Patent Number: 053274717
Section: summary

Cross-Reference to Related Application: This application is a continuation of International Application PCT/DE91/00168, filed Feb. 27, 1991. The invention relates to a nuclear reactor fuel assembly, having a bundle of fuel rods being aligned parallel to one another and containing nuclear fuel; a fuel assembly head being disposed above the bundle and having a top with a handle thereon; a bottom plate being disposed under the bundle and being permeable to liquid coolant; a coolant tube being disposed in the bundle, being parallel to the fuel rods and being open on both of its ends, the coolant tube having a lower end with a lower end piece and being retained by the lower end piece on the bottom plate; and the coolant tube having an upper end piece protruding from below into a recess of the fuel assembly head. Such a fuel assembly is known from Published European Application No. 0 307 705 A1. The end piece at the upper end of the coolant tube in that known fuel assembly is a platelike end screen with flow openings for the liquid coolant. A bolt which is parallel to the coolant tube is located on the end screen and protrudes from underneath into a recess in the fuel assembly head, where it is loosely guided without any further connection. The end piece at the lower end of the coolant tube is also an end screen. Disposed on the outside of that end screen is a threaded bolt which is also parallel to the longitudinal direction of the coolant tube, but it also reaches through a leadthrough in a gridlike perforated bottom plate in the fuel assembly foot, where it is screwed firmly to the fuel assembly foot with a nut. That nuclear reactor fuel assembly has gridlike spacers in a plurality of cross-sectional planes between its head and its foot. The spacers have meshes or holes, through each of which one fuel rod containing nuclear fuel extends. Some of the fuel rods are so-called bearing rods having threaded bolts on both ends, with which they are firmly screwed to the fuel assembly head with a nut and to the fuel assembly foot directly. The other fuel rods reach loosely through leadthroughs in the fuel assembly head and foot with bolts located on both ends of the rods. All of the fuel rods are fixed with a compressionally biased helical spring, and each helical spring is seated on the threaded bolt or bolt on the inside of the fuel assembly head and is supported on that end in the applicable fuel rod. The invention also relates to a fuel assembly for a boiling water reactor with fuel rods of different lengths. One such fuel assembly is known from U.S. Pat. No. 4,675,154 and includes a fuel assembly case that laterally surrounds the fuel assembly and is open at the top and bottom. A coolant tube extends axially inside the case and has at least one opening on each of its top and bottom end for the flow of liquid coolant. A cover plate provided with coolant outlets on the upper end of the case and a bottom plate provided with coolant inlets on the lower end of the case cover the two open ends of the case. Spacers that are at right angles to the water tube and include support ribs, extend over the cross section of the fuel assembly case in predetermined axial positions. T spacers are held on the coolant tube through suitable means (such as stops). A number of fuel rods is also provided, each of which is laterally supported on the support ribs of a plurality of spacers and need not be of uniform length. In the case of such boiling water fuel assemblies, it is customary to construct some fuel rods as retaining rods and to screw them to the bottom and cover plates with corresponding upper and lower closure caps. If fuel rods of different lengths are provided, then the shorter fuel rods are screwed only to the bottom plate by their lower end, while their upper end is held in position by corresponding spacers. However, at least some of the retaining rods are provided on their upper closure caps with retainers for a handle, which bear the entire weight of the fuel assembly. Additional helical springs on such upper closure caps can press the cover plate against the screw connections on the upper end pieces of the retaining rods. Such long fuel assemblies that are constructed as retaining rods are part of the load-bearing or supporting fuel assembly skeleton and are essentially under tension. Between the handle, the cover plate and the bottom plate, they produce a force-locking, torsion-proof connection, they carry the weight of the fuel assembly and also absorb the various forces that arise upon expansion of material from the radiation and heating that prevail during operation. A force-locking connection is one which connects two elements together by force external to the elements, as opposed to a form-locking connection which is provided by the shapes of the elements themselves. Since the plates are pierced by bores, some of which form the water inlets and water outlets and some of which receive the closure caps of the fuel rods, they must be dimensioned with a correspondingly great thickness, and therefore there is only limited space available for the flow cross section of the inlets and outlets. As a result, there is a considerable pressure difference, especially at the upper cover plate, since some of the water that is pumped axially through the fuel assembly during operation evaporates, so that the mixture of water and steam would require larger outlet cross sections in order to maintain an unimpeded flow. If the fuel assembly case is removed axially upward from the load-bearing or supporting skeleton, then the fuel rods become accessible and can be inspected from the side. However, in the event of more extensive inspection, maintenance and possible replacement of fuel rods, the cover plate must be unscrewed and the load-bearing or supporting skeleton must accordingly be dismantled. Since the screw connections generally seize after relatively long service, considerable forces are needed to release the fuel rods. Such forces can be brought to bear only at the upper closure caps of the retaining rods and must be transmitted to the screw connections of the lower closure caps by torsion on the retaining rods. Great mechanical stability and accordingly a certain minimum thickness are therefore necessary for the rods. However, if a change is to be made from a fuel assembly having rods which are disposed in eight rows and eight columns (which is known as an 8.times.8 assembly) to an 11.times.11 fuel assembly with the same external dimensions, then thinner rods must be used, having a mechanical stability that cannot meet the requirements made for retaining rods, especially with respect to the forces necessary to release the screw connection. For economical reasons, it is generally advantageous to use a larger number of thinner rods, instead of fewer but thicker fuel rods. It is accordingly an object of the invention to provide a nuclear reactor fuel assembly with a load-bearing or supporting coolant tube, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which improves such nuclear reactor fuel assemblies, in particular by avoiding load-bearing or supporting rods with threaded bolts located on both ends that are firmly screwed to both the head and foot of the fuel assembly. In the novel structure for the load-bearing or supporting parts of the fuel assembly skeleton, loading and forces are better distributed among the load-bearing or supporting parts, which creates new possibilities for retaining the fuel rods (and in particular shorter and/or thinner fuel rods), for simplifying assembly and disassembly of the fuel assembly, and/or for creating more favorable water inlets and water outlets with optimized pressure drops. The invention takes the fact that the coolant tube is a relatively stable, low-wear element as its point of departure. It is suitable for transmitting relatively strong forces and practically need not be removed for inspecting or repairing the fuel assembly. With an adequately stable connection to the bottom plate, the entire weight of the fuel assembly can therefore be borne, which makes retaining rods superfluous and makes load-bearing or supporting screw connections to fuel rods unnecessary for either the bottom plate or the top plate. A rigid connection for force transmission between the coolant tube and the bottom plate can therefore be non-detachably constructed, and a seized screw connection of the tube need not be loosened for inspections. The cover plate can be practically relieved of load-bearing or supporting forces and then serves only to cover the water channel, as an upper stop for unscrewed fuel assemblies, which are then retained only by the spacers in the coolant flow, and to maintain the desired flow. With the cover plate removed, the fuel rods can be inserted or replaced without further screwing operations. With the foregoing and other objects in view there is provided, in accordance with the invention, a nuclear reactor fuel assembly, comprising at least one coolant tube with an upper end having an opening formed therein and an upper end piece and a lower end having an opening formed therein and a lower end piece; a bottom plate being supported by said coolant tube and joined to said lower end piece in a dimensionally rigid manner, said bottom plate having inlet openings formed therein for liquid coolant; a cover plate retaining said upper end piece and having outlet openings formed therein for a liquid/steam mixture of coolant; gridlike spacers defining meshes therein, means for retaining and attaching said spacers on said coolant tube between said plates; and fuel rods each being guided through a respective one of said meshes, being filled with nuclear fuel and being joined to at most one of said plates. In accordance with another feature of the invention, there is provided a stop body mounted on said upper end piece, said cover plate being detachably retained on said stop body, and a lower closure cap disposed on said bottom plate, said fuel rods including longer and shorter fuel rods, and only said shorter fuel rods being retained by said lower closure cap on said bottom plate. As compared with known fuel assembly structures, the upper end piece of the coolant tube accordingly gains special significance, because it is no longer freely or only loosely guided in a recess relative to the fuel assembly head, but instead is retained on the fuel assembly head. The entire weight of the fuel assembly is then supported by the coolant tube when the handle on the fuel assembly head is raised. If additional forces arise at the fuel assembly head during manipulation (such as additional forces of gravity due to the application of the hoisting tool), they can be absorbed by springs between the head and the foot part, without uncontrollably loading the coolant tube. With the objects of the invention in view, there is also provided a nuclear reactor fuel assembly, comprising a bundle or cluster of mutually parallel aligned fuel rods containing nuclear fuel; a fuel assembly head being disposed above the bundle and having a top with a handle; a bottom plate being disposed under the bundle and being permeable to liquid coolant; and a coolant tube being disposed in the bundle, being parallel to the fuel rods and having upper and lower open ends; the coolant tube having a lower end piece on the lower end being supportingly retained on the bottom plate for retaining the coolant tube on the bottom plate; and the coolant tube having an upper end piece on the upper end being supportingly retained on the fuel assembly head and protruding from below into a recess formed in the fuel assembly head. In accordance with another feature of the invention, the bottom plate and the fuel assembly head are supports for the coolant tube, one of the end pieces is longitudinally displaceably supported on one of the supports, and including a stop body on the fuel assembly head defining a maximal spacing between the supports, and at least one spring disposed between the supports for pressing one of the supports against the stop body. In accordance with a further feature of the invention, there is provided a stop shoulder formed onto the one displaceably supported end piece, the shoulder defining a minimal spacing between the fuel assembly head and the bottom plate. In accordance with an added feature of the invention, there is provided a spring is disposed on one of the fuel rods. In accordance with an additional feature of the invention, the spring is disposed on one of the pieces, preferably on the upper end piece. In accordance with yet another feature of the invention, the fuel assembly head has a part being formed onto the handle, being secured against rotation, and being screwed on from above, and the upper end piece is passed through the part. In accordance with yet a further feature of the invention, the upper end piece carries a socket pin, the fuel assembly head carries a cover plate having coolant outlet openings formed therein, and the socket pin extends through the cover plate and carries a detachably mounted stop body for the cover plate. In accordance with yet an added feature of the invention, the cover plate is mounted on the upper end piece, and including a supporting mechanical connection securing the bottom plate to the lower end piece, a spring under pressure pressing the cover plate against the stop body, and the fuel rods having upper ends with upper closure caps, the upper closure caps being loosely guided on or not touching the cover plate in an appropriate receiving position. As a special advantage, at least all of the fuel rods that extend practically up to the cover plate in the head of the fuel assembly carry lower closure caps on their lower ends, with which these fuel assemblies then stand, unscrewed, on the bottom plate. Accordingly, the closure caps need merely be loosely introduced into suitable plug-type connections on the bottom plate or they need merely be placed on the surface of the bottom plate pointing toward the cover plate, without using special retainers. A lower end piece of the coolant tube is also provided, which is rigidly joined to the bottom plate so that all of the forces exerted upon the bottom plate are practically transmitted to the coolant tube. A corresponding upper end piece of the coolant tube, which also supports the cover plate, is detachably joined to the cover plate and/or to a handle secured to the cover plate. Therefore, with the objects of the invention in view, there is additionally provided a fuel assembly of a boiling water reactor, comprising a fuel assembly case laterally surrounding the fuel assembly and having an interior and open upper and lower ends at the top and bottom; a cover plate covering the upper end of the case and having coolant outlets formed therein; a bottom plate covering the lower end of the case and having coolant inlets formed therein; a coolant tube extending in axial direction in the interior of the case and having upper and lower ends, each of the upper and lower ends having at least one opening formed therein for the passage of water, the coolant tube having a lower end piece joined to the bottom plate with a rigid connection transmitting substantially all forces exerted upon the bottom plate to the lower end piece and to the coolant tube, and the coolant tube having an upper end piece, and a releasable connection supporting the upper end piece at the cover plate; a plurality of spacers standing practically or substantially perpendicular to and retained on the coolant tube at predetermined axial positions, the spacers containing support ribs; a plurality of fuel rods being parallel to the case, being respectively supported on the support ribs of a plurality of the spacers and having lower ends, some of the fuel rods extending practically or substantially as far as the cover plate; and lower closure caps standing unscrewed on the bottom plate and being disposed on the lower ends of at least all of the fuel rods extending substantially as far as the cover plate. In accordance with another feature of the invention, at least two of the spacers are penetrated by all of the fuel rods, the fuel rods include first and second groups of fuel rods, each of the fuel rods at least in the first group extend from the bottom plate as far as the cover plate and carry one of the lower closure caps on the lower end, the bottom plate has an upper surface facing toward the cover plate, the lower closure caps stand on the upper surface of the bottom plate, the fuel rods of the first group have upper ends, and including upper closure caps on the upper ends being loosely disposed on the cover plate at given receiving positions. In accordance with a further feature of the invention, each of the fuel rods in the second group is shorter than a fuel rod in the first group and carries one of the lower closure caps on the lower end being retained on the bottom plate, the fuel rods of the second group have upper ends, and including upper closure caps on the upper ends being disposed in the vicinity of one of the spacers and being spaced apart from the cover plate. In accordance with an added feature of the invention, there is provided a plug connection joining the lower closure caps of the fuel rods of the second group to the bottom plate. In accordance with an additional feature of the invention, the coolant outlets in the cover plate are outlet openings disposed between the given receiving positions of the fuel rods of the first group and enlarged outlet openings disposed in a projection of the shorter fuel rods. In accordance with again another feature of the invention, the fuel rods of the second group are each disposed along diagonals of the cross section of the case. In accordance with again a further feature of the invention, the case has walls, some of the fuel rods are adjacent the walls, and each of the fuel rods adjacent the walls belongs to the first group. In accordance with again an added feature of the invention, the support ribs of the spacers define first through eleventh rows and first through eleventh columns of meshes between the support ribs, counting from the wall inward, and the fuel rods of the second group are each disposed in the third row or the third column. In accordance with again an additional feature of the invention, the upper end piece of the coolant tube passes through the cover plate from below and is screwed to the cover plate and/or to the handle from above. In accordance with still another feature of the invention, the handle is undetachably secured to the cover plate. In accordance with still a further feature of the invention, there is provided a part formed onto the bottom plate into which the lower end piece of the coolant tube is introduced, and a securing bolt securing the lower end piece to the part, to protect against relative rotation of the bottom plate and the coolant tube. In accordance with still an added feature of the invention, the cover plate is resiliently supported against the bottom plate. In accordance with still an additional feature of the invention, the coolant inlets in the bottom plate are flow openings having cross sections creating a uniform flow through the fuel assembly with a pressure loss being negligible as compared to a pressure loss at the coolant outlets in the cover plate. In accordance with a concomitant feature of the invention, threadless closure caps are disposed on the upper and lower ends of all of the fuel rods. 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 nuclear reactor fuel assembly with a load-bearing or supporting coolant tube, 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.