ML11152A170
| ML11152A170 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar, Sequoyah  |
| Issue date: | 05/12/2011 |
| From: | Tennessee Valley Authority |
| To: | Office of Nuclear Reactor Regulation |
| lingam s p | |
| Shared Package | |
| ML11152A171 | List: |
| References | |
| TAC MC4717, TAC MC4718, TAC MC4730, GL-04-002 | |
| Download: ML11152A170 (33) | |
Text
TENNESSEE VALLEY AUTHORITY SEQUOYAH NUCLEAR PLANT, UNITS 1 AND 2 WATTS BAR NUCLEAR PLANT UNIT 1 WATTS BAR NUCLEAR PLANT, UNIT 1 NRC Meeting NRC Meeting Regarding Generic Letter 2004-02 Responses Rockville, Maryland May 12, 2011
Agenda
- Introduction
- Review of TVA Small Break Loss-of-Coolant Rod Krich Accident Water Level Calculations
- Sequoyah Nuclear Plant (SQN)
Sump Strainer Submergence Chris Carey
- Watts Bar Nuclear Plant (WBN)
Sump Strainer Submergence
- Review of WBN Sump Strainer Structural Robert Kirkpatrick Robert Kirkpatrick Review of WBN Sump Strainer Structural Integrity Calculation
- Applicability of Sump Strainer Structural Integrity Calculation to SQN/Plans for p
Dave Lafever Integrity Calculation to SQN/Plans for Insulation Remediation
- Schedule for Final Request for Additional Information Response Submittal Kara Stacy Information Response Submittal
- Closing Remarks Rod Krich 2
Background
- Demonstrate Adequate Sump Performance during Small Break Loss-of-Coolant Accident (SBLOCA)
- NRC Concern is Tall Sump Strainers will be Partially Submerged when Engineered Safety Features (ESF) Pumps begin to take Suction from Sump
- Condition could Result in Less Water Flowing through Strainers than being Drawn by ESF Pumps and Cause Loss of Emergency Core Cooling System (ECCS) and/or Containment Spray (CS)
Cooling System (ECCS) and/or Containment Spray (CS) 3
SQN Sump Strainer Submergence (continued)
ESF Design
y g
g Storage Tank (RWST)
RWST L L
L l
Aligned to Sump on RWST Low-Low Level
- CS Actuates on High-High Containment Pressure (2 psig)
- Containment Air Return Fans Force Steam and Hot Air in Lower Compartment through Ice Condenser
- Steam is condensed
- Hot Air is cooled
- Melt Water from Ice is produced
- Containment Design Channels All Water from CS and Steam 4
g Condensation/Ice Melt Back to Sump
SQN Sump Strainer Submergence (continued)
SBLOCA Characteristics
- Break Flow Small enough that High Head ECCS Pumps Maintain Pressurizer Level
- ECCS Pumps Provide Additional Water to RCS to Maintain Constant RCS Water Volume as RCS Cools Requirement Identified by NRC in Discussions with TVA/WBN, Unit 1
- RCS Pressure Remains above Accumulator Pressure Pre ents Acc m lator and Lo Head Safet Injection Prevents Accumulator and Low Head Safety Injection
- Break Could be Located Such that Liquid Portion of Break Flow is Contained Inside Reactor Cavity (does not Fill Sump)
This Break Location is not a Design Basis Accident Location
Not in Hot or Cold Leg Pipe This Break would not Generate Debris that could be Transported to Sump 5
- CS Actuates for Even Smallest Breaks due to Buildup of Steam and Hot Air in Lower Compartment of Containment
SQN Sump Strainer Submergence (continued)
Original SQN SBLOCA Sump Level Calculation
- Used Only Portion of Water Available in RWST between RWST Used Only Portion of Water Available in RWST between RWST Minimum Full Level and RWST Low Level
- Assumes Reactor Cavity Filled prior to RWST Low Level Assumes Reactor Cavity Filled prior to RWST Low Level
- Only Occurs if CS does not Actuate
- Did not Account for Contribution to Sump Water Volume from Steam
- Did not Account for Contribution to Sump Water Volume from Steam Condensing and Ice Melting 6
SQN Sump Strainer Submergence (continued)
Revised SQN SBLOCA Sump Level Calculation
- Still includes Adverse Instrument Errors
- Change increases Water Volume in Containment by 35,000 gallons
- Change Results in Less Water in Reactor Cavity at RWST Low Level and More Water in Sump 7
SQN Sump Strainer Submergence (continued)
Revised SQN SBLOCA Sump Level Calculation C l l ti D t i
d L B
d A
t f I th t ld b
- Calculation Determined Lower Bound on Amount of Ice that would be Melted by Steam from Break
- Amount of Steam Released from Break based on Saturated Conditions at ou t o Stea e eased o
ea based o Satu ated Co d t o s at RCS Pressure of 600 psia, Accumulator Pressure
- Released Steam Assumed to Condense on Ice and Flow to Sump as Saturated Water at Containment Pressure of 16 4 psia Saturated Water at Containment Pressure of 16.4 psia Lowest Containment Pressure that CS would be in Operation
- Melt Water from Ice also Assumed to Flow to Sump as Saturated Water at e t ate o
ce a so ssu ed to o
to Su p as Satu ated ate at Containment Pressure of 16.4 psia Minimizes Amount of Ice that is Melted
- Ice Melt Predicted by this Method is Low Compared to Better Estimate Modeling of Ice Condenser by TVAs version of CONTEMPT 8
SQN Sump Strainer Submergence (continued)
Revised SQN SBLOCA Sump Level Calculation
- Explicitly Evaluated following Breaks inside Reactor Cavity
- Explicitly Evaluated following Breaks inside Reactor Cavity
- 100 gpm - Just above Break Size that would be Considered LOCA (i.e., Greater than Normal Makeup Capability)
- 1200 gpm - Bounding Flow Rate for One-Train of ECCS when RCS remains Pressurized above Accumulator Pressure 2500 gpm Bounding Flow Rate for Two Trains of ECCS when RCS remains
- 2500 gpm - Bounding Flow Rate for Two-Trains of ECCS when RCS remains Pressurized above Accumulator Pressure
- Breaks Larger than 2500 gpm cannot be Maintained above Accumulator Injection Pressure
- Accumulator Injection increases Water Volume in Containment 9
SQN Sump Strainer Submergence (continued)
Insights from Revised SQN SBLOCA Sump Level Calculation
- About 1 12 gallon of Melt Water Flows to Sump for every gallon removed
p g
p Pumps in Operation
- Two CS Pumps increase Holdup of RWST Water in Refueling Canal
- More Ice Melt due to Longer Time required to Drain RWST to Low Level 10
SQN Sump Strainer Submergence (continued)
Revised SQN SBLOCA Sump Level Calculation Results
- When Two CS pumps are in Operation
- When Two CS pumps are in Operation 100 gpm - 5.73 feet (4.36 feet of strainer submerged)
- 1200 gpm - 5.83 feet (4.45 feet of strainer submerged)
- 2500 gpm - 5 94 feet (4 57 feet of strainer submerged) 2500 gpm - 5.94 feet (4.57 feet of strainer submerged)
- Above Sump Water Level are Greater than Previously Determined SBLOCA Sump Level of 2.5 feet at RWST Low Level p
- Increase due to Crediting Additional Water Volume in RWST Accounting for Melt Water Addition to Sump Addi S
Ti D
d t
C l l ti hi h d
Adding Some Time Dependency to Calculation, which reduces Water Holdup in Reactor Cavity 11
SQN Sump Strainer Submergence (continued)
Revised SQN SBLOCA Sump Level Calculation Results
- Full Submergence of Tall Strainers Occurs < 4 7 minutes after RWST Full Submergence of Tall Strainers Occurs < 4.7 minutes after RWST Low Level due to Continued Operation of CS Pumps with their Suctions Aligned to RWST
- Due to Low Sump Flow Rate (< 2,500 gpm) and Short Time Period, no Significant Debris Accumulation Occurs prior to Full Submergence
- Debris Load for SBLOCA is < 25 percent of Debris Load for LBLOCA p
No Potential for Significantly Loading Strainers
- When CS Pumps are aligned to Sump (at RWST Low-Low Level), Sump Water Level is 8.5 feet or 1 foot above top of Tall Strainers (sump flow
> 5,100 gpm and < 12,500 gpm)
WBN, Unit 1, Sump Strainer Submergence Overview
- All Water Levels will Fully Submerge Strainer
- All Water Levels will Fully Submerge Strainer
- Minimum SBLOCA Water Level increased to 5.78 feet from 5.48 feet
- SBLOCA Water Level increases over Time
- With Minimum Level at ECCS Switchover
- Hold Up in RCS due to decreasing Temperature Accounted for in Calculation 13
WBN, Unit 1, Sump Strainer Submergence (continued)
Assumptions
- All Assumptions are Conservative and Taken to Minimize Sump Water All Assumptions are Conservative and Taken to Minimize Sump Water Level
p
- Minimum Injection from RWST
- No Accumulator Injection
- Reactor Coolant System (RCS) Break Location between Reactor Vessel and Biological Shield Wall
- Maximum RCS Makeup due to Fluid Shrinkage p
g
- Minimum Ice Melt 14
WBN, Unit 1, Sump Strainer Submergence (continued)
Assumptions
- Fluid Assumed at Maximum Temperature of 105°F to Minimize Water Mass in Tank
- Beginning Water Level at Minimum Operating Level
- ECCS and CS Switchovers to Sump at Low and Low-Low Water Level Setpoint at Upper Analytical Limits
- No Accumulator Injection
- No Fluid from Accumulators Credited in SBLOCA Water Level Calculation
- RCS Break Location between Reactor Vessel and Biological Shield Wall
- Break Location Chosen so all ECCS Injection will be held up in Reactor Cavity 15
WBN, Unit 1, Sump Strainer Submergence (continued)
Assumptions
- RCS Pressure and Temperature Assumed to Decrease at (Bounding)
Rates Defined by 2-inch Line Break
- Long-Term Shrinkage Limited to Accumulator Check Valve Pressure of 600 psia
- Minimum Ice Melt
- Ice Melt Previously Only included up to RHR Switchover
- Ice Melt Now included until CS Switchover to Sump
- Ice Melt Water and Vapor Exit Temperatures Conservatively taken to minimize Ice Melt and increase Sensible Heat Addition e ce e t a d c ease Se s b e eat dd t o 16
WBN, Unit 1, Sump Strainer Submergence (continued)
Analysis
- Methodology not Changed between Revisions
- Methodology not Changed between Revisions
- RCS Holdup due to Fluid Shrinkage now Explicitly Calculated
- Calculation Accounts for Holdup Quantities in RCS, ECCS, and CS Piping, Containment Atmosphere, and Physical Locations in Containment
- Fluid Volume Contributed from RCS Break and Ice Melt is Calculated using WBN Containment Dynamic Response Model based on WCAP 8282 WCAP-8282
- Sump Water Volume then Calculated by Interpolation using Fluid Volume from Model 17 Volume from Model
WBN, Unit 1, Sump Strainer Submergence (continued)
Summary
- Assumptions are Conservative and Documented in Calculation 18
WBN, Unit 1, Sump Strainer Structural Integrity Overview
- Initial Design Basis Debris Loaded Thin Bed Test Performed at Alden Initial Design Basis Debris Loaded Thin Bed Test Performed at Alden Laboratories, July 2010, resulted in Unacceptably High Head Loss
- Further Testing Reduced Debris Head Loss g
- But only by Removing Min-K Insulation
- As-Tested Head Loss had Negative Impact on Strainer Assembly As Tested Head Loss had Negative Impact on Strainer Assembly Structural Integrity
q p
Net Positive Suction Head (NPSH) Requirements 19
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
Overview
- Reducing Head Loss and Acceptance of WBN Unit 1 Strainer Testing Reducing Head Loss and Acceptance of WBN, Unit 1, Strainer Testing Results Requires Additional Analysis and Modifications
- Three Approaches Considered for Addressing Test Results
- 1. Develop New Clean Strainer Head Loss (CSHL) Calculation using Computational Fluid Dynamics (CFD) Modeling of Strainer to Reduce CSHL
- 2. Determine Maximum Structural Qualification of Strainer Assembly
- 3. Modification to Reduce Debris Head Loss
- Pursuing Three Approaches Led to Following Solutions to increased Head Loss Head Loss
- 1. Replacement of Plenum Cover Plate with Larger Orifice Sizes to Reduce CSHL based on Results of New Calculation 2
M i
St t
l Q lifi ti f St i
A bl
- 2. Maximum Structural Qualification of Strainer Assembly
- 3. Removal of Min-K Insulation to Reduce Debris Head Loss 20
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
CSHL Reduction
- CFD Analysis Performed to Evaluate CSHL CFD Analysis Performed to Evaluate CSHL
- Preliminary CFD Analysis Calculated CSHL of > 5 feet
- CSHL is above Value Allowed by Design Specification used to Procure
- CSHL is above Value Allowed by Design Specification used to Procure Strainer Assembly
- High CSHL Results from the existing 5-inch and 5.5-inch Orifices used for g
g Flow Balancing through 23 Strainer Modules 21
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
CSHL Reduction
- Optimal Orifice Size Determined Using CFD Method by Incrementally Optimal Orifice Size Determined, Using CFD Method, by Incrementally Adjusting Orifice Diameter over Multiple Model Executions
- Optimal Orifice Sizes Reduce Head Loss while Maintaining Flow Balance
- New Orifice Diameters Range from 6.5 inches to 8 inches
- New Clean Strainer Head Loss is < 2 feet New Clean Strainer Head Loss is 2 feet
- Since Orifices are Holes Cut in Plenum Top Cover Plates, Replacement of Orifices Requires Replacement of Cover Plates
- Design Change for Plenum Cover Plate Replacement will be Issued in Support of Implementation and Completion during Fall 2012 Outage 22
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
CSHL Reduction
- Plenum Cover Plates are shown below Plenum Cover Plates are shown below 23
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
Structural Qualification
- WBN, Unit 1, Structural Calculation Currently Qualifies the Strainer WBN, Unit 1, Structural Calculation Currently Qualifies the Strainer Assembly for a Debris-Laden Head Loss of 3.65 feet
- Additional Preliminary Study Calculation Confirms WBN, Unit 1, Strainer is Structurally Qualified Up to a Head Loss of 5.7 feet
- New Total Head Loss is 3.83 feet, including Debris Loading at 120°F
- After Modifications Implemented 24
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
Min-K Insulation Removal
- Min-K Insulation was Primary Source of Head Loss during Recent Min K Insulation was Primary Source of Head Loss during Recent Strainer Testing
- Test of Record, without Min-K, had Significantly Less Head Loss
- WBN will Remove All Min-K from Applicable Locations inside Containment
- Min-K Insulation Currently used in Areas where Close Commodity Clearances Exist with Hot Piping 25
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
Min-K Insulation Removal
- 15 Locations where Min-K will be removed 15 Locations where Min K will be removed
- Locations Distributed throughout Lower Containment WBN M i t i St i t C t
l f I l ti i
C t i t th h
f
- WBN Maintains Strict Control of Insulation in Containment through use of Design Drawings
- Therefore, All Min-K Locations are known
- Walkdowns Performed during recent Spring 2011 Outage to Confirm Information on Drawings
- Volume of Min-K Varies from 0.03ft3 to 0.94ft3 depending on Location 26
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
Min-K Insulation Removal 27
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
Min-K Insulation Removal 28
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
Min-K Insulation Removal 29
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
Min-K Insulation Removal
- Some Locations may be replaced with an Industry Approved Some Locations may be replaced with an Industry Approved Non-Fibrous Type Insulation That does not Invalidate Sump Testing
- For Other Locations, if the Approved Material does not Provide Adequate Insulating Value, Modifications will be Required to Resolve Commodity Clearance Issues Clearance Issues
- Design Changes for Insulation Changes and Reroutes to Resolve Commodity Clearances will be issued in Support of Implementation and Completion during Fall 2012 Outage 30
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
Summary Existing Configuration Modified Configuration Existing Configuration Modified Configuration Orifice Diameters (inches) 5.0 and 5.5 6.5, 7.0, 7.5, and 8.0 Debris Head Loss (feet)*
0.03 1.88 Clean Strainer Head Loss (feet) 3 62 1 95 Clean Strainer Head Loss (feet) 3.62 1.95 Total Strainer Head Loss (feet) 3.65 3.83 Structural Qualification (feet) 3.65 5.7**
RHR NPSH Margin (feet) 9 4 11 0 For 120°F conditions Temperature Corrected 2010 Test Head Loss is 1 09 feet at 190°F Maximum RHR NPSH Margin (feet) 9.4 11.0 CS NPSH Margin (feet) 5.53 4.74 For 120 F conditions. Temperature Corrected 2010 Test Head Loss is 1.09 feet at 190 F Maximum Sump Temperature.
- Value will Change Slightly as it is based on 5.0- and 5.5-inch Orifice Diameters. New Value will Support Modified Total Strainer Head Loss.
31
WBN, Unit 1, Sump Strainer Structural Integrity (continued)
Summary
- Initial Design Basis Debris Loaded Thin Bed Test Resulted in Initial Design Basis Debris Loaded Thin Bed Test Resulted in Unacceptably High Head Loss
- High Test Head Loss will be resolved by
- 1. Implementing Design Change to Replace Plenum Cover Plate with Larger Orifice Sizes in Fall 2012 Outage
- 2. Completing Reanalysis of Structural Qualification of Strainer Assembly with New Orifices
- 3. Implementing Design Change to Remove Min-K Insulation in Fall 2012 p
g g
g Outage 32
Applicability of Sump Strainer Structural Integrity Calculation to SQN/Plans for Insulation Remediation
- SQN does not Contain Fibrous Insulation No Insulation Remediation Required
- SQN Individual Strainer Modules are Similar to WBN, Unit 1, Strainer Modules Similar Structural Margin Results Expected for Basic Module Elements
- SQN Plenum Assemblies are More Compact with Different Plenum Support Arrangement than WBN Plenum Assemblies 123 inches X 69 inches - SQN versus 304 inches X 130 inches - WBN, Unit 1
- SQN Configuration Specific Evaluations Required