ERMSAR 2012, Cologne March 21 – 23, 2012 Analysis and interpretation of the LIVE-L6 experiment A....
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ERMSAR 2012, Cologne March 21 – 23, 2012 Analysis and interpretation of the LIVE-L6 experiment A. Palagin, A. Miassoedov, X. Gaus-Liu (KIT), M. Buck (IKE), C.T. Tran, P. Kudinov (KTH), L. Carenini (IRSN), C. Koellein, W. Luther (GRS) V. Chudanov (IBRAE) Presented by A.Palagin
ERMSAR 2012, Cologne March 21 – 23, 2012 Analysis and interpretation of the LIVE-L6 experiment A. Palagin, A. Miassoedov, X. Gaus-Liu (KIT), M. Buck (IKE),
Analysis and interpretation of the LIVE-L6 experimentM. Buck
(IKE),
L. Carenini (IRSN),
V. Chudanov (IBRAE)
Presented by A.Palagin
Outline
Introduction
Application of different code systems to the LIVE-L6 test
Simulation of the LIVE-L6 test by the CONV code (KIT)
Post-test calculations of the LIVE-L6 experiment (USTUTT-IKE)
Calculations with MEWA module of system code ATHLET-CD
Calculations with CFD code ANSYS-CFX
Calculations with ICARE module of ASTEC code (IRSN)
Calculations with AIDA module (GRS)
Calculations with the PECM model (KTH)
Summary and conclusions
Introduction
The thermophysical behaviour of a corium pool in reactor pressure
vessel of a pressurised water reactor is of principal importance
for the prediction of core melt down accident development. This
concerns, in particular, the influence of major critical phases and
timing on the accident progression in terms of assessing the
possibility to remove the released heat by external vessel
cooling.
The general objective of the LIVE program at KIT is to study
phenomena resulting from core melting experimentally in large-scale
3D geometry with emphasis on the transient behaviour.
The presented work considers the analysis and interpretation of the
LIVE-L6 experiment, in which the molten pool (non-eutectic melt
KNO3-NaNO3) was separated by horizontal copper plate in order to
study the layering effect.
Different codes and models were used for post-test calculations:
This includes CFD code CONV, fast running models implemented in the
severe accident codes ASTEC (ICARE module) and ATHLET-CD (MEWA
module), CFD code ANSYS-CFX, AIDA module (GRS), PECM models
implemented in Fluent.
ERMSAR 2012, Cologne March 21 – 23, 2012
The experimental facility
Core of the LIVE test facility is a 1:5 scaled hemispherical bottom
of a typical pressurized water reactor.
The inner diameter of the test vessel is 1 m and the wall thickness
is 25 mm. The material of the test vessel is stainless steel.
The heating system consists of 6 heating planes at different
elevations with a distance of about 45 mm. Each heating plane
consists of a spirally formed heating element with a distance of
~40 mm between each winding. All heating planes together can
provide a power of about 18 kW.
The vessel wall is equipped with 17 instrumented plugs at different
positions along 4 axes. Each plug has 5 thermocouples (0, 5, 10,
15, 20 mm from the vessel wall). It is possible to place up to 80
thermocouples in the melt to measure its temperature at different
positions.
The LIVE test vessel Scheme of live test facility LIVE volumetric
heating system
ERMSAR 2012, Cologne March 21 – 23, 2012
LIVE-L6 test conduct
In the L-6 test the non-eutectic 80 mole% KNO3 – 20 mole% NaNO3
melt composition was used.
The total volume of melt was 210 l (68 kg of KNO3 plus 324 kg of
NaNO3) which corresponds to 43.5 cm of the pool height.
In the L-6 test horizontal copper plate of 2 mm thickness located
at the level of 33.3 cm separated melt in two parts. That was done
in order to develop the approach to the analysis of layering and
focusing effects that may take place during severe accident in
Reactor Pressure Vessel.
Evolution of heating power
Main results of LIVE-L6 test
Noticeable is the decreasing of temperature in the upper part of
the melt when approaching the cooper plate from above. Such
temperature gradient means that there is certain heat flux from the
upper part of melt (which has no heat sources) to the very vicinity
of the copper plate. This effect may be explained by redistribution
of heat fluxes coming from the lower (heated) melt part in the
upper part due to convection.
Melt temperature evolution averaged over azimuth angle
Melt temperature vertical profiles
Application of different code systems to the LIVE-L6 test
Simulation of the LIVE-L6 test by the CONV code (KIT)
CONV is 2D/3D thermohydraulic CFD code for the simulation of heat
transfer due to conduction and convection in complex geometry,
crust formation, etc. It was developed at IBRAE (Nuclear Safety
Institute of Russian Academy of Sciences, Moscow) within the
framework of the International RASPLAV project and additionally
improved within the ISTC #2936 and #3876 Projects.
For the modelling of heat generating viscous liquid in gravity
field with consideration of the buoyancy force in a Boussinesq
approximation the efficient difference scheme is applied to solve
unsteady 3D Navier-Stocks equations in natural "velocity-pressure"
variables on fully staggered orthogonal grids for Cartesian
coordinates.
The Large Eddy Simulation (LES) scheme with no SGS closure (i.e.
with implicit filtering) was realized in the code.
A frozen version of the CONV code has been transferred from IBRAE
to KIT within the framework of bilateral information exchange
agreement in order to simulate the LIVE experiments
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Simulation of the LIVE-L6 test by the CONV code (KIT)
In 3D calculation the cubic meshing nodalization 128×128×256 was
used with 256 nodes in the vertical direction. The vertical nodes
were condensed in the vicinity of the cooper plate in order to
describe in more details the heat exchange between the upper and
lower melt volumes.
Materials distribution at 18 kW phase of the test
Measured and calculated temperature evolution during first two
phases of the test. TC location: horizontal coordinate 37.4 cm,
vertical coordinate 25 cm
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Simulation of the LIVE-L6 test by the CONV code (KIT)
Vertical temperature distribution at the distance of 36 cm from the
vessel symmetry axis
Measured and calculated crust thickness
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Post-test calculations of the LIVE-L6 experiment (USTUTT-IKE)
Calculations with MEWA module of system code ATHLET-CD
The MEWA module is being developed and integrated in German system
code ATHLET-CD for simulation of late phase core melting. MEWA
describes the processes of late phase core degradation, the
behavior of corium in the lower head, including debris formation,
coolability, debris re-melting and molten pool behavior up to
failure of the RPV.
For the description of molten pools an approach based on a
representative model is applied. The underlying conceptual picture
divides the pool into a boundary layer along the cooled wall, where
the melt flows down, a stratified region in the central lower part
of the pool and a turbulent, isothermal region in the upper part.
For the heat transfer in the boundary layer, the model of Chawla
& Chan is used.
The temperature distribution in the central part of the pool is
determined from a one-dimensional energy conservation equation in
axial direction, assuming that mass flowing down in the boundary
layer is balanced by a corresponding upward mass flow in the
central pool part.
Heat transfer to the surface of the upper mixed layer is described
by empirical correlations. For a possible overlying metallic layer
a point model is used, i.e. only an average temperature of the
whole layer is calculated.
The heat transfer through the layer is described by empirical
correlations.
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Post-test calculations of the LIVE-L6 experiment (USTUTT-IKE)
Calculations with MEWA module of system code ATHLET-CD
Measured and calculated melt temperatures at 5 cm (MT1), 15 cm
(MT9) and 25 cm (MT21) elevations
Measured and calculated heat flux distributions for steady state at
different heating levels.
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Post-test calculations of the LIVE-L6 experiment (USTUTT-IKE)
Calculations with MEWA module of system code ATHLET-CD
Comparison of crust thickness distribution at the end of the test
LIVE L-6: experiment (left) and MEWA calculation (right)
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Post-test calculations of the LIVE-L6 experiment (USTUTT-IKE)
Calculations with CFD code ANSYS-CFX
In order to support the analysis and interpretation of the
experiments and through this the further development of simplified
models to be used in severe accident codes, complementary analyses
have been started at IKE using the commercial CFD code
ANSYS-CFX.
CFD calculations offer a more detailed analysis that can provide
better insight into details of the physical processes, which can be
used for checking of model assumptions (e.g. stratification, thin
boundary layer, upper layer with strong mixing) and the improvement
of empirical laws in more simplified approaches.
A structured mesh (100k on the finest one, refine factor of 3) was
used, with enhanced local treatment on the crust side. The
calculations were carried out in 2D cylindrical symmetry (one cell
on azimuthal coordinate). The SST (shear stress transport model)
turbulence model was applied, which is a blend of the k-ε and the
k-ω models.
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Post-test calculations of the LIVE-L6 experiment (USTUTT-IKE)
Calculations with CFD code ANSYS-CFX
Comparison of melt temperature (left) and heat flux (right)
profiles calculated by CFX for different heating powers with
experimental measurements
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Calculations with ICARE module of ASTEC code (IRSN)
The ASTEC code has a modular structure, each of its modules
simulating a reactor zone or a set of physical phenomena. The ICARE
module is used to describe in-vessel core degradation, core thermal
hydraulics and molten pool behaviour in the lower head during
severe accident.
The ASTEC lower plenum model, implemented in the V2.0 version, can
simulate up to 3 dense corium layers, depending on the metal-oxide
phases separation and their stratification. Each layer is
considered having a homogeneous composition. The molten pool in the
LIVE-L6 test was therefore treated as two layers in the ICARE
calculation, one for the melt positioned below the separating plate
and another to simulate the melt above the separating plate.
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Calculations with ICARE module of ASTEC code (IRSN)
Temperature evolution of the lower melt layer
Temperature evolution of the upper melt layer
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Calculations with ICARE module of ASTEC code (IRSN)
Vertical profiles of non-dimensional temperature difference
Non-dimensional heat flux distribution at vessel wall
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Calculations with AIDA module (GRS)
The module AIDA (Analysis of the Interaction between Core Debris
and the RPV during Severe Accident) is a coupled integral
simulation model for:
the thermal behavior of molten core material in the lower
head
the crust formation
the cooling of the melt by a transient two-phase flow through a gap
between crust and (RPV)
as well as for the structural response of the RPV wall.
The scope of AIDA modeling is restricted to the lower head and the
RPV wall.
Within the model all physical phenomena with short relaxation
periods are modeled by correlations. For processes with long
relaxation periods, such as the heat conduction through the RPV
wall or the crust formation in the meltdown, differential equations
are used.
For the case when a metallic layer is formed above the oxidic melt,
a point model is used which determines the average temperature of
this layer.
The heat conduction in the crust is considered one-dimensional and
stationary. The melt–crust system is thermally connected to vessel
wall, with the latest being modeled by a two dimensional
differential equation.
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Calculations with AIDA module (GRS)
Experimental (averaged over 36 TC) and computed mean temperatures
of the melt
Computed crust thickness during constant heat power phases
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Calculations with the PECM model (KTH)
The PECM (Phase-change Effective Convectivity Model) has been
developed for simulation of melt pool natural convection heat
transfer. Using a similar approach employed in the PECM for
simulation of turbulent natural convection heat transfer, the metal
layer PECM has been also developed for simulation of mixed
convection heat transfer in the metallic layer atop of oxidic melt
pool. Both mentioned PECM models were applied for simulation of the
LIVE-L6 experiment in this work.
The instantaneous liquid melt velocities are not employed in PECM,
therefore solving Navier-Stokes equations is not required. This
allows the PECM to be computationally efficient. Only energy
conservation equation is solved using the Fluent code solver. The
turbulent heat fluxes in different directions are transferred to
the cooled boundaries by the means of directional characteristic
velocities which represent energy splitting in the melt pool.
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Calculations with the PECM model (KTH)
Transient temperatures of PECM simulation and LIVE-L6
experiment
Vertical temperature profiles of the PECM simulation and
experiment
ERMSAR 2012, Cologne March 21 – 23, 2012
Application of different code systems to the LIVE-L6 test
Calculations with the PECM model (KTH)
PECM and experimental crust thickness
The PECM simulation and experimental heat fluxes at the end states
of 18 kW and 10 kW heating
ERMSAR 2012, Cologne March 21 – 23, 2012
Summary and conclusions
The thermophysical behaviour of a corium pool in pressure vessel of
a Pressurised Water Reactor (PWR) in the course of core melt down
accident is of principal importance for the prediction of its
development.
The general objective of the LIVE program at KIT is to study the
phenomena resulting from core melting experimentally in large-scale
3D geometry with emphasis on the transient behaviour.
In the presented work the analysis and interpretation of the
LIVE-L6 experiment were described. A range of different codes and
models is used for post-test calculations and comparative analyses.
This includes fast running models implemented in the severe
accident codes ASTEC (ICARE module) and ATHLET-CD (MEWA module),
CFD code CONV, CFD code ANSYS-CFX, AIDA module (GRS) and PECM
models implemented in Fluent.
Generally, all the codes show satisfactory agreement with the
experimental data. However, certain discrepancies (underestimation
of the upper pool temperature, overestimation of the crust
thickness over the separating plate, etc.) were revealed.
The cross-comparison of different codes calculation results,
analysis of discrepancies and possible recommendations for the
models improvement are foreseen within a benchmark calculation
study.
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