The relationship between risk analysis and event analysis – PSA based Event Analysis

1. The relationship between risk analysis and event analysis – PSA based Event Analysis P. De Gelder (AVN) PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

2. Content • Background • General considerations • PSAEA Methodology • Some issues for further consideration • Conclusions PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

3. Background • Importance of analysis of operational events • Originally “Deterministic” approach • In-depth analysis, root causes, aggravating factors, measures to avoid reoccurrence • Later also Probabilistic approach • E.g. well known ASP Programme of USNRC • Now commonly referred to as PSAEA = PSA based Event Analysis PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

4. Content • Background • General considerations • PSAEA Methodology • Some issues for further consideration • Conclusions PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

5. PSAEA – General considerations – Types of events • Types of event • Real initiating event • Potential initiating event • Condition event (unavailability of component or system) • Direct or transposed: • Direct event • Transposed event (occurred in NPP X; analysed for NPP Y) PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

6. PSAEA – General considerations – Screening • Screening policy depends on objectives of the PSAEA programme • Screening can be based on: • Scope of the PSA • Number and nature of challenged barriers • Occurrence of common mode failures • Additional unavailability of safety equipment • Duration of the event • … PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

7. Content • Background • General considerations • PSAEA Methodology • Some issues for further consideration • Conclusions PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

8. PSAEA methodology – Understanding the event (1/2) • Clear understanding of: • Initial conditions of the reactor • Any demand for reactor trip • Any demands on (safety) systems and operator actions • … • Timeline diagram, where needed • Several event phases • Event covering more than one plant operational state PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

9. PSAEA methodology – Understanding the event (2/2) • Representation on component level: • Initial conditions • Components known to be failed, degraded or under maintenance at initiation of the event • Components recovered later by the operator • Potential for common mode failures • … PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

10. PSAEA methodology – Modelling the event • “Mapping of the event on the PSA model”: • Identifying the event trees to be used • No inappropriate simplifications in the model? • Basic events to be modified • Making model modifications (e.g. operator recovery actions) • Modifications to data • … PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

11. PSAEA methodology – Quantification • Typical steps: • Preliminary quantification • Investigation on more detailed modelling (recovery actions, …) • Final quantification • “Failure memory approach”: • All failures modelled as such • Successes are “ignored” (nominal failure probability is applied) • Typical severity measures: • Conditional core damage probability (CCDP) • Instantaneous core damage frequency (ICDF) PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

12. PSAEA methodology – “What if” analyses • Optional task: Allows “sensitivity” analysis • Variations in plant operational state • Unavailable equipment • Common cause failures • Operator interventions • … • Can yield additional insights for defining corrective actions PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

13. PSAEA methodology – Analysis and interpretation of results • Identification of dominant contributors • Sensitivity of results (hypotheses, data, …) • Uncertainties PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

14. PSAEA methodology – Conclusions and reporting • Final report on the event: • Overview of the analysis performed • Modelling steps • Highlights of conclusions • Risk estimates for the event • Identification of dominant contributors • OEF: suggestion of safety improvements • Avoid reoccurrence • Reduce associated risk • Within large PSAEA programme: statistical processing or trend watching • Provides also feedback on PSA model PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

15. Content • Background • General considerations • PSAEA Methodology • Some issues for further consideration • Conclusions PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

16. Discussion – Input from OECD/NEA activities • OECD/NEA activities: • WGOE • WGRisk • Joint WGOE/WGRisk workshop (Brussels; March 2001) • Report NEA/CSNI/R(2003)11 • WGRisk report “Use and Development of PSA in NEA Member Countries” • PSAEA is now applied in many countries • Report NEA/CSNI/R(2002)1 • CSNI Technical Opinion Paper No. 6: PSA based Event Analysis (2004) PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

17. Benefits of PSAEA • Some quantitative measures of risk importance • Particularly suited for evaluating events with multiple failures and/or unavailabilities • Insights in lines of defence left for that event • Helps to determine appropriate level of attention to be devoted to that event • Helps to evaluate adequacy of potential corrective measures • Can be used for trending analysis PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

18. Development and harmonisation of methodology (1/2) • Harmonisation on level of detail • Originally simplified models • Now almost exclusively full plant specific PSAs • Some differences still exist, for instance: • Long lasting events • impact over whole time span or over one year • Events applicable to many plants • Impact to be multiplied by number of plants PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

19. Development and harmonisation of methodology (2/2) • Be careful about comparing “numbers” for different PSAEA programmes • Also trend analysis not always straightforward: • Changes in screening criteria • Transition from simplified to more sophisticated models • Periodic updates of PSA • Potential issue for development: • Extension to the use of level 2 PSA PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

20. Complementarity of approaches • Complementarity of deterministic and probabilistic approaches • Detailed “deterministic” analysis needed as input for PSAEA • Complementary focuses of both approaches • Not all events easy to quantify probabilistically (e.g. organisational or safety culture issues, …) • Under discussion: Event reporting to be risk-informed? • e.g. INES reporting (although keeping in mind that INES is for communication purposes) PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

21. Input from international exchange of information • Annual Technical Meeting of PSAEA • Hosted by AVN (this year: November 9 and 10) • Wide participation of PSAEA practioners • Reported towards WGOE and WGRisk through meeting minutes • Summary of insights: • See AVN presentation at PSAM 8 (New Orleans, May 2006): “Risk-based Precursor Analysis in the Nuclear Industry - Experiences on the National and the International Scene” (M. Hulsmans, AVN) PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

22. Input from international exchange of information - Achievements of precursor analysis • Structured quantitative assessment of potential consequences of the event itself and other relevant scenario’s (what-if) • More objective – possibly quite different – perception of risk significance • Insights in structure of the risk • Dominant factors (POS/configuration/IE/equipment/operator actions) • Useful for definition and evaluation of measures to prevent reoccurrence / mitigate consequences • Performance indicators, trending of risk • Feedback to PSA (e.g. model, IE frequency minutes) PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

23. Content • Background • General considerations • PSAEA Methodology • Some issues for further consideration • Conclusions PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)

24. Conclusions • PSAEA now widely used for event analysis • Important to keep complementary deterministic and probabilistic approaches • Possibility for future common work between OECD/NEA WGOE and WGRisk PSAEA – CNRA Conference on OEF (Köln, 29-31/05/2006)