This invention relates to nuclear engineering and may be used in structures of nuclear fuel assemblies, especially those used in PWR and BWR nuclear reactors. A fuel assembly comprises spacing grids comprising cells formed by orthogonal crossing plates. An insertable spacing element is installed in each cell, which is designed for fixing the fuel rod passing through the cell. In the spacing grids that are arranged between the first spacing grid and the last spacing grid downstream the coolant flow at least some cells, through which the fuel rods pass, are provided with deflectors designed for mixing the coolant flow. The insertable spacing element comprises a shell, which has a cross-section in the form of an octagon formed by four facets that are rounded and convex in the direction from the center of said element and by four facets located therebetween that are shaped and concave toward the center of the said element. The result is that cell stiffness is increased, and hydraulic resistance to a coolant flow is reduced.