Cfd Study on Coolant Mixing in Vver-440 Fuel Rod Bundles and Fuel Assembly Heads

A CFD model of VVER-440 fuel assembly heads was developed based on the technical documentation of a full-scale test facility built in the Kurchatov Institute, Moscow. Steady-state and transient calculations were performed to validate the model with a measurement set and investigate the effects of the spatial discretization, choosing of different turbulence models and use of different inlet boundary conditions. Inlet boundary conditions were determined both with COBRA subchannel code and with a fuel rod bundle CFD model that was built for this special purpose. The sensitivity studies showed that grid of about 8 million cells and BSL Reynolds Stress model are suitable to receive accurate prediction for the signal of the in-core thermocouples. The results were also compared against the experimental data. The results of the rod bundle calculations received with both subchannel and CFD codes differ significantly from the experimental data in some peripheral points. Whereas the results of the assembly head model are in relatively good agreement with the measured data. Best prediction for the signal of the in-core thermocouple was achieved with transient calculation using inlet boundary conditions generated by the CFD fuel rod bundle model. These calculations pointed out the difference between the outlet average temperature of the coolant and the temperature at the housing of the in-core thermocouple. Besides, the significant role of the outflow from the central tube was also proven. The transient runs revealed intensive coolant mixing with relatively large temperature fluctuations.