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1 IAEA Training Course on Safety Assessment of NPPs to Assist Decision Making Incident Evaluation and Ranking of Operational Events Lecturer Lesson Lesson IV IV3_II.13 Workshop Information IAEA Workshop City, XX XX - City -XX, Country Month, Year Year

2 Contents 1. Introduction. 2. International programs on probabilistic incident analysis. 3. NRC s precursor analysis procedure. 4. CSN s precursor analysis procedure. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 1

3 Introduction Probabilistic Incident Analysis One of the first applications of PSA. Survey of possible consequences of an incident -- Other than those that occurred. Survey of alternate evolution of the incident. Most probable core damage paths. Complement of root cause analysis. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 2

4 Introduction Obtain a quantitative measure of the risk associated with an event. Incident ranking Time evolution of the incidents Survey of other possible scenarios with severe risk Detection of plant vulnerabilities Obtain risk-based indicators Objectives Feedback to PSA models and tools IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 3

5 Introduction Computerised PSA model. PSA computer code (e.g. RiskSpectrum, IRRAS, CAFTA, SPSA, etc.), allowing fast requantification. PSA model: Reliability model of plant systems and initiating event evolutions. Incident screening process Tools IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 4

6 Introduction Procedure Systematic conduct of the analyses. Traceability of the analysis. Quality assurance. EdF 6 stages: 1. Investigation of actual incident consequences. 2. Investigation of potential incident analysis. 3. Modelling within the PSA model. 4. Quantification. 5. Importance measures calculation. 6. Interpretation of results. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 5

7 International Programs Regulatory Bodies US: NRC ASP program, assesses the importance of events since 1983; use of high level models with plant-specific data; NUREG/CR Germany: GRS Two generic studies initially (1988); systematic analysis of incidents occurred since 1993 started in France: IPSN/IRSN On-going program on the selection and investigation of incidents occurred since 1992; probabilistic quantification since Belgium: AVN On-going program of probabilistic incident analysis since Hungary: VEIKI Research institution performs analysis for the regulatory body since IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 6

8 International Programs Czech Rep.: Rez Precursor analysis program starting in Spain: CSN Activity since 1994; systematic analysis on behalf of the Incident Revision panel since Utilities France: EdF Uses generic PSA models to classify incidents and identify weak points, since Sweden: SwedPower Systematic activity for some Swedish plants (Vattenfall). Hundary: PAKS NPP Probabilistic incident analysis within the Operating Experience group, since (1999). IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 7

9 NRC s Precursor Analysis Procedure Started 1979 at Oak Ridge. First report published in Generic models, revised in 1988 and again in On going activity that assesses plants incidents. Trend analysis. Prior to 1999 IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 8

10 NRC s Precursos Analysis Procedure Standardised Plant Analysis Risk (SPAR) Models: Detailed models that have undergone a QA program in agreement with the utilities. Used for other risk-informed regulatory activities. 36 detailed models out of 72 already completed; 6 compared to plantspecific IPE results. Extensions: Since 1999 Shutdown and low power: 1 BWR - Grand Gulf - and 1 PWR - Surry. Level 2: about 10 models foreseen; scope to be defined. External events (seismic/fire) methodology to be defined. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 9

11 NRC s Precursor Analysis Procedure NRC Screening Criteria Unexpected core damage initiators (LOOP, SGTR, and small-break LOCA). All events in which a reactor trip was demanded and a safety-related component failed. All support system failures, including failures in cooling water systems, instrument air, instrumentation and control, and electric power systems. Any event in which two or more failures occurred. Any event or operating condition that was not predicted or that proceeded differently from the plant design basis; and Any event that, based on the reviewers experience, could have resulted in or significantly affected a chain of events leading to potential severe core damage. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 10

12 NRC s Precursos Analysis Procedure Incident selection by the Incident Revision Panel (IRP); Tools: Expert-based assessment of the safety importance of the incident. Preliminary quantification (importance measures). Computer help tool (under development). Information interchange required by the analysis may involve several individuals. Detailed analysis of the incident, including quantification of alternative scenarios. Discussion of the results within the IRP meeting: Classification as precursor incident (10-6 ). Classification as significant event (10-4 ). Rank within the indicator program. CSN Organisation IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 11

13 CSN s Precursor Analysis Procedure Indicators NRC Significant precursor events: >10-4. IPSN/IRSN Requires separate and detailed assessment of precursor incidents with measure >10-4. CSN Based on the number of incidents by probability interval. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 12

14 NRC s Precursos Analysis Procedure Use of Precursor Analyses Within CSN Further regulatory activities: in-depth assessment, specific inspections or requests to the utility. Support to the Operating Experience Group in: Evaluation of the Operating Experience. Trend Analysis, comparison among plants. Comparison with similar analyses done by the utilities. Training of plant personnel (examinations for operators licenses). Feedback to PSA - Living PSA. Complement other existing PSA applications in procedures/methodology/models/tools. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 13

15 CSN s Precursor Analysis Procedure Incident Classification Initiators Transient that leads to a reactor trip. Conditions Unavailability of systems or components of the plant that leads to a degradation of the plant safety for a given duration.. Potential Initiators Increase in the probability of occurrence of an initiating event due to operational reasons or equipment failure. Complex incidents may involve more than one phase. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 14

16 CSN s Precursor Analysis Procedure Failure Memory Approach Failures that actually occurred are modelled as logical failures (to apply Boolean logic rules). Partial failures increment the failure probability of the component. Initiating events that actually occurred are quantified separately. Potential initiating events increment the probability of occurrence. Success of component actuation and/or operator actions are modelled with their nominal probability. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 15

17 CSN s Precursor Analysis Procedure Modification of the PSA Model: Recovery Actions (NRC) A TRC-type correlation is used, For an available time t, the probability of operator error is estimated as: IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 16

18 CSN s Precursor Analysis Procedure Risk Measures Conditional core damage probability (Initiating events) Set initiating event probability to 1. Instantaneous core damage frequency (Condition events) Set the conditions of the incident by setting the failed components to logical failure and/or modifying probability of others as appropriate. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 17

19 NRC s Precursor Analysis Procedure CSN s Precursor Analysis: PSA Model PSA model setup (base model quantification). Gather information about the PSA model. Calculation of the baseline core damage frequency: Used as a reference for condition events. Exclude test and maintenance unavailabilities. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 18

20 CSN s Precursor Analysis Procedure IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 19

21 NRC s Precursor Analysis Procedure 1. Collect information on the incident. 2. Classify the incident. Task 1: Pre-analysis Activities 3. Establish specific objectives for the analysis. Task 2: Understanding the Event 1. Review available documentation. 2. Identify incident phases. 3. Develop timeline diagram. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 20

22 NRC s Precursor Analysis Procedure Timeline diagram for the reactor trip due to the trip of 2 out of 3 RCP. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 21

23 NRC s Precursor Analysis Procedure Task 3: Modelling of the event Identify the applicable event trees. Check that the models are adequate. Identify the basic events in the PSA model that must be modified in order to map the event: Initiating events. Components that failed (even partial failures). Special events. Common-cause basic events. Test and maintenance activities. Make model modifications and identify the appropriate data settings. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 22

24 NRC s Precursor Analysis Procedure Task 4: Quantification 1. Preliminary quantification to assess the importance of recovery actions. 2. Modelling of recovery actions. 3. Request and use of additional detailed information. 4. Final quantification. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 23

25 NRC s Precursor Analysis Procedure Results: Risk measure of the incident (Conditional core damage probability or instantaneous conditional core damage frequency). Cutset list: Identification of dominant sequences. Calculation of the importance measures to identify critical equipment. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 24

26 NRC s Precursor Analysis Procedure Task 5: What if? analysis Sensitivity analysis for different status of the plant: Variations of plant operational state. Unavailable equipment. Common cause failures. Generally poor operator performance. Operator and system failures. Modelling of a similar event in a different location. Modelling with a missed test. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 25

27 NRC s Precursor Analysis Procedure Task 6: Analysis and interpretation of results 1. Identify the dominant contributors to the risk from the event. 2. Investigate the sensitivity of the results obtained to reasonable (achievable) changes in the values of the data used.. 3. Studying the effect of analysis uncertainties. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 26

28 NRC s Precursor Analysis Procedure Task 7: Conclusions and reporting The estimated risk from the event: The best-estimate conditional core damage probability. Instantaneous core damage frequency (condition events). What if scenarios that were more risk significant. Conclusions from the task of Analysis and Interpretation of results. Feedback to the PSA model. IAEA Training Course of Safety Assessment of NPPs to Assist Decision Making 27