Patent Number: 
Section: claims

1. A fuel assembly for a nuclear boiling water reactor, wherein the reactor comprises a plurality of fuel assemblies, and a plurality of control rods, each control rod being insertable in a respective control rod position between the fuel assemblies, the fuel assembly includes a longitudinal center axis and further comprises;a plurality of elongated fuel rods, each fuel rod comprising nuclear fuel enclosed by a cladding, the fuel rods being held in predetermined positions relative one another, andan elongated channel box forming an outer casing of the fuel assembly and enclosing the fuel rods, the channel box having inner sides, facing the fuel rods, and outer sides, each inner side and each outer side having a longitudinal center line extending substantially in parallel with the center axis and along the length of the channel box, protrusions are provided on the channel box to protrude from a respective one of at least two of the outer sides, on each of said at least two outer sides a plurality of said protrusions being provided and being distributed at a first distance from each other along the center line of the respective one of the at least two outer sides, said first distance being at least 50 mm, wherein the plurality of protrusions are configured to provide a minimum second distance between the outer side and an adjacent control rod, and to enable the control rod to easily slide over and on top of the plurality of protrusions; andwherein the protrusions have a curved shape, the curved shape facilitating the sliding of a control rod blade of the control rod over and on top of the protrusions. 2. A fuel assembly according to claim 1, wherein the plurality of protrusions are distributed along substantially the full length of the outer sides of the channel box. 3. A fuel assembly according to claim 1, wherein the plurality of protrusions are distributed along the center line of each of the outer sides of the channel box. 4. A fuel assembly according to claim 1, wherein each of the plurality of protrusions protrude 0.5-1.5 mm from the outer sides of the channel box. 5. A fuel assembly according to claim 1, wherein each of the plurality of protrusions protrude 0.8-1.2 mm, from the outer sides of the channel box. 6. A fuel assembly according to claim 1, wherein said first distance is about 1000 mm. 7. A fuel assembly according to claim 1, wherein the plurality of protrusions are evenly distributed along the center line of the outer sides of the channel box. 8. A fuel assembly according to claim 1, wherein the channel box comprises four walls extending substantially parallel with the center axis, the walls comprising said inner sides and said outer sides, respectively. 9. A fuel assembly according to claim 1, wherein the channel box has a substantially square cross section seen in the direction of the center axis. 10. A fuel assembly according to claim 1, wherein the fuel assembly further comprises an elongated support member extending in the direction of the center axis, the support member having a cruciform cross section seen in the direction of the center axis, wherein the support member is secured to the inner sides of the channel box through a plurality of weld joints along the center lines. 11. A fuel assembly according to claim 10, wherein the support member comprises four wings, each wing being secured to a respective inner side of the channel box. 12. A fuel assembly according to claim 10, wherein the support member divides the fuel rods of the fuel assembly into four equal sub-groups. 13. A fuel assembly according to claim 10, wherein the support member is hollow, forming a substantially vertical channel through which water can flow upwardly through the fuel assembly.