Reflooding of a degraded core with ICARE/CATHARE V2

Reflooding of a degraded core with ICARE/CATHARE V2 Florian Fichot1 - Fabien Duval1 - Nicolas Trégourès1 Céline Béchaud2 - Michel Quintard3 - Magali Zabiégo1 1 Institut de Radioprotection et de Sûreté Nucléaire (IRSN) 2 Electricité de France (EdF) 3 Institut de Mécanique des Fluides de Toulouse (IMFT) 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Reflooding of a debris bed (porous medium) in a PWR damaged core Liquid water Solid phase •Average debris size between 1 and 4mm (TMI-2) • Internal power generation (residual power ~a few MW) • High temperature (greater than 2000 K) A few hundreds of degrees 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 Severe accident issues: Possibility to quench the debris bed ? Integrity of the vessel ? 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Two-phase flow in a porous medium  Specific momentum and energy conservation equations (up-scaling method) Momentum balance equation  Generalised two-phase Darcy law • Relative permeability and passability for viscous and inertial forces • Capillary force term • Inertial friction term between the gas and the liquid phase Empirical correlations Energy balance equation • Thermal non-equilibrium between the three phases considered • Heat transfer coefficients determined from the distribution and the geometry of the phases ICARE/CATHARE V2 modeling (3D,3T) (1) 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 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

SLG SGL Nucleate boiling Film boiling Selection of the flow regime: phase distribution map a SGL+SLG configuration a=0.5 0.8  Transition zone SGL configuration SLG configuration Solid phase temperature (K) T burn-out Tmsf(P) 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 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Tini debris = 594 K Fdebris = 3.18 mm Porosity ~ 0.4 Tutu et al. IC/CAT V2 (Trégourès et al.) Tutu, Ginsberg et al. "Debris bed quenching under bottom flood conditions" - 1984 - NUREG/CR3850. Validation (1D) 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. Trégourès et al. "Multi-dimensional numerical study of core debris bed reflooding under severe accident conditions" - NURETH10 - 2003 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Homogeneous beds (porosity, particle diameter and power distribution) Void fraction profile • Same debris particles • Same porosity • Same power, chosen to lead to dry-out in the 1D bed • Saturated debris bed at time 0 1D 2D 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) 1D debris bed 2D debris bed Accurate CHF calculation in large debris beds depends on correct prediction of 2D/3D two-phase flow. Dry-out: 1D-2D comparison 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Water injection Lower head geometry • Dry, overheated debris bed • P = 60 bar, Tini max = 1300 K F = 2 mm, Porosity = 0.4 • Power = 200 W/Kg (homogeneous) • No debris oxidation Water injected into the downcomer (simulation of the safety injection system) Initial temperature map 2D debris bed reflooding : Initial state ICARE/CATHARE V2 simulation 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

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

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

2D 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 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

ICARE/CATHARE V2 calculation • same lower head geometry • same conditions (porosity, pressure…) • ZrO2 + UO2 : 90% Zr : 10% • Reflooding with Zr oxidation • Effect of the debris initial temperature  2D debris bed reflooding : Zr oxydation effect (1) 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 Intensity of the oxidation process depends on the quench front velocity and on the debris temperature 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Center bed temperature with time Tini = 1350 K Tini = 1350 K Cumul. H2 prod. with time Tini = 1050 K Kg Start of reflooding Start of reflooding Tini = 1050 K 2D debris bed reflooding : Zr oxydation effect (2) Tini = 1050 K • Oxidation slower than quench front progression • Reaction quickly stopped due to quenching Tini = 1350 K • Much faster oxidation reaction • Strong H2 increase after start of reflooding (delay corresponds to the time to reach higher temperatures within the bed) 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Tini = 1050 K Tini = 1350 K Non oxidized Fully oxidized 2D debris bed reflooding : Zr oxydation effect (3) Time = 900 s  system fully quenched • A small part of metallic debris has been oxidized • A limited region is fully 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 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Initial temperature map Reflooding of a reactor-like vessel (1) ICARE/CATHARE V2 calculation Initial state Simplified PWR vessel Hot, partially oxidized rods No debris Main models activated Thermal exchanges Rod and mixture oxidation Molten material relocation Reflooding Reflooding  Standard CATHARE2 laws for still-standing rods  Porous medium model for debris particles 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Reflooding of a reactor-like vessel (2) Main events • 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) Void fraction 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

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 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Building of a general reflooding model  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 Validation  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 ? Work under way 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Tomography Tomography of PHEBUS-FPT1 rod bundle after degradation (cross section) 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Upscaling Modelling two-phase flow in a large porous medium requires the use of averaged equations for the momentum and energy conservation. 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 » 11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005

Reflooding of a degraded core with ICARE/CATHARE V2