CFD calculations of the coolant mixing in Pressurised Water Reactors
Background
Numerical investigations on coolant mixing in Pressurised Water Reactors (PWR) have been performed at the HZDR for almost a decade. The work was aimed at describing the mixing phenomena relevant for both safety analysis, particularly in steam line break and boron dilution scenarios, and mixing phenomena of interest for economical operation and the structural integrity.
With the setup of the ROCOM test facility, a unique data base has been created to be used for the validation of Computational Fluid Dynamics (CFD) codes for the application to turbulent mixing in nuclear reactors. Benchmark problems based on selected experiments were used to study the effect of different turbulent mixing models under various flow conditions, to investigate the influence of the geometry, the boundary conditions, the grid and the time step in the CFD analyses. In doing the calculations the Best Practice Guidelines for nuclear reactor safety calculations have been followed.
Validation Matrix
A selection of the performed work:
 stationary and transient flow and mixing studies of the coolant in the PWR Konvoi and the ROCOM test facility with CFX4 and CFX5 during

 boron dilution transients (startup of the first coolant pump)
 main steam line break scenarios
 density driven flows after an inherent dilution with ECC injection (generic experiments at the ROCOM test facility)
 participation in the ISP43 CFX4, (Figure 1)
 participation in the OECD benchmark on VVER1000 reactors, (Figure 2)
 flow and mixing studies in the VVER440 and VVER1000 type reactors (Figure 3)
 post test calculation of pump startup experiments at the Gidropress VVER1000 reactor mockup
 TRIO_U validation on a ROCOM experiment using a LES approach
Results of the ROCOM CFD Analysis
The CFD calculations were carried out with the CFDcodes CFX4 and CFX5 on the Rossendorf 100processor RedHat LINUX cluster (dual CPU compute nodes XEON, 3.2 GHz, ~1.3 Gflops, each containing 2 GBytes RAM).
Using the blockstructured code CFX4 internals were modelled using the porous media approach and additional body forces. Sensitivity studies showed, that the kε turbulence model together with the higher order HYBRID discretization scheme give the best results.
Within CFX5 it was possible, to model all internals of the RPV of ROCOM in detail. A production mesh with 7 Million elements was generated (Figure 4). Detailed and extensive grid studies were made. It was shown, that a detailed model of the perforated drum in CFX5 give the best agreement with the experiments. Sensitivity studies showed, that the SST turbulence model and the automatic wall functions together with higher order discretization schemes should be used if possible.
In the case of stationary mixing, the maximum value of the averaged mixing scalar at the core inlet was found in the sector below the inlet nozzle, where the tracer was injected (Figures 710). There is a good agreement between the measurement and the CFD calculations, especially in the averaged global mixing scalar at the core inlet (Figure 7). At the local position of the maximum mixing scalar the time course of the measurement and the calculations is also in good agreement (Figure 8).
At the startup case of one pump due to a strong impulse driven flow at the inlet nozzle the horizontal part of the flow dominates in the downcomer (Figures 5 and 6). The injection is distributed into two main jets, the maximum of the tracer concentration at the core inlet appears at the opposite part of the loop where the tracer was injected.
Figure 1 Babcock 2x4loop  Figure 2 WWER1000 
Figure 3 WWER440  Fig. 4 Detailed hybrid grid model CFX5 (ROCOM) 
Fig. 5 CFDresults of the startup of the first pump after 9s  Fig. 6 Startup of the first pump streamlines representing the flow field 
Fig. 7 Time dependent averaged mixing scalar at the core inlet  Fig. 8 Time dependent local mixing scalar at the core inlet, near wall position  


Fig. 9 Mixing scalar at azimuthal positions of the core inlet near the wall, (t=10.0 s)  Fig. 10 Plateau averaged mixing scalar at the core inlet 
Publications
Stieglitz, R.; Mull, T.; Höhne, T.; Rieger, U.; Buettner, K.
Section Reports: 2012 Annual Meeting on Nuclear Technology  Part 5
atw  International Journal for Nuclear Power 58(2013)1, 4344
Höhne, T.; Grahn, A.; Kliem, S.; Rohde, U.; Weiss, F.P.
Numerical simulation of the insulation material transport in a PWR core under loss of coolant conditions
Nuclear Engineering and Design 258(2013), 241248
Höhne, T.; Kliem, S.; Rohde, U.
Buoyancy driven mixing studies of natural circulation flows using ROCOM experiments and CFD
Chemie Ingenieur Technik 83(2011)8, 12821289
Höhne, T.; Schaffrath, A.
Fachsitzung: „CFDMethoden für sicherheitsrelevante Fragestellungen“
atw  International Journal for Nuclear Power 56(2011)7, 419423
Loginov, M. S.; Komen, E.; Höhne, T.
Application of LargeEddy Simulation to Pressurized Thermal Shock: assessment of the accuracy
Nuclear Engineering and Design 240(2011), 20342045
Moretti, F.; Melideo, D.; Del Nevo, A.; D’Auria, F.; Höhne, T.; Lisenkov, E.; Bucalossi, A.; Gallori, D.
Experimental investigation of invessel mixing phenomena in a VVER1000 scaled test facility during unsteady asymmetric transients
Nuclear Engineering and Design 241(2011)8, 30683075
Höhne, T.
Anwendung von CFDMethoden für den Kern sowie den Primärkreislauf von LWR
atw  International Journal for Nuclear Power 8/9(2009), 546548
Da Silva, M. J.; Thiele, S.; Höhne, T.; Vaibar, R.; Hampel, U.
Experimental studies and CFD calculations for buoyancy driven mixing phenomena
Nuclear Engineering and Design 240(2010)9, 21852193
Höhne, T.
CFDsimulation of the VVER thermal hydraulic benchmark V1000CT2 using ANSYS CFX
Science and Technology of Nuclear Installations 2009(2009), Article ID 835162
Moretti, F.; Melideo, D.; Del Nevo, A.; DAuria, F.; Höhne, T.; Lisenkov, E.
CFD validation against slug mixing experiment
Science and Technology of Nuclear Installations (2009), Article ID 436218
Kliem, S.; Höhne, T.; Rohde, U.; Weiß, F.P.
Experiments on slug mixing under natural circulation conditions at the ROCOM test facility using high resolution measurement technique and numerical modeling
Nuclear Engineering and Design 240(2010)9, 22712280
Rohde, U.; Höhne, T.; Krepper, E.; Kliem, S.
Application of CFD codes in nuclear reactor safety analysis
Science and Technology of Nuclear Installations 2010(2010), Article ID 198758
Höhne, T.; Kliem, S.; Vaibar, R.
Experimental and numerical modeling of transition matrix from momentum to buoyancydriven flow in a pressurized water reactor
Journal of Engineering for Gas Turbines and Power  Transactions of the ASME 131(2009)1, 012906
Kliem, S.; Hemström, B.; Bezrukov, Y.; Höhne, T.; Rohde, U.
Comparative evaluation of coolant mixing experiments at the ROCOM, Vattenfall, and Gidropress test facilities
Science and Technology of Nuclear Installations 2007(2007), 25950
Höhne, T.
Numerical simulation of coolant mixing in a pressurized water reactor with different CFD methods based on complex meshes
International Journal of Nuclear Energy Science and Technology 3(2007)4, 399412
Höhne, T.; Kliem, S.
Modeling of a BuoyancyDriven Flow experiment in Pressurized Water Reactors using CFDMethods
Nuclear Engineering and Technology 39(2007)4, 327336
Moretti, F.; Melideo, D.; DAuria, F.; Höhne, T.; Kliem, S.
CFX simulations of ROCOM slug mixing experiments
Journal of Power and Energy Systems 2(2008)2, 720733
Höhne, T.; Kliem, S.; Rohde, U.; Weiss, F.P.
Boron Dilution Transients during natural circulation flow in PWR experiments and CFD simulations
Nuclear Engineering and Design 238(2008), 19871995
Rohde, U.; Höhne, T.; Kliem, S.; Hemström, B.; Scheuerer, M.; Toppila, T.; Aszodi, A.; Boros, I.; Farkas, I.; Muehlbauer, P.; Vyskocil, V.; Klepac, J.; Remis, J.; Dury, T.
Fluid mixing and flow distribution ín a primary circuit of a nuclear pressurized water reactor Validation of CFD codes
Nuclear Engineering and Design 237(2007)1517, 16391655
Cartland Glover, G. M.; Höhne, T.; Kliem, S.; Rohde, U.; Weiss, F.P.; Prasser, H.M.
Hydrodynamic phenomena in the downcomer during flow rate transients in the primary circuit of a PWR
Nuclear Engineering and Design 237(2007)7, 732748
Höhne, T.; Kliem, S.; Weiß, F.P.
Modeling of a buoyancydriven flow experiment at the ROCOM test facility using the CFDCode ANSYS CFX
atw  International Journal for Nuclear Power 3(2007), 168174
Kliem, S.; Sühnel, T.; Rohde, U.; Höhne, T.; Prasser, H.M.; Weiß, F.P.
Experiments at the mixing test facility ROCOM for benchmarking of CFDcodes
Nuclear Engineering and Design 238(2008), 566576
Rohde, U.; Höhne, T.; Kliem, S.
Using CFD to simulate turbulent mixing in nuclear reactor pressure vessels
ANSYS Solutions 7(2006)2, 2728
Vallee, C.; Höhne, T.; Prasser, H.M.; Sühnel, T.
Experimental investigation and CFD simulation of horizontal air/water slug flow
Kerntechnik 71(2006)3, 95103
Kliem, S.; Kozmenkov, Y.; Höhne, T.; Rohde, U.
Analyses of the V1000CT1 benchmark with the DYN3D/ATHLET and DYN3D/RELAP coupled code systems including a coolant mixing model validated against CFD calculations
Progress in Nuclear Energy 48(2006), 830848
Höhne, T.; Kliem, S.; Bieder, U.
Modeling of a buoyancydriven flow experiment at the ROCOM test facility using the CFDcodes CFX5 and TRIO_U
Nuclear Engineering and Design 236(2006)12, 13091325
Rohde, U.; Kliem, S.; Höhne, T.; Karlsson, R.; Hemström, B.; Lillington, J.; Toppila, T.; Elter, J.; Bezrukov, Y.
Fluid mixing and flow distribution in the reactor circuit  Part 1: Measurement data base
Nuclear Engineering and Design, 235(2005), 421443
Hertlein, R.; Umminger, K.; Kliem, S.; Prasser, H.M.; Höhne, T.; Weiß, F.P.
Experimental and Numerical Investigation of Boron Dilution Transients in Pressurized Water Reactors
Nuclear Technology, vol. 141,January 2003, pp. 88107
Prasser, H.M.; Grunwald, G.; Höhne, T.; Kliem, S.; Rohde, U.; Weiss, F.P.
Coolant mixing in a PWR  deboration transients, steam line breaks and emergency core cooling injection  experiments and analyses
Nuclear Technology 143 (2003) 3756
Höhne, T.
Untersuchung der Kühlmittelvermischung in Druckwasserreaktoren
atw 12, S. 774775
Höhne, T.; Grunwald, G.; Prasser, H.M.
Experimental Investigations on the FourLoop Test Facility ROCOM
Kerntechnik 65/56, S. 212215
Cooperations and References
 strategic partnership with ANSYS CFX, Otterfing
 UCL Belgium
 Kompetenzverbund Kerntechnik Germany
 CFD network Germany
 TU Dresden
 ETH Zürich, Switzerland
 AREVA Erlangen, Germany
 CEA Grenoble, France
 University Pisa, Italy
 Gesellschaft für Reaktorsicherheit GRS, Garching and Cologne, Germany
 TÜV Nord/ TÜV Süd, Germany
 VGB Powertech
 EON Kernkraft, Hannover, Germany