Patent Number: 054209021
Section: summary

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation of International application Ser. No. PCT/DE91/00733, filed Sep. 17, 1991. SPECIFICATION The invention relates to a fuel assembly having a bundle or cluster of mutually parallel fuel rods each being guided at a plurality of axial positions by a mesh, or mesh opening, of a grid-like spacer and being laterally supported there by a support spring. Fuel assemblies contain a cluster of fuel rods that are parallel to one another and in boiling water reactors they are disposed around a coolant pipe, which typically has a rectangular cross section with flat pipe walls. The cluster is covered at the top and bottom by a plate having openings for the passage of coolant flowing from bottom to top, and in the case of a boiling water reactor is surrounded laterally by a fuel assembly channel, which typically has a rectangular cross section and practically flat channel walls. The lateral spacing of the fuel elements in the cluster is fixed at a plurality of axial positions of the fuel rods by grid-like spacers. Each fuel rod is guided through a mesh of the grid and is laterally supported there by a support spring. Such a spacer is formed by ribs extending transversely to the rods and being aligned parallel to the rods. It is surrounded on the outside by outer peripheral ribs and on the inside, in the case of a boiling water reactor having a central coolant pipe, by inner peripheral ribs. The ribs may be rectilinear and may penetrate one another, producing polygonal grid meshes. However, they may also be constructed as tubes in particular, which are welded together and annularly surround the fuel rods. The ribs typically have tabs on their upper edge, which serve as flow guide surfaces and in particular in the case of the peripheral ribs, face into interstices between adjacent fuel rods. In pressure water reactors, the axial position of the spacers is dictated by fastening them to guide tubes, while in boiling water reactors having coolant pipes, the spacers may be held on the coolant pipe by stops secured to the pipe walls. The stops abut against the upper and lower edges of both the inner peripheral ribs and adjacent inner ribs, to provide an adequate stop surface area. The inner ribs that extend between the fuel rods form bearing surfaces in each mesh for at least one support spring which presses the fuel rod against other retaining elements, such as rigid knobs. For the coolant flowing from bottom to top between the fuel rods, the ribs, knobs and springs present undesirable hindrances that prevent a uniform flow. In order to provide the best possible utilization of the existing cross section, an attempt is made in boiling water reactors to put the fuel rods as close as possible to the coolant pipe or fuel assembly channel. Then, however, the peripheral ribs form further flow hindrances. For the sake of good fuel utilization, an attempt is also made to distribute the fuel to as many fuel rods as possible, so that the rods are therefore made thin. That means that the interstices between the fuel rods are small as well, and therefore the structural elements of the spacer, which cannot be made arbitrarily thin because of the required mechanical strength, have an increasingly disruptive influence on the coolant flow. If a change is made from clusters with eight or nine rows and columns of fuel rods to configurations with 11 rows of fuel rods in the same fuel assembly cross section, for example, then care must be taken to provide a lower flow resistance by means of a suitably flow-aiding or streamlined construction of the spacer, if the necessary coolant throughput is to be maintained. It is accordingly an object of the invention to provide a fuel assembly with a flow-aiding spacer, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which has an adequately low flow resistance. With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel assembly, comprising a cluster of mutually parallel fuel rods; a fuel assembly channel laterally surrounding the cluster of fuel rods and having a substantially rectangular cross section and flat channel walls; grid-like spacers having meshes formed therein each receiving a respective one of the fuel rods for guiding the fuel rods in a plurality of axial positions; at least one support spring laterally supporting each respective one of the fuel rods in the mesh guiding the fuel rod; each of the spacers having inner ribs being aligned parallel to the fuel rods and outer peripheral ribs opposite the channel walls, at least some of the inner ribs being fastened to the peripheral ribs, and the outer peripheral ribs being joined together only by the inner ribs. With the objects of the invention in view, there is also provided a fuel assembly for a boiling water reactor, comprising an approximately central coolant pipe; a cluster of mutually parallel fuel rods surrounding the coolant pipe and defining interstices therebetween; a channel laterally surrounding the cluster of fuel rods and having walls; a grid-like spacer having meshes for guiding the fuel rods, outer peripheral ribs opposite the walls of the channel, inner peripheral ribs substantially resting on the coolant pipe, and inner ribs joining the outer ribs to the inner peripheral ribs, the outer peripheral ribs and the inner peripheral ribs having upper edges; and support springs laterally supporting the fuel rods in the meshes, the support springs being rings of tabs disposed on the respective upper edges of the outer peripheral ribs and the inner peripheral ribs and bent into the interstices, all of the tabs having upper edges disposed in an upper plane, all of the ribs having lower edges disposed in a lower plane, and the outer peripheral ribs being lower than the inner peripheral ribs between the tabs. In pressurized water reactors, the corners of the spacer must be spaced in such a way that the spacers of adjacent fuel assemblies are prevented as much as possible from catching on one another during maneuvers in the reactor. In boiling water reactors, it is difficult to assure that the corner rods will be adequately bathed with coolant. The invention therefore proposes (particularly for boiling water reactors) spacers with "open corners". In other words, the outer ribs do not directly meet one another at the corners. Instead, according to the invention, they are joined together only through inner ribs, so that a gap is created in the boundary formed by the outer ribs. In order to attain this object, the invention also takes its point of departure from the flow resistance presented by the support springs and proposes one support spring for each mesh guiding a fuel rod. The support spring has an upper and a lower bearing surface that rests on a front side facing toward the rod, of a rib surrounding the applicable mesh. These two bearing surfaces are joined to two flat legs, each adjoining a bearing surface, through a resilient middle part that is bent a single time and faces toward the fuel rod. These bearing surfaces each merge into one end of the spring and are fastened to one another in such a way that they encompass the rib. This brings about not only a first contact with the rib but also a long spring travel, which results in high elasticity and adequate contact force. In order to prevent over-stretching of the spring legs when the fuel rods are inserted and to prevent deformation of the bent middle part, the front side of the rib has a safety stop, in the form of a protrusion facing toward the resilient middle part. Published European Application No. 0 330 013 A1, corresponding to U.S. Pat. No. 5,035,853, has already disclosed a spring with upper and lower bearing surfaces, a middle part joining the bearing surfaces through flat legs, and a safety stop, but there the middle piece between the flat legs has an undulating form with one or more arched portions, facing toward the front side of the rib. The arched portions can at the same time act as a safety stop. However, a singly bent muddle part according to the invention is substantially more flow-aiding or streamlined, and therefore the flow resistance at the protrusion facing toward the resilient middle part on the front side of the rib is virtually negligible. It is normal for the outer ribs in boiling water reactors to be supported on the channel walls through knobs that are formed by two halves which are mirror images of one another and that are provided on each of the corners in rectangular fuel assemblies. In other words, each end of a rib adjacent to another rib at a corner has two such half-knobs, in the prior art. In contrast, the invention provides only one half-knob for each such rib end. In other words, the two halves of such a knob are separated from one another by a long, flat middle part of the rib. As a result, the flow resistance of the knobs is virtually halved, while the contact force is virtually unchanged. In reactors (especially boiling water reactors) with spacers which have the aforementioned tabs on their tops, the size of the tabs is limited in a practical sense by the space between adjacent fuel rods into which the tabs are bent. That prevents limitations in terms of the geometry of the inner rib, since the spacer is held on the coolant pipe there. If the height of the remaining ribs is kept low, then the flow resistance is also correspondingly low. The invention provides for the outer peripheral ribs and the inner peripheral ribs to each extend with their lower edges down to a lower plane and with the upper edges of the tabs carried by the peripheral ribs extending up to an upper plane. However, between these tabs, the height of the outer peripheral ribs is less than the height of the inner peripheral ribs. As a result, the tabs on the outer peripheral ribs are lengthened. In other words, their base is wider and the interstices between the tabs are correspondingly smaller than at the inner rib. There is accordingly still sufficient room between the tabs of the higher inner rib for the stops carried by the pipe walls to engage the tabs and come to a stop there, without requiring that the stops protrude so far into the interstice between the fuel rods that they also come to a stop at the inner ribs. In such a construction, it suffices to provide corresponding stops only in the middle of the pipe walls in each case. That lessens the flow resistance at the stops. Particularly with rectangular coolant pipes, the corners formed by abutting peripheral ribs are weak points, mechanically. In particular, the tabs mounted there rip away all the more easily as they have to be made narrower, in view of the available space between the fuel rods. In this case the invention therefore provides that at least two adjacent tabs of the inner ribs (for instance, the corner tabs of abutting peripheral inner ribs) have locations at which they can be laterally welded to one another. This creates a reinforcement of the inner ribs (for instance, a reinforced corner). Further problems arise because the inner peripheral ribs must be welded to inner ribs on one hand, but on the other hand they must offer space for accommodating spacer elements that support the inner ribs against the pipe walls. The invention provides for constructing upper and lower peripheral parts of the inner rib as contact parts, which spring back toward the fuel rods (that is, they point away from the pipe wall), and onto which a tubular or can-like inner rib forming the mesh of an adjacent fuel rod is welded or fastened on in some other suitable way. Between the upper and lower peripheral parts, a middle part of the peripheral rib is provided, on which a spacer element that supports the peripheral rib against the pipe wall is disposed. In particular, this spacer element may be a spring. The spacer elements are advantageously each disposed between the stop in the middle of a pipe wall and the corners of the coolant pipe, so that four upper and four lower stops and eight spacer elements (springs) are sufficient. 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 with a flow-aiding spacer, 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.