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Engineering Perspectives on First Year of New Reactor Construction
Mohamed Shams, Ph.D., PE
Structural Engineering BC
US NRC, Office of New Reactors
July 23, 2013
Purpose
• Present NRC staff view, from an engineering perspective (civil/structural), on construction inspection efforts and license amendment requests.
Background
• AP1000 Rev. 19 certification on December 30, 2011– Vogtle Units 3 & 4 license issuance February 10, 2012 – Summer Units 2 & 3 license issuance on March 30, 2012
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License Amendment Requests
Modular Construction
• Stud Spacing• Structural Module Details
Nuclear Island RC
Structures, Shield
Building & Basemat
• Nuclear Island Wall Reinforcement Criteria
• Development Length for Basemat Reinforcement
• Basemat Shear Reinforcement Design Details
• Basemat Shear Reinforcement Spacing• Basemat Concrete and Rebar Details
Turbine Building
• Bracing System and UBC vs. IBC• Battery Room and Electrical Equipment
Changes• Layout & Floor Elevation Changes
Containment• Additional Electrical Penetrations• Chemical and Volume Control System• Primary Sampling System
Issued 17 License Amendments
Basemat Technical IssuesShear Reinforcement (T-heads)
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• Inspection of basemat rebar identified two issues for the concrete slabs below the elevator and sump pits– Headed shear
reinforcement not demonstrated to comply with ACI 349-01 Appendix B
– Shear reinforcement spacing exceeded d/2
Source: Headed Reinforcement Corporation (HRC) Website
Typical T-head Installation
Headed Reinforcement
Licensing Basis– Shear stirrups have T-
headed anchors at each end
– T-headed anchors are used where rebar mechanical anchorage is required
– Criteria of ACI 349 Chapter 12 are applied for rebar development
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Source: AP1000 DCD, Rev 19
Commitments / Guidance
• ACI 349, Section 12.6, Mechanical Anchorage– Appendix B, Anchorage to Concrete
• ASTM A970• Staff open to alternative approaches to demonstrate adequate
anchorage– Manufacturer literature– Research and testing– Alternative models (Strut-and-tie)– Other relevant codes (e.g., ACI 318)
Objective - Ensure adequate anchorage for ductile rebar behavior
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Headed Reinforcement
Resolution– Approved License
Amendment Requests• ACI 318-11 provisions
for headed reinforcement
• Increase elevator and sump pits thickness
– Revise shear design of walls
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Basemat Technical Issues• Basemat Hooked Reinforcement
During a March 2012 inspection, NRC staff identified a deviation in the detailed/installed nuclear island basemat rebar relative to the typical reinforcement depicted from the AP1000 DCD.
Hooked Reinforcement
Licensing Basis• Design and analysis procedures for seismic Category I
structures are in accordance with ACI 349.• Foundation walls act integrally with the basemat to provide
stiffness and distribute foundation loads.• Basemat designed as a two-way slab.• Basemat considered as a diaphragm, as such is part of the
lateral-force resisting system.
Commitments / Guidance
• ACI 349-01– Chapter 13 – Negative moment reinforcement must
be developed at support – Chapter 12 – #14 bar splices are prohibited – Chapter 21
• Ductile connections• Cannot invoke excess reinforcement provision (12.5.3.4)
for elements in lateral force-resisting system.
Objective - Ensure ductile behavior of connection of perimeter walls to the basemat
12
Basemat technical Issues
Resolution– Re-constitution of
reinforcement hooks in basemat design.
– Approved License Amendment Requests• Increase in basemat
concrete compressive strength to decrease required development length.
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Lessons Learned• Provide quantitative technical justifications for LARs • Adhere to codes and standards in all areas of the design • Detailed design must satisfy both the DCD and code
commitments• Perform detailed engineering analysis sufficiently in
advance of installation• Institute consistency checks• Balance specificity and flexibility in licensing basis