The existence of boundary walls will impact flow and heat transfer to some extent. The boundary walls decrease the lateral velocities and change the flow direction to form some eddies, so the flow instability increases. Besides, they stop the mass transfer between the inner areas and outside. For the spacer grids of cluster fuel assembly, in order to confirm the scale of simulation object or experiment noumenon, the first thing is to research the influence depth of boundary effect, and try to eliminate its influence to the concerned object or area. This paper is focused on the spacer grids of 7×7 scale, 5×5 scale and 3×3 scale, respectively. Computed with CFD method, the lateral velocities of corner areas and typical subchannels are compared. And then, the influence depth of boundary effect is confirmed as the distance of a range of fuel rods. Therefore, for the concerned subchannel of CFD simulation, there must be no less than a range of fuel rods in the periphery to mitigate, so the result is not affected by the boundary effect.

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