1. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz DYN3D/ATHLET AND ANSYS CFX CALCULATIONS OF THE OECD VVER-1000 COOLANT TRANSIENT BENCHMARK S. Kliem, T. Höhne, U. Rohde Forschungszentrum Dresden-Rossendorf Institute of Safety Research Y. Kozmenkov IPPE Obninsk “Assurance of NPP with WWER” Podolsk, 29 May-1 June, 2007

2. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Introduction (1) OECD/NEA Benchmark for VVER-1000 2 Phases –Calculation of a start-up experiment “Switch- on of one main coolant pump while the other three are in operation” –Calculation of coolant mixing experiments at low reactor power (isolation of one steam generator at running pumps) Reference plant: NPP Kozloduy-6 (Bulgaria)

3. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Introduction (2) Our institute is taking part in the calculations of the benchmark Phase 1: coupled neutron kinetic/thermal hydraulic system code DYN3D/ATHLET Phase 2: commercial computational fluid dynamics code ANSYS CFX

4. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz DYN3D –Excellent validation basis for hexagonal and square FA geometry

5. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz DYN3D/ATHLET Coupling

6. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Phase 1 Core and loop positions Loop to be switched on

7. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Phase 1 Velocity in the loops (cold leg)

8. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Phase 1 Temperatures in the loops (cold leg)

9. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Problem Initial state: three active loops Final state: four active loops Open: How to model the transition inside the system code The old question:

10. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Lower plenum Simplified empirical mixing model Assumptions –Inside the pressure vessel, there is an azimuthal equalisation of the flow rates from the single loops. –The flow shifts from the loop position to the sector position. –The redistribution of the flow of all active loops results in a zero net shift. –The described sector formation is present in the vessel until the core inlet plane. Implementation –Recalculation of the positions of the sectors and the FA belonging to the single sectors

11. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Upper plenum Upper plenum nodalization at the elevation of the hot leg nozzles

12. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Validation of lower plenum mixing model No experiments but CFD A model of the vessel was developed and used for Phase 2 (stationary mixing experiment) One transient calculation –modelling of the transport of a perturbation (e.g. temperature) –four passive scalars (one for each loop at the inlet positions into the vessel) of infinite length –transported with the fluid and are subject of turbulent dispersion, but do not affect the flow field –Individual transport equation for each scalar –Result: time and space dependent contributions of the flow of all loops to the distribution of the perturbation at each fuel element position in the core inlet plane

13. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Model of the VVER-1000 reactor q An exact representation of the inlet region, the downcomer below the inlet region, the 8 spacer elements in the downcomer and the lower plenum structures is necessary q The mesh contained 4.7 Mio. tetrahedral elements (IC4C) CFX-5 Grid

14. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Modeling the Porous Regions 1 2 Porous Regions: (1) Elliptical Sieve Plate (2) Perforation region of support tubes Elliptical sieve plate Support columns Perforated columns The Lower Plenum structure –Elliptical perforated core barrel plate –163 partly perforated support columns –Each column is associated to a fuel assembly

15. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Validation of lower plenum mixing model Velocity in the loops

16. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Phase 1 – Initial state ParameterMeasured dataAccuracyDYN3D/ATHLET K eff --0.999200 Core power, MW824±60 MW823.85 Upper Plenum pressure, MPa 15.6±0.3 MPa15.606 Temperature CL1, K555.6±2.0 K554.83 Temperature CL2, K554.6±2.0 K553.50 Temperature CL3, K554.4±2.0 K554.42 Temperature CL4, K555.3±2.0 K554.93 Temperature HL1, K567.1±2.0 K566.06 Temperature HL2, K562.1±2.0 K560.96 Temperature HL3, K550.8±2.0 K550.53 Temperature HL4, K566.2±2.0 K566.06

17. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Phase 1: Transient Measured and calculated upper plenum pressure

18. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Phase 1: Transient Measured and calculated coolant temperatures in loop 3

19. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Phase 1 Calculated normalized fuel assembly power values greatest changes

20. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Phase 2 Available data –Stationary temperature distribution at the core inlet –Derived during recalculation from core outlet measurements under some assumptions Peculiarity –Non-symmetrical connection of the loops on the vessel

21. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Phase 2 Relative core inlet temperatures

22. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Phase 2 Steady state results using three different turbulence models –Shear stress turbulence –Largy eddy simulation –Detached eddy simulation

23. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Phase 2 Deviations between DES-calculation and measurement DEV(i)=CFD(i)-EXP(i)

24. Institute of Safety Research Member Institution of the Scientific Association Gottfried Wilhelm Leibniz Conclusions Calculation of both phases of the VVER-1000 Coolant transient benchmark Phase 1: DYN3D/ATHLET calculation –Use of a simplified mixing model at the interface between system code and core model –proof of the applicability by comparison with a transient CFD calculation using ANSYS CFX Phase 2: ANSYS CFX calculation –Good agreement in temperature distribution –Small changes during the variation of the turbulence models