Coupling of the CFD code ANSYS CFX with the 3D neutron kinetic core model DYN3D for VVER applications

The CFD code ANSYS CFX has been coupled with the neutron-kinetic core model DYN3D. ANSYS CFX calculates the fluid dynamics and related transport phenomena in the reactor’s coolant and provides the corresponding data to DYN3D. In the fluid flow simulation of the coolant, the core itself is modeled within the porous body approach. DYN3D calculates the neutron kinetics and the fuel behavior including the heat transfer to the coolant. The physical data interface between the codes is the volumetric heat release rate into the coolant. In the prototype that is currently available, the coupling is restricted to single-phase flow problems. In the time domain an explicit coupling of the codes has been implemented so far.
Steady-state and transient verification calculations for a small-size test problem confirm the correctness of the implementation of the prototype coupling. The test problem was a mini-core consisting of seven real-size VVER-1000 fuel assemblies. Comparison was performed with the DYN3D stand-alone code. In the steady state, the effective multiplication factor obtained by the DYN3D/ANSYS CFX codes shows a deviation of 0.2 pcm from the DYN3D stand-alone solution. The transient test case simulated the withdrawal of the control rod from the central fuel assembly at hot zero power in the same mini-core. Power increase during the introduction of positive reactivity and power reduction due to fuel temperature increase are calculated in the same manner by the coupled and the stand-alone codes. The maximum values reached during the power rise differ by about 3 MW at a power level of 240 MW. These differences are caused by the use of different flow solvers.
After this verification a steady-state full power calculation for a full VVER-1000 reactor was carried out in order to show the applicability of the new code system to real problems. A CFX grid consisting of about 1.3 106 nodes was created. The main difference to a pure DYN3D calculation with its 1D thermal hydraulic model is the presence of a lateral coolant flow at a velocity in the order of 1 cm/s from the circumference of the core centre. It is driven by the acceleration of the liquid in the centre due to the stronger heating. This flow increases the exchange of heat in the lateral direction by advection and leads to a ‘smearing’ of the step-like temperature profile that has been found in a pure DYN3D calculation.