ML12221A268
| ML12221A268 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 08/09/2012 |
| From: | FirstEnergy Nuclear Operating Co |
| To: | NRC/RGN-III |
| Atif Shaikh | |
| References | |
| Download: ML12221A268 (48) | |
Text
Shield Building Crack Investigation and Root Cause Presentation Davis-Besse Nuclear Power Station August 9, 2012
Agenda Introduction
- Barry Allen, Site Vice President - Davis-Besse Shield Building Condition Evaluation
- Ken Byrd, Director - Site Engineering Shield Building Root Cause Investigation
- Jon Hook, Design Engineering Manager Shield Building Corrective Actions
- Ken Byrd, Director - Site Engineering Closing Comments
- Barry Allen, Site Vice President - Davis-Besse Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 2
Shield Building Condition Evaluation Ken Byrd, Director - Site Engineering Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 3
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Background===
Mid-cycle outage to replace Reactor Pressure Vessel Head Access opening required in concrete Shield Building Opening dimensions 26.5 wide X 35.5 high Hydro-demolition method employed Previous opening in 2002 used similar method Size and orientation different than in 2002 Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 4
Shield Building Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 5
Shield Building Purpose of Shield Building
- Biological shielding
- Environmental protection for Containment Vessel
- Controlled release of Annulus atmosphere under accident conditions Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 6
Shield Building Flutes/Shoulders Davis-Besse Update Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 7
Shield Building Flute Shoulders Architectural Feature Shield
- The flute shoulders are a Building part of the Shield Building; concrete for shoulders and building shell was placed concurrently
- Evaluation of structural capacity of Shield Building does not credit flute shoulders
- Evaluated as a dead load in structural analysis Auxiliary Building Roof Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 8
Discovery Cracking found on October 10, 2011, during hydro-demolition NRC resident notified Condition Report written Restraint on restart established Team of experts to investigate issue mobilized Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 9
Investigation Impulse Response (IR) testing methodology used to investigate extent of crack Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 10
Investigation, continued Core bores taken to validate IR testing results, to determine crack depth and to determine crack width Investigation results were documented in the corrective action process, and the NRC was promptly notified of findings Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 11
Summary of Shield Building Condition Cracking is generic to flute shoulder regions and can be assumed to be present at any elevation in the flutes shoulders; cracking was observed to be more prevalent on the south side of the building Cracks are located near the outer reinforcing mat; no cracking observed in interior reinforcing mat Left boundary of RVCH opening Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 12
Summary of Shield Building Condition, continued Cracking exists at the top 20 feet of the Shield Building wall outside the flute shoulder region Two small regions adjacent to the Main Steam Line penetration have similar cracks
- The extent of these regions is localized and unique to these particular penetrations Cracks are very tight Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 13
Structural Evaluation Original 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 depressurization and missiles Impact of laminar cracks on original design
- Potentially reduce the bond strength between concrete and reinforcing steel
- Cracks of little impact unless reinforcing bars are spliced in the cracked region
- Shield Building remains adequate for safety function Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 14
Bounding Building Analysis Bond strength of reinforcement lap splices with adjacent cracks could not be quantified and were conservatively treated as non-existent in analysis Calculations performed to provide a bounding evaluation of the effect of cracking
- Vertical and horizontal reinforcement assumed ineffective for strength in flute shoulders, 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 Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 15
Summary of Calculation Results Shield Building meets strength requirements Any bond between the concrete and reinforcement in cracked regions would be an additional margin of safety Shield Building is capable of performing all safety functions with margin Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 16
Shield Building Root Cause Investigation Jon Hook, Manager - Design Engineering Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 17
Root Cause Overview Established independent team of experts Established a comprehensive Failure Modes Analysis Investigated the design, materials, construction methods, and present day operational conditions Performed concrete tests Performed analyses Identified root cause Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 18
Root Cause Overview, continued Performance Improvement International (PII)
- The PII team are experts in root cause investigation
- Team consist of Professional Engineers, PhDs, and university professors
- Performed more than 500 root causes Industry experts as well as assistance from FENOC Engineering Followed our established and proven root cause process Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 19
Shield Building Root Cause Fault Tree Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 20
Shield Building Concrete Concrete was subjected to a series of tests 36 concrete cores from the Shield Building tested Concrete properties were determined Test results confirmed the concrete is sound and can be ruled out Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 21
Shield Building Concrete, continued Typical concrete sample showing the laminar crack sheared the coarse aggregate Therefore, laminar crack occurred after the concrete achieved it strength Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 22
Shield Building Concrete, continued No evidence of micro cracks No signs of cyclic load mechanism No cyclic freeze-thaw mechanism No indication of fatigue or age related events Magnification at 100 Times Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 23
Shield Building Concrete Tests, continued 16 samples were tested for carbonation
- Average depth of carbonation is 8.57 mm (0.337 inches)
- Maximum average 11.7 mm ( 0.46 inches)
- Typical for concrete 40 years old Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 24
Shield Building Concrete - Conclusion Crack passed through the course aggregate
- Strong bond between the cement paste and the coarse aggregate; therefore, initial placement concerns can be ruled out
- Large tensile force is required to initiate the crack No micro cracks identified that would indicate freeze-thaw or cyclic events Chemical properties, carbonation, corrosion, etc, were all acceptable Based on the above, concrete can be ruled out as an initiating or contributing cause Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 25
Shield Building Configuration Cracking is predominantly located in the shoulder areas, the top 20 feet of the Shield Building, and near the Main Steam Line penetration block-outs; cracking concentrated on southern exposures Shoulder areas are regions of discontinuity Limited radial reinforcing steel in the shoulder areas High rebar density (6 spacing) located at the top of the Shield Building and around the Main Steam Line penetration construction block-outs Conclusion
- There is a correlation between the crack locations and the physical layout of the reinforcing steel that needed to be investigated Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 26
Shield Building Analytical Analyses Numerous computer analyses were performed for normal design conditions
- Self weight, wind loads
- Thermal analyses (summer hot and winter cold conditions)
- Fujita Category 2 tornado Stresses were significantly below the normal tensile capacity of the concrete Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 27
Shield Building Analytical Analyses, continued Analysis showed:
- Design stresses can not initiate the laminar crack
- Significant stresses beyond what is normally analyzed would be required to crack the concrete Investigate industry experience for similar conditions Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 28
Industry Experience with Laminar Cracks Ontario Ministry of the Environment study on 50 above ground water tanks in Ontario Water migrated into the concrete from the inside
- Inner layer of the wall freezes and expands
- Outer layer of wall contracts
- Creates high radial stress
- Results in laminar cracking
Conclusion:
Laminar cracking as a result of water freezing is a real potential Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 29
Shield Building Investigation into Water Intrusion/Freezing The effects of moisture intrusion and sub freezing temperatures was investigated as a possible cause The review of severe environmental conditions that the plant was exposed to was performed The most significant event recorded at the site and also in Ohio history was the storm of January 25-27, 1978 Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 30
Moisture Intrusion and Low Temperatures January 25-27, 1978, was the worst in terms of:
- Moisture
- Winds
- Temperature
- Duration
- Pressure Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 31
Moisture Intrusion and Low Temperatures, continued Scenario:
- Temperature near zero
- Sustained strong winds
- Moisture penetrated the Shield Building
- Moisture trapped in the outer layer of concrete crystallized
- Concrete expansion exceeded the tensile capacity of the concrete and propagated the crack Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 32
Moisture Intrusion and Low Temperatures, continued A complex computer model of the Shield Building was developed Concrete properties from the concrete core tests were used Laboratory tests showed moisture infiltration up to four inches Maximum radial stress in the shoulder area were approximately the tensile capacity of the concrete High stresses were located in areas of observed cracking Max 550 psi Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 33
Sensitivity Analysis - High Density of Rebar A complex computer model evaluated the affects of rebar spacing to determine the potential for developing cracks Evaluation showed laminar cracks could:
- Form in regions of closely spaced rebar and
- Less likely in areas were the rebar is spaced at 12 inches This analysis establishes that rebar spacing is a probable contributing factor Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 34
Summary of Analyses Normal design conditions result in low stresses which could not cause cracking Moisture and freezing could cause high stresses in the shoulder areas that results in cracking Analysis shows closely spaced reinforcing steel can be a contributor to laminar cracking Observed cracking coincides with the locations of high stress in the shoulder areas and in the areas of high density of rebar; cracking concentrated on southern exposures Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 35
Shield Building Root Cause Root Cause:
- Lack of water sealant on the concrete exterior Contributing Causes:
- Shoulder reinforcing details (discontinuity and no radial rebar)
- High density of rebar spacing
- High moisture, severe wind, and low temperature conditions Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 36
Shield Building Additional Actions Ken Byrd, Director - Site Engineering Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 37
Shield Building - Preventative Action Root Cause
- Lack of concrete sealant Preventative Action to Prevent Recurrence
- The exposed exterior surfaces of the Shield Building will be sealed
- Contractor has started and is expected to be completed by the end of September of this year Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 38
Shield Building - Additional Actions The Root Cause has established several additional Corrective Actions
- Complete Impulse Response (IR) examinations on the Shield Building wall
- Perform IR mapping on another structure (Auxiliary Building) to confirm assumptions of our analyses
- Develop and implement a test program to establish capacity in an area of laminar cracks
- Develop a Long-Term Monitoring program Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 39
Additional Actions - IR Mapping Complete IR examinations on the Shield Building wall and an independent structure
- All accessible areas of the Shield Building wall were mapped
- Over 60,000 individual readings were obtained to fully characterize the condition of the building Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 40
Additional Actions - IR Mapping, continued The IR validated our original assessment that the laminar cracks are generally confined to:
- The shoulder areas
- Top of the Shield Building
- Near one corner of the Main Steam Line penetration Impulse Response reading on an independent structure validated that laminar cracks are not present Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 41
Additional Actions - IR Mapping, continued Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 42
Additional Actions - Testing Tests were developed and conducted at two nationally recognized universities Professors are industry experts and are American Concrete Institute (ACI) Committee members Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 43
Additional Actions - Testing, continued Two different methods were used to create laminar cracks in the samples to be tested Results were independently verified Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 44
Additional Actions - Testing, continued Testing results
- Full capability of reinforcement is maintained in regions with longer splice lengths (upper portion of Shield Building)
- Results showed near to full capability of reinforcement in regions with shorter splice lengths Testing conclusions
- The tests provide high confidence of the capability of the rebar located in regions of laminar cracking
- Testing confirms the assumptions made in structural calculation prior to restart were very conservative Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 45
Additional Actions - Long Term Monitoring Establish a Long-Term Monitoring Program
- FENOC has established a long-term monitoring plan that includes:
- Monitoring existing core bores for crack propagation
- Inspection of the integrity of the Shield Building coatings
- Inspection of the integrity of other safety related building coatings Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 46
Summary The corrective actions established will:
- Prevent moisture from entering the Shield Building and freezing
- Provide comprehensive characterization of the laminar crack
- Establish the capacity of the rebar in the area of laminar crack
- Provide long term monitoring of the shield building Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 47
Closing Comments Barry Allen, Site Vice President Shield Building Crack Investigation and Root Cause Presentation Public Meeting August 9, 2012 48