Numerical Simulation Study on Axial Mixing Characteristics of Fuel Assembly

Abstract Because the power in the reactor is distributed in cosine, the CHF is usually not located at the exit. In order to accurately predict the CHF point and increase the accuracy of the reactor safety analysis, it is necessary to conduct in-depth research on the axial mixing coefficient distribution of the fuel assembly. This study used the CFD method to investigate the flow mixing of the rod bundle and spacer grid of fuel assembly. Establish a geometric model for the rod bundle and spacer grid, select the SST model for calculation, and obtain the temperature distribution of sub-channels at different axial positions under rated operating conditions. Using the sub-channel analysis program in conjunction with numerical simulation data, determine the mixing coefficients at different axial positions of rod bundle and spacer grid. The variation law of mixing coefficient along axial position is summarized. The results show that for the rod bundle, the mixing coefficient decreases with the increase of axial length, and shows a tendency of approaching 0. The location of the spacer grid has an important effect on the mixing coefficient. The mixing coefficient of the spacer grid has a significant change compared with that of the rod bundle, but the effect decreases with the increase of the distance. In this study, a method was established to calculate the mixing coefficient of different axial positions, and the variation law of the mixing coefficient along the axial position was obtained, which laid a foundation for the development of the mixing model reflecting the axial difference, and helped to improve the accuracy of safety analysis.