Patent Number: 044787863
Section: claims

1. A fuel assembly for a boiling water reactor, said fuel assembly including a plurality of vertical fuel rods enclosed within a fuel channel separate from said fuel rods, said fuel channel being of substantially rectangular cross-section, wherein said fuel rods are positioned with the aid of a plurality of spacer devices, said fuel channel surrounding an elongated stiffening device for said fuel channel, said stiffening device being separate from said fuel rods and extending along a predominant portion of the length of said fuel rods, the vertical center line of said stiffening device lying in the vicinity of the vertical center line of said fuel channel, said stiffening device having four stiffening wings arranged in vertical planes and having angular distances of about 90.degree. between themselves, said stiffening wings extending along a predominant portion of the length of said fuel rods, each of said stiffening wings being separate from said fuel rods and mechanically connected to the three remaining stiffening wings in the vicinity of said vertical center line of said stiffening device, said stiffening wings being mechanically connected to corresponding walls of said fuel channel to prevent said fuel channel from deforming outwardly during reactor operation, said wings constituting partitions between four bundles of fuel rods and each of said wings comprising at least one vertical internal water passageway extending along said predominant portion of the length of said fuel rods, said internal passageways communicating with at least one inlet opening for water provided at the lower end of said stiffening device, at least one mechanical connection in the vicinity of said vertical center line being in the upper portion of said stiffening device, whereby water while flowing upward through said internal water passageways is prevented from contacting said fuel rods, thus providing a minimized void content and a maximized moderator effect along said internal water passageways. 2. Fuel assembly according to claim 15, wherein at least one of said water passageways is arranged in hydraulic communication with the surroundings of said fuel assembly via a perforated portion of a fuel channel wall. 3. Fuel assembly according to claim 1, wherein the total average cross-section of said water passageways of said stiffening device comprises dominant part of the total average cross-section of said stiffening device. 4. Fuel assembly according to claim 1, wherein each of said stiffening wings comprises two elongated, vertical, mutually parallel and mutually spaced-apart sheet-metal portions, defining said at least one water passageway. 5. Fuel assembly according to claim 4, wherein said stiffening device at a plurality of different levels is provided with a plurality of hydraulic connections between said bundles. 6. Fuel assembly according to claim 1, wherein each of the walls of said fuel channel has an inwardly-folded mid-portion which forms a U-shaped projection, directed towards the vertical centre line of the fuel channel, said projection being mechanically connected to a vertically extending, central water tube. 7. Fuel assembly according to claim 1, wherein a plurality of vertical water tubes are arranged in a cruciform configuration and, at each of a plurality of separate level sections, provided with a plurality of transversely extending mechanical connecting elements, said water tubes together with said connecting elements constituting said stiffening device. 8. Fuel assembly according to claim 7, wherein said fuel rods are divided into four equal bundles by said stiffening wings, the fuel rods of each bundle being positioned by a plurality of groups of spacer devices arranged between the upper and lower ends of said bundles, each of said groups of spacer devices positioning in said fuel channel all of said fuel rods, each of said groups being composed of four separate spacer devices, each spacer device comprising a plurality of spacer grid cells, the total number of spacer grid cells in each of said spacer devices being sufficient to accommodate a minor portion of the number of said fuel rods, the spacer devices belonging to one and the same group of spacer devices being disposed at least at two different levels within said fuel channel with each spacer device at a corresponding corner of said fuel channels. 9. Fuel assembly according to claim 7, wherein a plurality of said mechanical connecting elements have surfaces which are obliquely arranged in relation to vertical planes in such a way that upwardly-flowing water acquires a horizontal speed component by streaming along said surfaces. 10. Fuel assembly according to claim 7, wherein each of said transversely extending connecting elements consists of a substantially straight, horizontal bar and that the bars in a first pair of such bars are arranged at a first level and at one side each of a first group of water tubes, the bars in a second pair of such bars being arranged at a second level and at one side each of a second group of water tubes, the horizontal projection of said first pair making an angle of 90.degree. with the horizontal projection of said second pair, each water tube portion, located between two bars included in a pair, being locally flattened to provide plane contact surfaces for mechanical contact with the bars of the pair. 11. Fuel assembly according to claim 1, wherein the walls of said fuel channel are each formed with a vertically extending stiffening ridge, which is mechanically connected to one of said wings. 12. Fuel assembly according to claim 11, wherein said ridge consists of an inwardly-folded wall portion. 13. Fuel assembly according to claim 11, wherein said ridge comprises a vertical strip. 14. Fuel assembly according to claim 13, wherein said vertical strip has an outwardly-facing plane strip surface, the distance of which from the vertical centre axis of the fuel channel is greater than the corresponding distance for the other outwardly-facing plane surfaces of the channel wall in which the strip is included thereby providing a suitable sliding surface for a control rod.