[en] The results of the numerical calculations of the impact of the mixing grid designed by NCCP on flow are presented. The phase mixing in three directions is discussed: from fuel surface to flow, between fuel assembly subchannels and around fuel rod, and in azimuth direction. The mixing grids were demonstrated to have the impact on steam being generated on fuel rod in two ways. They are the carry-out of steam across fuel surface with varying velocity at the grid location and steam flowing along fuel surface with its injection to flow at flow breakaway. By using of grids, the fuel rods are divided in two groups according to flow pattern, with a beneficial effect produced by the grid only on one group. The steam flow along fuel surface results in azimuth non-uniformity, and in case of a compact group of hot fuel rods the margin to CHF is reduced. A comparison with the results obtained for a plate-type mixing grid is presented. (authors)