Improvement of Grid-directed Cross Flow in CUPID Subchannel Scale Analysis Module based on CFD Calculation

Mixing vane is a structure applied in rod bundle geometry, which enhances the coolability of nuclear fuel rods. In thermal-hydraulic analysis field, one of the essential issue is simulating the lateral flow due to mixing vane and spacer grid geometry. Grid-directed cross flow model is the model simulates lateral flow due to mixing vane in subchannel scale analysis code, CTF. This model is implemented in CUPID subchannel scale analysis in previous study. There was lack of physical basis for defining the lateral convection factor, which is the factor represents the intensity of lateral flow in CUPID subchannel scale analysis. Also in order to justify defining lateral convection factor equally for all subchannels, CFD analysis of MATiS-H experiment was used for calculating lateral convection factor. Lateral convection factor was various for each type of subchannels. Subchannels near the CE-type guide tube geometry were also considered for calculation of lateral convection factor. PSBT thermal mixing test was simulated for validation of modified lateral convection factor in CUPID code. Compared with the calculation that used uniform lateral convection factor like previous model, the new calculation result with modified lateral convection factor predicted well the PSBT thermal mixing test. Especially it predicts the location of the subchannel which had the highest temperature. APR1400 whole core simulation with modified lateral convection factor was compared with previous calculation. Power distribution from calculation result of nTRACER code was used. Calculations of 8^th subchannel with modified and uniform lateral convection factor was compared, and whole core calculations were also compared. Minimum DNBR from modified lateral convection factor was 0.5% less than minimum DNBR from uniform lateral convection factor. In previous study, grid-directed cross flow model was only implemented in single phase calculation. PSBT void measurement test was validated for justify the grid directed cross flow model from CUPID code in two phase simulation. Grid-directed cross flow model was implemented only in the subchannels in single phase liquid flow regime or bubble flow regime. Implementing the griddirected cross flow model decreased the void fraction and set back boiling. Calculation result with grid-directed cross flow model well predicted the PSBT void measurement test than the calculation without mixing vane model. There were over predictions at test cases include high void fraction result. Therefore, implementation of grid-directed cross flow model in subchannels with high void fraction is needed. Also, experimental data about the bubbly flow near the mixing vane geometry was suggested for the model for simulating two phase flow near mixing vane in subchannel scale analysis