ML21217A302
| ML21217A302 | |
| Person / Time | |
|---|---|
| Issue date: | 08/09/2021 |
| From: | Michael Benson, Ching Ng, David Rudland NRC/NRR/DNRL |
| To: | |
| Hiser A | |
| Shared Package | |
| ML21217A247 | List: |
| References | |
| 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
- 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