ML19161A108
| ML19161A108 | |
| Person / Time | |
|---|---|
| Issue date: | 06/10/2019 |
| From: | Office of Nuclear Regulatory Research |
| To: | |
| Robert Tregoning | |
| Shared Package | |
| ML19161A105 | List: |
| References | |
| Download: ML19161A108 (8) | |
Text
Initial NRC Comments on EPRI 1022873, Improved Basis and Requirements for Break Location Postulation U.S. Nuclear Regulatory Commission June 11, 2019
EPRI Proposal: Four-Phase Approach 1)
Initial Screening: Screen out locations with low consequences of rupture.
2)
Degradation Mechanism Review: Identify relevant damage mechanisms and screen out locations which can reasonably be demonstrated to have a slow rate of propagation, based on operating experience and literature review.
3)
Mitigation/Management: Screen out locations which have damage mechanisms that can propagate rapidly, if it can be demonstrated that these mechanisms can be effectively mitigated.
4)
Probabilistic Risk Assessment: Assess risk of failure for remaining locations/mechanisms against acceptance criteria.
Overall Comments
- Development of a risk-informed approach seems feasible, but the five principles of risk-informed decision making would need to be specifically evaluated.
- Report broadly describes the proposed method, but specific details need to be developed and these will be important for NRC to determine acceptability.
- The scope of a risk-informed approach will likely need to address all hazards, including the seismic hazard curve.
- Key assumptions and sources of uncertainty should be identified and justified for each phase (or an alternative to RG 1.174 criteria proposed).
- Current approach is risk-informed but a deterministic approach that incorporates risk-informed aspects may also be possible.
Informal Comments on Phase 1:
Initial Screening
- Additional detail should describe how failure consequence is determined and causal variables are addressed
- System operating conditions
- Postulated break/crack locations
- Failure type (e.g., crack or rupture)
- Potential dynamic and environmental targets
- Vulnerability and integrity of potential targets
Informal Comments on Phase 2:
Degradation Mechanism Review
- Aspects of the review need to be formalized
- Considering the full spectrum of postulated pipe failures
- Determining the potential for rupture for each failure mechanism
- Developing acceptance criteria for applicable slow-growing, non-rupture generating mechanisms
- Impact of existing or additional performance measurement strategies (e.g., periodic inspections, operational experience review) is an important consideration
- Confirm active degradation mechanisms
- Address potential consequences
Informal Comments on Phase 3:
Mitigation/Management
- Appropriate crediting of mitigation and aging management programs is an important consideration to demonstrate that a degradation mechanism is effectively managed to ensure negligible failure risk.
- Effect on risk resulting from inspection sampling strategies need to be considered
- Existing or additional performance measurement strategies may be needed to confirm that a degradation mechanism is effectively managed.
Informal Comments on Phase 4:
Risk Assessment
- Technical adequacy of the probabilistic risk assessment will need to be established.
- Risk associated with aspects which were screened out in Phases 1-3 may need to be considered.
- Performance measurement strategies may be needed to confirm key assumptions.
Path Forward
- NRC is amenable to considering a revision of current position on break location, including CUF criteria.
- It may be possible to use a deterministic or risk-informed approach as the basis for this revision.
- Approach selected should consider industry need and targeted margin.
- Important to establish process for a proposed revision.
- Define NRC and industry roles in basis development.
- Identify implementation plan for licensees/applicants.