Engineering Perspectives on First Year of New Reactor Construction

1. 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

2. Purpose • Present NRC staff view, from an engineering perspective (civil/structural), on construction inspection efforts and license amendment requests.

3. 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

4. License Amendment Requests Issued 17 License Amendments

5. Basemat Technical Issues Shear Reinforcement (T-heads) • 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 Typical T-head Installation Source: Headed Reinforcement Corporation (HRC) Website

6. 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 Source: AP1000 DCD, Rev 19

7. 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

8. ACI 349 Model

9. 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

10. 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.

11. 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. Hooked Reinforcement

12. 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

13. 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.

14. 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