Thermal Hydraulic System Codes Performance in Simulating Buoyancy Flow Mixing Experiment in ROCOM Test Facility

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1 Thermal Hydraulic System Codes Performance in Simulating Buoyancy Flow Mixing Experiment in ROCOM Test Facility ABSTRACT Eugenio Coscarelli San Piero a Grado Nuclear Research Group (GRNSPG), University of Pisa Via Livornese , 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 , San Piero a Grado, Pisa, Italy sergii.lutsanych@gmail.com, f.dauria@ing.unipi.it 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)

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 Conditioning phase Temperature [ C] Core inlet Phase I UP SG-1 outlet (affected) SG-1 inlet (affected) Phase II Ph. II-a Ph. II-b Ph. II-c 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 SG-2, -3, -4 inlet (intact) UH SG-2, -3, -4 outlet (intact) 150 Time [s] maximum overcooling ECC injection Figure 1: Measurement loop temperature in the PKL test G3.1 [4]

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 Temperature, [ o C] Mass flow rate, [kg/s] Relative Density [-] Density [kg/m 3 ] Inlet/Outlet Pressure [MPa]

4 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

5 DOWNCOMER SECT 7 HL3 HL2 SECT 6 CL3 SECT 8 CL2 SECT 5 CL4 SECT 1 R1 R2 R3R4 R5 R6 SECT 4 CL1 CORE INLET HL4 SECT 2 HL1 SECT 3 Figure 3: ROCOM nodalization sketch. Top view axial layer R2 DRUM R1 R5 R4 R3 R6 DOWNCOMER 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]). LOWER PLENUM Porosity Table 3: Volumetric porosity distribution in LP and at CSP Axial layer R1 R2 R3 R4 R5 (barrel zone) R6 (DC zone) 1 (LP bottom) (drum zone) (Bypass) CORE INLET (CORE SUPPORT PLATE) Porosity 4 (CSP)

6 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.

7 Temperature [ C] SPATIAL AVERAGE OF TEMPERATURE DISTRIBUTION IN DONWCOMER 240 CALC-Ref TRACE. 235 CALC-Ref C2. EXP Temperature [ C] SPATIAL AVERAGE OF TEMPERATURE DISTRIBUTION CORE INLET CALC-Ref TRACE. CALC-Ref C2. EXP Time [s] 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 Transition zone TRACE-V5 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)

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. Temperature [ C] TRACE-V5p2 CATHARE2 EXP azimuthal sector Axial layer position [m] TRACE-V5p2 CATHARE2 EXP Temperature [ C] (a) (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.

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

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. REFERENCES [1] C. Agnoux et. all, OECD-PKL2 Project Solving Thermal Hydraulic Safety Issues for Current PWR and New PWR Design Concepts through Experiments in the Integral Test Facility PKL, Final Report, PTCTP-G/2012/en/0004, AREVA NP GmbH, [2] E. Coscarelli, A. Del Nevo, F. D Auria, Qualification of TRACE V5.0 Code against Fast Cooldown Transient in the PKL-III Integral Test Facility, Science and Technology of Nuclear Installations, Volume 2013, June 2013, pp [3] E. Coscarelli, AN INTEGRATED APPROACH TO ACCIDENT ANALYSIS IN PWR, PhD. Thesis, University of Pisa, Pisa, Italy, July [4] A. Del Nevo, E. Coscarelli, A. Kovtonyuk, F. D Auria, Analytical Exercise on OECD/NEA/CSNI PKL-2 Project Test G3.1: MSBL Transient in PKL-III Facility: Phase 2 Post-test Calculation, UNIPI/GRNSPG, TH/PKL-2/02(10) Rev. 1, Pisa, Italy, March [5] A. Bousbia Salah, J. Vlassenbroeck, Assessment of the CATHARE 3D capabilities in predicting buoyant driven flow under asymmetric cooling loops conditions, Nuclear Engineering and Design, in press. [6] S. Kliem, R. Franz, OECD PKL2 Project Final Report on the ROCOM tests HZDR\FWO\2012\03, [7] United States Nuclear Regulatory Commission, TRACE V5.0 Theory Manual, Field Equations, Solution Methods, and Physical Models, [8] General Description of the CATHARE 2 V25_2mod8.1 System Code, DEN/DANS/DM2S/STMF/LMES/RT/12-008/A, G. Lavialle, [9] S. Lutsanych, E. Coscarelli, Thermal Hydraulic System Codes Performance in Simulating Buoyance Flow Mixing Experiment in ROCOM Test Facility, GRNSPG Internal Seminar, Pisa, Italy, July [10] R. Macian-Juan, J. H. Mahaffy, Numerical diffusion and the tracking of solute fields in system codes Part II. Multi-dimensional flows, Nuclear Engineering and Design, Vol. 179, 1998, pp