Engineering Perspectives on First Year of New Reactor Construction

Mohamed Shams, Ph.D., PEStructural Engineering BC

US NRC, Office of New ReactorsJuly 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

6Source: 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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ACI 349 Model

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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

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Basemat technical IssuesResolution

– 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