ML14253A303
| ML14253A303 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 09/10/2014 |
| From: | Tennessee Valley Authority |
| To: | Andrew Hon Watts Bar Special Projects Branch |
| Hon A | |
| References | |
| Download: ML14253A303 (16) | |
Text
H d l Hydrology September 10, 2014
Agenda g
- Introduction and Purpose Joe Shea g
- Background Joe Shea
- TVAs Approach Gary Mauldin
- Schedule Gary Mauldin
- Conclusion Joe Shea
- Questions Gary Mauldin/Joe Shea Shea 2
Hydrology Background
- Describe TVAs Approach to resolving NRC questions regarding dam stability analyses as part of the Watts Bar Nuclear Plant (WBN) hydrology evaluation
- Describe D ib TVA TVAs plans l tto submit b it an update d t tto th the pending di WBN U Unitit 1 Li License Amendment Request and WBN Unit 2 Final Safety Analysis Report that:
- Reflect revised approach to evaluating dam stability as part of TVAs TVA s nuclear plant hydrology analyses
- Revise the hydrology modeling tools used by TVA to evaluate probable maximum flood scenarios
- Discuss schedule for submitting updated information Shea 3
Hydrology Background
- TVA proposed to revise the hydrologic analyses for Watts Bar site
- TVA andd NRC met on JJuly l 22 22, 2014 to di discuss portions i off the h submittal.
b i l Th The discussion surrounded TVAs dam stability analyses.
In 2012 2012, TVA completed re-evaluation of the design basis Probable Maximum Flood (PMF) affecting the TVA Nuclear Sites As a result of the re-evaluation, PMF levels increased from 734.9 to 739.2 at Watts Bar The increase in PMF levels resulted in a corresponding increase in the headwater and tailwater levels seen at the dams of the WBN and Sequoyah (SQN) plants.
The increase in headwater and tailwater required the structural stability of the dams to be validated Shea 4
Hydrology Background
- July 22, 2014 meeting with NRC (continued):
During an audit of TVAs TVA s Updated Final Safety Analysis Report (UFSAR) submittal in April of 2014, the NRC staff noted that the no dam failure assumption in the PMF analysis for a licensing basis change was not supported.
Based on TVAs TVA s understanding of the issue, issue the criteria used to evaluate the acceptability of dams impacting WBN and SQN for PMF will be clearly defined and should be consistent with current industry techniques
- TVA has performed analysis of applicable dams using current industry techniques Details will be discussed in subsequent slides TVA is designing and implementing improvements to certain dams consistent with current industry techniques Shea 5
Timeline of Design Basis Hydrology Events Shea 6
Tennessee River System Overview
- The existing WBN and SQN licensing basis regarding dams and hydrology are based on dam safety criteria in place at the time.
Those techniques, while meeting the requirements of the licensing basis, do not meet the documentation expectations of today
- TVA has determined that the most robust way to demonstrate the stability of its dams and associated hydraulic analysis is to convert to modern industry methodology TVA can then h document d allll its i PMP assumptions, i
methods, and conclusions in a way that will best facilitate the NRCs review to support the licensing timeline for Watts Bar Unit 2 This methodology will also more clearly demonstrate the margins in PMF for WBN and SQN.
- Simulated Open Channel Hydraulics (SOCH) is not an industry recognized standard tool.
Mauldin 7
TVAs Approach
- Two major elements of converting TVA to modern industry techniques:
1)) TVA has developed,p in its RO Dam Safetyy pprogram, g ap procedure for evaluatingg dams that is consistent with other federal dam safety agencies
- 2) TVA will convert to the United States Army Corps of Engineers Hydrologic Engineering Center River Analysis System (HEC-RAS) tool for performing hydraulic analysis.
- The new TVA analytical approach is to perform hydraulic analysis using HEC-RAS.
Using the developed nuclear PMF elevations, elevations TVA will evaluate stability of upstream dams credited in the current licensing basis using TVA RO procedures.
Any dam not meeting the requirements of TVAs current procedures based on nuclear PMF inputs will not be credited in its hydraulic PMF analyses HEC-RAS will be re-run crediting only those dams meeting current TVA RO dam stability requirements Mauldin 8
TVAs Approach
- The basis for TVA to demonstrate dam safety under PMF conditions will be the combination of upgraded dam stability criteria and HEC-RAS crediting only fully compliant dams:
Current approach will require completing in-process modifications and additional modifications. Design and implementation of the new modifications is underway Evaluate warning g times to ensure the updated p PMF analysis y continues to meet designg basis Evaluate the seismic events associated with flooding to ensure updated PMF analysis continues to meet design basis.
basis Current seismic design basis is not revised as part of this updated submittal Mauldin 9
TVAs Approach
- Submittal (Approach and Actions):
TVA proposes a submittal that moves to a full HEC-RAS model, and requests NRC staffs review of that model.
Stability assumptions of model:
o Stability for PMF to modern industry standards analysis complete for initial flood numbers (will require validation after HEC-RAS model runs are completed) o 11 dams are credited in the evaluation - Cherokee, Douglas (main/concrete dam),
Fontana, Hiwassee, Norris, South Holston, Watauga, Nottely, Chatuge, Blue Ridge, Chickamauga Mauldin 10
TVAs Approach
- Submittal (Approach and Actions) continued:
o 4 dams are not credited in the analysis - Boone, Fort Patrick Henry, Apalachia, Melton Hill o TVA will upgrade 4 reduced margin dams to dam stability standards
- Additional dam modifications will be p performed - Watts Bar and Tellico non-overflow necks, Fort Loudoun non-overflow, Watts Bar west saddle dam, Watts Bar east flood wall, Douglas saddle dam Dam stability calculations will be available for audit consistent with TVAs other upgraded calculations (Cherokee and Douglas).
Mauldin 11
TVAs Approach
- Submittal (Approach and Actions) - continued:
Assumptions of HEC-RAS model V&V model - main stream and tributaries Input assumptions for main river consistent with SOCH model Tributary reservoir routing Assumptions consistent with July 2012 submittal Main Stem Geometry Storm Data Runoff data Methodology for discharge rating Mauldin 12
TVAs Approach
- Basis (Why Approach Works):
Removes any question about the stability of dams credited in the PMF analysis Move to a full HEC-RAS model provides more consistent and reliable results, which will not result in modifications to the plant plant.
HEC-RAS tool provides a robust analysis using current industry acknowledged methodologies Facilitates timely NRC review using analysis methodology familiar to the staff Tool is consistent with TVA analysis for its Fukushima submittals
- Result:
PMF level is bounded by July 2012 License Amendment Request submittal level Mauldin 13
Schedule
- Complete p initial evaluation of affected dams - Complete p
- Commence Independent p Evaluation of reduced margin g dams - Complete p
- Complete development and implementation of modifications to reduced margin dams - Prior to Fuel Load
Conclusion
- NRC approval of TVAs proposal to transition to a HEC-RAS based model for evaluating dam stability will maintain the 2012 proposed design basis flood levels and facilitate the licensing of Watts Bar Unit 2.
2 Shea 15
Questions