An Approach to Evaluation of Uncertainties in Level 2 PSAs

1. An Approach to Evaluation of Uncertainties in Level 2 PSAs • T. Ishigami, J. Ishikawa, K. Shintani, • M. Mayumi and K. Muramatsu • Japan Atomic Energy Research Institute • OECD/NEA/CSNI/WGRISK Workshop, Cologne, • March 29 - 31, 2004

2. Introduction • Background • PSA application study at JAERI on safety goals, emergency planning, basic technical study for legal system for compensation of nuclear damage, and so on • The first phase PSA at JAERI in 1990 did not address AM, source terms for energetic events, and uncertainty • Uncertainty is one of the most important issues in PSA application • Purpose of This Study • To develop an uncertainty analysis method for Level 2 PSA JAERI

3. Type of Uncertainties • Parameter uncertainty • Model uncertainty • Completeness uncertainty Parameter and model uncertainties are addressed in this study Study on Uncertainty Evaluation at JAERI • Development and improvement of computer codes • Assessment of uncertain parameters • Development of uncertainty analysis method for source terms JAERI

4. Framework of uncertainty analysis for Level 2 PSA Analysis step Uncertain parameters Core damage frequency analysis ・Component failure rates etc. Resources ・Existing PSA results ・Analysis method (DET, ROAAM) ・Experiment - Steam explosion- Release of fission products from fuel SAPHIRE Accident progression analysis Uncertainty analysis ・CET branch probabilities etc. PREP/SPOP CET ・Release rates of fission products from fuel etc. Source termanalysis Frequency of exceeding X THALES2 95% 50% Computer codes SAPHIRE: System analysis CET: Containment ET analysis THALES2: Sever accident analysis PREP/SPOP:Uncertainty propagation analysis 5% Source term, X Conceptual figure of uncertainty analysis results JAERI

5. Computer Codes • SAPHIRE (USNRC) - Analysis of core damage frequency with ET/FT model - Capability of uncertainty analysis • CET Analysis Code (JAERI) - Analysis of containment function failure probability - Object-oriented programming • THALES2 (JAERI) • Integrated severe accident analysis code • Analysis of thermal hydraulics and fission product transport • PREP/SPOP (JRC Ispra) - Analysis of parameter uncertainty propagation through a model - Monte Carlo or Latin Hypercube sampling method JAERI

6. Assessment of Uncertain Parameters- Core Damage Frequency and Accident Progression Analyses - JAERI

7. Example of Assessment of Uncertain Parameter- Probability of Containment Failure due to Ex-Vessel Steam Explosion at PWR - • Possible phenomena Steam explosion at cavity Load energy brought into containment Cavity wall failure Loss of containment integrity at penetration Piping tenseness Movement of reactor vessel “Load energy > Critical load energy ” 　　“Loss of containment integrity” • Preceding analysis for Japanese APWR (Nuclear Safety Research Association) • Point estimate with DET method • Survey of research results • Structural analysis JAERI

8. Present Analysis for 4-loop PWR • Approach • Use of the DET for APWR • Similar physical processes at both plants • Type of containment is PCCV at both plants • Reevaluation of physical quantities depending on plants • Flow rate of molten core at large scale RV failure • Critical load energy resulting in containment failure → These quantities were scaled according to reactor powers of the two plants (APWR: 4,451MWt, PWR: 3,411MWt) • New Feature • Evaluation of uncertainty in containment failure probability caused by uncertainties in • Branch probabilities of “reactor vessel failure mode (large/small)” and “Occurrence of triggering (Yes/No)”, and • Critical load energy resulting in containment failure • Aleatory and epistemic uncertainties were addressed JAERI

9. Melt Flow Rate Melt Internal Energy Melt Mass in Premixture Energy Conversion Rate RV Failure Failure Mode Triggering Load Energy Probability E1 Small Scale Small Low No E2 Low Yes E3 Medium ・・・・・・・・・・ ・・・・・・・・・・ Lower Energy High High Large Higher Energy Large Scale En Decomposition Event Tree JAERI

10. Epistemic PDF Critical load energy Aleatory and Epistemic Uncertainties Aleatory uncertainty : Randomness or stochastic properties Epistemic uncertainty : Lack of our knowledge, Possible to reduce Treatment in present analysis • Uncertainties in the branch probabilities : Epistemic • Uncertainty in the critical load energy (capacity) of the containment: Aleatory and epistemic Aleatory JAERI

11. 1.0 5% 50% 95% 0.8 Cumulative probability of containment failure 0.6 0.4 0.2 0.0 0 200 400 600 800 Load energy (MJ) Load energy (MJ) Probability Distribution of Load Energy and Containment Failure Probability Aleatory Epistemic Scenarios with higher load energy caused from larger size of RV failure contribute to containment failure JAERI

12. 1 0.9 Aleatory 0.8 0.7 Aleatory+Epistemic (Solid line) 0.6 Cumulative probability 0.5 0.4 0.3 Epistemic (Dotted line) 0.2 0.1 0 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 Containment failure probability Uncertainty Analysis Result of Containment Failure Probability (conditional probability) • Uncertainty range : 0 – 0.006 (95th) • Epistemic uncertainty is dominant JAERI

13. X1 Y X2 Uncertainty Analysis Method for Source Terms Repeated calculation Code (THALES2) ・・・ Input Parameters Output JAERI

14. Classification of sequences for uncertainty analysis (BWR Mark-II) JAERI

15. Uncertain Parameters in THALES2 and Parametric Model Uncertain parameters is denoted by＊ THALES2 ＊ ＊ ＊ ＊ -Failure pressure Deposition rate Heat transfer coefficient Release rate coefficient Input ・・ ・・・ ・・・ ・・・ -Size of rupture Thermal-Hydraulic behavior FP behavior in RV Environmental release ・・・ Release from fuel Model Release fraction from fuel Data Calculated Release fraction from RV Release fraction to the environment • Pressure • Temperature Parametric Model ＊ ＊ Release fraction from RV Release fraction from fuel Release fraction to the environment × ・・・ ＝ JAERI

16. Comparison of the two Methods - Rather fundamental (Some are related to experimental data) - Not fundamental (To be obtained from a model) Characteristics of uncertain parameters • Direct Use of Code Parametric Model - Less dependent on time or sequence - Dependent on time and sequence - Individual representative sequence in a set of sequences classified by state of safety systems - Possible to compare the results with different model results • - a set of sequences classified by physical state (Zr-oxidation level, RCS pressure) • - Difficult to assess the results Accident sequences to be analyzed Calculation time - Long - Short JAERI

17. Approach to Assessment of Uncertain Parameters • Survey preceding PSAs (NUREG-1150) and comparative study of THALES2 and MELCOR to determine uncertain parameters • Select parameters in THALES2 relating to the above uncertain parameters - Representative parameters are selected to reduce the number of uncertain parameters e.g. One correction factor for deposition rate in RCS for all the deposition mechanisms • Determine the uncertainties by surveying recent experimental and analytical research results as well as preceding PSA study - Experiments on FCI (FARO, COTELS, …) - Experiment on FP release from fuel (VEGA,…) etc. JAERI

18. An example of uncertain items and associated parameters in THALES2 (BWR Mark-II) JAERI

19. Summary • Study on developing uncertainty analysis method for Level 2 PSA at JAERI includes • Development and improvement of computer codes • Assessment of uncertain parameters • Development of uncertainty analysis method for source terms • To quantify uncertain parameters - Recent experimental and analytical research results are surveyed - Analytical method such as DET is used to evaluate uncertainty in containment failure probability due to steam explosion, where aleatory and epistemic uncertainties are considered • Uncertainty analysis method for source terms is - Direct use of THALES2 with Monte Carlo simulation, where - Uncertain parameters in THLES2 are assessed by surveying recent research results JAERI