ML21217A302

02B - NRC - Matl Tech Exchg - BWRVIP-100

ML21217A302
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Issue date:	08/09/2021
From:	Michael Benson, Ching Ng, David Rudland NRC/NRR/DNRL
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Download: ML21217A302 (7)

Text

NRC Perspective on BWRVIP-100 Nonconservatism Michael L. Benson, Ching Ng, and David Rudland Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Industry/NRC Materials Programs Technical Information Exchange August 10, 2021

History of Core Shroud Cracking

• Relatively severe cracking discovered in the 1990s

• NRC issued Generic Letter 94-03

• NRC published NUREG-1544

- Available via websearch

- Summary of the cracking experience

- Summary of industry response, included inspection and repair activities Industry/NRC Materials Programs Technical Information Exchange 2

Aging Management of Core Shroud

• BWRVIP-76, Revision 1-A

- proprietary

• BWRVIP-100, Revision 1-A

- proprietary

• Inspection and flaw evaluation guidelines for the core shroud

• ASME Code,Section XI inspection requirements for Core Support Structures

• Generic Aging Lessons Learned reports for license renewal and subsequent license renewal Industry/NRC Materials Programs Technical Information Exchange 3

BWRVIP-100 Nonconservatism

• Affects toughness and fluence threshold assumptions

• Actual toughness could be lower than currently assumed in licensee calculations

• Failure mode could transition from ductile to brittle earlier in life than previously assumed in licensee calculations

• Licensee inspection frequencies for the core shroud could be too long Industry/NRC Materials Programs Technical Information Exchange 4

Staff Flaw Evaluation

• Conservative assumptions

- Shroud wall thickness

- Crack size (through-wall and 360° surface flaw)

- Loading conditions (i.e., accident conditions)

• Staff calculations indicate that the shroud can tolerate a relatively severe flaw Industry/NRC Materials Programs Technical Information Exchange 5

Risk-Informed Decision Making

• NRRs LIC-504 process for dispositioning emergent safety concerns

• Applies key principles of risk-informed decision-making

• Staff weighs options for NRC management consideration

- Immediate action

- Longer-term action

- No action

• The staff does not recommend immediate action Industry/NRC Materials Programs Technical Information Exchange 6

Conclusions

• Licensees should update their inspection programs to account for the new information

• LIC-504 document describes complete NRC risk-informed evaluation of the significance

- Flaw analysis and risk analysis

- Currently under internal review

- May be made public in the future

• NRC is currently assessing options for appropriate oversight of industry actions on this matter Industry/NRC Materials Programs Technical Information Exchange 7