stage I shutdown activities will be allowable only after flood levels and weather conditions, as determined by TVA's RO, have returned to a condition in which 27 hours3.125e-4 days 0.0075 hours 4.464286e-5 weeks 1.02735e-5 months of warning will again be available.

2.4.14.9.5 Hydrologic Basis for Ta

e+-&atesWarning Times and Elevations Figur-e 2.

110, in four. pats, shows how tar-get forec*asi÷-e-Ast flood*

A-d elvation

-s

-At the W*t-t 1-

s Bar plant have been determined to assure adequat, 'wanig times. The fleods shown are the fastest r-ising probable maximumn fleeds at the site. Onl.y the pr-incipal storm in which the PM4P occur-s has been shewn. TheseFigure 2.4-110 (Sheet 1) and Figure 2.4-110 (Sheet 2) for winter and summer respectively, show target forecast flood warning time and elevation at WBN which assure adequate warning times. The fastest rising probable maximum flood for the winter at the site is shown in Figure 2.4-110 (Sheet 1A). Figure 2.4-110 (Sheets lB and 1C) show the adopted rainfall distribution for the 21,400 square mile storm and the 7,980 square mile storm, respectively. An intermediate flood with average basin rainfall of 10 inches (rainfall heavy at the end) is shown in Figure 2.4-110 (Sheet 1D). Figure 2.4-110 (Sheet 2A) shows the 7,980 square mile fastest rising probable maximum flood for the summer with heavy rainfall at the end. The 7,980 square mile adopted rainfall distribution is shown in Figure 2.4-110 (Sheet 2B). An intermediate flood with average basin rainfall of 10 inches heavy at the end is shown in Figure 2.4-110 (Sheet 2C). All of these storms have been preceded 3three days earlier by a -- three-day storm having 40% of the-mainPMP storm rainfall.

Figure 2.1 110 (A,B,C) shows the winter-PN4P which would produce the fastest risingflo

.which would cross plant gr-ade and var-iations caused by changed tim.e distribution. The fastest rising flood occurs during a PMP when the 6six-hour increments increase throughout the storm with the maximum 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months occurring in the last period. Figure 2.4-110 (A)(Sheet )A) shows the essential elements of this storm which provides the basis for the warning plan. In this flood 8-.78.6 inches of rain would have fallen 31 hours3.587963e-4 days 0.00861 hours 5.125661e-5 weeks 1.17955e-5 months (27 + 4) prior to the flood crossing Elevationelevation 727.0 ft and would produce Elevation 711elevation 715.5 ft at the plant.

Hence, any time rain on the ground results in a pedie-forecast plant elevation of4-l74.52715.5 ft a Stage I shutdown warning will be issued. Examination of Figure 2.4-110 (B and C) show that following this procedure in these non critical floods w ould result in a lapsed time of 17 and 19 hours2.199074e-4 days 0.00528 hours 3.141534e-5 weeks 7.2295e-6 months between when 8.7 inches had fallen and the flood would exceed Elevation 927(Sheets l B and 1 C) show that following this procedure in these floods would result in longer times to reach 2.4-72

WBNP-elevation 727.0 ft after Stage I warning was issued. These times would be 41 and 46.5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months (includes 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months for forecasting and communication) for Figure 2.4-110 (Sheet 1B) and (Sheet IC), respectively. This compares to the 31 hours3.587963e-4 days 0.00861 hours 5.125661e-5 weeks 1.17955e-5 months for the fastest rising flood Figure 2.4-110 (Sheet 1A). Stage I warning would be issued for the storm shown in Figure 2.4-110 (Sheet ID) but would not reach a Stage II warning as the maximum elevation reached is 721.92 ft which is well below elevation 727.0 ft.

An additional -382.6 inches of rain must fall promptly for a total of 3-.511.2 inches of rain to cause the flood to exceed Ekeatienelevation 727.0 ft. In the fastest rising flood, igtifeigure 2.4-110 (A)(Sheet 1A), this rain would have fallen in the next 86.0 hours0 days 0 hours 0 weeks 0 months . A Stage II warning would be issued within the next 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months . Thus, the Stage 11 warning would be issued 86.0 hours0 days 0 hours 0 weeks 0 months after issuance of a Stage I warning and 4-92 1.0 hours0 days 0 hours 0 weeks 0 months before the flood would exceed Elevtienelevation 727.0 ft. In the slower rising floods, Figure 2.4-110 (B and C)(Sheets lB and 1C), the time between issuance of a Stage I warning and when the 4-1411.2 inches of rain required to put the flood to Elevatie elevation 727.0 ft would have occurred, is 4-8-7.0 and 4-25.0 hours0 days 0 hours 0 weeks 0 months respectively. This would result in issuance of a Stage II warning not 1essmore than 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months later or 2-530.0 or 3337.5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months , respectively, before the flood would reach Elevatefielevation 727.0 ft.

The summer flood, shown by Figure 2.4-110 (D4)(Sheet 2A), with the maximum lone-day rain on the last day provides controlling conditions when reservoirs are at summer levels. At a time 31 hours3.587963e-4 days 0.00861 hours 5.125661e-5 weeks 1.17955e-5 months (27 + 4) before the flood reaches Elevation levation 727.0 ft, 44-.69.3 inches of rain would have fallen. This 44-.69.3 inches of rain under these runoff conditions would produce Elevation 726.5elevation 720.6 ft, so this level becomes the Stage I target. An additional 04 ineh2.0 inches of rain must fall promptly for a total of 4--.1 1.3 inches of rain to cause the flood to exceed Elevtienelevation 727.0 ft. In this fastest rising summer flood, Figure 2.4-110

(-)(Sheet 2A), this rain would have fallen in the next heui,4.5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months . A Stage II warning would be issued within the next 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months . Thus, the Stage II warning would be issued One heiri'.5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months after issuance of a Stage I warning and 2-622.5 hours5.787037e-5 days 0.00139 hours 8.267196e-6 weeks 1.9025e-6 months before the flood would exceed E4vationelevation 727.0 ft.

The above criteria all relate to forecasts which use rain on the ground. In actual practice quantitative rain forecasts, which are already a part of daily operations, would be used to provide advance alerts that the need for shutdown may be imminent. Only rain on the ground, however, is included in the procedure for firm warning use.

Because the above analyses used fastest possible rising floods at the plant, all other floods will allow longer warning times than required for physical plant shutdown activities.

In summary, the predieted target levekrforecast elevations which will assure adequate shutdown times are:

Forecasts Elevations at Watts Bar Season Stage I shutdown Stage II shutdown Winter (Oe.tber-1 April 15) *-4.5715.5 ft 727.0 ft Summer (April 16 September-3

26-.720.6 ft 727.0 ft 2.4-73

WBNP-2.4.14.9.6 Communications Reliability Communication between projects in the TVA power system is via (a) TVA-owned microwave network, (b) Fiber-Optics System, and (c) by commercial telephone. In emergencies, additional communication links are provided by Transmission Power Supply radio networks. The four networks provide a high level of dependability against emergencies. Additionally, RO have available satellite telephone communications with the TVA hydro projects upstream of Chattanooga (listed in Section 2.4.14.9.2).

The hydr-ologic nctwor-k for the water-shed above Watts Bar-that would be available inflo emer-gencies if commerceial telephon cmuications are lost includes 61 r-ainfall gages (17 at power. insallations and 54 satellite gages). The Resero.

ir-OperationsRO is linked to the TVA power system by all five communication networks. The data from the satellite gages are received via a data collection platform-satellite computer system located in the Reservek Opertiaons office. These are distributed over the watershed such that reaseonabie flocd for.ecasting can bc done from this data while the balance cf data is being secured fomt reminig hydrologic networ-k stationRO office.

The pr-eferred, ccmplete cov'erage of the water-shed employs numerouts r-ainfall and stream flow link to those loeetions are rceutine radio, radio satellite and th reomme-rreial telephone syste networks. Mn an emergency, available radio communic-ationis wouild be called upon to assist.

The var-ious networ-ks prov'ed to be capable o~f proeviding the r-ain and stream flow low dat needed fo*r reliable fer*1 easts in the large floods of 1957, 1963, 1973, and 1981.

2.4.14.10 Basis for Flood Protection Plan in Seismic-Caused Dam Failures Floods resulting from. com..b.ined. seismic and flood events can closely approach plant grade, thus reuiin emer-gency mneasur-es. The 1998 r-eanalysis showed that only onie seismic dam failur-e cmiation coincident with a flood the SFE failur-e of Norris, Cheroekee and DouiglasPlant grade would be exceeded by three of the five candidate seismic failure combinations evaluated, thus requiring emergency measures. The seismic dam failure combination, the SSE failure of Norris, Cherokee, Douglas and Tellico Dams concurrent with the 25-year flood would result in a oeed approaehing plant grade. As shown inTable 2.1 8, all other-candidate com..bination.s of events would create flood levelswell below plant grade Elevation 728.0. Dam failure during noen food periods would not present a problem at the plant as resulting flood levels for-all cande combinationswouldbe rwell boweW plant grade. The reanalysis showed that failure ofxth eentrtolling combination in a noni flod period and a summer flood guide levelsw produce Elevation 725.2 at the plant, 2.8 feet below plant grade. All other combinations in non flood periods would proeduce elevations much lower. The time from seismic occurrence to af-val ol failur-e surge at the plant in the cr-itical event is about 50 hourfs as shown in-Figur-e 2.1 111 n i-smaximum flood elevation of 731.17 ft at WBN. The OBE failure of Norris and Tellico Dams and the OBE failure of Cherokee. Douglas. and Tellico Dams concurrent with the one-half PMF would result in flood elevations above WBN plant grade. Table 2.4-14. shows the maximum elevations at WBN for the candidate combinations.

2.4-74

WBNP-The times from seismic failure to the time elevation 727.0 ft is reached at WBN in the three critical events is about 35, 27, and 44 hours5.092593e-4 days 0.0122 hours 7.275132e-5 weeks 1.6742e-5 months as shown in Figures 2.4-114, 2.4-115, and 2.4-116 for the Norris, Cherokee, Douglas and Tellico Dams SSE failure combination, the Norris and Tellico Dams OBE failure combination and the Cherokee, Douglas, and Tellico Dams OBE failure combinations, respectively. These times are adequate to permit safe plant shutdown in readiness for flooding.

Dam failure during non-flood periods was not evaluated, but would be bounded by the three critical failure combinations.

The warning scheme for safe plant shutdown is based on the fact that a combination of critically centered large earthquake, and rain produced flood conditions must coincide before the flood wave from seismically caused dam failures will approach plant grade. In flood situations, an extreme earthquake must be precisely located to fail Norris, Cherokee, and-Douglasand Tellico Dams before a flood threat to the site would exist. This would also be the case with the failure of Norris and Tellico. Cherokee and Douglas Dams failures could occur when the OBE is located midway between the dams which are just 15 miles apart.

The warning system utilizes TVA's RO flood forecast system to identify when flood conditions will be such that seismic failure of critical dams could cause a flood wave to approach Ele;atiefnelevation 728.0 ft at the plant site. These e.nditicns mbined with any oncern by TNYA Water Management that failure of a single u.pstr.eam. dam has o". urred orr is imminentln addition to the critical combinations, failure of a single major upstream dam will lead to an early warning. A Stage I warning is declared once failure of Norris, Cherokee, and Douglas(L) Norris.,

Cherokee, Douglas, and Tellico Dams or (2) Norris and Tellico Dams or (3) Cherokee, Douglas and Tellico Dams has been confirmed.

If loss of or damage to an upstream dam is suspected, effor4ts will be made by

,ydr,-

Operatio based on monitoring by TVA's RO, efforts will be made by TVA to determine whether dam failure has occurred. If the critical case has occurred or it cannot be determined that it has not occurred, Stage I shutdown will be initiated in time to assur. the 27 h"ur. flood pr.epar.ation pe*iod. Once initiated, the flood preparation procedures will be carried to completion unless it is determined that the critical case has not occurred.

Communications between the p!tWBN, dams, power system control center, and TVA Watef Management at Knex-i!!eRO are accomplished by TVA-owned microwave networks, fiber-optics network, radio networks, and commercial and satellite telephone service. These systems are described in UFSAR Section 9.5.2.3.

2.4.14.11 Special Condition Allowance The flood protection plan is based upon the minimum time available for the worst case. This worst case provides adequate preparation time including contingency margin for normal and anticipated plant conditions including anticipated maintenance operations. It is conceivable, however, that a plant condition might develop for which maintenance operations would make a longer warning time desirable. In such a situation the Plant Manager determines the desirable warning time. He contacts TVA's Water ManagementRO to determine if the desired warning time is available. If weather and reservoir conditions are such that the desired time can be 2.4-75

WBNP-provided, special warning procedures will be developed, if necessary, to ensure the time is available. This special case continues until the Plant Manager notifies TVA's Water ManagementRO that maintenance has been completed. If threatening storm conditions are forecast which might shorten the available time for special maintenance, the Plant Manager is notified by RO and steps taken to assure that the plant is placed in a safe shutdown modein-4the iFnimum time detSrmined available for-the theatening stm conditins.

REFERENCES 1.

National Weather Ser..ec, "Probable Maxmu an. TVA Pre-;ipitation fr Tennessee River-Basins up to 3,000 Square Miles in Afea and Durations to 72 Hour-s,"

Hydro.meteor.ologial Repo..

No. 15, 1969, with Addendum of June 1973Reference deleted.

U.S. Armv Carms of Eneineers. "Standar-d Proicet Fleod Determiniation.: Civil Works 2.

Engineer-Bu"leti 52 8, March 1952Reference deleted.

3.

SCS National Engineering Handbook, Section 4, Hydrology, July 1969.

4.

U.S. Weather Bureau, "Probable Maximum and TVA Precipitation Over The Tennessee River Basin Above Chattanooga," Hydrometeorological Report No. 41, 1965.

5.

Newton, Donald W., and Vineyard, J. W., "Computer-Determined Unit Hydrographs From Floods," Journal of the Hydraulics Division, ASCE, Volume 93, No. HY5, September 1967.

6.

Garrison, J. M., Granju, J. P., and Price, J. T., "Unsteady Flow Simulation in Rivers and Reservoirs," Journal of the Hydraulics Division, ASCE, Volume 95, No. HY5, Proceedings Paper 6771, September 1969, pages 1559-1576.

7.

Eagleson, Peter S., "A Distributed Linear Model for-Pe 1E atemen Dseharge,"

Proceeedings, The International Hydroelegy Sympesium, September-1967, Fort Collins, Color-ado, V*olunm 1Reference deleted.

Kulandaiswamy, V. C., "A Nonlinear-Approach t Runoff S.dies," Proceedings, The International Hydr-ology Symposium, September-1967, Fort Collins, Color-ado, V'olume 4-Reference deleted.

8.

9.

10.

11.

12.

ArdEis, C. V., Jr., "A Nonlinear-Channel Routing Model, "Proceeedings, Theme 1, May 1971 Flow Sympositm, st ment Sceiey of Amerc*ia, Pittsburgh, 97.Reference deleted.

Reference deleted in initial UFSAR.

Cristofano, E. A., "Method Of Computin. g Erosion Rate for-Failure of Earohfill Dams,"

Engineering and Research Center-, Bur-eau o.f.

Reclamation, Denver, 1 966Reference deleted.

"The Br-eaching of the Or-as Earth Dam in the State of Cear-a, North East Brazil," Water and W ater-tncineleernfl. ;'tueust : 6Y4Kererence deleted.

2.4-76

WBNP-

13.

National Climatic Center-,

Asheville, North Carolina, "Extreme Wind Study for Selected Stations in the Temessee Valley," pr.epared under Cntract No. TV 36522A, A,,,st 4-9-7-4Reference deleted.

14.

U.S. Army Corps of Engineers, "Computation of Freeboard Allowances for Waves in Reservoirs," Engineering Technical Letter No. 1110-2-8, August 1966.

15.

U.S. Army coastal Engineering Research Center, "Shore Protection Planning and Design,"

Third Edition, 1966.

16.

Anderson, Paul, "Substructure Analysis and Design," 1948.

17.

Hinds, Julian, Cregar, William P., and Justin, Joel D., "Engineering For Dams," Volume 11, Concrete Dams, John Wiley and Sons, Incorporated, 1945.

18.

Bustamante, Jurge I., Flores, Arando, "Water Pressure in Dams Subject to Earthquakes,"

Journal of the Engineering Mechanics Division, ASCE Proceedings, October 1966.

19.

Chopra, Anil K., "Hydrodynamic Pressures on Dams During Earthquakes," Journal of the Engineering Mechanics Division ASCE Proceedings, December 1967, pages 205-223.

20.

Zienkiewicz, 0. C., "Hydrodynamic Pressures Due to Earthquakes," Water Pressures Due to Earthquakes," Water Power, Volume 16, September 1964, pages 382-388.

21.

Tennessee Valley Authority, "Sedimentation in TVA Reservoirs," TVA Report No. 0-6693, Division of Water Control Planning, February 1968.

22.

Reference deleted in initial UFSAR.

23.

Price, J. T. and Garrison, J. M., Flood Waves From Hydrologic and Seismic Dam Failures,"

paper presented at the 1973 ASCE National Water Resources Engineering Meeting, Washington, D. C.

24.

Fisher, H. B., "Longitudinal Dispersion in Laboratory and Natural Systems" Keck Laboratory Report KH-R-12, California Institute of Technology, Pasadena, California, June 1966.

25.

Fisher, H. B., "The Mechanics of Dispersion in Natural Streams," Journal of the Hydraulics Division, ASCE Vol. 93, No HY6, November 1967.

26.

Yotsukura, N., "A Two-Dimensional Temperature Model for the Thermally Loaded River with Steady Discharge" Proceedings of the Eleventh Annual Environmental and Water Resources Engineering Conference, Vanderbilt University, Nashville, Tennessee, 1972.

27.

Almquist, C. W., "A Simple Model for the Calculation of Transverse Mixing in Rivers with Application to the Watts Bar Nuclear Plant," TVA, Division of Water Management, Water Systems Development Branch, Technical Report No. 9-2012, March 1977.

2.4-77

WBNP-

28.

Henderson, E. M., Open Channel Flow, MacMillen, 1966.

29.

Carlslaw, B. S. and J. C. Jaeger, Conduction of Heat in Solids, Oxford University Press, London England, 1959.

30.

Johnson, A. E., 1963, Application of Laboratory Permeability.

31.

Stoker, J. J., "Water Waves," Intersei 3..,3-341*Reference deleted.

ence Publisher. Inc., NTew York, 1966, Up.

32.

33.

Br-etschneider-, C. L., "Wave Refraction, Diffraction and Rfeto Chapter-F ofEsur and Coastline Hydrodynamic~s. MIT Hydroedynamic. Lab, Camnbr-idge-,

Massaehuse~ttsleference deleted.

Keulegan, G. H4., "Wave Damping Eff-ects of Fibroaus Screens," Rescareh Report H4 72 2.

1c, e 17Ps et-f-agmeer-s, -Viettstgur-g, tN-ttSSfSsfppf, 1I-st/ Keierence deleted.

34.

U.S. Army Corps of Engineers, Hydrologic Engineering Center, River Analysis System, HEC-RAS computer software, version 3.1.3.

35.

National Weather Service, "Probable Maximum and TVA Precipitation Estimates with Areal Distribution for Tennessee River Drainages Less Than 3,000 Square Miles in Area,"

Hydrometeorological Report No. 56, October 1986.

36.

U.S. Geological Survey, National Water Information System: Web Interface, USGS Surface-Water Data for the Nation, Website, http://waterdata.usgs.gov/usa/nwis/ws, accessed April 2006.

37.

Federal Emergency Management Agency (FEMA), "Federal Guidelines for Dam Safety:

Earthquake Analysis and Design of Dams," FEMA 65. May 2005.

2.4-78

ENCLOSURE 1 EVALUATION OF PROPOSED CHANGES ATTACHMENT 2 Proposed WBN Unit 1 UFSAR Tables

WBNP-Table 2.4-1 Location of Surface Water Supplies in the 58.9 Mile Reach of the Mainstream of the Tennessee River Between Watts Bar Dam (TRM 529.9) and Chichamaqua Dam (TRM 471.9)

Approximate Distance From Site Plant Name Use (MGD)

Location (Bank)

(River Miles)

Watts Bar Dam TRM 529.9 1.9 (Upstream)

Watts Bar Steam Plant TRM 529.9 R

1.9 (Upstream)

Watts Bar Nuclear Plant 40.000 ###

TRM 528 R

0 City of Dayton 1.78 TRM 503.8 R

24.2 (Downstream)

Sequoyah Nuclear Plant 1615.68 TRM 483.6 R

44.4 (Downstream)

East Side Utility 5.00 TRM 473 L

55.0 (Downstream)

Chickamagua Dam TRM 471 57.0 (Downstream)

Water usage is not metered. Flow Rate fluctuates as needed and is directed by power control center in Chattanooga.

Not active at this time. If plant is reactivated, new numbers will be needed.

1. 1. Unit 2 is not in operation at this time. When operational maximum combined intake will be -115 million gallons per day.

Tvye SupDIv Industrial Industrial Industrial Municipal Industrial Municipal Industrial 2.4-79

WBNP-Table 2.4-2 Facts About TVA Dams and Reservoirs (Page 1 of 2)

K1,12J00942 T6,686 22Y 40.200 0264 02121938 0/3001014 421421144 12111GB94 104 5

044 276 6.422 2.06 I104000475('4 1840

00043 180,300 21260 0540 07500 0090 2.101.0 4.12.0060 2,809,000 4.008,060 006080 P

T80 76,,6 00 00.0"20 1204 1220130 228100 09190 I1211210 00 9

407 110 7.710 COE 110.17000*

127 4606 82.,00 0.,0 4070 418 00 8740 839,00 1,002,0 1,1106,6 400100 00

66 92o 1

282.

At4 30870 0 0010 700 421214/161 224214924 9210J1026 412/10000 090 21 269 4 137 4547 00 l 10*O5404oo0 15 5 16 2115.600 9.208 50140 0176 12 07.1 64.700 040.200 007,200 706,010 0,0R6 0

20,8161 Tennessee0 2d.

29540 000 112101307 102721604 1160/1900 10J1821040 001 II 4749 70 9002 CO 100x4 600 32741 130'72 07,210 17.660 5000 000148 5000 742.660 1,009000 1,600000 828.74O 056P080 9

110,5006520 02ur60152.8 7.06,8

0.

04.450 142 1027/900 219261979 92121900 310210411 144 4

0440 001 0919 000 60,0,2045 15760001 00.0OO 10.060 54701 59544 0000 866.500 1,049.700 1,018,00 114.160 11067,.

9 110,0>OO,45*

040244 Te1900 00 21.,80 04.1 42121904 107I4;097 20/210198 4t022160 100 4

424.7 80 7.767 0120 1108006416 460 004 10.200 4760 6375-47100 1041-002.

461.100 522 525 1N06.680 0

30601412a826 781006 00 20.780 7441212100163 121910140 92221940 0221952 110 4

4710 120J 5.460 COP 6070360x5t 000 7007 01,050 0,500 7750 16004 1071 702,O0 702700O 9241600 045.700 30006'r 9

208t6B, 762604 30 11.010 104 12121900 12127040 01ll10042 40240044 162 5

0299 1250w 2400 000 600000,70 9500o 7217 17,500 10,346 1710 74560 1400 186.0C0 1,175000 1.010,00 170000 T0~,606 9

Po01lo41o4n 7Te686 00 0.530 410 760100 6,021040 1160/1040 1070609 162 4

603 0 109262 4.100 000 902300440 00 82 0 7820 14.600 4.423 6070 11560 4130 264.00*

0900 w

600.000 111.000 0,6062e 9

0082la0128 0564697 N,

1,016 094 712111941 221421040 042.2/I104 1121712143 62 2

690 150 1604 220 522 06 015 1,160 60 12720-124000 7127, 002.A 57,920 N25 522.

061,868 4

1-8(F0

1) 800--

0,908 0H406 00 9748 445 711521976 22621040 0621040104241904 141 21277758 3717 379 CO 8,22A 222 1049 5.410 1,30O 1765.0 112050 15410 006.400 440060 7094O0 040000 03H 0,888 4

0082256 52968 50 160 05 712/12 41 22142/104 14601004 1282/1002 17 1

14107 150 265010 1,23t 170 1760 0,760 210 19190 192400 10407 177,060 247,500 2406 60 52,600 1206,868 4

010o48 l70m2 07048 00 595 224 000/1920 1221527911 120421912 92210141.4 5

110 l35 640 CO 0,22 75 410 1.640 110 6200 90010 4090 04.000 90.700 19.900 1900O 00088 4

04842}

098 00 522 218 0 02O1922 10200101 10021923 13201911 70 7

244 60 410 0

220 2,22 002 00A 522 1022 111520 5201 000A000

.1 2

A 07,,

6228 5T27 00 232 244 112022152502]

122721030 7001910 7

/1200100 11 2

507 115 2360 0

0,22.

713 61.1 0.223 162 1151.0 1690700 15674 121,460 1095£0 162.400 16500 1010 1

50082" 1o021y

02.

214 112 221721041 10041042 121001056 121021004 16 1

212 107 2.000 lIP 5,2A 202 02 13 921139 0 2170 t620 119000 1711 112,700 114.000 162,700 61,500 H01,8866 4

M080708 0001C~nc 100043 0

215 9001000 00101904 7602804 12122/900 10 4

201 103 12.20 CO 7544204570 54 15034 51690 1.651 1920-10160 10 20-000 126.040 002.

522.

ClP7h 4

50T, 02inh 00 2.0 21 122121010 22421076 1021006 9.02101-110 2

104 405

-800 006 002 12300u 6602 50.00 2960 1- 0 10345 1D2D 0 1

.412.0m 0212.00_

2.0-0.000 1.111000 0Pin 4

1804 L081 06 2.127 111.0 0621001 110004167 (o2

20) 2o)

(o2 03

177282 3006 00E (02 337 0070 05.600 2.200 810 B1114 B1100 00.47404.0 004.00 14070 260855 4

PF,5024 2

1N 00 1.512 907 1220042 11021944 2007943 22221054 404 0

070 460 23-5 00

-,A 270 2371 10.20 1,0 16530 11000 17000 004.00 1043.-00 1.101.100 514.O0 9540 4

60246 1J 0 6,042a 00 2,042 840 22021940 022191941 062271404 8221004 211 4

124 2155 1,405 00 520 471 1121 26.070 3.170 0020 10060 09240 074.00O 1461,000 1.409.500 1,084,O0 1F,2.6 I

00erok44 0o60o 00 040 02.672750W40 12201041 421921942 7061002 244 4

520 1740'- 4.10 COER 522 040 7005 20.1.0 2.421 10450 141540 10.1 747.600 1.541.0 1.422.-00 49.440 1o04 4

P6F 69 pa 800P921* 00 I

1.603 189 510121921 130.'71050 10101600 20'22J1902 41 2

82 60 701 00 522 0224 710 0403 409 125000 22*6000 22580- 002 26.400 522.

222.

001000 4

D05ry 0

1.454 124 7

0402, 080601 00~

0N.940 155 002226050 1221226/150 00920000 0'721003 90 9

184 104 1.502 P000 522.

7010 12'04 4.170 014 13040 104040 10J00 127.950 190.400 160.,500 15.e60 74040 4

01121 08401000 00r 10 22.1 54210102 11-01701 0D12 1 011151 04 1

448 441 1-607

.6 002.

2E 1 9

II 0ID0-1 103 0 14040 01140 511.-00 10.00

.2.00 450,600 14h9-4 0518298 0457844 30 450 001 7.. 2410102D 12.1104-4-10 008009 66 2

061 000 90, 66 102a 163 104 10.440 413145241 1540 01950 524.300 77.002 148.50O 156.300

1289a 7

E.00

-u2 TN472310 4l 16 80001900 0060219102

-01912 1 112 4

040 19 441055 CO1.

51 7 46 10 2410-19040 70 D

410-000 702 379 520 W3tau 2

0 t2 Elk 08 2

00 1,6 2143 1227975 120/1016 0,0910 2 00-1972 31 0

411 15 840 0

222A 220 1400 1,700 1,460 7950 60540 8400 218a 4

0200 4O,400 405.10 00lky 4

oR-1 H -

00 0.19 0

7

-0019-21 (0

10 120 1

460 O

13 26 0

044 4100 Hull.

I2 2.4-80

WBNP-Table 2.4-2 Facts About TVA Dams and Reservoirs (Page 2 of 2)

9) All,n 91 wq1.s A -

21-. W-pt 1.ft4 m"

F-lls wchI d,

1-

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Valey6by1-d) E E-,, R:

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g) -l d

1.

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r G-1a F -~l by -

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bt dNl~Uk (.-brd) by Urhs ro E-~

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al4 g a 200 fl. -,

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-ug Pvr 1u 7nu*

3 ý1l. up 1UP-ln R-er -n 56 m-e up -* P-1 R-m v)*eU Army.,.~ I E,

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83 9 1.* d 1092o.83 2.4-81

WBNP-Table 2.4-3 TVA Dams - River Mile Distances to WBNP (Page 1 of 2)

Distance from River Structure/River Mouth River Mile(a)

WBNP (mi.)

Tennessee River Chickamauga Dam 471 57 Hiwassee River 499.5 28.5 WBNP 528 Watts Bar Dam 530 2

Clinch River 568 40 Little Tennessee River 601 73 Fort Loudoun Dam 602 74 Holston River 652 124 French Broad River 652 124 Hiwassee River 0

28.5 Ocoee River 34.5 63 Apalachia Dam 66 94.5 Hiwassee Dam 76 104.5 Nottely River 92 120.5 Chatuge Dam 121 149.5 Ocoee River 0

63 Ocoee #1 Dam 12 75 Ocoee #2 Dam 24 87 Ocoee #3 Dam 29 92 Toccoa River 38(b) 101 Toccoa River 0

101 Blue Ridge Dam 15(b) 116 Nottely River 0

120.5 Nottely Dam 21 141.5 Clinch River 0

40 Melton Hill Dam 23 63 Norris Dam 80 120 Little Tennessee River 0

73 Tellico Dam 0.5 73.5 2.4-82

WBNP-Table 2.4-3 TVA Dams - River Mile Distances to WBNP (Page 2 of 2)

Distance from River Structure/River Mouth River Mile(a)

WBNP (mi.)

Chilhowee Dam 33.5 106.5 Calderwood Dam 43.5 116.5 Cheoah Dam 51.5 124.5 Fontana Dam 61 134 Holston River 0

124 Cherokee Dam 52 176 French Broad River 0

124 Douglas Dam 32 156 a) Approximated to the one-half river mile based on U.S. Geological Survey Quadrangles river mile designations.

b) Estimated river mile. River miles not provided for Toccoa River on U.S. Geological Survey Quadrangles.

2.4-83

WBNP-Table 2.4-4 Facts about TVA Dams Above Chickamaupa Project Spillway Type Outlet Works Spillway Crest Elevation Top of Gate Capacity, cfs at Gate Elevation Top Apalachia Blue Ridge Boone Chatuge Cherokee Chickamauga Douglas Fontana Fort Loudoun Fort Patrick Henry Hiwassee Melton Hill Norris Nottely South Holston Tellico Watauga Watts Bar a) At elevation 1752.

b) At elevation 1985.

Ogee, radial gates Ogee, tainter gates Ogee, radial gates Concrete chute, curved weir, vertical-lift gates Ogee, radial gates Concrete gravity, vertical-lift fixed roller gates Ogee, radial gates Ogee, radial gates Ogee, radial gates Ogee, radial gates Ogee, radial gates Ogee, radial gates Ogee, drum gates Concrete chute, curved weir vertical-lift gates Uncontrolled morning-glory with concrete-lined shaft and discharge tunnel Ogee, radial gates Uncontrolled morning-glory with concrete-lined shaft and discharge tunnel Ogee, radial gates 1257 1675 1350 1923 1043 645 970 1675 783 1228 1503.5 754 1020 1775 1742 773 1975 713 1280 1691 1385 1928 1075 685.44 1002 1710 815 1263 1526.5 796 1034 1780 N/A 815 N/A 745 135,900 39,000 141,700 11,700 255,900 436,300 312,700 107,300 392,200 141,700 88,300 115,600 55,000 11,500 41,200(a) 117,900 41, 2 00 (b) 560,300 2.4-84

WBNP-Table 2.4-5 Facts About Non-TVA Dams and Reservoirs Drainage Area Distance from Mouth LI*U.

Maximum

Height, 111' Area of Lake Length(ft.)

(ac.)

Length of Lake L(mil.1 Proiects Maior Dams Calderwood Cheoah Chilhowee Nantahala Santeetlah Thorpe (Glenville)

River Total1

Storage, (ac.-ft.)

41,160 35,030 49,250 138,730 158,250 Construction Started Little Tennessee Little Tennessee Little Tennessee Nantahala Cheoah West Fork Tuckasegee 1,856 1,608 1,976 108 176 36.7 43.7 51.4 33.6 22.8 9.3 9.7 232 225 91 250 212 150 Minor Dams Bear Creek East Fork Tuckasegee Cedar Cliff East Fork Tuckasegee 916 750 1,373 1,042 1,054 900 740 600 390 382 810 536 595 1,690 1,605 2,863 1,462 4.5 70,810 4.6 34,711 2.4 6,315 8

10 8.9 4.6 7.5 Mission (Andrews)

Queens Creek Wolf Creek East Fork Tuckasegee Hiwassee Queens Creek Wolf Creek East Fork Tuckasegee West Fork Tuckasegee 75.3 80.7 292 3.58 15.2 24.9 54.7 455 4.8 215 2.4 165 476 121 61 37 176 106.1 1.5 1.7 10.9 3.1 38.0 50 78 180 1.46 0.5 2.2 283 817 10,056 1928 1916 1955 1930 1926 1940 1952 1950 1924 1947 1952 1952 1949 1927 140 385 39 1.4 1,797 61 Walters (Carolina P&L) 254 870 9

0.5 183 5.5 25,390 Pigeon 200 340 (1) Volume at top of gates.

2.4-85

WBNP-Table 2.4-6 Flood Detention Capacity - TVA Proiects Above Watts Bar Nuclear Plant FloodStoraae January 1 (ac.-ft.)

Project Tributary Boone Cherokee Douglas Fontana Norris South Holston Tellico Watauga Main River Fort Loudoun Watts Bar Total 75,800 749,400 1,082,000 514,000 1,113,000 252,800 120,000 152,800 Flood Storage March 15 (ac.-ft.)

48,200 749,400 1,020,000 514,000 1,113,000 220,000 120,000 152,800 Flood Storaqe Summer (ac.-ft.)

12,900 118,100 237,500 73,000 512,000 106,000 32,000 108,500 111,000 379,000 4,549,800 111,000 379 000 4,427,400 30,000 165,000 1,395,000 2.4-86

WBNP-Table 2.4-7 Peak Streamflow of the Tennessee River at Chattanooga. TN (USGS Station 03568000) 1867 - 2007 (Page 1 of 5)

Water Year(a)

Date Discharge (cfs) 1867 3/11/1867 459,000 1874 5/01/1874 195,000 1875 3/01/1875 410,000 1876 12/31/1875 227,000 1877 4/11/1877 190,000 1878 2/25/1878 125,000 1879 1/15/1879 252,000 1880 3/18/1880 254,000 1881 12/03/1880 174,000 1882 1/19/1882 275,000 1883 1/23/1883 261,000 1884 3/10/1884 285,000 1885 1/18/1885 174,000 1886 4/03/1886 391,000 1887 2/28/1887 181,000 1888 3/31/1888 178,000 1889 2/18/1889 198,000 1890 3/02/1890 283,000 1891 3/11/1891 259,000 1892 1/17/1892 252,000 1893 2/20/1893 221,000 1894 2/06/1894 167,000 1895 1/12/1895 212,000 1896 4/05/1896 269,000 1897 3/14/1897 257,000 1898 9/05/1898 167,000 1899 3/22/1899 273,000 1900 2/15/1900 159,000 1901 5/25/1901 221,000 1902 1/02/1902 271,000 1903 4/11/1903 210,000 2.4-87

WBNP-Table 2.4-7 Peak Streamflow of the Tennessee River at Chattanooaa. TN (USGS Station 03568000) 1867-2007 (Page 2 of 5)

Water Year(a)

Date Discharge (cfs) 1904 3/25/1904 144,000 1905 2/11/1905 146,000 1906 1/26/1906 140,000 1907 11/22/1906 222,000 1908 2/17/1908 163,000 1909 6/06/1909 163,000 1910 2/19/1910 86,600 1911 4/08/1911 198,000 1912 3/31/1912 190,000 1913 3/30/1913 222,000 1914 4/03/1914 105,000 1915 12/28/1914 185,000 1916 12/20/1915 197,000 1917 3/07/1917 341,000 1918 2/02/1918 270,000 1919 1/05/1919 189,000 1920 4/05/1920 275,000 1921 2/13/1921 213,000 1922 1/23/1922 229,000 1923 2/07/1923 188,000 1924 1/05/1924 143,000 1925 12/11/1924 138,000 1926 4/16/1926 92,900 1927 12/29/1926 249,000 1928 7/02/1928 184,000 1929 3/26/1929 248,000 1930 11/19/1929 180,000 1931 4/08/1931 125,000 1932 2/01/1932 192,000 1933 1/01/1933 241,000 1934 3/06/1934 215,000 2.4-88

WBNP-Table 2.4-7 Peak Streamflow of the Tennessee River at Chattanooga, TN (USGS Station 03568000) 1867 - 2007 (Page 3 of 5)

Water Year(a)

Date Discharge (cfs) 1935 3/15/1935 175,000 1936 3/29/1936 234,000 1937 1/04/1937 204,000 1938 4/10/1938 136,000 1939 2/17/1939 193,000 1940 9/02/1940 89,400 1941 7/18/1941 58,200 1942 3/22/1942 72,300 1943 12/30/1942 235,000 1944 3/30/1944 201,000 1945 2/18/1945 115,000 1946 1/09/1946 225,000 1947 1/20/1947 186,000 1948 2/14/1948 225,000 1949 1/06/1949 179,000 1950 2/02/1950 192,000 1951 3/30/1951 140,000 1952 (b)

(b) 1953 2/22/1953 107,000 1954 1/22/1954 185,000 1955 3/23/1955 118,000 1956 2/04/1956 187,000 1957 2/02/1957 208,000 1958 11/19/1957 189,000 1959 1/23/1959 110,000 1960 12/20/1959 108,000 1961 3/09/1961 178,000 1962 12/18/1961 190,000 1963 3/13/1963 219,000 1964 3/16/1964 122,000 1965 3/26/1965 180,000 2.4-89

WBNP-Table 2.4-7 Peak Streamflow of the Tennessee River at Chattanooga, TN (USGS Station 03568000) 1867-2007 (Page 4 of 5)

Water Year(a)

Date Discharge (cfs) 1966 2/16/1966 104,000 1967 7/08/1967 120,000 1968 12/23/1967 148,000 1969 2/03/1969 121,000 1970 12/31/1969 186,000 1971 2/07/1971 90,700 1972 1/11/1972 116,000 1973 3/18/1973 267,000 1974 1/11/1974 181,000 1975 3/14/1975 148,000 1976 1/28/1976 67,200 1977 4/05/1977 191,000 1978 1/28/1978 115,000 1979 3/05/1979 145,000 1980 3/21/1980 168,000 1981 2/12/1981 50,800 1982 1/04/1982 133,000 1983 5/21/1983 116,000 1984 5/9/1984 239,000 1985 2/02/1985 81,000 1986 2/18/1986 66,200 1987 2/27/1987 109,000 1988 1/21/1988 74,100 1989 6/21/1989 173,000 1990 2/19/1990 169,000 1991 12/23/1990 185,000 1992 12/04/1991 146,000 1993 3/24/1993 113,000 1994 3/28/1994 202,000 1995 2/18/1995 99,900 1996 1/28/1996 145,000 2.4-90

WBNP-Table 2.4-7 Peak Streamflow of the Tennessee River at Chattanooqa, TN (USGS Station 03568000) 1867 - 2007 (Page 5 of 5)

Water Year(a)

Date Discharge (cfs) 1997 3/04/1997 138,000 1998 4/19/1998 207,000 1999 1/24/1999 91,400 2000 4/05/2000 137,000 2001 2/18/2001 86,100 2002 1/24/2002 184,100 2003 5/8/2003 241,000 2004 9/18/2004 160,000 2005 12/13/2004 153,000 2006 1/23/2006 63,800 2007 1/09/2007 66,300 (a) Water Year runs from October 1 of prior year to September 30 of year identified.

(b) Not reported.

[36]

2.4-91

WBNP-Table 2.4-8 Weir Lenqth Description and Coefficients of Discharge For Areas 3 and 4 (Sheet I of 1)

Weir Parameters Coefficient of Discharge "C" in Q=CLH3/2 3.0 Watershed Area 3

Descriotion Area bounded by Reactor and Turbine Buildings to the west, embankment to the north, and railroad tracks to the east and south.

Length Feet 450(1)

Description of Control Main plant track, elevation =

728.0 555 Transformer yard track, elevation = 728.25 590 East condenser tube access track elevation = 728.22 3.0 3.0 4

Area consisting of the switchyard area west of the condenser tube access track and south of the Turbine Building.

1220 Perimeter road, elevation =

728.0 3.0 Area consisting of 540(2) the switchyard area east of the condenser tube access track and south of the transformer yard track (1) Actual crest length is 600 ft. Length reduced by flow through fence.

East and south 3.0 end of switchyard area, elevation = 728.0 150 ft. to conservatively account for decreased (2) Actual crest length is 1080 ft. Length reduced 50% to conservatively account for decreased flow through fence.

2.4-92

WBNP-Table 2.4-9 Drainage Area Peak Discharae (Sheet 1 of 1)

Drainage Area Maximum Description (leain Watershed Area' Channel formed by the west end of the switchyard and the adjacent embankment.

2 Natural drain with flow to the west from the perimeter road to the access road.

3 Pool bounded by Reactor and Turbine Buildings to the west, embankment to the north, and railroad tracks to the east and south.

Flow reach extending from the main railroad track to a section between the east Reactor Ruilding and embankment to the north.

4 Pool consisting of the switchyard area west of the condenser tube access track and south of the Turbine Building.

Pool consisting of the switchyard area east of the condenser tube access track and south of the transformer yard track.

North Pool bounded by the of Site embankment to the south of the access railroad to the east.

Pool bounded by the dike to the east, the access highway to the west, and the construction access road to the south.

15.92 728.85 Backwater was computed in the channel from the road leading to the chemical holdup ponds to the gatehouse and Office Building. The estimated peak discharge at the road (15.92 acres) was 570 cfs. Discharges at upstream locations were decreased proportional to drainage area.

67.0 728.81 Backwater was computed from the crossing the perimeter road, to the Reactor, Diesel Generator, and Waste Wvaporation System Buildings. The estimated peak discharge at the access road was 6053 cfs. This includes flow from area 2 (67 acres), flow over the construction access road north of the site (100 acres), and flow from 60 acres to the northwest of the site. Discharges at upstream locations were decreased proportional to drainage area.

22.9 728.87 The inflow hydrograph was routed with a starting elevation of 728.0 and outflow over the main and transformer yard railroad tracks.

10.0 728.79 Backwater was computed from the main railroad track to the Reactor Building. The estimated peak discharge at the railroad was 420 cfs. Discharges at upstream locations were decreased proportional to drainage area.

11.4 728.50 The inflow hydrograph was routed with a starting elevation of and outflow to the south and west over the perimeter road, and to the east over the condenser tube access track.

6.2 728.75 The inflow hydrograph was routed with a starting elevation of 728.0 and outflow to the south and east toward the perimeter road.

50 725.67 The inflow hydrograph was routed with a starting elevation of 716 (invert of double 96-inch pipe) and outflow through the double 96-inch pipe.

100 735.282 The inflow hydrograph routed with a starting elevation of 727.1 (invert of 81-by 59-inch pipe arch) and outflow through the pipe arch and over the construction access road.

1. Watershed areas 1 - 4 are shown on Figure 2.4-40a; 150-acre area north of site shown on Figure 2.4-40b.

2. Maximum elevation reached at the construction access road.

2.4-93

WBNP-Table 2.4-10 Seasonal Variations of Rainfall (PMP)

Antecedent 3-Day PMP (in.)

(in.)

21,400 Dry Interval Ratio to Main Storm 7,980 Sq.-

Sq.-Mi.

Before PMP 7,980 Sq -Mi.

21,400 Sq.-Mi.

Month (Percent)

Mi. Basin Basin (Days)

Basin Basin March 40 8.14 6.71 3

20.36 16.78 April 40 8.08 6.44 3

20.20 16.11 May 40 7.96 6.10 3

19.92 15.27 June 40 7.81 5.63 3

19.53 14.09 July 30 5.72 3.87 2/

19.07 12.92 August 30 5.72 3.87 21 19.07 13.09 September 30 6.09 4.47 2/

20.30 14.92 Source: HMR Report 41 2.4-94

WBNP-Table 2.4-11 Probable Maximum Storm Precipitation and Precioitation Excess (Page 1 of 2)

Antecedent Storm Main Storm Index Unit Areaa

Rain, Excessb

Rain, Excessc No.

Name (Inches)

(Inches)

(inches)

(Inches) 1 Asheville 6.18 2.91 18.12 15.44 2

Newport, French Broad 6.18 3.67 18.42 16.43 3

Newport, Pigeon 6.18 2.91 19.26 16.58 4

Embreeville 6.18 3.67 15.30 13.31 5

Nolichucky Local 6.18 3.67 15.42 13.43 6

Douglas Local 6.18 4.43 17.16 15.94 7

Little Pigeon River 6.18 3.81 21.12 19.13 8

French Broad Local 6.18 3.81 19.38 17.39 9

South Holston 6.18 4.60 12.12 10.90 10 Watauga 6.18 3.67 12.96 10.97 11 Boone Local 6.18 3.81 13.86 11.87 12 Fort Patrick Henry 6.18 4.60 14.34 13.12 13 Gate City 6.18 4.60 12.30 11.08 14 & 15 Total Cherokee Local 6.18 4.60 15.42 14.20 16 Holston River Local 6.18 4.60 16.74 15.52 17 Little River 6.18 3.81 20.82 18.83 18 Fort Loudoun Local 6.18 3.81 17.28 15.29 19 Needmore 6.18 2.73 20.22 17.54 20 Nantahala 6.18 2.73 20.94 18.26 21 Bryson City 6.18 2.91 20.04 17.36 22 Fontana Local 6.18 2.91 19.56 16.88 23 Little Tennessee Local -

6.18 2.91 22.50 19.82 Fontana to Chilhowee Dam 24 Little Tennessee Local -

6.18 2.91 19.26 16.58 Chilhowee to Tellico Dam 25 Watts Bar Local above 6.18 3.81 15.84 13.85 Clinch River 26 Norris Dam 6.18 4.60 13.56 12.34 27 Melton Hill Local 6.18 4.27 15.42 14.01 33 Local above mile 16 6.18 4.43 15.42 14.01 34 Poplar Creek 6.18 4.43 14.88 13.47 35 Emory River 6.18 4.43 12.78 11.37 36 Local Area at Mouth 6.18 4.43 14.94 13.53 2.4-95

WBNP-Table 2.4-11 Probable Maximum Storm Precipitation and Precipitation Excess (Continued)

(Page 2 of 2)

Index Unit Areaa No.

Name 37 Watts Bar Local below Clinch River 38 Chatuge 39 Nottely 40 Hiwassee Local 41 Apalachia 42 Blue Ridge 43 Ocoee No. 1, Blue Ridge to Ocoee No. 1 44A Hiwassee River Local at Charleston 44B Hiwassee River Local mouth to Charleston 45 Chickamauga Local Average above Chickamauga Dam Antecedent Storm Main Storm

Rain, Excessb

Rain, Excessc (inches

tInches)

(Inches)

(inches) 6.18 4.43 14.28 12.87 6.18 6.18 6.18 6.18 6.18 6.18 6.18 6.18 6.18 6.18 2.91 2.91 2.73 3.81 2.91 2.91 3.81 4.27 4.27 3.85 21.12 18.66 18.18 18.18 22.14 18.42 15.48 14.52 13.56 16.25 18.44 15.98 15.50 16.19 19.46 15.74 13.49 13.11 12.15 14.39

a. Unit area corresponds to Figure 2.4-9 numbered areas.

b. Adopted antecedent precipitation index prior to antecedent storm varies by unit area, ranging from 0.78-1.29 inches.

c. Computed antecedent precipitation index prior to main storm, 3.65 inches.

2.4-96

WBNP-Table 2.4-12 Historical Flood Events Unit Area 1

Basin French Broad at Asheville 2

French Broad Newport Local 3

Pigeon at Newport 7

Little Pigeon at Sevierville 9

South Holston Dam 10 Watauga Dam 17 Little River at Mouth 18 Fort Loudoun Local 23 Chilhowee Local 24 Tellico Local 26 Norris Dam 27 Melton Hill Local 42 Blue Ridge Dam 44A Hiwassee at Charleston (RM 18.9)

Flood 4/05/1957 5/03/2003 3/13/1963 3/17/1973 3/28/1994 3/28/1994 5/06/2003 3/17/2002 5/06/2003 3/12/1963 3/16/1973 3/18/2002 3/12/1963 3/17/1973 1/14/1995 3/17/1973 3/17/1973 3/16/1973 5/06/2003 3/17/1973 5/06/2003 3/17/2002 3/16/1973 3/29/1951 3/27/1965 3/16/1973 Rain (in.)

5.53 5.66 5.31 4.68 5.60 6.19 7.18 4.61 6.19 3.12 3.33 4.41 3.64 3.61 6.97 6.26 6.81 6.97 6.19 7.34 7.84 5.00 6.66 5.70 6.04 7.36 Runoff (in.)

2.30 1.44 2.47 2.20 2.33 2.92 2.68 3.46 3.85 1.55 1.29 1.55 2.16 1.84 3.75 3.82 3.14 3.24 3.13 3.56 3.72 2.90 4.85 1.61 3.52 5.84 2.4-97

WBNP-Table 2.4-13 Unit Hydroaraph Data (Page 1 of 2)

Unit Area GIS Drainage Area Duration Number Name 1

2 3

4 5

6 7

8 9

10 11 12 13 14&15 Asheville Newport, French Broad Newport, Pigeon Embreeville Nolichucky Local Douglas Local Little Pigeon River French Broad Local South Holston Watauga Boone Local Fort Patrick Henry Gate City Total Cherokee Local (sq. mi.)

(hrs.)

Qp Cp Tp W50 W7 5 TB 944.4 6

14,000 0.21 12 39 15 168 913.1 6

43,114 0.66 12 10 4

48 667.1 6

30,910 0.65 12 8

4 90 804.8 4

33,275 0.65 12 10 7

80 378.7 6

11,740 0.44 12 14 6

90 835 6

47,207 0.27 6

8 5

60 352.1 4

17,000 0.75 12 10 6

66 206.5 6

8,600 0.20 6

13 6

60 703.2 6

15,958 0.53 18 25 17 96 468.2 4

37,002 0.74 8

6 3

32 667.7 6

22,812 0.16 6

13 7

90 62.8 6

2,550 0.19 6

12 7

66 668.9 6

11,363 0.56 24 34 26 108 854.6 6

25,387 0.42 12 20 10 54 16 Holston River Local 17 Little River 18 Fort Loudoun Local 19 Needmore 20 Nantahala 21 Bryson City 22 Fontana Local 23 Little Tennessee Local-Fontana to Chilhowee Dam 24 Little Tennessee Local-Chilhowee to Tellico Dam 25 Watts Bar Local above Clinch River 26 Norris Dam 27 Melton Hill Local 289.6 378.6 323.4 436.5 90.9 653.8 389.8 404.7 650.2 295.3 2912.8 431.9 6

4 6

6 2

6 4

6 8,400 0.27 9

11,726 0.68 16 20,000 0.29 6

9,130 0.49 18 3,130 0.38 8

26,000 0.43 10 17,931 0.14 4

16,613 0.58 12 18 15 10 22 16 13 14 10 12 96 7

96 5

36 12 126 11 54 7

60 7

28 4

84 6

22,600 0.49 12 6

11,063 0.18 6

15 8

54 10 4

90 6

6 43,773 0.07 6

12,530 0.14 6

18 19 6

102 10 90 2.4-98

WBNP-Table 2.4-13 Unit Hydroqraph Data (Continued)

(Page 2 of 2)

Unit Area GIS Drainage Area Duration (sq. mi.)

(hrs.)

Number Name Qp Cp T p W5 0 W7 5 TB 33 34 35 36 37 Local above mile 16 Poplar Creek Emory River Local area at Mouth Watts Bar Local below Clinch River 37.2 135.2 868.8 29.3 408.4 2

2 4

2 6

4,490 0.94 6

2,800 0.61 20 36,090 0.39 8

3,703 0.99 6

16,125 0.19 6

38 Chatuge 189.1 1

19,062 0.24 2

39 Nottely 214.3 1

44,477 0.16 1

40 Hiwassee Local 565.1 6

23,349 0.58 12 41 Applachia 49.8 1

5,563 0.26 2

42 Blue Ridge 231.6 2

11,902 0.40 6

43 Ocoee No. 1 Local 362.6 6

17,517 0.23 6

44A Hiwassee at Charletson 686.6 6

9,600 0.59 30 44B Hiwassee at Mouth 396.0 6

16,870 1.00 18 45 Chickamauga Local 792.1 6

32,000 0.38 9

Definition of Symbols Qp = Peak discharge in cfs Cp =Snyder coefficient Tp= Time in hours from beginning of precipitation excess to peak of unit hydrograph W

= Width in hours at 50% of peak discharge W75= Width in hours at 75% of peak discharge TB = Base length in hours of unit hydrograph 3

2 26 13 1T1 6

3 2

10 4

3 2

1 1

11 6

4 1

10 7

12 8

39 23 11 6

14 7

48 90 84 48 90 37 12 96 23 60 36 108 78 36 2.4-99

v t e Letter Sequence Request
TAC:MC4717, (Approved, Closed)
Initiation Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request, Request Acceptance... Supplement, Supplement, Supplement, Supplement, Supplement, Supplement, Supplement, Supplement, Supplement, Supplement Administration Withholding Request Acceptance Questions 3 June 2005 10 February 2006 10 February 2006 25 November 2008 3 December 2008 29 September 2009 23 December 2009 7 January 2010 17 June 2010 28 June 2011 Answers 11 April 2006 11 April 2006 23 February 2009 3 March 2009 19 May 2011 12 May 2011 Results Approval Other: ML052490347, ML052500298, ML052850352, ML061010180, ML062910513, ML062910522, ML063060251, ML073370317, ML073380152, ML090720869, ML101590556, ML11154A093, ML11230B250, ML14283A526, ML15191A183
MONTHYEAR2008-11-25 [Table View]

ML12236A164

Text

Subject:

SUMMARY

3.0 TECHNICAL EVALUATION

4.0 REGULATORY EVALUATION

5.0 ENVIRONMENTAL CONSIDERATION

SUMMARY

Reference:

Reference:

3.0 TECHNICAL EVALUATION

4.0 REGULATORY EVALUATION

5.0 ENVIRONMENTAL CONSIDERATION

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