outlets 21 through holes in the top tie plate in the uppermost fuel unit in the stack to illustrate how the lattice positions of the fuel rods are displaced between the top tie plate and the bottom tie plate fig3 e shows a horizontal section d 2 d 2 through the fuel unit 3 d on half its height in fig3 f a horizontal section d 3 d 3 through the fuel unit immediately below the top tie plate is shown a fuel rod displaces its lattice position one step in the clockwise or the counterclockwise direction within the ring to which it is associated it is especially shown how the fuel rods 4 b 4 c 4 d 4 e in the inner ring are displaced to the next lattice position one step in the counterclockwise direction in the inner ring the embodiment described so far is based on an orthogonal lattice with top tie plates and bottom tie plates identical as regards lattice positions however the invention may very well be applied also if the lattice is irregular and nor do the lattice positions need to be identical in the top tie plates and the bottom tie plates the fuel rods may also be inclined to differing degrees in the same fuel unit such embodiments may be preferable for example.