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

S. Kliem, A. Grahn, U. Rohde. FZD; J. Schuetze, Th. Frank, ANSYS Germany GmbH, Germany

20th Symposium of AER on VVER Reactor Physics and Reactor Safety (2010, Hanasaari, Espoo, Finland)
Reactor dynamics and safety analysis


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 two small-size test problems confirm
the correctness of the implementation of the prototype coupling. The first test problem was a
mini-core consisting of nine real-size fuel assemblies with quadratic cross section.
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 9.8 pcm from the DYN3D stand-alone solution. This difference can be attributed to the use
of different water property packages in the two codes. 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 1 MW
at a power level of 50 MW. Beside the different water property packages, these differences are
caused by the use of different flow solvers.
The same calculations were carried for a mini-core with seven real-size fuel assemblies with
hexagonal cross section in order to prove the applicability of the coupled code to cores with
hexagonal fuel assemblies. The differences between the results of coupled calculations and
those of the stand-alone DYN3D code are in the same range as for the quadratic mini-core.

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