11 th International QUENCH Workshop - Karlsruhe - October 25-27, Reflooding of a degraded core with ICARE/CATHARE V2 Florian Fichot 1 - Fabien Duval 1 - Nicolas Trégourès 1 Céline Béchaud 2 - Michel Quintard 3 - Magali Zabiégo 1 1 Institut de Radioprotection et de Sûreté Nucléaire (IRSN) 2 Electricité de France (EdF) 3 Institut de Mécanique des Fluides de Toulouse (IMFT)

11 th International QUENCH Workshop - Karlsruhe - October 25-27, Context  Thermal non-equilibrium between the liquid, vapor and the solid phase  Complex flow pattern due to calefaction phenomenon  Often treated as a two-phase flow in a porous medium  Multi-dimensional effects Average debris size between 1 and 4 mm (TMI-2) Internal power generation (residual power ~a few MW) High temperature (greater than 2000 K) Solid phase A few hundreds of degrees Liquid water Severe accident issues: Possibility to quench the debris bed ? Integrity of the vessel ? Reflooding of a debris bed (porous medium) in a PWR damaged core

11 th International QUENCH Workshop - Karlsruhe - October 25-27, ICARE/CATHARE V2 modeling (3D,3T) (1) Energy balance equation Thermal non-equilibrium between the three phases considered Heat transfer coefficients determined from the distribution and the geometry of the phases Two-phase flow in a porous medium  Specific momentum and energy conservation equations (up-scaling method) Relative permeability and passability for viscous and inertial forces Capillary force term Inertial friction term between the gas and the liquid phase Momentum balance equation  Generalised two-phase Darcy law Empirical correlations Fichot et al. "The impact of thermal non-equilibrium and large- scale 2D/3D effects on debris bed reflooding and coolability" - Accepted for publication (Nuclear Engineering and Design)- 2006

11 th International QUENCH Workshop - Karlsruhe - October 25-27, ICARE/CATHARE V2 modeling (3D,3T) (2) Up-scaling method (averaging of the local conservation equations)  Knowing: phase distribution (Solid-Liquid-Gas or Solid-Gas-Liquid) void fraction, porosity, particle diam.  Heat transfer fluxes (Qsl, Qsg, Qlg) can be derived from simplified representations of the porous medium TBo Film boiling Nucleate boiling SGL SLG Selection of the flow regime: phase distribution map SGL+SLG configuration SLG configuration Transition zone SGL configuration  0.8 T burn-out Tmsf(P) Solid phase temperature  =0.5  (K)(K)

11 th International QUENCH Workshop - Karlsruhe - October 25-27, Validation (1D) Tutu et al. IC/CAT V2 (Trégourès et al.) Tini debris = 594 K  debris = 3.18 mm Porosity ~ 0.4 Steam production and reflooding time in good agreement except for high mass flow rates. The transition from film boiling to nucleate boiling seems to be correctly reproduced. Needs of improvements for high mass flow rates. Main lack: droplet transport. Similar conclusions for top reflooding in spite of a less satisfactory behavior of the model. Tutu, Ginsberg et al. "Debris bed quenching under bottom flood conditions" NUREG/CR3850. Trégourès et al. "Multi-dimensional numerical study of core debris bed reflooding under severe accident conditions" - NURETH

11 th International QUENCH Workshop - Karlsruhe - October 25-27, Dry-out: 1D-2D comparison 1D debris bed 2D debris bed Same debris particles Same porosity Same power, chosen to lead to dry-out in the 1D bed Saturated debris bed at time 0 Homogeneous beds (porosity, particle diameter and power distribution) 1D 2D Void fraction profile Void fraction distribution very different because of the liquid circulation (no region with strong steam counter-current in 2D) 2D dry-out power is higher than the classical 1D prediction (~1.5) Accurate CHF calculation in large debris beds depends on correct prediction of 2D/3D two-phase flow.

11 th International QUENCH Workshop - Karlsruhe - October 25-27, D debris bed reflooding : Initial state ICARE/CATHARE V2 simulation Water injection Initial temperature map Dry, overheated debris bed P = 60 bar, Tini max = 1300 K  = 2 mm, Porosity = 0.4 Power = 200 W/Kg (homogeneous) No debris oxidation Water injected into the downcomer (simulation of the safety injection system) Lower head geometry

8 2D debris bed reflooding : Void fraction distribution Slope of the lower head  Water flow along the wall without any counter-current effect Water penetration from the top limited by the strong steam flow  Formation of a liquid pool at the top of the bed and of a dry bubble in the center Progressive quenching of the bubble No sharp quench front but continuous transition from a dry region to a saturated and eventually cooled one

9 2D debris bed reflooding : Temperature field Same observations in terms of temperature distribution Colder temperatures along the wall  Faster quenching of the bed periphery Progressive quenching of the dry bubble

11 th International QUENCH Workshop - Karlsruhe - October 25-27, D debris bed reflooding : particle diameter effect 1 mm particles, porosity = 0.4 Water accumulation at the top of the bed (lower permeability of the bed) 2 mm particles, porosity = 0.4 The injected water flows directely down to the bottom of the bed

11 th International QUENCH Workshop - Karlsruhe - October 25-27, D debris bed reflooding : Zr oxydation effect (1) Intensity of the oxidation process depends on the quench front velocity and on the debris temperature Reflooding effects on oxidation Steam supply on hot metallic debris  Oxidation enhancement Fast cooling of the particles  Oxidation reduction Sequential effects at a given location ICARE/CATHARE V2 calculation same lower head geometry same conditions (porosity, pressure…) ZrO 2 + UO 2 : 90% Zr : 10% Reflooding with Zr oxidation Effect of the debris initial temperature 

11 th International QUENCH Workshop - Karlsruhe - October 25-27, D debris bed reflooding : Zr oxydation effect (2) Tini = 1350 K Tini = 1050 K Start of reflooding Center bed temperature with time Tini = 1050 K Oxidation slower than quench front progression Reaction quickly stopped due to quenching Tini = 1350 K Much faster oxidation reaction Strong H 2 increase after start of reflooding (delay corresponds to the time to reach higher temperatures within the bed) Start of reflooding Tini = 1350 K Tini = 1050 K Cumul. H 2 prod. with time Kg

11 th International QUENCH Workshop - Karlsruhe - October 25-27, D debris bed reflooding : Zr oxydation effect (3) Tini = 1050 K Tini = 1350 K Time = 900 s  system fully quenched A small part of metallic debris has been oxidized A limited region is fully oxidized Fully oxidized Non oxidized Time = 700 s  intermediate state Complete oxidation of a narrow region Center part non oxidized (steam starvation condition) Oxidation front downstream of the quench front Non uniform distribution of oxidized zones Final state: full oxidation of the center part of the debris bed

11 th International QUENCH Workshop - Karlsruhe - October 25-27, Reflooding of a reactor-like vessel (1) Initial temperature map Initial state Simplified PWR vessel Hot, partially oxidized rods No debris Main models activated Thermal exchanges Rod and mixture oxidation Molten material relocation Reflooding  Standard CATHARE2 laws for still-standing rods  Porous medium model for debris particles ICARE/CATHARE V2 calculation

11 th International QUENCH Workshop - Karlsruhe - October 25-27, Reflooding of a reactor-like vessel (2) Fuel rod heat-up Melting and relocation of the control rod materials Water injection at the top of the downcomer (starts at t = 100 s) Fuel rod dislocation depending on time and temperature criteria (t  220 s AND T  1300 K)  debris bed generation Debris collapse on a porosity criterion (p  0.6) Main events Void fraction

11 th International QUENCH Workshop - Karlsruhe - October 25-27, Summary 3D non-equilibrium model implemented in ICARE/CATHARE V2 Reflooding of a debris bed can be calculated Debris oxidation can be taken into account 2D significant effects on dry-out, reflooding and oxidation Correct behavior when reflooding a damaged core-like medium  rods + debris  collapse

11 th International QUENCH Workshop - Karlsruhe - October 25-27, Work under way  Continuous transition from rod geometry to debris geometry  Possibility to treat more realistic configurations Post-doc work based on the study of the PHEBUS-FP tomography Link between temperature and specific parameters of the state of the bundle (solid "particle" size, porosity size) Improvement of the heat transfer coefficient calculation Improvement of the flow map Building of a general reflooding model  Definition of experimental needs for the 2D model validation  Synthesis of the experiments already performed Need of a 2D, high temperature debris bed reflooding experiment SARNET WP 11.1 : IKE (DEBRIS facility), VTT (STYX facility) Could give answers ? Validation

11 th International QUENCH Workshop - Karlsruhe - October 25-27, Tomography Tomography of PHEBUS-FPT1 rod bundle after degradation (cross section)

11 th International QUENCH Workshop - Karlsruhe - October 25-27, Upscaling The strongly anisotropic porous medium is represented by an equivalent continuous medium at the macroscopic scale. Effective transport properties characterize the small-scale physical processes The upscaling technique selected is the « volume averaging » Modelling two-phase flow in a large porous medium requires the use of averaged equations for the momentum and energy conservation.