215. 1

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

eugenio.coscarelli@ing.unipi.it

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

sergii.lutsanych@gmail.com, f.dauria@ing.unipi.it

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).

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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.

Figure 1: Measurement loop temperature in the PKL test G3.1 [4]

150

170

190

210

230

250

-500 0 500 1000 1500 2000 2500 3000 3500 4000 4500

Temperature [°C]

Time [s]

TF OP ME11/1 TF DK ME 19 TF HS1 DE-EIN TF HS2 DE-EIN TF HS3 DE-EIN TF HS4 DE-EIN TF KS1 DE-AUS TF KS2 DE-AUS TF KS3 DE-AUS TF KS4 DE-AUS TF UP OBEN

Phase I Phase IIConditioning

phase

Core inlet

SG-1 outlet (affected)

UH

UP

SG-2, -3, -4 outlet (intact)SG-2, -3, -4 inlet (intact)

SG-1 inlet (affected)

Ph. II-a Ph. II-cPh. II-b

maximum

overcooling ECC injection

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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 (three-

dimensional) 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 1 2-4

Temperature, [o

C] 153 236.1

Mass flow rate, [kg/s] 5.743 1.328

Relative Density [-] 1.12 1.00

Density [kg/m3] 915.9 819.9

Inlet/Outlet Pressure [MPa] 3.8 3.8

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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

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Figure 3: ROCOM nodalization sketch. Top view

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

CL1

CL2

CL4

CL3

HL2

HL1

HL3

HL4

SECT 1

SECT 2SECT 3

SECT 4

SECT 5

SECT 6SECT 7

SECT 8

DOWNCOMER

CORE INLET

R1R2

R4R5R6

R3

R1

R2DRUM

R3R4

R5R6

LOWER PLENUM

CORE REGION

UPPER PLENUM

DOWNCOMER

radial ring

axial layer

BYPASSDRUM-CSP

Axial layer R1 R2 R3 R4 R5 (barrel zone)R6

(DC zone)

LOWER PLENUM

Porosity

1 (LP bottom) 1.0 1.0 1.0 1.0 1.0 1.0

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

CORE INLET (CORE SUPPORT PLATE)

Porosity 4 (CSP) 0.33375 0.34816 0.34816 0.15754 0 1.0

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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.

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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).

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)

0 10 20 30 40 50 60 70 80195

200

205

210

215

220

225

230

235

240

Time [s]

Tem

pera

ture

[°C

]SPATIAL AVERAGE OF TEMPERATURE DISTRIBUTION IN DONWCOMER

CALC-Ref TRACE.

CALC-Ref C2.

EXP

0 10 20 30 40 50 60 70 80180

190

200

210

220

230

240

Time [s]

Tem

pera

ture

[°C

]

SPATIAL AVERAGE OF TEMPERATURE DISTRIBUTION CORE INLET

CALC-Ref TRACE.

CALC-Ref C2.

EXP

TRACE CATHARE

Transition zone

Transition zone

EXP

TRACE-V5 CATHARE2

Transition zone

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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.

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.

(a)

1 2 3 4 5 6 7 8180

190

200

210

220

230

240

azimuthal sector

Tem

pera

ture

[°

C]

TRACE-V5p2

CATHARE2

EXP

(b)

180 190 200 210 220 230 240

-1

-0.8

-0.6

-0.4

-0.2

0

Temperature [°C]

Axia

l la

yer

positio

n

[m]

TRACE-V5p2

CATHARE2

EXP

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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

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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.

REFERENCES

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Internal Seminar, Pisa, Italy, July 2013.

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