Reflooding of a degraded core with ICARE/CATHARE V2

11th International QUENCH Workshop - Karlsruhe - October 25-27, 2005 1

Reflooding of a degraded core with Reflooding of a degraded core with ICARE/CATHARE V2ICARE/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)


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


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


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 boilingNucleate boiling

SGLSLG

Selection of the flow regime: phase distribution map

SGL+SLGconfiguration

SLGconfiguration

Transitionzone

SGLconfiguration

0.8

T burn-out Tmsf(P)Solid phase temperature

=0.5

(K)


Validation (1D)

Tutu et al.

IC/CAT V2 (Trégourès et al.)

Tini debris = 594 Kdebris = 3.18 mmPorosity ~ 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" -

1984 - NUREG/CR3850.

Trégourès et al. "Multi-dimensional numerical study of core debris bed reflooding under severe accident conditions" - NURETH10 - 2003


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)

1D2D

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.


2D 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

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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

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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


2D 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…)

• ZrO2 + UO2 : 90% Zr : 10%

• Reflooding with Zr oxidation• Effect of the debris initial

temperature



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 H2 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. H2 prod. with timeK

g



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


Reflooding of a reactor-like vessel (1)

Initial temperature map

Initial state

Simplified PWR vesselHot, partially oxidized rods

No debris

Main models activated

Thermal exchangesRod and mixture oxidationMolten material relocation

Reflooding

Reflooding Standard CATHARE2 laws for still-standing rods Porous medium model for debris particles

ICARE/CATHARE V2 calculation


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


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


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


Tomography

Tomography of PHEBUS-FPT1rod bundle after degradation

(cross section)


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.

Documents

Reflooding of a degraded core with ICARE/CATHARE V2