Patent Number: 
Section: claims

1. A fuel assembly for a nuclear power boiling water reactor, comprising:a fuel channel extending in and defining a length direction of the fuel assembly and defining a central fuel channel axis extending in said length direction;fuel rods positioned such that they are surrounded by said fuel channel, each fuel rod having a central fuel rod axis extending substantially in said length direction; andwater channels positioned such that they are surrounded by said fuel channel, the water channels being configured and positioned for, during operation, allowing non-boiling water to flow through the water channels, each water channel having a central water channel axis extending substantially in said length direction,wherein said fuel rods comprise a first group of fuel rods and a second group of fuel rods,wherein each fuel rod in said first group is a full length fuel rod that extends from a lower part of the fuel assembly to an upper part of the fuel assembly,wherein each fuel rod in said second group is a part length fuel rod that extends from said lower part of the fuel assembly and upwards, but does not reach as high up as said full length fuel rods,wherein said water channels of the fuel assembly comprise at least three and no more than three water channels, each of which has a cross-sectional area that is at least twice as large as the average cross-sectional area of the fuel rods,wherein the three water channels are positioned with no further water channel having its central axis closer to the central fuel channel axis than the central water channel axis of each the three water channels, andwherein there are at least five second group fuel rods positioned with their central fuel rod axes being closer to the central fuel channel axis than any of the water channel axes of the water channels of the fuel assembly. 2. A fuel assembly according to claim 1, wherein no first group fuel rod is positioned with its central fuel rod axis closer to the central fuel channel axis than the central fuel rod axis of any of said at least five second group fuel rods. 3. A fuel assembly according to claim 1, comprising at least seven second group fuel rods positioned with their respective central fuel rod axes closer to the central fuel channel axis than any of the water channel axes of the water channels. 4. A fuel assembly according to claim 3, wherein no first group fuel rod is positioned with its central fuel rod axis closer to the central fuel channel axis than the central fuel rod axis of any of said at least seven second group fuel rods. 5. A fuel assembly according to claim 3, wherein each second group fuel rod has a length that is less than 0.50 times the length of said first group fuel rods. 6. A fuel assembly according to claim 5, wherein no fuel rod longer than 0.50 times the length of said first group fuel rods is positioned with its central fuel rod axis closer to the central fuel channel axis than the central fuel rod axis of any of said second group fuel rods. 7. A fuel assembly according to claim 1, wherein each second group fuel rod has a length that is less than 0.50 times the length of said first group fuel rods. 8. A fuel assembly according to claim 7, wherein no fuel rod longer than 0.50 times the length of said first group fuel rods is positioned with its central fuel rod axis closer to the central fuel channel axis than the central fuel rod axis of any of said second group fuel rods. 9. A fuel assembly according to claim 1, wherein the cross-sectional area of each water channel is between 3.0 and 10.0 times the cross-sectional area of each one of said fuel rods. 10. A fuel assembly according to claim 9, wherein the cross-sectional area of each water channel is between 4.0 and 8.0 times the cross-sectional area of each one of said fuel rods. 11. A fuel assembly according to claim 1 wherein the fuel assembly has fuel rods of different cross-sectional areas and wherein the cross-sectional area of each water channel is between 3.0 and 10.0 times the average cross-sectional area of the fuel rods. 12. A fuel assembly according to claim 11 wherein the fuel assembly has fuel rods of different cross-sectional areas and wherein the cross-sectional area of each water channel is between 4.0 and 8.0 times the average cross-sectional area of the fuel rods. 13. A fuel assembly according to claim 1, wherein each of said at least three and no more than three water channels has a circular cross-section, at least in the lower part of the fuel assembly where the second group fuel rods are arranged. 14. A fuel assembly according to claim 1, wherein the fuel assembly comprises no more than 19 fuel rods having a distance between the central fuel rod axis and the central fuel channel axis less than the distance between the central water channel axis of at least one of said three water channels and the central fuel channel axis. 15. A fuel assembly according to claim 14, wherein the fuel assembly comprises no more than 16 fuel rods having a distance between the central fuel rod axis and the central fuel channel axis is less than the distance between the central water channel axis of at least one of said three water channels and the central fuel channel axis. 16. A fuel assembly according to claim 15, wherein the fuel assembly comprises no more than 14 fuel rods having a distance between the central fuel rod axis and the central fuel channel axis is less than the distance between the central water channel axis of at least one of said three water channels and the central fuel channel axis. 17. A fuel assembly according to claim 1, wherein the fuel assembly comprises a substantially regular pattern of fuel rod positions, wherein each one of said three water channels is positioned such that it replaces four fuel rods in this substantially regular pattern. 18. A fuel assembly according to claim 1, wherein the fuel assembly comprises 65-160 fuel rods. 19. A fuel assembly according to claim 18, wherein the fuel assembly comprises 100-120 fuel rods. 20. A fuel assembly according to claim 19, wherein the fuel assembly comprises 105-113 fuel rods. 21. A fuel assembly according to claim 20, wherein the fuel assembly comprises 109 fuel rods. 22. A fuel assembly according to claim 1, wherein the fuel assembly further comprises 2-8 additional second group fuel rods, each of which has a length of between 0.59 and 0.79 times the length of said first group fuel rods. 23. A fuel assembly according to claim 22, wherein the fuel assembly further comprises 4-6 additional second group fuel rods, each of which has a length of between 0.59 and 0.79 times the length of said full length fuel rods. 24. A fuel assembly according to claim 1, wherein the fuel assembly comprises at least 70 full length fuel rods. 25. A fuel assembly according to claim 24, wherein the fuel assembly comprises at least 80 full length fuel rods. 26. A fuel assembly according to claim 25, wherein the fuel assembly comprises at least 90 full length fuel rods. 27. A fuel assembly according to claim 1, further comprising:a lower tie plate, positioned below the fuel rods, wherein a lower end of each of said three water channels is attached to said tie plate;an upper lifting device positioned above the fuel rods and including a handle for gripping and lifting a bundle of fuel rods;a plurality of spacer grids for holding the fuel rods, at least most of the spacer grids being attached to said three water channels; andattachment rods attached at their lower ends to the upper part of said three water channels and at their upper ends to said upper lifting device.