Simulation of turbulent mixing rate in simulated subchannels of a reactor rod bundle

Subchannel analysis codes are widely used for the thermal-hydraulic design of nuclear reactor rod bundle. The effectiveness of subchannel analysis codes depends on turbulent mixing between these subchannels. Turbulent mixing has no direct contribution to the axial mass flow rate through subchannel but it will cause exchange of momentum and energy between the neighboring subchannels. Thus, it is important to evaluate the turbulent mixing coefficient for reactor rod bundle as it is a significant factor in the lateral energy and momentum equation for subchannel analysis codes like COBRA IIIC, COBRA-IV and MATRA LMR-FB. With the rapid developments in computational fluid dynamics and computer performance, three-dimensional analyses of turbulent flows occurring in the nuclear rod bundle have become more prominent. Several numerical analyses have already been attempted to investigate the flow behavior in rod bundles of different reactors. Much of these are dedicated to find out the structure of turbulence in rod bundle but a few analyses has been done to evaluate the magnitude of the turbulent mixing coefficient. In view of this, CFD analyses were carried out to determine the turbulent mixing coefficient in the simulated sub-channels of the reactor rod bundle. Previous studies on the structure of turbulence reveals that it is highly anisotropic. Hence, the Reynolds Stress Model (RSM), finer mesh and near wall distance ( y + ≤ 2) is required to capture turbulent mixing phenomena. The validation of results is done by comparing with subchannel mixing experiments.

A DNBR Analysis to Patterns of Subchannel Control Volume

The sensitivity of DNBR values to approaches of subchannel control volume selection has been investigated for the TS01 and TS03 out of HIPER17 CHF tests. In rod bundle analysis, most of the subchannel analyses used the coolant-centered subchannel nodal layout. But, it has been known that the rod-centered subchannel analysis showed good results for high quality critical heat flux. In this study, it was demonstrated that the rod-centered subchannel analysis showed more effective and conservative results than the coolant-centered subchannel analysis with the DNBR value, the quality, and the mass velocity. Also, it was verified that when the both results yielded from each subchannel analysis were compared, the subchannel locations at which DNBR occurred were similar.