CSNI Workshop on Evaluation of Uncertainties in Relation to Severe Accidents and Level II PRA Cadarache, France, November 7-9 2005 Assessment of Phenomenological Assessment of Phenomenological Uncertainties in Level 2 PRAs Uncertainties in Level 2 PRAs Hossein P. Nourbakhsh and Thomas S. Kress Hossein P. Nourbakhsh and Thomas S. Kress Advisory Committee on Reactor Safeguards (ACRS) Advisory Committee on Reactor Safeguards (ACRS) U.S. Nuclear Regulatory Commission U.S. Nuclear Regulatory Commission
“The value of what one knows is The value of what one knows is “ doubled if one confesses to not doubled if one confesses to not knowing what one does not knowing what one does not know” know” Arthur Schopenhauer Arthur Schopenhauer 2 2
OUTLINE OUTLINE � Overview of PRA Process Overview of PRA Process � � Phenomenological Uncertainties in Severe Phenomenological Uncertainties in Severe � Accident Progression Analysis Accident Progression Analysis � Uncertainties Associated with Source Term Uncertainties Associated with Source Term � Analysis Analysis � Risk Importance Measures for Risk Importance Measures for � Phenomenological Issues Phenomenological Issues 3 3
Elements of the PRA Analytical Elements of the PRA Analytical Process Process Core-Damage Accident Frequency Core-Damage Accident Frequency Analysis Analysis ----- Level 1 Plant Damage States Accident Progression and Accident Progression and Containment Performance Analysis Containment Performance Analysis Accident Progression Bins Source Term Analysis Source Term Analysis ----- Level 2 Source Term Groups Offsite Consequences Analysis Offsite Consequences Analysis ----- Level 3 Consequence Measures Risk Integration Risk Integration 4 4
Severe Accident Progression Severe Accident Progression Phenomenological Issues & Containment Failure Mechanisms Phenomenological Issues & Containment Failure Mechanisms Containment Failure Plant Damage State Mechanisms Attributes Phenomenological Issues Isolation Failure Status of Isolation Failure Status of Containment at Containment at Onset of Core Onset of Core Damage Thermally Induced Bypass Thermally Induced Damage Bypass SGTR SGTR Status of RCS Thermally Induced Direct Containment Status of RCS Thermally Induced Direct Containment (pressure, …) Hot Leg Failure Heating (DCH) Hot Leg Failure Heating (DCH) Rapid Steam Pressure (pressure, …) Rapid Steam Pressure Rises and Missiles Rises and Missiles In-Vessel In-Vessel Ex-Vessel Steam Explosion Ex-Vessel Steam Explosion Electric Power Steam Explosion Electric Power Steam Explosion Failure by Direct Status Failure by Direct Status Contact with Core Contact with Core Debris Debris Bottom Head Core Debris Bottom Head Core Debris Failure Coolability Failure Coolability Status of Reactor Status of Reactor Basemat Meltthrough Basemat Meltthrough Core Cooling Core Cooling System Core-Concrete Core-Concrete System Hydrogen Interaction Hydrogen Interaction Generation Overpressurization Generation Overpressurization Status of Failure Failure Status of Containment Hydrogen Containment Hydrogen Systems (sprays, Combustion Combustion Systems (sprays, igniters, …) igniters, …) Severe Accident Recovery Actions Severe Accident Recovery Actions 5 5
Conditional Probability of Accident Conditional Probability of Accident Progression Bins for Internal Events at Zion Progression Bins for Internal Events at Zion Conditional Probability of APB A 1 0.1 0.01 0.001 Isolation Failure Early CF Late CF Bypass No CF Accident Progression Bin (APB) Updated Evaluations NUREG-1150 Results 6 6
Conditional Probability of Conditional Probability of Accident Progression Bins for Internal Accident Progression Bins for Internal Events at Peach Bottom Events at Peach Bottom Conditional Probability of APB A 1 0.1 0.01 0.001 0.0001 Early WWF Early DWF Late WWF Venting Late DWF No CF Accident Progression Bin (APB) Updated Evaluations NUREG-1150 Results 7 7
Source Term Analysis Source Term Analysis Source Term Issues and In- -Containment Removal Mechanisms Containment Removal Mechanisms Source Term Issues and In In-Containment Source Term Issues Containment Core Heat Up & Fuel Gap Activity Containment Core Heat Up & Fuel Failure Mode and Gap Activity Cladding Failure Release Failure Mode and Cladding Failure Mechanism Release Mechanism Status of Containment Retention in the Early In-Vessel Status of Containment Retention in the Systems (sprays, water in RCS Release into the Systems (sprays, water in RCS cavity/pedestal, …..) Containment (or cavity/pedestal, …..) Bypass Release) In-Vessel Core Release from Fuel In-Vessel Core In-Containment Removal Mechanisms Release from Fuel Degradation into the RCS Degradation into the RCS Natural Aerosol Natural Aerosol Deposition Deposition Release Due to Release Due to HPME Scrubbing in BWR HPME Scrubbing in BWR RPV Bottom Head Suppression Pools RPV Bottom Head Suppression Pools Failure Failure Ex-Vessel Release Due to Release Due to Release into the Aerosol Removal by Ex-Vessel SE Aerosol Removal by Ex-Vessel SE Containment Sprays Sprays Core-Concrete Releases Due to Scrubbing of CCI Core-Concrete Releases Due to Scrubbing of CCI Interactions CCI Releases by Interactions CCI Releases by Overlaying Water Overlaying Water Late In-Vessel Late Aerosol Removal by Late Aerosol Removal by Release into the Revolatilization Ice Condenser Revolatilization Ice Condenser Containment Environmental Source Term Environmental Source Term 8 8
Uncertainty Distributions for Early In- -Vessel Vessel Uncertainty Distributions for Early In Releases into the Containment Releases into the Containment PWR, Low RCS Pressure (NUREG/CR- -5747) 5747) PWR, Low RCS Pressure (NUREG/CR 9 9
Risk Importance Measures for Risk Importance Measures for Phenomenological Issues Phenomenological Issues � It is desirable to assign some ranking of “risk It is desirable to assign some ranking of “risk � importance” among various phenomenological importance” among various phenomenological issues that are considered in a plant PRA model. issues that are considered in a plant PRA model. � Risk importance measures for phenomenological Risk importance measures for phenomenological � issues can be useful for assessing potential issues can be useful for assessing potential accident management strategies as well as for accident management strategies as well as for developing research priorities to reduce the developing research priorities to reduce the overall uncertainty. overall uncertainty. 10 10
Risk Importance Measures Commonly Risk Importance Measures Commonly Used for Ranking PRA Basic Events or SSCs Used for Ranking PRA Basic Events or SSCs + / R Risk Achievement Worth, RAW RAWi = R i i+ / R 0 Risk Achievement Worth, RAW RAWi = R 0 - ) / R Fussell – – Vesely, FV Vesely, FV FVi = (R 0 -R R i i- ) / R 0 Fussell FVi = (R 0 - 0 - / R Risk Reduction Worth, RRW Risk Reduction Worth, RRW RRWi = R i RRWi = R i- / R 0 0 where: where: + = overall risk with the probability of basic event i set to 1 ( + R R i = overall risk with the probability of basic event i set to 1 (the event the event i has occurred or the equipment is failed), has occurred or the equipment is failed), - R i - = overall risk with the probability of basic event i set to 0 (the event is (the event is R = overall risk with the probability of basic event i set to 0 i impossible or the equipment is totally reliable), and impossible or the equipment is totally reliable), and R R 0 0 = overall base = overall base- -case risk case risk 11 11
Risk Importance Measures for Post Core- - Risk Importance Measures for Post Core damage Mitigation Systems in AP1000 Design damage Mitigation Systems in AP1000 Design 1 0 0 1 0 R A W 1 R R W 0 .1 C a v it y F lo o d in g C o n t a i n m e n t I s o l a t io n R C S D e p r e s s u riz a t io n H y d r o g e n I g n it e rs P C S S y s t e m 12 12
Risk Importance Measures of Severe Risk Importance Measures of Severe Accident Phenomena in AP1000 Design Accident Phenomena in AP1000 Design 1 0 0 1 0 1 0 . 1 C F b y D i ff u s i o n F l a m e C F b y H y d r o g e n D F L I n - V e s s e l M e l t R e te n t i o n C F b y E a r l y D D T C F b y In t . D D T P h e n o m e n o l o g i c a l I s s u e 13 13
Summary Summary � An assessment of the phenomenological An assessment of the phenomenological � uncertainties associated with Level 2 PRAs PRAs uncertainties associated with Level 2 was presented. was presented. � Development of risk importance measures Development of risk importance measures � for phenomenological issues was also for phenomenological issues was also discussed. discussed. 14 14
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