Experimental research of geometrical parameters influencing factor on fuel assembly mixing characteristics

The thermal diffusion coefficient (TDC) is a dimensionless characteristic parameter to reflect the overall mixing characteristics in bundle channels. The effects of bundle geometries and spacer grids axial arrangement on TDC have not been systematically studied. Experimental study on the mixing characteristics of fuel assembly with 5×5 full-length bundles were carried out in two structural forms. The geometric parameters, mainly are the bundles radial power distribution and the form of spacer grids axial arrangement, which influence on the TDC of fuel assembly has been emphatically analysed. The experimental results show that the current FLLICA subchannel software cannot accurately simulate the transverse macroscopic flow in the subchannel matrix space. Without further optimization of subchannel software, the hot-cold bundles radial power distribution has a great influence on the TDC value of fuel assembly. The TDC value of fuel assembly is closer to true value when the hot-cold rods are centrally symmetrical. The mid-span mixing grid (MSMG) has a small enhancement effect on the thermal diffusion coefficient, and its contribution to the pressure drop of bundles is very low.