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TM
Davis-Besse Nuclear Power Station
Nuclear Regulatory CommissionPublic MeetingJanuary 5, 2012
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Davis-Besse Shield Building - NRC Informational Presentation 2TM
Agenda
Introduction
– Barry Allen , Site Vice President – Davis-Besse
17th Mid-Cycle Outage Summary
– Brian Boles , Director – Site Operations
Shield Building Condition Evaluation
– Ken Byrd , Director – Site Engineering
Closing Comments
– Barry Allen , Site Vice President – Davis-Besse
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Davis-Besse Shield Building - NRC Informational Presentation 3TM
Desired Outcomes
Demonstrate FENOC’s overriding priority and commitmentto ensure the safe and reliable operation of Davis-Besse
Recap 17 Mid-Cycle Outage Activities
Describe the discovery and investigation of the ShieldBuilding concrete cracking
Describe the evaluations performed to verify ShieldBuilding capability for operating conditions
Provide summary of actions going forward
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Davis-Besse Shield Building - NRC Informational Presentation 4TM
17th
Mid-Cycle OutageSummary
Brian Boles – Director, Site Operations
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Davis-Besse Shield Building - NRC Informational Presentation 5TM
Davis-Besse Plant Status
Currently Operating at 100% Reactor Power
– 950 Megawatts
31 Days On-Line
122 Days Until 17th Refueling Outage
Reactor, Primary and Secondary systems operating as
designed
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Davis-Besse Shield Building - NRC Informational Presentation 6TM
Davis-Besse Mid-Cycle Outage Summary
Safety Upgrades
– Replaced Reactor Head
– Replaced Integrated Control System
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Davis-Besse Shield Building - NRC Informational Presentation 7TM
Davis-Besse Outage Summary
ReliabilityImprovements
– Replaced StationBattery Chargers
– Installed new stylePower OperatedRelief Valve
– Replaced powercables
– Replaced CoreCooling Motors
– Performed MotorOperated and AirOperated Valvemaintenance
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Davis-Besse Shield Building - NRC Informational Presentation 8TM
Davis-Besse Outage Summary
Operating MarginImprovements
– Replaced selectedService Water Piping
– Drained and cleanedTurbine Plant Cooling
Water System – Performed Cooling
Tower maintenance
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Davis-Besse Shield Building - NRC Informational Presentation 9TM
Davis-Besse Outage Summary
Inspection Requirements
– Completed 10 year
Inservice inspection – Inspected all fuel
assemblies
– Performed Integrated
Leak Rate Testof Containment
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Davis-Besse Shield Building - NRC Informational Presentation 10TM
Outage Conclusions
Mid-Cycle Outage conducted in a safe manner
FENOC continues to invest in safety and reliability
improvements
As a result of work performed during the Mid-CycleOutage, Davis-Besse is operating safely and efficientlywith improved margins to safety.
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Davis-Besse Shield Building - NRC Informational Presentation 11TM
Shield Building ConditionEvaluation
Ken Byrd – Director Site Engineering
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Davis-Besse Shield Building - NRC Informational Presentation 12TM
Background
Mid-cycle outage to replace Reactor Pressure Vessel Head
Access opening required in concrete Shield Building
Opening dimensions 26.5’ w X 35.5’ h
Hydro-demolition method employed
Previous opening in 2002 used similar method
Size and orientation was different
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Davis-Besse Shield Building - NRC Informational Presentation 13TM
Shield Building
Purpose of ShieldBuilding
– Biological Shielding – Environmental protection
for containment vessel
– Controlled release of
annulus atmosphereunder accident conditions
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Davis-Besse Shield Building - NRC Informational Presentation 14TM
Shield Building
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Davis-Besse Shield Building - NRC Informational Presentation 15TM
Shield Building
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Davis-Besse Shield Building - NRC Informational Presentation 16TM
Shield Building Flutes
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Davis-Besse Shield Building - NRC Informational Presentation 17TM
Shield Building Flutes
Architectural Feature
– The flute shoulders arepart of the ShieldBuilding. Concrete forshoulders and buildingshell was placedconcurrently.
– Evaluation of structuralcapacity of ShieldBuilding does not creditflute shoulders.
– Evaluated as a dead loadin structural analysis.
AuxiliaryBuildingRoof
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Davis-Besse Shield Building - NRC Informational Presentation 18TM
Shield Building
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Davis-Besse Shield Building - NRC Informational Presentation 19TM
Discovery
Cracking found on October 10 during hydro-demolition
Indications reported
– Full length of left side of opening
– Top and bottom of opening on left side
– Indications appeared to be near the first (outer) rebar mat
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Davis-Besse Shield Building - NRC Informational Presentation 20TM
Initial Response
NRC Resident Inspector notified
Condition Report written
Restraint on restart established
Mobilized team of experts to investigate issue
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Davis-Besse Shield Building - NRC Informational Presentation 21TM
Initial Investigation
Decision made to “chip back” concrete to observeindication
No indications remained on bottom or left side of openingafter chipping
Chipping at top of opening revealed the crack extendedbeyond the construction opening
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Davis-Besse Shield Building - NRC Informational Presentation 22TM
Upper Opening Indication Investigation
Construction Technology Laboratory (CTL) ImpulseResponse (IR) testing methodology used to investigateextent of crack
IR was employed as a tool to provide a non-destructivemeans to identify regions of interest for furtherinvestigation
Core bores were taken to confirm the boundary of crackingbased on IR results
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Davis-Besse Shield Building - NRC Informational Presentation 23TM
Initial Investigation Around Construction Opening
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Davis-Besse Shield Building - NRC Informational Presentation 24TM
Investigation of Extent of Condition
Extent of Cracking Condition was investigated using IRtesting and Core Bores.
The results of the investigation were documented in theCorrective Action Process as follows:
– October 10, Initiated CR and notified NRC – Unexpected concretecrack within Shield Building temporary opening
– October 24, Initiated CR and notified NRC – Additional crackidentified in Shield Building architectural flute shoulder area
– October 26, Initiated CR and notified NRC – Cracking identified inShield Building at Main Steam Line Penetration region
– October 31, Initiated CR and notified NRC – Shield Building IRindications above Elevation 780
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Davis-Besse Shield Building - NRC Informational Presentation 25TM
Impulse Response Inspection
IR Testing Performed:
– 15 of 16 flute shoulders
– 6 of 8 flutes – 8 of 8 shell regions between flute shoulders
– Main Steam Line Rooms and Fan Equipment room
– Vertically to the top of Shield Building wall
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Davis-Besse Shield Building - NRC Informational Presentation 26TM
Shield Building Core Bores
70 core bores performed
Core bore pairs were usedto validate the IR results
Measured cracks were
adjacent to the ShieldBuilding outer rebar mat
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Davis-Besse Shield Building - NRC Informational Presentation 27TM
Shield Building Core Bores
All measured cracks were
very tight Eight deep core bores were
taken that reached theinterior reinforcing steel
mat. Confirmed crackingonly in outer rebar mat.
– 2 in flute shoulders
– 4 in flutes – 2 in Main Shell Region
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Davis-Besse Shield Building - NRC Informational Presentation 28TM
Shield Building Visual Examinations
Surface examinations of all areas investigated for spalledconcrete, corrosion/staining, surface bulging and looseconcrete
– Surface examination results revealed no indications of concretesurface distress impacting the structural integrity of the ShieldBuilding
– Condition Report initiated to document a case of exposedreinforcement and spalling in the corner of a shoulder.
Rebar inspected at the construction opening in goodcondition
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Davis-Besse Shield Building - NRC Informational Presentation 29TM
Laboratory Evaluation of Shield Building
Concrete
Initial Laboratory
evaluation of two corebores performed by CTLGroup
Results of tests indicate
– Concrete is in good conditionwith no apparent deficienciesthat would affect itscontinued service
– Carbonation is at a depthconsistent with the age of theconcrete
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Davis-Besse Shield Building - NRC Informational Presentation 30TM
Summary of Shield Building Condition
Cracking is generic to flute shoulder regions and can be assumedto be present at any elevation in the flute shoulders. Crackingobserved to be more prevalent on the south side of the building.
Cracking exists at the top 20 feet of the Shield Building walloutside the flute shoulder region.
Two small regions adjacent to the Main Steam Line penetrationshave similar cracks. The extent of these regions is localized andunique to these particular penetrations.
Cracks are located near the outer reinforcing mat. No crackingobserved in interior reinforcing mat.
Cracks are very tight.
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Davis-Besse Shield Building - NRC Informational Presentation 31TM
Structural Evaluation
Shield Building Design
– Building designed and constructed with significant reinforcement
– Significant Margin under design basis loads – Design Basis
– Earthquake 6 – 6.5 on Richter magnitude scale
– Tornado winds of 300 miles per hour
– Tornado wind-driven debris
– Tornado depressurization
Impact of laminar cracks on design
– Potentially reduce the bond strength between concrete andreinforcing steel
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Davis-Besse Shield Building - NRC Informational Presentation 32TM
Bond Strength
Force transfer between reinforcement and concrete
– Adhesion (small contribution)
– Friction (moderate contribution) – Mechanical bearing of lugs (most contribution)
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Davis-Besse Shield Building - NRC Informational Presentation 33TM
Bond Strength
Bond stress about 50% of capacity under Design BasisLoad
Concrete much stronger than specified (>6000 psi vsspecified 4000 psi)
Test results indicate a margin of at least 50% after initiationof a splitting crack.
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Davis-Besse Shield Building - NRC Informational Presentation 34TM
Bounding Building Analysis
Bond strength with adjacent cracks can not be quantifiedand is conservatively treated as non-existent in analysis.
Calculations performed to provide a bounding evaluation ofthe effect of cracking.
– Vertical and horizontal reinforcement ineffective for strength in fluteshoulders, two steam line penetration areas and in regions at top of
shield building. Any bond between reinforcement and concrete in crack
regions provides additional margin.
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Davis-Besse Shield Building - NRC Informational Presentation 35TM
Bounding Building Analysis
Calculations are consistent with the original design andmethodology.
Calculations were performed by highly qualified andexperienced Bechtel engineering team.
FENOC Engineering staff performed detailed owner’sacceptance review.
Independent third party engineering review was performedby Sargent & Lundy.
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Davis-Besse Shield Building - NRC Informational Presentation 36TM
Root Cause
Root Cause determination in progress
– Director Fleet Performance Improvement
– Davis-Besse Root Cause Evaluator – MPR Associates, Inc.
– Performance Improvement International
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Davis-Besse Shield Building - NRC Informational Presentation 37TM
Root Cause
Root Cause Team Activities
– Visual Inspections
– Review of IR and Core Bore Inspection Results – Review of Construction Records
– Analysis of Core/Rebar Samples
– Detailed Modeling of Shield Building
Expected Results of Root Cause
– How, Why and When Cracking Occurred
– Long Term Monitoring Program
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Davis-Besse Shield Building - NRC Informational Presentation 38TM
Summary of Evaluation Results
Shield Building meets strength requirements.
Meets serviceability as demonstrated by the structure’s
past performance. Any bond between the concrete and reinforcement in
cracked regions would be an additional margin of safety.
Shield Building is capable of performing its safetyfunctions.
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Davis-Besse Shield Building - NRC Informational Presentation 39TM
Closing Comments
Barry Allen – Site Vice President
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Davis-Besse Shield Building - NRC Informational Presentation 40TM
Summary of Actions Going Forward
FENOC has committed to the following ongoing actions:
Determine Root Cause of cracks in the Shield Building
Perform interim monitoring of the Shield Building duringthis run cycle
Perform additional monitoring of the Shield Building during
the 2012 refueling outage Develop Long-Term Monitoring Requirements
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Davis-Besse Shield Building - NRC Informational Presentation 41TM
Conclusions
The Shield Building condition has been extensivelyinvestigated to establish a thorough understanding of theextent of cracking.
Conservative calculations demonstrate the Shield Buildingmeets strength requirements.
The Shield Building is structurally sound, and is fully
capable of performing its safety function.
Davis-Besse is being operated safely.