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1 IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making Diablo Canyon NPP Risk-Informed In-service Inspection Lecturer Lesson Lesson IV IV 3_11.3 3_11.3 Workshop Information IAEA Workshop City, XX XX - City -XX, Country Month, Year Year

2 Purpose of In-service Inspection To identify conditions, such as flaw indications, that are precursors to leaks and rupture, which violate pressure boundary integrity principles. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 2

3 RI-ISI ISI benefits Enhance or maintained plant safety (CDF/LERF) Enhanced component reliability for high safety significance components (HSSCs) Reduce nondestructive exams (NDE) Reduced man-rem exposure Other unquantifiable benefits Reduced costs of engineering analysis (flaw evaluations, etc.) Reduced outage time Reduced chance of complicating plant operations (scaffolding, leakage, etc.) IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 3

4 ASME Section XI Enhanced by Risk- Informed ISI ASME Section XI Process Risk-Informed ISI Consequence Class 1, 2, and 3 Exercising of PSA Model (CDF, LERF, others) Failure Probability High design stress and fatigue locations augmented by random selection Calculating pipe failure prob. by considering design, experience and operations IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 4

5 Overall Risk-Informed ISI Process Consequence Evaluation Expert Panel Categorization Scope and Segment Definition Risk- Evaluation Element/ NDE Selection Implement Program Structural Element Failure Probability Assessment Feedback Loop IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 5

6 Segment Definition Full Scope Definition All Class 1, 2, and 3 piping systems in ASME Section XI Piping fluid systems modeled in PSA Various balance of plant (non-nuclear code class) fluid systems of importance Systems included under scope of Maintenance Rule determined to be risk-significant Systems included in program are reviewed by expert panel for concurrence Partial Scope Definition Subset of piping classes such as ASME Class 1 piping only (includes piping exempt from current requirements) IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 6

7 Segment Definition Segment defined based on: Piping which have same consequence (loss of train A of RHR, loss of RWST, inside or outside containment consequences) Where flow splits or joins (traditional PSA modeling points) Includes piping to a point in which a pipe failure could be isolated (e.g., check valve, MOV, AOV, no credit for manual valves) Pipe size changes Failure probability expected to be markedly different due to material properties Iterative process with Consequence Evaluation IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 7

8 Segment Definition Subdivided system into piping segments Assigned numerical identifier Based upon similar consequence Marked P&Ids & field isometrics Determined failure modes effects analysis (FMEA) Without operator action With operator action IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 8

9 Consequence Evaluation Both direct and indirect (spatial) effects are considered PSA is used to quantify impact Consistent with EPRI PSA Applications Guide Calculations for CDF and LERF Conditional probability/frequency given piping failure Considers multiple impacts Initiating event impact Single/multiple component/train/system impacts Combinations of impacts IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 9

10 Direct Effects Evaluation Failure effect based on disabling segment function leak PRA and system information used to determine if piping failure causes: An initiating event (e.g. LOCA, Reactor Trip) Loss of train or system Loss of multiple trains or systems Combination of the above IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 10

11 Overview Of Indirect Effects Evaluation Purpose of Evaluation To review any issues in identifying potential indirect effects/consequences from piping failures Identify indirect effects that would differentiate piping segments from each other IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 11

12 Indirect Effects Considerations Flooding, spraying, dripping should be primarily addressed by the PSA internal flooding analyses for all plant areas Pipe Whip, jet impingement concern is primarily for highenergy fluid system piping IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 12

13 Indirect Effects Process Prewalkdown Review existing documents which examine the local effects of pipe breaks for the systems in the risk-informed ISI program Identify other systems/trains affected by a failure in each area Identify plant areas for plant walkdown Document evaluation Develop walkdown sheets for key areas Walkdown Perform walkdown and document results, actions, issues Post Walkdown Evaluate results Resolve actions IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 13

14 Failure Probability Assessment Process Industry failure experience Identification of potential failure modes and causes Specific-plant information layout, materials, operating conditions and experience Use of tools or data to calculate failure probability Estimation of leak and break probabilities by engineering team IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 14

15 Failure Probability Assessment Process INDUSTRY EXPERIENCE IDENTIFICATION OF POTENTIAL FAILURE MODES AND CAUSES PLANT INFORMATION (LAYOUT, MATERIALS, OPERATING CONDITIONS PLANT OPERATING EXPERIENCE) Engineering Team -ISI/NDE Engineering -Materials Engineering -Design Stress Engineering (Engineering Mechanics) -Plant System Engineer ENGINEERING TEAM ESTIMATED LEAK AND BREAK PROBABILITIES CALC. TOOL IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 15

16 RI-ISI ISI Expert Panel Process RISK EVALUATION -IMPACT -RRW -RAW - INDIRECT EFFECTS PRESSURE BOUNDARY FAILURE PROBABILITY - MECHANISM - PROBABILITY -BASIS OTHER CONSIDERATIONS -CONTAINMENT PERFORMANCE -EXTERNAL EVENTS -SHUTDOWN RISK -OTHER SCENARIOS -MAINTENANCE/OPERATION INSIGHTS -DESIGN BASIS/DEFENSE-IN-DEPTH -OTHER DETERMINISTIC INSIGHTS EXPERT PANEL HIGH AND LOW SAFETY SIGNIFICANT PIPING SEGMENTS IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 16

17 Mapping of Surry Segments on Structural Element Selection Matrix HIGH FAILURE IMPORTANCE SEGMENT 3 OWNER DEFINED PROGRAM 1 (a) SUSCEPTIBLE LOCATIONS (100%) (b) INSPECTION LOCATION SELECTION PROCESS LOW FAILURE IMPORTANCE SEGMENT 4 ONLY INSPECTION SYSTEM PRESSURE LOCATION TEST & VISUAL SELECTION EXAMINATION PROCESS 2 LOW HIGH SAFETY SAFETY SIGNIFICANT SIGNIFICANT SEGMENT SEGMENT IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 17

18 Presentation Format Overview of RI-ISI approach Detailed comparison Scope and segment definition Consequence evaluation Failure probability assessment process Risk evaluation Selection of elements and NDE methods (expert panel) Change in Risk calculations RI-ISI implementation (not addressed here) IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 18

19 EPRI-RI RI-ISI ISI Process Determine Scope Perform Segment Consequence Analysis Perform Segment Damage Mechanism Analysis Perform Service Review Adjust Element Selection Determine Segment Risk Category Select Elements for Inspection and Element Inspection Methods Perform Risk Impact Assessment Performance Monitoring Finalize Program IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 19

20 Segment Definition Segment definition guidelines (similar in both methodologies) Piping which have same consequences Where flow splits or joins Pipe size changes Change in piping material Isolation capability EPRI uses the above plus same failure mechanism criterion IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 20

21 Consequence Evaluation Deterministic evaluation of piping failure-induced impact (both methodologies) Direct impact (e.g. loss of a train) Indirect impact (e.g. damage caused by flooding, jet impingement) Multiple impacts (e.g. initiating events + Accident mitigation) Probabilistic evaluation EPRI uses a bounding worst case evaluation (using matrix or calculation) WOG uses surrogate(s) to quantify condition CDF (CDP) and LERF (LERP) for spectrum of failure modes (leak, disabling leak, double ended break) utilizing internal events PSA model IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 21

22 Structural Reliability Assessment Both methodologies evaluate potential for pipe failure EPRI qualitatively classifies potential for pipe rupture as High, Medium, or Low based on degradation mechanisms, in-service data, expert knowledge (no code). WOG uses SRRA code (stays with the user) to quantify leak/rupture frequency/probability based on in-service data, potential failure mechanisms, and plant specific information (e.g. layout, materials, operating and conditions, etc.) IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 22

23 Risk Evaluation EPRI uses risk matrix to separately categorize piping segments in the high, medium, or low classifications using prescriptive criteria for the consequence and rupture potential elements (risk is not calculated). It uses plant staff to review the results and concur with the risk ranking results WOG methodology uses standard approaches for CDF/LERF calculation (ie. Frequency * CCDP) and risk ranking process (RAW and RRW). Additionally, expert panel discussions are held to review PSA results and include other potential risk contributors (e.g. shutdown risk, external events, etc.) WOG methodology allows credit for aumented programs IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 23

24 Element Selection Both methodologies inspect for cause EPRI methodology uses prescriptive rules (fixed percentages) to determine the population of elements to be inspected WOG methodology uses a combination of prescriptive and statistical rules to determine the population of elements to be inspected. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 24

25 WOG Matrix HIGH FAILURE IMPORTANCE SEGMENT 3 OWNER DEFINED PROGRAM 1 (a) SUSCEPTIBLE LOCATIONS (100%) (b) INSPECTION LOCATION SELECTION PROCESS LOW FAILURE IMPORTANCE SEGMENT 4 ONLY INSPECTION SYSTEM PRESSURE LOCATION TEST & VISUAL SELECTION EXAMINATION PROCESS 2 LOW SAFETY SIGNIFICANT SEGMENT HIGH SAFETY SIGNIFICANT SEGMENT IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 25

26 EPRI Risk Matrix Consequence Assessment Failure Potential CONSEQUENCE CATEGORY CCDP and CLERP Potential Assessment NONE LOW MEDIUM HIGH HIGH LOW (Cat. 7) MEDIUM (Cat. 5) HIGH (Cat. 3) HIGH (Cat. 1) Pipe Rupture Potential MEDIUM LOW (Cat. 7) LOW (Cat. 6) MEDIUM (Cat. 5) HIGH (Cat. 2) DEGRADATION CATEGORY LOW LOW (Cat. 7) LOW (Cat. 7) LOW (Cat. 6) MEDIUM (Cat. 4) IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 26

27 Change in Risk Calculations EPRI methodology uses a progressively more quantitative evaluation to assess the changes in Risk Qualitative Bounding Simplified Complex WOG methodology calculates the change in Risk based on the change in pipe failure frequency (probability) due to the proposed change in the Inspection program. The calculations are consistent with those performed to calculate the Risk. IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making 27