Patent Number: 062367013
Section: claims

1. A fuel assembly for a boiling water reactor which is adapted, during operation of the reactor, to allow coolant to flow upwards through the fuel assembly, and wherein the fuel assembly comprises a plurality of fuel rods, each of which having an upper end and a lower end, a steam conducting channel through which steam flows through the fuel assembly, and  a fuel channel (7) surrounding the fuel rods, characterized in that the fuel assembly comprises a rotary cell (13a-13d) in which the fuel rods (4, 20) are arranged in a number of concentric rings (15, 16) with a substantially circular shape, and said steam conducting channel (17a-17d) is arranged in the centre of the concentric rings, and that at least certain of the fuel rods in the rings are arranged such that their upper ends are displaced in relation to their lower ends in a tangential direction such that the flowing coolant is brought to rotate around the steam conducting channel. 2. A fuel assembly according to claim 1, characterized in that all the fuel rods in the rotary cell are inclined in the same direction and this direction is either clockwise or anticlockwise around the steam conducting channel. 3. A fuel assembly according to claim 1, characterized in that the fuel assembly comprises a plurality of rotary cells (13a-13d) and that each one of the rotary cells is at least partially surrounded by a flow-guiding structure (8a, 9a, 7b). 4. A fuel assembly according to claim 3, characterized in that at least part of the flow-guiding structure consists of a tubular member (8a) arranged in the central part of the fuel assembly with its longitudinal direction substantially parallel to the longitudinal direction of the fuel assembly. 5. A fuel assembly according to claim 3, characterized in that the flow-guiding structures comprise guide members (10) to mechanically strengthen and contribute to the rotation of coolant around the steam conducting channels. 6. A fuel assembly according to claim 3, characterized in that at least part of the flow-guiding structure consists of an elongated member (9a), the longitudinal direction of which is substantially parallel to the longitudinal direction of the fuel assembly and which is arranged at the fuel channel (7). 7. A fuel assembly according to claim 6, characterized in that said elongated member (9a) and the fuel channel (7) together form a channel (9b) which allows the passage of coolant. 8. A fuel assembly according to claim 1, characterized in that the fuel assembly comprises a plurality of fuel units (3a-3e) stacked on top of each other, each one comprising a top tie plate (5), a bottom tie plate (6), a plurality of fuel rods (4, 20) extending between the top tie plate and the bottom tie plate, and that at least certain of the fuel units (3b-3e) comprise an empty volume which constitutes part of said steam conducting channel (17a-17d, 25a, 25d). 9. A fuel assembly according to claim 8, characterized in that it comprises a fuel unit (3d) in which at least certain of the fuel rods are arranged such that the first end is displaced in a radial direction outwards from the centre of the ring in relation to the lower end. 10. A fuel assembly according to claim 8, characterized in that at least some fuel unit (3a-3e) has reduced corner portions (7b, 7c) and that the corner portions are provided with corner bars (12). 11. A fuel assembly according to claim 8, characterized in that it comprises a lower fuel unit (3a) with a first lattice and an upper fuel unit (3c, 3e) with a second lattice which differs from the first lattice in that at least some of the positions in the lattice have been displaced outwards in a radial direction. 12. A fuel assembly according to claim 11, characterized in that the majority of the fuel rods (20) in the upper fuel unit (3c, 3e) have a diameter (d.sub.1) which is smaller than the diameter (d.sub.2) of the majority of the fuel rods (4) in the lower fuel unit (3a).