The flow characteristics of spacer grid and rod bundle channels play an important role in the frame designof fuel assembly.In this paper,one unabridged spacer grid of a 5 ×5 fuel assembly is investigated.Its 3D geometryand unstructured grids are established,and a CFD code is adopted to simulate the flow characteristics of rod bundlechannels and the heat transfer characteristics of the fuel rod.Then,the effects of mixing vane,dimple,spring andband on the flow field of the fuel assembly are analyzed.The results show that (1) the spacer grid can significantlyenhance the flowing mixture and the heat transfer in the fuel assembly; (2) the mixing vane has the most signifi-cant effect on the flow field,and the downstream lateral flow of the mixing vane forms along the vane stretch direc-tion with a lateral flow length of more than 9 times greater than the grid length; and (3) the dimple and the stringcause the lateral flow to form only near their location; specifically,the lateral flow of the dimple extends from theband to the dimple front,the lateral flow of the single spring runs along its bending direction,and as for the dualspring,there exists a weak lateral flow in its joint part with the band.