Patent Number: 052326586
Section: summary

The invention relates to a fuel assembly for a boiling water reactor, having an elongated box extending between a top part and a bottom part, outer surfaces of the box being flat and defining a polygonal outer cross section of the box with rounded corners, and a bundle of fuel rods disposed in the box, the fuel rods being aligned parallel to one another and to the longitudinal axis of the box and being disposed beside one another in rows parallel to the walls of the box. Boiling water reactors contain a core configuration that has a flow of coolant, which also serves as a moderator, from bottom to top, and a plurality of fuel assemblies disposed side by side and parallel to one another in the flow. Each fuel assembly includes an elongated box extending between a top part and a bottom part. The top and bottom parts have flow openings for the coolant. In the interior of the box, there is a bundle of fuel rods, which are aligned parallel to one another and to the longitudinal axis of the box and are disposed side by side in rows parallel to the walls of the box. In interstices between individual fuel assembly boxes, there are absorber elements, which are moved axially into and out of the interstices in order to control the nuclear reaction. The fuel assemblies themselves are packed as tightly as possible in a core zone, but the absorber elements define a minimum spacing between the fuel assemblies. Care must also be taken to ensure that the coolant is introduced through the bottom parts into the interior of the fuel assemblies at a considerable pressure, and the box walls must therefore have a certain mechanical strength, so that during reactor operation they will not bulge outward excessively, which could hinder the mobility of the absorber rods. On the other hand, the box walls should attenuate the neutron flow in the core zone as little as possible, so that limits exist for both the minimum thickness of the box wall and for the selection of the stable material that is required. Usually, the core zone of a boiling water reactor has a geometry in which the outer surfaces of the fuel assembly boxes define a polygonal box cross section with rounded corners. As already mentioned, the interstice between the outer surfaces of adjacent fuel assemblies is defined by the absorber elements disposed at that location, and minimum dimensions also arise for the radius of curvature of the rounded corners, which are determined by measuring lances or other reactor components. If the box therefore has a uniform wall thickness, then the geometry of the reactor core and the wall thickness required for mechanical strength also defines the available space in the interior of the box for the fuel rod bundle. The fuel rods are disposed as regularly as possible in the interior of the box. For instance, in a common commercial reactor type with a square box cross section, they are disposed in nine lengthwise rows and nine crosswise rows at right angles to the lengthwise rows. The lengthwise and crosswise rows each extend parallel to the box walls. However, from the standpoint of a low mean linear rod power, it is advantageous to make the individual fuel rods thinner and instead to provide more fuel rods, for example 10 or even 11 lengthwise and crosswise rows of fuel rods. However, configurations of this kind with 10.times.10 or 11.times.11 fuel rods, with the conventional square basic pattern, have a disadvantageous small spacing between the fuel rods of one row (called the "fuel rod pitch"), and also have a disadvantageous wider spacing of the outermost fuel rod row from the inner wall of the fuel element box, unless the corner fuel rods disposed in the corners are moved impermissibly close to the rounded corners. Fuel element boxes have already been proposed that still have ribs provided on the outside in the region of the rounded corners, in order to increase the stability of the box, although they do have a square internal cross section and a relatively small wall thickness. However, that does mean that less neutron-absorbing material overall is needed for the box, and since, as already explained, there is a predetermined radius of curvature of the rounded corners, the difficulties described, which arise at the transition from fuel assemblies with 9.times.9 fuel rods to fuel assemblies with 10.times.10 or 11.times.11 rods, are not overcome. It is accordingly an object of the invention to provide a fuel assembly for a boiling water reactor with a reinforced box, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which does so by improving the geometry of the fuel assembly. With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel assembly for a boiling water reactor, comprising an elongated box extending between a top part and a bottom part, having box walls with flat outer surfaces defining a polygonal outer cross section with rounded corners, an interior and a longitudinal axis, a bundle of fuel rods being disposed in the box and aligned parallel to one another and to the longitudinal axis of the box, the fuel rods being disposed beside one another in rows parallel to the box walls, the box walls having reinforcements protruding into the interior in the vicinity of the rounded corners defining a relatively thickened or increased wall thickness, and the box walls having a relatively reduced wall thickness between the reinforcements. In accordance with another feature of the invention, the bundle of fuel rods has no fuel rod at intersections of two of the rows of fuel rods adjacent to two of the box walls bordering one another at the rounded corners. In accordance with a further feature of the invention, two of the fuel rods in the rows within the bundle are spaced apart by a minimum spacing being equal to a spacing between two fuel rods in a row parallel to one of the box walls. In accordance with an added feature of the invention, the fuel rods of two adjacent rows are staggered relative to one another in the direction of the box walls. In accordance with an additional feature of the invention, the relatively reduced wall thickness is less than approximately 2.4 mm and preferably between 1.5 and 1.7 mm, and the relatively increased wall thickness in the vicinity of the rounded corners is between approximately 2.5 and 3.0 mm and preferably 2.7 to 2.9 mm. With the objects of the invention in view, there is also provided, in a boiling water reactor having a core zone with a given geometry and a plurality of fuel assemblies, each of the fuel assemblies comprising an elongated box having box walls with flat outer surfaces defining a polygonal outer cross section with rounded corners, an interior and a longitudinal axis, the outer cross section being defined by the given geometry of the core zone, a bundle of fuel rods being disposed in the box and aligned parallel to one another and to the longitudinal axis of the box, the fuel rods being disposed beside one another in rows parallel to the box walls, the box walls having reinforcements protruding into the interior in the vicinity of the rounded corners defining a relatively increased wall thickness, and the box walls having a relatively reduced wall thickness between the reinforcements. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a fuel assembly for a boiling water reactor with a reinforced box, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.