MODIFICATIONS TO A PRA MODEL TO ADDRESS MULTIPLE SPURIOUS OPERATIONS (MSOs)

1. MODIFICATIONS TO A PRA MODEL TO ADDRESS MULTIPLE SPURIOUS OPERATIONS (MSOs) .

2. Presented at the ANS PSA 2008 Topical MeetingSeptember 7–11, 2008Knoxville, Tennessee

3. Richard Anoba, Anoba Consulting Services, LLCDavid Miskiewicz, Progress Energy Carolinas, Inc

4. Introduction • Increasing use of PRAs to develop Fire PRA Models for NFPA-805 Application • Increasing demands to address fire-induced multiple spurious operations (MSOs)

5. What is a MSO? • A MSO is unique to fire initiating events such that it involve fire-induced failure of multiple electric-powered components that lead to undesired end states • Spurious closure/opening of MOVs, AOVs, SOVs, etc due to fire-induced hot short. • Spurious starting/stopping of pumps, etc due to fire-induced hot short

6. Simplified Hot Short Model

7. How do we model MSOs? • NUREG/CR-6850 • ANS Fire PRA Standard • Expert Panel Reviews • Fire Safe Shutdown Analysis (Appendix R) • No specific “how to” guidance

8. Modeling Challenges • PRA model alone will not capture all possible MSOs • Apendix R captures single spurious operation • Expert panel can capture additional MSOs based on Appendix R safe shutdown functions and systems

9. Modeling Challenges • Requires commprehensive review of all inputs/guidance plus creativity of PRA analyst • The modeling of MSOs in the PRA model presents some unique issues that the PRA analysts must deal with • Fault tree is a static model, while a fire scenario is time-dependent

10. Modeling Challenges • The duration of a hot short event that causes the MSO might be on the order of 10 to 20 minutes after which the fire-induced fault becomes a short-to-ground • At this point power is removed from the component and the hot short event is terminated.

11. Modeling Challenges • Competing MSO failure modes for the same component may be mutually exclusive • Alternatively, MSO failure modes that have to occur later in time may be invalid

12. NUREG/CR-6850 Component Selection (Key MSO Steps) • The NFPA-805 project typically includes a task to develop the component list to be considered for inclusion into the Fire PRA model using NUREG/CR-6850 • Identify electrically dependent components in the PRA model and in Appendix R.

13. NUREG/CR-6850 Component Selection (Key MSO Steps) • Reconciliation of differences between the Appendix R Analysis and the PRA • Addition of components whose potential spurious actuations (considering multiples) could challenge the event mitigation capability.

14. NUREG/CR-6850 Component Selection (Key MSO Steps) • Addition of instrumentation important to human response • Inclusion of components whose failure, by itself, could cause high consequence events as defined in the NUREG/CR-6850 • Addition of components identified by the expert panel review team

15. Expert Panel Reviews • Focus on Appendix R or Safe Shutdown functions and systems (2 Spurious operations) • Many identified MSOs are also applicable to Fire PRA • PRA analyst needs to expand focus to PRA functions and systems

16. A - Loss of Reactivity Control • 1- Boron Dilution • 2- Uncontrolled Cooldown

17. B - Loss of Reactor Coolant System (RCS) Inventory Control • 1- Reactor Coolant Pump Seal LOCA • 2- Stuck Open Pressurizer PORV • 3- Spurious Opening of Head/High Point Vents • 4- Spurious Opening of Letdown Line • 5- Diversion flowpaths for RWST inventory • 6- Pump dead head

18. C- Excessive RCS Injection • 1- Spurious HPI injection in excess of letdown capability with failure of Pressurizer PORV

19. D- Loss of RCS Pressure Control • 1- Spurious Auxiliary Pressurizer Spray • 2- Spurious Pressurizer Heater Actuation • 3- Spurious Start of RCP with subsequent pump heat • 4- Spurious normal Pressurizer spray with RCPs running

20. E- RCS Overcooling • 1- Spurious Turbine Bypass Valve actuation • 2- Failure of MSIVs to close with spurious opening of downstream condenser/atmospheric dump valves • 3- Spurious opening of MSIV bypass valve • 4- Failure to isolate SG Blowdown Valves

21. F- Loss of Decay Heat Removal • 1- Spurious isolation of MFW flow path • 2- Spurious start of AFW system

22. G- Loss of Support System • 1- Loss of Electrical Power • 2- Loss of Component Cooling Water System • 3- Loss of Salt Water Cooling System • 4- Loss of HVAC • 5- Loss of Instrument Air

23. Characterization and Evaluation of MSOs • A- MSO already addressed in the PRA • B- MSO from Appendix R excluded since PRA function not impacted • C- New MSO from various steps in the component selection task • D - New MSO from Appendix R • .

24. Characterization and Evaluation of MSOs • E - New MSO from expert panel review • F - MSO from Appendix R and incorporated into the PRA

25. RCS System Pressurizer PORVs and Vent Valves

26. Safety Injection System - Recirculation Sump Valves

27. Component Cooling Water System

28. Safety Injection System - High Head Injection

29. Example MSO Evaluation

30. Spurious Opening of Pressurizer PORVs Causes Small LOCA

31. Spurious Opening of Reactor Vessel or Pressurizer Vent Valves Causes Small LOCA

32. Spurious Opening of SG Blowdown Valves Causes Uncontrolled Secondary Depressurization

33. Summary • Although, the PRA already addresses many of the MSO issues, there remain a handful of issues that require model modification • Extensive modifications to the PRA model may be required to adequately address MSO

34. Summary • Time-dependent nature of a fire scenario can cause mutually exclusive failure mode combinations and/or invalid failure modes. These need to be addressed. • New failure scenarios may require additional supporting deterministic analyses