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Thermal Hydraulic System Codes Performance in Simulating
Buoyancy Flow Mixing Experiment in ROCOM Test Facility
Eugenio Coscarelli
San Piero a Grado Nuclear Research Group (GRNSPG), University of Pisa
Via Livornese 1291
56122, San Piero a Grado, Pisa, Italy
[email protected]
Sergii Lutsanych, Francesco D’Auria
San Piero a Grado Nuclear Research Group (GRNSPG), University of Pisa
Via Livornese 1291
56122, San Piero a Grado, Pisa, Italy
[email protected], [email protected]
ABSTRACT
The MSLB (Main Steam Line Break) accident scenario is one of the severe abnormal
transients that might occur in a NPP (Nuclear Power Plant). The main concerns of the MSLB
are the potential return to power condition and the occurrence of PTS (Pressurized Thermal
Shock) as a consequence of both rapid depressurization of the secondary circuit and the
entrainment of cold water into the core region. Assessment of these issues is the main
objective of integrated experimental tests carried out in the PKL-III and ROCOM facilities.
The first test rig is aimed to simulate thermal-hydraulic phenomenology at the system level,
whereas supporting ROCOM test facility is focused on the coolant mixing phenomenon that
took place in the Reactor Pressure Vessel (RPV). Combination of these two typologies of
experiments (integral effect test (IET) and separate effect test (SET)) provides appropriate
experimental data for CFD and TH-SYS (Thermal Hydraulic-SYStem) codes validation
against the relevant thermal hydraulic phenomena that occur during the MSLB.
The main purpose of this study is to evaluate the capability of two TH-SYS codes
TRACE V5 and CATHARE2 V2.5 to predict reasonably buoyancy driven mixing phenomena
that affects the IVF (In-Vessel Flow) and the distribution of coolant temperature at the core
inlet using 3-D porous media approach. Test 1.1 that had been carried out in ROCOM facility
was selected to investigate the coolant mixing inside the RPV under flow conditions typical
for a MSLB scenario. Averaging analysis of integral behaviour of the experimental and
calculated temperature distributions inside the RPV has been performed.
1
INTRODUCTION
In the framework of the OECD-PKL 2 project an integrated experimental test was
carried out in the PKL-III and ROCOM facilities to simulate a MSLB accident scenario [1].
The main purpose was to create a reliable database for validation of a CFD and TH-SYS
computer codes ([2], [3] and [4]).
In view of the test goals (pressurized thermal shock and recriticality phenomena), the
results of PKL G3.1 test provide boundary conditions for the complementary tests on coolant
mixing phenomenon in the RPV (ROCOM test facility).
215. 1
215. 2
In the current studies, the ROCOM test 1.1 was considered to assess both TRACE V5
and CATHARE2 V2.5 thermal-hydraulic system codes capability to predict sensibly the
buoyancy driven mixing phenomenon using a 3-D porous media approach.
2
OUTLINE OF THE ROCOM TEST
2.1
ROCOM test facility
The ROCOM (ROssendorf COolant Mixing) separate test effect facility models the
primary circuit of a German KONVOI-type PWR in a linear scale of 1:5 [6]. The main
purpose of the facility is to investigate a wide spectrum of coolant mixing scenarios that may
occur inside the primary circuit of a typical PWR.
The set of experiments carried in the facility provides valuable experimental data for
code validation (mainly CFD but also TH-SYS codes). Considerable attention was given to
the facility components which significantly influence the velocity field, such as the core barrel
with the lower core support plate and core simulator, as well as the perforated drum in the
lower plenum. The core basket consists of 193 aluminium tubes.
In current design the pressure vessel is equipped with a plane vessel head, which can be
replaced by a spherical head according to the original reactor. The upper plenum does not
contain any internals.
2.2
Buoyancy driven flow mixing experiment (ROCOM test 1.1)
In the framework of the OECD PKL 2 Project five complementary tests were conducted
at the ROCOM test facility. Two of the most severe thermal hydraulic conditions of the PKL
G3.1 test (maximum overcooling and ECC injection) were considered in ROCOM
experiments.
The tests ROCOM 1.1, 2.1 and 2.2 are dedicated to the overcooling phase of the PKL
test G3.1, whereas the tests ROCOM 1.2 and 1.3 are connected to the ECC injection phase.
Phase I
Conditioning
phase
250
Phase II
Ph. II-a
Ph. II-b
Ph. II-c
TF OP ME11/1
TF HS3 DE-EIN
TF KS3 DE-AUS
Temperature [°C]
TF DK ME 19
TF HS4 DE-EIN
TF KS4 DE-AUS
TF HS1 DE-EIN
TF KS1 DE-AUS
TF UP OBEN
TF HS2 DE-EIN
TF KS2 DE-AUS
UH
230
SG-2, -3, -4 outlet
(intact)
SG-2, -3, -4 inlet (intact)
UP
SG-1 inlet
(affected)
210
Core inlet
190
SG-1 outlet
(affected)
170
150
-500
0
500
1000
maximum
overcooling
1500
2000
2500
3000
3500
ECC injection
Figure 1: Measurement loop temperature in the PKL test G3.1 [4]
Time [s]
4000
4500
215. 3
Summary of the characteristics of ROCOM tests are reported in Table 1 (in the table
represent the relative density between the perturbed and the unperturbed flow).
Table 1: ROCOM test matrix
The main objective of the ROCOM test 1.1 [6] is to investigate the 3-D (threedimensional) flow behaviour inside the reactor pressure vessel during the maximum shrinkage
of the fluid flow which characterizes the first phase of the MSLB scenario (Phase 1 of the test
G3.1, see Figure 1). The boundary conditions were straightly derived from the corresponding
PKL experiment G3.1 (Table 2). Quasi-stationary flow conditions were derived from the time
point that corresponds to the minimum coolant temperature in the PKL experiment (Figure 1).
The temperature distribution inside downcomer is an important thermal hydraulic
parameter that should be considered to understand the turbulent mixing phenomenon. This
phenomenon is caused by dissimilar flow rates and coolant temperatures at the vessel inlets.
Another relevant physical aspect concerns the sector formation as a consequence of the
asymmetrical loop behaviour of the coolant flow.
ROCOM test 1.1 provides information about the following phenomena:
 position of the transition region between established quasi-homogeneous and
unperturbed temperature distributions in the downcomer;
 azimuthal temperature distribution at the core inlet.
Table 2: Boundary conditions of the ROCOM test 1.1
Loop
o
Temperature, [ C]
Mass flow rate, [kg/s]
Relative Density [-]
Density [kg/m3]
Inlet/Outlet Pressure [MPa]
1
153
5.743
1.12
915.9
3.8
2-4
236.1
1.328
1.00
819.9
3.8
215. 4
3
MODELING OF ROCOM TEST FACILITY
The main idea during the nodalization set up of the ROCOM test facility was to make a
consistent model suitable to reproduce installation with a high resolution nodalization scheme.
Consequently, the emphasis in modeling was put on exhaustive replication of the vessel.
The porous medium concept was utilized in simulation of the three dimensional flow
inside the RPV taking advantages of the features of the 3-D modules implemented in
CATHARE2 and TRACE V5 system codes ([7] and [8]). The porous media formulation uses
the concept of volume porosity and directional surface porosity. Volume porosity (of scalar
nature) is defined as the ratio of the volume occupied by the fluid(s) to the mesh cell volume,
while the directional surface porosity (of vector nature) are defined as the ratio of the free
flow surface area to the mesh cell surface in the three main directions.
Both CATHARE2 and TRACE V5 nodalizations (Figure 2) consist of the one 3-D
vessel component with boundary conditions imposed at the connections with the hot and cold
legs of the reactor coolant system (RCS). The computational grid of the 3-D module (Figure
3) is composed of the 6 radial rings, 8 azimuthal sectors and 16 axial layers for the TRACE
vessel component, whereas CATHARE2 3-D module is discretized in the axial direction
using 18 cells (number of radial rings and azimuthal sectors are preserved). Cylindrical
coordinate system was used in both nodalizations.
The LP (lower plenum) region was modeled in both cases taking into account the real
hemispherical shape. In order to reproduce more accurately the mixing processes in the LP,
the sieve drum was simulated. To reflect the flow distribution inside the hemispherical region,
the volume porosity was considered as a function of radial and axial nodes positions (Figure
4). The volumetric porosity distribution in the LP region is shown in Table 3.
Figure 2: The general layout of ROCOM facility
215. 5
DOWNCOMER
HL3
HL2
SECT 6
SECT 7
CL3
CL2
SECT 8
SECT 5
R1
SECT 1
R2 R3
R4R5
R6
SECT 4
CL1
CL4
SECT 3
HL4
SECT 2
HL1
CORE INLET
Figure 3: ROCOM nodalization sketch. Top view
R6
R5
DOWNCOMER
R4
R3
R2DRUM
axial layer
R1
UPPER PLENUM
CORE REGION
BYPASS
DRUM-CSP
LOWER PLENUM
radial ring
Figure 4: Fluid domain in TRACE V5 (a) and CATHARE2 (b) reference models of the
ROCOM facility
The perforated drum was defined by surface porosity in the radial direction given by
ratio of the total area of the holes to the area of the drum’s cylindrical surface (
).
The CSP (core support plate), located at the entrance of the core inlet, was modeled
using volumetric porosity equal to the surface porosity variable in the radial direction to
reproduce the distribution of the 193 core channels as summarized in Table 3 ([3] [9]).
Table 3: Volumetric porosity distribution in LP and at CSP
Axial layer
R1
R2
R3
R4
R5 (barrel zone)
R6
(DC zone)
1.0
1.0
1.0
1.0
1.0
1.0
LOWER PLENUM
1 (LP bottom)
Porosity
2 (drum zone)
1.0
1.0
0.9048
0.7155
0.4331
0.0493
3 (Bypass)
0.5109
0.5158
0
0
0
0
0.34816
0.34816
0.15754
0
1.0
CORE INLET (CORE SUPPORT PLATE)
Porosity
4 (CSP)
0.33375
215. 6
4
SIMULATION OF ROCOM TEST 1.1
The results obtained by TRACE V5 and CATHARE2 system codes are based on the use
of aforementioned computational models (Section 3). These calculated results were compared
with the available experimental data from the ROCOM Test 1.1.
The test 1.1 was performed in ROCOM facility at thermodynamic state characterized by
the working fluid at the atmospheric temperature. The density differences were produced by
mixing sugar into the water. Computational analysis presented in the current paper was
performed at the real thermodynamic conditions (the pressure and temperatures of the test rig
correspond to the PKL experiments).
The numerical investigation was performed following 100 seconds of null transient with
the aim of reaching the stationary and initial conditions of the ROCOM Test 1.1. During null
transient the mass flow rates at each cold leg were set to the nominal values of ROCOM test
facility.
The isobaric pressure 3.8 MPa was imposed to keep the system at the same condition of
the PKL at the time of interest (t=609 s). The transient started with the injection (5.743 kg/s)
of coolant at 153°C temperature in the damaged loop, whereas the mass flow rate to the rest
of the loops (1.328 kg/s) and the liquid temperature (236.1°C) were preserved.
4.1
Experimental and calculated results comparison
The strategy followed to assess the numerical results against the experimental data was
based on spatial and temporal comparison. The experimental to numerical comparison was
performed using an integral averaging on the spatial and temporal scale of the temperature
distribution in the downcomer as well as at the core inlet.
The spatial averaging of the experimental and calculated temperatures was performed
over all the measurement points (implemented thermocouples and computational meshes
respectively) in the downcomer and at the core inlet. This comparison was aimed to analyze
TRACE V5 and CATHARE2 results from the macroscopic point of view neglecting the
effects of local turbulent mixing.
Moreover the mentioned thermal mixing cannot be detected by the TH-SYS codes
merely because of the numerical diffusion in the solver schemes implemented into the two
codes. These schemes are called “stability enhancing two step scheme (SETS)” for TRACE
and “semi-implicit” for CATHARE2.
In Figure 5 the averaged temperature evolution inside the downcomer and at the core
inlet is shown. CATHARE2 slightly underestimates coolant temperature in the DC, whereas
TRACE overestimates it. This could be explained by higher thermal mixing in the first case
and lower thermal mixing in the second case.
At the core inlet both TH-SYS codes show almost the same behaviour with a higher (to
some extent) coolant temperature at the end of considered transient. Obtained results could be
justified by the numerical diffusion induced by the cold plum injection. The numerical
diffusion reduces the level of mixing which consequently results in higher temperature values
at the end of the test (Figure 5).
The temporal averaging was aimed to consider a quasi-stationary flow. The flow probes
were averaged on the range from 73 to 83 seconds for both experiment and the computation.
215. 7
SPATIAL AVERAGE OF TEMPERATURE DISTRIBUTION IN DONWCOMER
SPATIAL AVERAGE OF TEMPERATURE DISTRIBUTION CORE INLET
240
240
CALC-Ref TRACE.
CALC-Ref C2.
EXP
235
230
Temperature [°C]
Temperature [°C]
230
225
220
215
210
CALC-Ref TRACE.
CALC-Ref C2.
EXP
220
210
200
205
190
200
195
0
10
20
30
40
50
60
70
80
180
0
Time [s]
10
20
30
40
50
60
70
80
Time [s]
Figure 5: ROCOM experiment 1.1: averaged temperature evolution inside the DC and at the
core inlet for reference CATHARE2 and TRACE V5 nodalizations
During the transient, coolant stratification was observed in the downcomer.
Experimental results show the sharp transition zone between the mixing region and the
unperturbed zone, whereas in simulation the transition zone has rather dispersive and smooth
nature (Figure 6).
The following transition zones can be compared:
 horizontal separation is characterized by the transition from hot to mixed water;
 vertical separation is characterized by the transition from the affected cold leg’s
stream to water in the upper downcomer (top measurement points).
Transition zone
EXP
TRACE
CATHARE
Transition zone
TRACE-V5
Transition zone
CATHARE2
Figure 6: ROCOM experiment 1.1: temperature time averaged value in the DC outer plane
(time averaging interval: t = 73 s to t = 83 s)
215. 8
In order to contemplate more details about the in-vessel coolant stratification and
mixing phenomenon, the two supplementary “geometrical cuts” were performed. The first cut
(horizontal) represents temperatures in all the azimuthal sectors of the unrolled downcomer.
In Figure 7(a) is shown comparison of the calculated and averaged experimental data for each
azimuthal sector of the DC. The horizontal length of cold plum in the computation is
comparable to that in the experiment. For both CATHARE2 and TRACE V5 simulations the
stream is more diffused in the downcomer.
The second cut (vertical) represents temperature distribution in the one azimuthal sector
(neighbouring to the sector of a break leg connection) versus axial elevation (Figure 7(b)).
TRACE-V5 simulation predicts the 20 cm height of separation zone between the unperturbed
(hot water) and the mixed layer, whereas CATHARE2 estimates the 30 cm thickness of the
transition region. Noteworthily, the simulated cold water jet flow is less concentrated and
more diffused in the DC compared to the experimental results.
240
0
Axial layer position [m]
Temperature [°C]
230
220
210
200
TRACE-V5p2
CATHARE2
EXP
190
180
1
2
3
4
5
azimuthal sector
6
(a)
7
TRACE-V5p2
CATHARE2
EXP
-0.2
-0.4
-0.6
-0.8
-1
8
180
190
200
210
220
Temperature [°C]
230
240
(b)
Figure 7: ROCOM experiment 1.1: downcomer time averaged temperature horizontal cut (a),
vertical cut (b)
4.2
Nodalization sensitivity analysis
In order to assess the influence of nodalization scheme on calculated results, the
simulation of quasi-steady state test 1.1 was performed with finer vessel noding regarding the
reference nodalization (Section 3). In both CATHARE and TRACE fine nodalizations are
used the same number of radial rings and axial levels like in the respective reference
nodalizations. However, the number of azimuthal meshes was increased from 8 to 24.
In the case of refined mesh the azimuthal nodalization was executed in agreement to the
corresponding angular locations of the vessel inlet and outlet. The mentioned case was aimed
to study the effect of numerical diffusion in predicting of the thermal mixing in the DC
region. Noteworthily, the refined TRACE nodalization comprises of 2304 computational cells
(768 cells in reference model), whereas CATHARE2 consists of 2592 computational cells
(864 cells in reference nodalization).
Based on spatial temperature distribution analysis in the DC and at the core inlet it can
be concluded that in the case of CATHARE2 the mesh refinement tends to reduce the
discrepancy between experimental data and calculated results. However, in the case of
TRACE the discrepancy tends to increase. Results on Figure 8 provide a qualitative
evaluation of the influence of the numerical diffusion on the averaged temperature
distribution in the DC and at the core inlet by changing the nodalization schemes.
215. 9
Figure 8: ROCOM experiment 1.1: averaged temperature evolution inside the DC and at the
core inlet in case of reference and fine mesh nodalizations (24 azimuthal meshes)
Particularly, it should be mentioned that even if the DC averaged temperature curve
computed by the fine computational scheme tends to match the experimental one, it cannot be
explicitly considered as a general sign of good code performance, and could be misleading.
Change of the nodalization from coarse to fine leads to decrease of the numerical diffusion.
Therefore, it is always advisable to apply methods with as low as possible diffusion and
attempt to model the turbulence phenomenon by using appropriate, physically based
computational models ([3], [9] and [10]).
5
CONCLUSIONS
The progressive assessment and validation of the 3-D component features embedded in
the TH-SYS codes like CATHARE2 and TRACE V5 is mandatory step on a way to replace
the old approaches for simulation of the multi-dimensional effects with the use of 1-D
elements. In the current study, calculations of the coolant mixing phenomena in the
downcomer and the core lower plenum zones under asymmetric buoyant cooling loop
conditions have been carried out for OECD/PKL-2 ROCOM test 1.1.
Noteworthily, both CATHARE2 and TRACE V5 mixing process mechanisms are
related mainly to the truncation error of the numerical scheme (numerical diffusion). It could
be explained by the lack of the turbulent diffusion/viscosity models for multi-dimension flow
conditions for both TH-SYS codes.
Calculated integral parameters, like the average temperature inside the DC and at the
core inlet, show acceptable (from the qualitative point of view) agreement with the
experimental results.
The sector formation at the core inlet, as well as the position of the transition region
between established quasi-homogeneous and unperturbed temperature zones in the
downcomer, have been reproduced by the CATHARE2 and TRACE V5 codes. Experimental
results show the sharp transition zone between the mixing region and the unperturbed zone,
whereas in the simulations the transition zone is rather dispersive and smooth.
Calculations with the fine mesh nodalizations show better agreement with experimental
results in the case of CATHARE2, whereas in the case of TRACE the discrepancy tends to
increase. Actually, increase of the node numbers causes decrease of the numerical diffusion
[10].
Supplementary, in order to finalize the assessment process of the considered TH-SYS
codes, quantitative analysis must be performed. It should be also emphasized that the current
215. 10
results concern the mixing under buoyant low flow rates conditions in a scaled facility. Their
applicability to NPP scale has to be further investigated experimentally and analytically as
well [5].
ACKNOWLEDGMENTS
The authors gratefully acknowledge the advanced experimental work done by the
HZDR team. Furthermore, the authors want to express their gratitude to Dr. Giorgio Galassi,
Dr. Luben Sabotinov, Dr. Alessandro Del Nevo and Dr. Anis Bousbia Salah for their valuable
suggestions and constructive discussions.
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