INITIATING EVENTS GROUPING

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IAEA.PNRA.05.05. Stetkar

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INITIATING EVENTS GROUPING

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1. IAEA.PNRA.05.05 Stetkar & Associates 1 INITIATING EVENTS GROUPING by John W. Stetkar Presented to National Training Course on Probabilistic Safety Assessment Islamabad, Pakistan May 2 - 6, 2005 IAEA Project C7-PAK/9/028-001

2. IAEA.PNRA.05.05 Stetkar & Associates 2 GROUPING OF INITIATING EVENTS PLANT THERMAL / HYDRAULIC RESPONSE FUNCTIONAL SUCCESS CRITERIA SYSTEM SUCCESS CRITERIA SUPPORT SYSTEM IMPACTS OPERATOR ACTIONS LEVEL 2 PSA IMPACTS

3. IAEA.PNRA.05.05 Stetkar & Associates 3 NUMERICAL SCREENING

4. IAEA.PNRA.05.05 Stetkar & Associates 4 NUMERICAL SCREENING MOST MODERN PSAs INCLUDE FAIRLY "STANDARD" SET OF INITIATING EVENTS FROM FRONTLINE SYSTEM FAILURES (TRANSIENTS, LOCAs, ETC.) NUMERICAL ARGUMENTS TYPICALLY USED TO LIMIT SCOPE OF SUPPORT SYSTEM INITIATING EVENTS AND EXTERNAL INITIATING EVENTS NUMERICAL CRITERIA ARE OFTEN NOT JUSTIFIED OR ARE NOT CONSISTENT WITH QUANTIFICATION OF OTHER PSA INITIATING EVENTS SIGNIFICANT PROBLEM IN SOME REVIEWS

5. IAEA.PNRA.05.05 Stetkar & Associates 5 NUMERICAL SCREENING CANNOT BE JUSTIFIED ONLY BY ESTIMATED INITIATING EVENT FREQUENCY MUST CONSIDER RISK CONSEQUENCES LEVEL 1 PSA MODELS / SUCCESS CRITERIA OPERATOR ACTIONS LEVEL 2 PSA CONSEQUENCES RISK CONTRIBUTORS HIGH FREQUENCY / LOW CONSEQUENCES MEDIUM FREQUENCY / MEDIUM CONSEQUENCES LOW FREQUENCY / HIGH CONSEQUENCES

6. IAEA.PNRA.05.05 Stetkar & Associates 6 NUMERICAL SCREENING - LIVING PSA IMPROVED PSA MODELS AND NUMERICAL RESULTS REFINED THERMAL / HYDRAULIC ANALYSES REFINED SUCCESS CRITERIA OPERATOR RECOVERY ACTIONS IMPROVED DATA SCREENING BASED ON PRELIMINARY MODELS / RESULTS OFTEN NOT VALID FOR FINAL MODELS / RESULTS MUST CONSISTENTLY REEVALUATE SCREENING CRITERIA AFTER EVERY PSA UPDATE

7. IAEA.PNRA.05.05 Stetkar & Associates 7 NUMERICAL SCREENING ** GENERAL RULE ** QUANTIFY THE INITIATING EVENT LET THE PSA MODELS CONFIRM ITS ACTUAL SIGNIFICANCE

8. IAEA.PNRA.05.05 Stetkar & Associates 8 NUMERICAL SCREENING PERSPECTIVES The 7 astronauts aboard the space shuttle Challenger would have been very thankful for a realistic assessment of the risk from low temperature failures of "passive" O-rings. The 229 passengers and crew aboard Swissair Flight 111 would have been very thankful for a realistic assessment of the risk from pilot performance and ground personnel decisions during a specific type of small, localized fire scenario.

9. IAEA.PNRA.05.05 Stetkar & Associates 9 NUMERICAL SCREENING PERSPECTIVES The 69 passengers and crew (52 children) aboard Bashkir Airlines Flight 2937 and the 2 DHL cargo plane pilots would have been very thankful for a realistic assessment of the integrated risk from maintenance of collision avoidance systems, air traffic control personnel actions, and pilot responses to automatic alarms. The 7 astronauts aboard the space shuttle Columbia would have been very thankful for a realistic assessment of the risk from impacts by "insignificant" pieces of insulation.

10. IAEA.PNRA.05.05 Stetkar & Associates 10 INITIATING EVENT GROUPS

11. IAEA.PNRA.05.05 Stetkar & Associates 11 GROUPING PRINCIPLES GROUPS IMPROVE PSA MODELING AND QUANTIFICATION EFFICIENCY GROUPS DECREASE PSA DETAIL COMBINE INDIVIDUAL INITIATING EVENTS INTO FUNCTIONAL GROUPS GROUP FREQUENCY = SUM OF INDIVIDUAL INITIATING EVENT FREQUENCIES GROUP PSA IMPACTS = MOST LIMITING COMBINED IMPACTS FROM ALL INDIVIDUAL INITIATING EVENTS

12. IAEA.PNRA.05.05 Stetkar & Associates 12 GROUPING PRINCIPLES (continued) FOCUS ON INTEGRATED IMPACTS FROM EACH INITIATOR PLANT RESPONSE PSA SUCCESS CRITERIA IMPACT ON PSA SYSTEMS IMPACT ON PSA OPERATOR ACTIONS "CONSERVATIVE CONDENSATION" COMBINE LOWER FREQUENCY, LESS SEVERE INITIATORS WITH HIGHER FREQUENCY, MORE SEVERE INITIATORS USE WITH CAUTION

13. IAEA.PNRA.05.05 Stetkar & Associates 13 GROUPING PRINCIPLES *** CAUTIONS *** DO NOT FOCUS ONLY ON TURBINE TRIP OR REACTOR TRIP SIGNALS AS BASIS FOR GROUPING DO NOT COMBINE SUPPORT SYSTEM INITIATING EVENTS WITH FRONTLINE INITIATORS BEWARE OF LEVEL 2 PSA IMPACTS – NEED TO SUBDIVIDE GROUPS FOR LATER ANALYSES IF THERE ARE ANY QUESTIONS, QUANTIFY INITIATING EVENTS SEPARATELY

14. IAEA.PNRA.05.05 Stetkar & Associates 14 CONSEQUENTIAL IMPACTS INITIATING EVENT STARTS SERIES OF POSSIBLE RESPONSES EQUIPMENT SUCCESSES / FAILURES OPERATOR ACTIONS DO NOT COMBINE INITIATOR AND CONSEQUENCES THE FOLLOWING CONSEQUENTIAL CONDITIONS ARE NOT INITIATING EVENTS STATION BLACKOUT ATWS INDUCED LOCAs OVERCOOLING

15. IAEA.PNRA.05.05 Stetkar & Associates 15 TRANSIENTS AND LOCAS TOO MUCH EMPHASIS ON LOCAS SIZE, LOCATION, FREQUENCY COMPLEX SUCCESS CRITERIA TOO LITTLE EMPHASIS ON TRANSIENTS BROAD INITIATING EVENT GROUPS TREATMENT OF TRANSIENT-INDUCED IMPACTS SCOPE OF SUPPORT SYSTEM INITIATING EVENTS FULL-SCOPE LEVEL 1 PSA RESULTS OFTEN DOMINATED BY TRANSIENTS AND SUPPORT SYSTEM FAILURES

16. IAEA.PNRA.05.05 Stetkar & Associates 16 TRANSIENT-INDUCED IMPACTS PRIMARY OVERPRESSURE SECONDARY OVERPRESSURE OVERCOOLING ATWS MAKEUP / LETDOWN REACTOR COOLANT PUMP SEAL FAILURE ENVIRONMENTAL / PHYSICAL DAMAGE CONTAINMENT

17. IAEA.PNRA.05.05 Stetkar & Associates 17 LEVEL 2 PSA IMPACTS ENERGY RELEASE INTO CONTAINMENT CONTAINMENT HEAT REMOVAL CONTAINMENT BYPASS CONTAINMENT ISOLATION

18. IAEA.PNRA.05.05 Stetkar & Associates 18 COMMON PROBLEMS / CONSIDERATIONS LOSS OF MAIN CONDENSER VACUUM LOSS OF MAIN CONDENSATE LOSS OF MAIN FEEDWATER LOSS OF TURBINE BYPASS STEAM RELIEF FEEDWATER CONTROL / ISOLATION VALVE FAILURES PARTIAL / TOTAL LOSS OF MAIN FEEDWATER PARTIAL / TOTAL LOSS OF STARTUP / STANDBY FEEDWATER POSSIBLE IMPACT ON EMERGENCY FEEDWATER

19. IAEA.PNRA.05.05 Stetkar & Associates 19 COMMON PROBLEMS / CONSIDERATIONS (continued) BOUNDARY BETWEEN OFFSITE AND ONSITE POWER GRID / TRANSMISSION LINES / SWITCHYARDS MAIN / AUXILIARY / STANDBY TRANSFORMERS IN-PLANT BUSWORK POTENTIAL POWER RECOVERY IMPROPER GROUPING OF AC AND DC POWER FAILURES AC AFFECTS MOTIVE POWER DC AFFECTS INSTRUMENTATION, ACTUATION SIGNALS, AUTOMATIC / MANUAL CONTROLS

20. IAEA.PNRA.05.05 Stetkar & Associates 20 COMMON PROBLEMS / CONSIDERATIONS (continued) IMPROPER GROUPING OF ELECTRIC POWER FAILURES WITH FRONTLINE SYSTEM FAILURES IMPROPER GROUPING OF COOLING WATER FAILURES WITH "MOST IMPORTANT" COOLING LOADS IMPROPER GROUPING OF HARDWARE FAILURES WITH EVENTS THAT INCLUDE CREDIT FOR OPERATOR ACTIONS IMPROPER GROUPING OF INTERFACING SYSTEM LOCAs WITH LOCAs INSIDE CONTAINMENT

21. IAEA.PNRA.05.05 Stetkar & Associates 21 NUMBER OF INITIATING EVENTS IMPROVED PSA SOFTWARE AND HARDWARE REDUCE NEED FOR INITIATING EVENT GROUPING INCREASED USE OF RISK-INFORMED PSA APPLICATIONS OFTEN REQUIRES MORE DETAILED ANALYSIS OF SPECIFIC INITIATING EVENTS NUMBER OF INITIATING EVENTS / GROUPS DEPENDS ON PLANT-SPECIFIC CONSIDERATIONS MANY MODERN PSAs QUANTIFY 25 TO 50 SEPARATE INTERNAL INITIATING EVENTS (GROUPS + INDIVIDUAL INITIATORS)

22. IAEA.PNRA.05.05 Stetkar & Associates 22 ELECTRIC POWER INITIATING EVENT EXAMPLE

23. IAEA.PNRA.05.05 Stetkar & Associates 23 EXAMPLE ELECTRIC POWER SYSTEM

24. IAEA.PNRA.05.05 Stetkar & Associates 24 EXAMPLE ELECTRIC POWER SYSTEM OFFSITE POWER CONNECTIONS 220KV TRANSMISSION LINES (3) 220KV SWITCHYARD (2 BUSES) MAIN TRANSFORMER (18KV / 220KV) MAIN GENERATOR OUTPUT BREAKER (220KV) SERVICE TRANSFORMER (220KV / 6KV) STARTUP / STANDBY TRANSFORMER (220KV / 6KV)

25. IAEA.PNRA.05.05 Stetkar & Associates 25 EXAMPLE ELECTRIC POWER SYSTEM IN-PLANT 6KV BUSES "NON-ESSENTIAL" BUSES SERVICE BUS 2160 SERVICE BUS 2161 COMMON BUS 2160 COMMON BUS 2161 "SAFETY-RELATED" BUSES SAFEGUARDS BUS 2160 SAFEGUARDS BUS 2161

26. IAEA.PNRA.05.05 Stetkar & Associates 26 EXAMPLE ELECTRIC POWER SYSTEM NORMAL POWER ALIGNMENT SERVICE BUSES NORMAL SUPPLY FROM SERVICE TRANSFORMER BACKUP SUPPLY FROM STARTUP / STANDBY TRANSFORMER VIA COMMON BUSES COMMON BUSES ONLY SUPPLY FROM STARTUP / STANDBY TRANSFORMER SAFEGUARDS BUSES NORMAL SUPPLY FROM STARTUP / STANDBY TRANSFORMER VIA COMMON BUSES BACKUP SUPPLY FROM DIESEL GENERATORS

27. IAEA.PNRA.05.05 Stetkar & Associates 27 EXAMPLE ELECTRIC POWER SYSTEM INITIATING EVENT CONSIDERATIONS LOSS OF OFFSITE POWER INITIATING EVENT REGIONAL GRID 220KV TRANSMISSION LINES 220KV SWITCHYARD OFFSITE POWER RECOVERY MODELS AND DATA FAILURE OF MAIN TRANSFORMER EQUIVALENT TO MAIN TURBINE-GENERATOR TRIP FAILURE OF GENERATOR OUTPUT BREAKER DISABLES SERVICE TRANSFORMER SUPPLY

28. IAEA.PNRA.05.05 Stetkar & Associates 28 EXAMPLE ELECTRIC POWER SYSTEM INITIATING EVENT CONSIDERATIONS FAILURE OF SERVICE TRANSFORMER LOSS OF NORMAL POWER TO SERVICE BUSES 2160 AND 2161 SERVICE BUSES REQUIRE ACTIVE POWER TRANSFER FROM COMMON BUSES POWER SUPPLY CANNOT BE RECOVERED QUICKLY

29. IAEA.PNRA.05.05 Stetkar & Associates 29 EXAMPLE ELECTRIC POWER SYSTEM INITIATING EVENT CONSIDERATIONS FAILURE OF STARTUP / STANDBY TRANSFORMER LOSS OF NORMAL POWER TO COMMON BUSES 2160 AND 2161 SERVICE BUSES REQUIRE NORMAL POWER FROM SERVICE TRANSFORMER SAFEGUARDS BUSES REQUIRE POWER FROM DIESEL GENERATORS POWER SUPPLY CANNOT BE RECOVERED QUICKLY

30. IAEA.PNRA.05.05 Stetkar & Associates 30 EXAMPLE ELECTRIC POWER SYSTEM INITIATING EVENT CONSIDERATIONS FAILURE OF SERVICE TRANSFORMER AND STARTUP / STANDBY TRANSFORMER ELECTRICALLY EQUIVALENT TO LOSS OF OFFSITE POWER COMMON BUSES AND SERVICE BUSES DEENERGIZED SAFEGUARDS BUSES REQUIRE POWER FROM DIESEL GENERATORS POWER SUPPLY CANNOT BE RECOVERED QUICKLY

31. IAEA.PNRA.05.05 Stetkar & Associates 31 EXAMPLE ELECTRIC POWER SYSTEM INITIATING EVENT CONSIDERATIONS FAILURE OF SERVICE BUS DISABLES ALL BUS LOADS POWER SUPPLY CANNOT BE RECOVERED QUICKLY FAILURE OF COMMON BUS DISABLES ALL BUS LOADS DISABLES ALTERNATE SUPPLY TO SERVICE BUS DISABLES NORMAL SUPPLY TO SAFEGUARDS BUS POWER SUPPLY CANNOT BE RECOVERED QUICKLY

32. IAEA.PNRA.05.05 Stetkar & Associates 32 EXAMPLE ELECTRIC POWER SYSTEM INITIATING EVENT CONSIDERATIONS FAILURE OF SAFEGUARDS BUS DISABLES ALL BUS LOADS POWER SUPPLY CANNOT BE RECOVERED QUICKLY

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