ML19161A108

NRC Comments on Break Location 6-11

ML19161A108
Person / Time
Issue date:	06/10/2019
From:	Office of Nuclear Regulatory Research
To:
Robert Tregoning
Shared Package
ML19161A105	List: ML19161A107 ML19161A108
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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.