Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP. Dr. Olivier Nusbaumer Probabilistic Safety Analysis Kernkraftwerk Leibstadt AG

Transcription

1 Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Dr. Olivier Nusbaumer Probabilistic Safety Analysis Kernkraftwerk Leibstadt AG Background Methodological Aspects Swiss Atomic Law Scope of an Industrial PSA Study Applications and Results Conclusions Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 1 Leibstadt Nuclear Power Plant largest Swiss power plant Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 2

2 Grundlagen der PSA Vergleichstabelle natürliche / vom Menschen erzeugte Risiken Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 3 Grundlagen der PSA Verlorene Lebensjahre in Bezug auf Ursachen Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 4

3 Vergleich der Sicherheit KKL mit Neuanlagen Entwicklung der CDF von Kraftwerkstypen Gen. I - III CDF Leibstadt Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 5 Background: PSA Complement the deterministic Design Basis Requirements Make use of probabilistic calculation tools (Fault Tree / Event Tree) and statistics (plant specific reliability data) Give answers as to: What can happen? How likely is it? What are the consequences? How large are the uncertainties? ( make uncertainty visible ) What are the dominant contributors? Improvement measures Level 0 PSA Level 1 PSA Level 2 and 3 PSA Uncertainty analysis Risk Informed Applications Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 6

4 Background: Modeling Postulation of an Initiating Event (IE) and its frequency f Modeling of the safety barriers (equipment and measures) Quantification of phenomenological events and damage level Level 0 PSA Level 1 PSA Level 2 and 3 PSA Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 7 Background: Levels of PSA Response to initiating events, Assessment of safety barriers Level 1 Frequency of core damage (CDF) Physical effects, consequences Level 2 Radiological consequences (source term) Athmospheric dispersion, potential and expected doses, dose-effect/risk relation Level 3 Frequency and quantities of environmental and health effects Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 8

5 Background: Approaches Deterministic (postulative) Events completely determined through causality chains Effect analysis of postulated causes Statistic (retrospective) Experience laws derived from a large number of similar observations Incorporation of the observations at system and event level Probabilistic (prognostic) Events determined by probability or frequency Use of observations at component level (axiom of Kolmogorov) Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 9 Background: Approaches 1 Postulation (deterministic) Probability of failure (cummulative) 0.5 Real leak spectrum (probabilistic) 0 Small leak Medium leak Large leak 2F- Break Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 10

6 Methodological Aspects: Level 1 Fault Trees are logical models of fault combinations that could cause a mitigating system to fail to perform its function when required Basis: all causes leading to system failure System modeling System reliability Event Trees depict the potential event sequences from initiating event to consequences Basis: plant response Modeling of accident progression Frequency of accident sequences Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 11 Methodological Aspects: Fault Trees Which of those designs is more reliable? (failure to inject water) A: DG DG B: DG DG Reliability Data Offsite power unavailability: 15 min / yr = 0.25 / 8760 = 2.85E-5 Pump failure (mech.): 2 / 100 demands = 2E-2 Diesel Generator failure: 1 / 100 demands =1E-2 Valve failure (mech.): 2.5 / 1000 demands = 2.50E-3 Double-valve failure (mech.): 1 / 100 = 1E-2 Check valve failure: 5 / = 5E-4 Transformer failure: 1E-8 Heat exchanger failure: 1E-8 Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 12

7 Methodological Aspects: Fault Trees A: DG DG Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 13 Methodological Aspects: Fault Trees B: Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 14

8 Methodological Aspects: Fault Trees Design A DG DG P(top) = 1.09E-2 Design B DG DG P(top) = 9.00E-4 Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 15 Methodological Aspects: Risk Importance Measures Risk Increase Factor (RIF / RAW) P( top) p( x) RIF( x) P( top) 1 Fussell-Vesely (FV) Fractional contribution of sequences in which component x is involved Measure of the involvement level of a given component Differential Importance Measure (DIM) P( top) DIM ( x) p( x) Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 16

9 Zuverlässigkeit von Basis Ereignissen Component reliability Startversagen: Q=q Dauerversagen: d( N0 N( t)) N( t) dt Q( t) 1 e t Q 1/ T (1 e T 0 Tailor t t ) dt 1 1 ( e T T Tailor 1) i2 T i! i 1 1/ 2 T 1 Q( t t) Q( t) Failure rate : lim 1 Q( t) t0 t Survived Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 17 Methodological Aspects: Reliability Data Generic or international data (observations) 5% 95% 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 Bayesian Update PSA model Plant specific observations: 6 failures out of demands = 6.0E-4 Data specialization Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 18

10 Methodological Aspects: Reliability Data Bayesian Law p( H E) p( E) p( E H ) p( H ) can be derived for continuous functions H: Hypothesis (here: λ) E: Evidence (observations) dλ Prior Posterior 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 λ 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 f ( E) d f ( ) ( E ) d ( E ) f ( ) d 0 p( E) p( E ) p( ) p( E) n! r n 1 r!( n r)! E r T T e r! for failure rates r for failures Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 19 with Methodological Aspects: Seismic Hazards Earthquake likelihood is given by an hazard curve Fragility is a function of the sustained earthquake magnitude ln( a a F( a, Q) m ) u ( Q) r 1 where: ø(): Gaussian cumulative function Q: confidence level (0..1) a m : median ground-acceleration capacity β u : uncertainty in capacity β r : randomness in earthquake and effects a: sustained ground motion level. Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 20

11 Methodological Aspects: Other types of data Other types of data assessment include Human Reliability Analysis (HRA) In Switzerland: THERP / SLIM Common Cause Failures (CCF) Also subject to Bayesian updates! Equipment unavailabilities Impacts (example: fire, airplane crash, wind, ) Initiating Event (IE) frequencies Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 21 Defense in depth Core Damage Frequency (CDF) Earthquake LOCA Initiator (T)LOOP Loss of BM Drainage RDB Load rejection Initiating Event Frequency [/yr] Reactor shutdown HP Core Cooling Eme. Depressurization LP Core Cooling Containment Cooling How reliable is the safety barrier =? Core Damage Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 22

12 Methodological Aspects: Event Trees Girl draws aside (recovery action) Break Road wet Result 98% 90% 99% 60% 52% OK 40% 70% 30% 24% 12% 1% 1% Accident 10% 10% 2% 2% Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 23 Methodological Aspects: Event Trees Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 24

13 Methodological Aspects: Level 2 Containment Response Accident progression and phenomenology Calculation of radiological consequences (source term) Uncertainty assessment Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 25 Methodological Aspects: Integral Risk Core Damage Frequency [yr -1 ] (non-cumulative) 1E-06 Accident A R i f ( IEi ) CPi End State likelihood K f(ie i ): Initiating event frequency CP i : Conditional Probability K i : Consequence α: Weighting factor ( 1) i 1E-07 Accident B Accident D 1E-08 Accident C Accident E Consequences [Bq] Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 26

14 Swiss Atomic Law Swiss Atomic Law (KEG) Law for peaceful use of atomic energy No claim about PSA in the text Swiss Atomic Ordinance (KEV) Came into effect in February 2005 Defines basic requirements on PSA Detailed in guidelines ENSI-A05 und A06 Safety goals (as IAEA and NRC) 1E-4 for Core Damage Frequency (CDF) 1E-5 for Large Early Release Frequency (LERF) Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 27 Regulatives Umfeld PSA in der Integrierten Aufsicht PSA ist (nur) ein Element der Integrierten Sicherheitsbewertung KEG KEV ENSI-A05 ENSI-A06 Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 28

15 Überblick über die KKLPSA Umfang einer PSA Systemmodelle Auslösende Ereignisse Umfangreiches Computer-Modell der Anlage Brandanalysen PSA = + Erdbebenanalysen Stufe 2 PSA (Freisetzung) Unfallabläufe Überflutungsanalysen Zuverlässigkeitsdaten Containmentverhalten von Komponenten Menschliche Zuverlässigkeit (SFA, SFV, SAMG) Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 29 Scope of an Industrial PSA Study Analysis Scope (ENSI-A05, Fullpower Internal, external and area events Level 1: Calculation of Core Damage Frequency (CDF) Level 2: Calculation of radiological consequences Low power and Shutdown (KKL: 12 Plant Operating States) Internal, external and area events Level 1: Calculation of Fuel Damage Frequency (FDF) Level 2: Calculation of radiological consequences (New!) Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 30

16 Scope of an Industrial PSA Study: Types of Events Internal Events Transients (24) Loss of Coolant Accidents (LOCA) (37) External Events Earthquakes, extreme winds, tornadoes, external flooding and aircraft crashes (20) Area Events (internal hazards) Fires (85) Flood (35) Turbine missile (1) 202 Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 31 Scope of an Industrial PSA Study Component failure modes: ~ Human actions: ~ 400 Fault trees: ~ 2000 Up to 80 depth levels Event Trees: ~ 300 Common Cause Failure Groups: ~350 Man-power Development & maintenance: 3 Man-Yr / Yr Applications: 1 Man-Yr / Yr >1M CHF / yr Documentation: ~ pages Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 32

17 Applications and Results Application scope (ENSI-A06, Evaluation of the Safety Level (CDF < 1E-5) Evaluation of the Balance of the Risk Contributors Evaluation of the Technical Specifications Evaluation of Changes to Structures and Systems Risk Significance of Components (FV 1E-3 or RIF 2) Evaluation of Operational Experience Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 33 Applications and Results: Risk Contributors Flood; 20% Turbine Missile; 0% All LOCA Events; 3% Transients and special initiators; 6% Fire; 22% Weir failure; 0% Airplane crash; 0% High winds and tornadoes; 1% Earthquakes; 47% Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 34

18 Root Cause Analysis Fukushima - First steps M-9 class earthquakes in the past 100 years Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 35 Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 36

19 Wave height distribution as for Red circle: inundation depth. Blue triangle: run up height. Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 37 Vergleich der Sicherheit KKL mit Neuanlagen Entwicklung der CDF von Kraftwerkstypen Gen. I - III CDF Fukushima (?) CDF Leibstadt Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 38

20 Conclusions PSA aim to realistically describe risk and safety levels; assess safety barriers Give insights about the performance of safety measures; indentify weak points Assess the relative important of accident sequences, optimize the use of available resources Enable safety assessment of operating aspects and operating experience Probabilistic Safety Assessment (PSA): Case Study Leibstadt NPP Folie 39