Synthesis of Analytical Activities for Direct Containment Heating ERMSAR-07, 2007, Karlsruhe

1. Synthesis of Analytical Activities for Direct Containment HeatingERMSAR-07, 2007, Karlsruhe R. Meignen, S. Mikasser - IRSN, France C. Spengler – GRS, Germany A. Bretault – EDF, France

2. Summary • Introduction DCH phenomena Scope of work Overview of FZK experimental activities Overview of analytical activities • Lumped parameter codes • JPA activity • Use of CMFD codes • Overview of DISCO-L1/FH02 results • Comparison of LP codes Major features of code results Major features of code models with respect to dynamical and thermal aspects • Conclusions and perspectives

3. Introduction DCH phenomena Scope of work Overview of FZK experimental activities Overview of analytical activities Lumped parameter codes JPA activity Use of CMFD codes

4. Direct Containment Heating phenomena • The vessel failure under pressure leads to the dispersion of the melt. Thermal and chemical transfers lead to a pressurization of the containment. • The major tasks for modeling are therefore : • Prediction of the dispersion • Fragmentation and coalescence processes • Size of fragments: • interfacial area for transfers • entrainment by the gas • Chemical transfers • Oxidation of fuel • Combustion of initially present and produced hydrogen

5. Scope of work in SARNET context - 1 • Most of the models where built for US reactor type geometries • However, model are very dependent on geometry  Quite limited confidence  Need at least an assessment on other reactors.

6. Scope of work in SARNET context - 2 • FZK engaged an experimental and analytical program for EPR geometry => DISCO. • IRSN-FZK collaboration for French PWR Reactors (P’4) • Objectives : • Better understanding of phenomena with the use of multi-dimensional multi-fluid codes • Assessment/improvement of ASTEC models • In SARNET WP13-2 a collaborative work of comparison of available 0-D code • CONTAIN : GRS, MAAP : EDF, ASTEC : IRSN • 1st phase : dynamical and thermal aspects

7. Overview of FZK experimental activity • First DISCO program with EPR geometry • Moderate ejection pressure, up to ~ 15 bars. • ~ 50 tests with cold simulant (water, Wood’s metal) • Visualization, pressure measurements, dispersion. • Several failure mode => central mode gives conservative results • 6 tests with hot simulant (Al2O3-Fe thermite) • 2 cases with direct path to containment. • Variable amount of initial H2 in cont. • Measurement of hydrogen characteristics: • Production through oxidation • Combustion

8. 5 6 3 4 2 7 Overview of FZK exp. Activity-2 • Second DISCO series in collaboration with IRSN with French 1300 MWe P’4 reactor geometry • Complex 3-D geometry, large pit, direct path to containment (5), access door (7) • Ejection pressure, up to ~ 20 bars. • ~ 15 tests with cold simulant :water, gallium alloy (d~6) • Only central failure • Some test with “2D” geometry for code qualification • 5 tests with hot simulant (Al2O3-Fe thermite) • Up to 6% initial H2 in containment • 1 test in neutral environment => LACOMERA L1, funded by EC • 1 test in “2D” geometry (no access door)

9. Lumped parameter code used in SARNET • ASTEC / RUPUICUV • Developed by IRSN and GRS • MAAP • Developed by US utilities, used by EDF • CONTAIN • Developed at SNL, used by GRS • Codes were used with a limited knowledge of the models and their adequacy/accuracy • IRSN calculations made by DSR (FAR) whereas code developed by DPAM (Cadarache) • GRS uses CONTAIN with the perspective to get familiar with the physics and develop COCOSYS. • Limited open literature for MAAP.

10. JPA task on lumped parameter code comparison • Objective : • - compare ASTEC modules with experiment (LACOMERA L1) and other codes : MAAP (EDF) and CONTAIN (GRS). • - propose modifications for ASTEC modelling if necessary • Status • Launched at mid-2005 • Received result calculations from GRS and EDF during nov. and dec. 2005. • Report on calculation results delivered • New modelling for ASTEC under investigation at IRSN and GRS

11. Use of CMFD codes • Two codes used : • AFDM (FZK) and MC3D (IRSN) • Oxidation quite parametric • No combustion in MC3D, 0-D model in AFDM • Global analysis and calculations • Necessarily quite rough meshes • 3-D tests possible with MC3D • Gives insights for scaling effects • Local analysis of specific phenomenon • Possible use of 3-d fine meshes, particularly for dynamical aspects. • Gives some insight for details in designs or reactor with no experimental data

12. DISCO/LACOMERA test L1 (FH02) Description and short analysis

13. 5 6 3 4 2 7 DISCO-L1/FH02 : Main characteristics • Geometry of P’4 french PWR. • Complex 3-D geometry • Direct path to containment (5) • Path to sub-compartments (6) • Bottom access (7) • Scale 1:16 • Fuel : Alumina/Fe thermite • Neutral environment • No chemical interaction

14. Dispersal results DISCO-L1/FH02 : short analysis •  Thermal efficiency • Using a simple thermal model: • Max possible pressure : 5.5 bar • Global efficiency ~25 % • Considering only dispersed fuel energy : • Containment + sub-comp : h = 55 % • Containment only : h ~ 100 %

15. DISCO-L1/FH02 : short analysis - 2 Particle diameter far smaller in containment than in sub-compartment. Explains : • The low efficiency in sub-compartment (also due to smaller volume) • The near thermal equilibrium in containment Particle mass distribution

16. sub DISCO-L1/FH02 : short analysis - 3 • Why is there a so big difference in particle diameter ? •  melt has to make a 90°turn to go to sub-compartments, but larger particles : • Indicates : • - a recoalescence process before flow separation. • - particle size determined by flow at respective nozzles, not necessarily representative of particles in pit and of dispersal process Cont.

17. Comparison of LP codes and results Major features of code Major code models with respect to dynamical and thermal aspects

18. 1-d. Overview of the JPA task on lumped parameter code comparison-2 • Short first conclusions from calculations : • Unsatisfactory results due to arbitrary fittings for all code calculations. • Predicitive capabilities quite dubious, at least for ASTEC • Need for improved modelling. • Use of the different models very unclear • Unadapted geometrical models ? • Weak understanding of the conditions of use of the models and choice of parameters ? Relevance of models ?

19. CPA RUPUICUV CONTAINMENT V = 11.429 m3 Zmin = 1.04 m Zmax = 3.94 m Diameter = 2.16 m V = 2.451 m3 Zmin = 0 m Zmax = 1.04 m Dmax = 1.81 m Dmin = 0.6 m COMPARTMENT CAVITY V = 0.148 m3 Zmin = 0 m Zmax = 0.2 m Dmax = 1.8 m Dmin = 0.6 m CORIUM PIT BOTTOM ACCESS Comparison of 0-D calculation : ASTEC (IRSN)° • Rough model quite inadequate to complex geometries Dispersion ratio = user input • Parametrical dispersion and particle size (heat transfer) • Geometry fitted to obtain adequated global dispersion • Complex heat transfer modelling as CPA does not handle particle flow: • CORIUM module as interface to compute the heat source input • Heat transfer with a correlation.

20. MAAP (Modular Accident Analysis Program) Commercial tool similar to ASTEC (but far older), developed for the US utilities, used in particular by EDF. Limited open literature and documentation. The DCH modelling has a rather low level of complexity. Melt dispersal from cavity evaluated with: two steps : melt followed by gas a correlation evaluated at time 0; a kinetic based on the gas blow-down time; Dynamical and thermal equilibrium between gas and debris. Geometry used in calculations not fully relevant Flow sections inadequate Intermediate volume necessary Comparison of 0-D calculation : MAAP

21. Only 54 % of fuel ejected from vessel The rest is unmelted => pb with definition of melt energy.  Computation of dispersion not relevant (although good results) But pressurisation well calculated with an homogeneous model Comparison of 0-D calculation : MAAP

22. Comparison of 0-D calculation : CONTAIN (GRS)° • Developed for the USNRC. DCH modelling was an important focus of CONTAIN with quite detailed models. •  CONTAIN is one of the reference tools for DCH • Melt and gas ejection from vessel incorporates a transient 2-j period. • The melt fragmentation/trapping processes is calculated using: • A setting of different classes for debris according to their size and their temperature. The shape of the spectrum is however parametric. • A process of fragmentation based on an entrainment rate correlation. (Alternatively, the user could also specify an entrained fraction correlation) • Inter-cell flow with a dynamical equilibrium assumption (slip velocity could be used). • Convective heat transfer with a standard correlation. • Radiative heat transfer to gases and structures.

23. Comparison of 0-D calculation : CONTAIN (GRS)° • Numerous discussions during meetings and in report concerning the use of dispersal models • Correct use of correlations ? • Coefficient from 100 to 1000, whereas a value of 5 is theroretically recommended. • Imposed spectrum of particle size • 10 groups • Not consistent with exp. difference between compartments and containment • Heat transfer reduction by factor of 3 Revised geometry in calculations with CONTAIN (GRS)

24. Melt dispersal computation • ASTEC and MAAP use complex correlations difficult to read and understand. Dimensionless numbers with obscur meaning. • ASTEC : • MAAP :

25. Melt dispersal computation-2 • GRS calculations with CONTAIN used the Whalley-Hewitt correlation: • Correlation for entrainement in 2-j film flow in tubes • Kc originally = 5, values from 100 to 1000 used (~100 recommended in documentation) • Dependancy of viscosity quite dubious. • No influence on the melt density !

26. Heat transfers • A local model using a correlation as in ASTEC and CONTAIN seems unadequate in O-D model (choice of velocity ? Sensitivity ?) •  Necessary fitting with large uncertainties for reactor applications • Equilibrium assumption in MAAP • Relevant regarding exp. analysis • Needs further checking • Important sensitivity in ASTEC • Contradiction with exp. Analysis • Investigations on-going.

27. Conclusions of JPA Task. • Use of existing correlations and models for dynamical aspects very uncertain: • Geometry in general not adapted • => Strong fitting hides the relevance. • => Need for geometry-specific models ? (already recognized with previous US studies, parameters in correlation varied by 1 order of magnitude depending of the reactor) • Necessity for an increased comprehension: • Dispersal processes • Fragmentation processes. •  DCH not really closed as long as calculation adequacy not clarified and validated. • Characteristic particle size should be different in each volume • Simplified heat transfer modelling necessary and possible • Homogenous model might be sufficient.

28. Perspectives • 2nd phase of task with computation of tests FH01 and FH03 with oxidation and combustion • Analysis of the building of a new model still on-going at IRSN. • Use of CFMD codes showed to be promising for the dispersal problem. • As an alternative : a specific correlation can be build from calculations (at reactor scale) for each reactor geometry (on-going at IRSN with MC3D)