DYN3D/ATHLET AND ANSYS CFX CALCULATIONS OF THE OECD VVER-1000 COOLANT TRANSIENT BENCHMARK

1. 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. 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. 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. DYN3D • Excellent validation basis for hexagonal and square FA geometry

5. DYN3D/ATHLET • Coupling

6. Loop to be switched on Phase 1 • Core and loop positions

7. Phase 1 • Velocity in the loops (cold leg)

8. Phase 1 • Temperatures in the loops (cold leg)

9. 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. 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. Upper plenum • Upper plenum nodalization at the elevation of the hot leg nozzles

12. 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. Model of the VVER-1000 reactor CFX-5 Grid • 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 • The mesh contained 4.7 Mio. tetrahedral elements (IC4C)

14. Modeling the Porous Regions Perforated columns Support columns 2 Elliptical sieve plate 1 The Lower Plenum structure • Elliptical perforated core barrel plate • 163 partly perforated support columns • Each column is associated to a fuel assembly Porous Regions: (1) Elliptical Sieve Plate (2) Perforation region of support tubes

15. Validation of lower plenum mixing model Velocity in the loops

16. Phase 1 – Initial state

17. Phase 1: Transient • Measured and calculated upper plenum pressure

18. Phase 1: Transient • Measured and calculated coolant temperatures in loop 3

19. greatest changes Phase 1 • Calculated normalized fuel assembly power values

20. 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. Phase 2 • Relative core inlet temperatures

22. Phase 2 • Steady state results using three different turbulence models • Shear stress turbulence • Largy eddy simulation • Detached eddy simulation

23. Phase 2 • Deviations between DES-calculation and measurement DEV(i)=CFD(i)-EXP(i)

24. 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