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

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. RohdeForschungszentrum Dresden-Rossendorf


Y. KozmenkovIPPE Obninsk

“Assurance of NPP with WWER”Podolsk, 29 May-1 June, 2007



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)



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



• DYN3D– Excellent validation

basis for hexagonal and square FA geometry

DYN3D3D Core Model

Steady State and TransientCartesian and Hexagonal

Geometry

NeutronKinetics

ThermalHydraulics

2 - neutron groupsdiffusion theory

3 - dimensionalnodal methodsversions for

quadratic assemblieshexagonal assemblies

1D 4-equations th. modelfor two phase flowradial heat conductionin fuel and cladheat transfer modelssafety parametersboron mixing models

calculation of cross sections

nodal powers

fuel temperaturescoolant densitiescoolant temperaturesboron concentrations

library of group constantsburnup distribution

cross sections of nodes



DYN3D/ATHLET

• Coupling



Phase 1

• Core and loop positions

Loop to be switched on



Phase 1

• Velocity in the loops (cold leg)



Phase 1

• Temperatures in the loops (cold leg)



Problem

• Initial state: three active loops• Final state: four active loops• Open: How to model the transition inside the

system code • The old question:



Lower plenum

Simplified empirical mixing modelAssumptions

– 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



Upper plenum

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

H o t leg n o zz le # 3

Ju n c tio n 2 Ju n c tio n 1

F ro m the co re o u tle t

A B H o t leg n o zz le # 2



C



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



Model of the VVER-1000 reactor

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)

CFX-5 Grid



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



Validation of lower plenum mixing model

Velocity in the loops



Phase 1 – Initial state

Parameter Measured data Accuracy DYN3D/ATHLET

Keff - - 0.999200

Core power, MW 824 ±60 MW 823.85

Upper Plenum pressure, MPa

15.6 ±0.3 MPa 15.606

Temperature CL1, K 555.6 ±2.0 K 554.83




Temperature HL1, K 567.1 ±2.0 K 566.06






Phase 1: Transient

• Measured and calculated upper plenum pressure



Phase 1: Transient

• Measured and calculated coolant temperatures in loop 3



Phase 1

• Calculated normalized fuel assembly power values

greatest changes



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



Phase 2

• Relative core inlet temperatures



Phase 2

• Steady state results using three different turbulence models– Shear stress turbulence

– Largy eddy simulation

– Detached eddy simulation



Phase 2

• Deviations between DES-calculation and measurement

DEV(i)=CFD(i)-EXP(i)



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

Documents

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