Background

1. SARNET WP5 - Level 2 PSAComparison between classical and dynamic reliability methods. Specification and results of a benchmark exercise on consequences of hydrogen combustion during in-vessel core degradationE. Raimond, T. DurinIRSN, BP 17 – 92265 Fontenay-aux-Roses

2. Background • SARNET WP 5.3 : Dynamic methods for level 2 PSA • Stage 1 – Review of existing approaches(cf. P.E Labeau – ERMSAR-05) • Stage 2 – Practical application in a benchmark exercise • Benchmark specification End 2005 / Mid 2006 • Solutions proposal by participants 2006 • Synthesis and additional contributions Mid-2007

3. Background • THE « CHALLENGE » TO PROVIDE A « SIMPLE » EXAMPLE THAT DEMONSTRATE THE LIMITATION OF CLASSICAL EVENT TREE METHODS AND GAIN OBTAINED BY DYNAMIC METHODS

4. PRESENTATION OF THE BENCHMARK EXERCISE

5. Benchmark description • A “basic transient” • A French 900 MWe PWR (3 loops, with Passive Autocatalytic Recombiners – PAR) operating at nominal power before the initiating event • Loss of coolant accident (LOCA) after a 3’’ break size on cold leg of RCS, • Failure of all water injection system and spray system • An ASTEC calculation provides basic information on: • The kinetic of core degradation process • The kinetic of hydrogen and vapor releases in containment • The delay before vessel rupture • The pressure evolution in containment (and atmosphere composition)

6. INFORMATION FROM ASTEC Maximum Hydrogen mass (100 % Zr oxydation) May be increased by steel oxydation Pressure evolution in containment (between 2 or 3 bar) Hydrogen mass released in containment Time for core degradation beginning Vessel Rupture time

7. Benchmark description For the basic transient (no water injection, no spray) the containment atmosphere is not flammable no combustion

8. The issue • After reparation, water injection and spray system are available after beginning of core degradation • QUESTION = what is the dominant risk of containment failure • Three independent events are considered : • Water injection • Spray system start • Ignition of H2-H20-Air mixture by recombiners • No chronological link between the events is assumed • Consequences of each event is described by analytical models

9. Water injection • For the benchmark, consequence of water injection is only hydrogen production. An analytical model has been proposed : Maximum Hydrogen mass (100 % Zr oxydation) May be increased by steel oxydation New hydrogen source term as a function of time reflooding Hydrogen mass released in containment without reflooding Vessel Rupture time Time for core degradation beginning

10. Spray system effect • In the benchmark, consequence of spray system start is atmosphere containment depressurization, cooling and composition modification • An analytical model has been proposed based on ASTEC results Pressure evolution in containment (between 2 or 3 bar) 1 bar New evolution of pressure in containment Time for core degradation beginning Vessel Rupture time

11. Ignition (by recombiners or other) • « Classical » physical criteria have been used to precise if combustion is possible and “probable” • Combustion can be total or local only • Multiple combustions can also occur … • Delay before combustion is unknown • Pressure peak in containment due to combustion are evaluated by PAICC model

12. List of used “physical” information • A representative ASTEC transient without spray and reflooding • Beginning of core degradation – Vessel Rupture • Hydrogen mass released in containment • Containment Pressure as a function of time • A simple law that allows to predict pressure evolution as a function of time after spray system start • A simple law that allows to predict H2 release after core reflooding • A simple law that allows to predict recombiners efficiency in function of H2 and H20 concentrations • Criteria for hydrogen combustion: Shapiro, ignition by recombiners • The probability of containment failure as a function of pressure peak

13. Stochastic events • Water injection • Probability 0.5 to have water injection before vessel rupture • uniform probability distribution • Spray system start • Probability 0.5 to have water injection before vessel rupture • uniform probability distribution • Ignition of hydrogen combustion • “Atmosphere flammability” is defined with a Shapiro diagram and a criteria for ignition by recombiners • (the atmosphere ignition within a short delay is very problable if the recombiners ignition criteria is achieved for average H2 concentration) • Local ignition (partial) have been taken into account

14. ?? • CAN THIS SIMPLE QUESTION BE SOLVED WITH A CLASSICAL EVENT TREE METHOD • ??

15. Difficulties for the “classical event tree” approach • The chronological links between events have to be defined • 12 situations are possible from the chronological point of view : • Spray water injection ignition • Spray water injection no ignition • Ignition Spray water injection • Ignition Spray no water injection • Spray ignition water injection • Spray ignition no water injection • Ignition water injection Spray • Ignition water injection no Spray • water injection ignition Spray • water injection ignition no Spray • water injection Spray ignition • water injection Spray no ignition

16. Difficulties for « classical event tree” approach • The treatment of all chronological issues is difficult. • !!!! The treatment of multiple combustions is impossible !!! • The only way for a classical approach is a conservative approach : • « TO CHECK THAT IN THE WORTH CASE, THE CONTAINMENT FAILURE RISK IS RESIDUAL »

17. 2 steps • STEP 1 – First implementation of the problem with dynamic or classical method, with simple analytical model for the physics • STEP 2 – Second implementation of the problem with complements in the analytical models for epistemic uncertainties • ?? Possible STEP 3 – Implementation of ASTEC modules instead of simple analytic model ??

18. SYNTHESIS OF THE BENCHMARK RESULTS

19. 10 PARTICIPANTS • IRSN • GRS • CEA • AREVA • VEIKI • ULB • (CSN) • LEI • UJV • INR

20. Five categories of solutions • Direct calculation • Classical event tree methods • Macro-event method with classic tools • Monte Carlo Dynamic Event Tree (MCDET) • Stimulus-Driven Theory of Probabilistic Dynamics (SDTPD)

21. Methods1- Direct calculation (CEA, INR) • Monte Carlo simulations (C++ or Fortran program): • time of water injection and spray system activation are randomly sampled • For each sequence: • The evolution of the system is calculated for each time step (1 second) • If the containment atmosphere is flammable, a delay before combustion is determined • Overpressure is calculated • Containment integrity is determined • + : simple method, the validation of the model is easy • - : few information for analysis, implementation in global event tree ?

22. Methods2- Classical event tree method (AREVA, VEIKI INR) • Specific calculations of containment composition evolution are performed for a few sequences (with EXCEL) • Results are used for quantification of probabilities of branching nodes in a containment event tree • Stochastic events become determinist (water injection) • Some assumptions are modified • + : easy to implement (because of the modified assumptions) • - : relevance of conservative results for practical applications ? • Specifications cannot be fulfilled and accordinglyresults are very different

23. Methods2- Classical event tree method (AREVA, VEIKI INR) Example (from VEIKI contribution)

24. Methods3- Macro-events methods (UJV, IRSN) • Based on software initially created for accident progression event tree (EVNTRE, KANT) • Division of the simulation time in short time intervals (60 seconds) • Use of the same event tree (macro-event) for each interval • Quite comparable to the direct calculation method with Monte-Carlo simulations (for IRSN) or to the MCDET-method (for UJV) • + : use of classic tools, easy to integrate in a global event tree • - :

25. Methods3- Macro-events methods (UJV, IRSN) • IRSN macro-event UJV macro-event • Each macro-event is duplicated in a global event tree

26. Methods4- MCDET-analysis (GRS) • Mix of Discrete Dynamic Event Trees (DDET) and Monte Carlo Simulation • Each DDET has the same structure with 22 sequences, only the random values change (Monte Carlo) +: allows a lot of sensibility analysis, epistemic uncertainties and stochastic events are considered singly

27. Methods 5 - Stimulus-Driven Theory of Probabilistic Dynamics (SDTPD) - (ULB, CSN) • General methodology used as a basis for a Monte Carlo simulation of dynamic reliability problems. • The SDTPD analysis is based on a particular formalism: • the “process variables” are the variables that describe the system evolution (6 variables for LEI and 9 variables for ULB) • the “stimuli” are the events that can happen during the simulation (5 stimuli are defined by ULB and 6 by LEI) • the “dynamics” are the different regimes of evolution of the continuous process variables.

28. Methods 5 - Stimulus-Driven Theory of Probabilistic Dynamics (SDTPD) - (ULB, CSN) • The evolution of each process variable is defined for each dynamics. • To each stimulus are associated: • a probability of activation and a probability of deactivation if needed, • an activation (and if needed deactivation) delay. • This method allows a more precise modeling of combustion: if the flammability conditions change, combustion can be canceled. This point was not clearly specified in the specification of the benchmark exercise, but it shows one of the interesting capacities of the SDTPD. • A given number of histories (simulations) are performed. For each simulation, the final result is the containment integrity (saved or not). This allows determining the containment failure probability.

29. RESULTS – STEP 1

30. RESULTS – STEP 2

31. COMMENTS • The difference between results has not yet been fully explained and should not be linked to the method but also to benchmark assumptions interpretation • The different contributions shows different methodologies with advantages / disadvantages. • The analysis/comparison of results has shown some needs in terms of guidance for the presentation of uncertainties

32. Some outlook • These results are an encouragement to continue the development of specific methods for dynamic reliability problems, including specific post-processing of results, especially for uncertainties. • Some participants are interested for a step 3 with a direct use of a severe accident code like ASTEC • Such application is seen (at IRSN) as an interesting way for examination of robustness of severe accident guide