Risk-Informed ISI of Nuclear Plant Piping
ASME has issued three Code Cases which provide alternatives to the Section XI inservice inspection (ISI) requirements for nuclear plant piping [Ref 1-3]. A project team headed by Structural Integrity Associates has direct experience with the application of two of the Cases, N-560 for Class 1 piping and N-578 for plant piping in general, both of which employ the EPRI Risk-Informed ISI Evaluation Procedure [4]. An N-560 application has recently received approval from the US Nuclear Regulatory Commission [5], resulting in substantial cost savings to the plant operator.
Technical Approach
As illustrated in the diagram below, the EPRI technical approach involves assigning segments of piping into risk categories based on the probability of a large break occurring in the piping and the consequences of a break in that segment. Probability of a break is based on an assessment of the susceptibility of the piping to the degradation mechanisms known to effect such piping.
The EPRI methodology provides quantitative screening criteria for determining the susceptibility of the piping to each mechanism based on the specific materials of construction, operating conditions and piping geometry. The consequence assessment utilizes conventional failure modes and effects criticality analysis (FMECA) techniques used in plant Probabilistic Risk Assessments, which are simplified for this application in the EPRI methodology. Once classified in this manner, piping elements (typically welds) are selected for inspection from the highest risk segments. When selected in this manner, Code Case N-560 permits a reduction of inspections from 25% to 10% of the total number of Class 1 welds. Code Case N-578 utilizes a similar selection technique for inspections of Class 2, Class 3 and non-code piping as well.
Typical Results
The following table summarizes results from two recent applications of N-560 (Class 1 piping only), one for a BWR and one for a PWR. As shown, the RI-ISI process resulted in the elimination of more than 70 examinations of Class 1 piping welds per ten-year interval for both applications. Yet it has been shown that the change in overall plant risk due to the decreased inspections is negligible. This is possible because the RI-ISI program focuses the inspections on:
1) regions most likely to be susceptible to the relevant degradation mechanisms, with inspection personnel, techniques and exam volumes that are specifically targeted to these forms of degradation, and
2) those damage susceptible locations with the greatest consequences of failure, thereby further optimizing the effectiveness of the examinations.
Significant Cost Savings Projected
The following chart illustrates the cost savings, per ten-year interval, projected by one utility which implemented the EPRI methodology using both Class 1 (N-560) and full-scope (N-578) programs. This chart shows that savings of over $1.3M are projected for the Class 1 portion of the program, with an additional $500K saved from application of RI-ISI to the balance of the plant piping.
The N-560 Class 1 program thus represents an excellent way for utility personnel to become familiar with a RI-ISI application at their plant. It provides the most effective savings, at the lowest implementation cost, since the EPRI methodology is much easier to apply to Class 1 systems alone. Moreover, if the utility subsequently decides to implement a full-scope program, using the N-578 approach, the N-560 work is directly applicable. Most importantly, application of this metthodology has been accepted by the NRC [5] at an operating plant.
Project Team
The EPRI RI-ISI team brings together three organizations with extensive experience in the applicable disciplines: Structural Integrity Associates for location specific degradation mechanism assessment, Duke Engineering & Services for FMECA/consequence assessment, and Inservice Engineering for segment risk ranking and preparation of the NRC alternative program submittal. The experience of this team permits a highly cost-efficient implementation process.
For more information on Risk-Informed ISI, contact SI.
References
- ASME B&PV Code Case N-560 - "Alternative Examination Requirements for Class 1, Category B-J Piping Welds" (Supplement 6 - Approval Date Aug. 9, 1996)
- ASME B&PV Code Case N-577 - "Risk-Informed Requirements for Class 1, 2, and 3 Piping, Method A" (Supplement 10 - Approval Date Sept. 2, 1997)
- ASME B&PV Code Case N-578 - "Risk-Informed Requirements for Class 1, 2, and 3 Piping, Method B (Supplement 10 - Approval Date Sept. 2, 1997)
- "Risk-Informed Inservice Inspection Evaluation Procedures" Gosselin, S.R. et al., Electric Power Research Institute, Palo Alto, CA, June 1996, EPRI TR-106706.
- U.S. Nuclear Regulatory Commission, "Safety Evaluation by the Office of Nuclear Reactor Regulation of Proposal to Use ASME Code Case N-560 as an Alternative to ASME Code, Section XI Table IWB-2500-1", Vermont Yankee Nuclear Power Station, Docket # 50-271, November, 1998
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