Coolant Mixing Intensity and Hydraulic Resistance of FA Mixing Grids

The results of tests of the water mixing efficiency and the pressure drop in a channel with VVER model fuel assemblies equipped with modernized mixing grids of five types are presented. To evaluate the grid efficiency, experiments were performed with assemblies equipped with mixing grids of two types: ‘swirling around fuel rods’ and ‘sequential sweep.’ The main difference of the modernized grid from the two types of mixing grids is the arrangement of two paddles instead of one in each trihedral cell. The investigation was conducted on the Trasser facility by feeding into the flow in front of the grids a fluid tracer and determining its concentration in selected cells of the assembly. It is shown that equipping the VVER model fuel assemblies with a modernized grid of the second type instead of the grid of the ‘sequential sweep’ type appreciably increases the efficiency of mixing of the coolant with almost identical coefficients of hydraulic resistance.