Patent Number: 048287920
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, a fuel assembly 10 has a support structure having grids 12 which retain fuel rods 14 according to a square lattice, two upper 15 and lower 18 end pieces and guide tubes holding the other components of the support structure in position. The top part only of the fuel rods is illustrated. The top part only of the fuel rods is illustrated. The guide tubes, forming tie rods, are distributed between two groups. The guide tubes 20 of the first group are fixed permanently to grids 12, as shown schematically by a cross (X) in the figure. The guide tubes of the first group are also connected permanently to a plate 22 formed with outlet openings for the coolant, forming the bottom of the upper end piece 16. The permanent connections may be of any appropriate type and selected depending on the nature of the material forming the pieces to be assembled together. Welding, threaded connections or connections by deformation of a thin wall may be used as is well known in the art. The guide tubes 20 of the first group are received in the lower end piece 18 in which they may slide vertically. The guide tubes 24 of the second group are rigidly fixed to the lower end piece 18. They project through cells in grid 12 in which they are slidably received. The guide tubes 24 pass through the bottom wall 22 of the upper end piece 16 and project inside piece 16. Their upper end is fixed by a permanent connection 26 to a perforated plate 28 mounted in a frame belonging to the upper end piece 16, above the bottom wall 22. As shown in FIG. 1, four springs 30 are located between the plate 28 and flanges 32 at the upper part of the frame of the upper end piece 16. The springs are under a precompression so as to exert on plate 28 a force biasing it into contact with the bottom wall 22. Plate 28 advantageously includes studs 34 for holding the springs 30 in position and, possibly, for limiting their amount of compression by abutting flanges 32. A first substructure, formed of the first group of guide tubes 20, grids 12 and the upper end piece 16, carries the fuel rods: it is subjected to about 90% of the hydraulic thrust received by the fuel assembly. A second substructure, comprising the lower end piece 18 and the guide tubes 24, receives about 10% of the hydraulic thrust. Consequently, springs 30 may be dimensioned so as to take up only 10% of the total thrust of the coolant on the fuel assembly. When the assemblies are loaded into the reactor, the lower end piece of each assembly rests on the core support plate 36. Springs 30 bias the upper end piece 16 upwardly with a force which is less than the weight of the first substructure and the rods which it carries; the first substructure consequently remains in abutment on the second substructure. The upper core plate 38 may then be positioned. The whole of the weight of the assembly will be applied to the core support plate, directly for the second substructure, through the second substructure in so far as the first is concerned. During start up and operation of the reactor, an upwardly directed thrust is exerted by the coolant. That force lifts the first substructure until the upper end piece 16 is in contact with the upper core plate. Then about 90% of the thrust will be directly applied to plate 38 by direct abutting connection rather than via resilient means. Due to the direct contact and the omission of the springs found in the prior art assemblies and which have their own resonance frequency, amplification of the vibration of the upper internal equipments (which the resilient means of conventional assemblies may generate on the fuel rod bundle 14) is avoided. Attenuation of the vibration of the fuel rods is an essential factor in improving the life of the sheath of the fuel rods and the life and efficiency of the springs provided in the grids for exerting on the fuel rods a force holding them in position. The fraction of the thrust of the coolant which is exerted on the second substructure tends to raise the lower end piece 18 and to lift it off from the core support plate 36. Since however the fraction of the total thrust which is exerted on the second substructure is small, springs 30 having a low precompression, typically between 100 and 200 decanewton, are sufficient for holding the lower end piece 18 in contact with the core support plate 36. That direct contact again attenuates the vibrations of the fuel rod bundle. Referring to FIGS. 2A, 2B and 3, the connection of the guide tubes 20 and 24 with the end pieces are illustrated. The guide tubes 20 are enlarged at their upper end into passages of bottom wall 22 formed with indentations and their lower portions are slidably received in the lower end piece 18. On the other hand, guide tubes 24 are crimped at their upper end in plate 24, slidably received in bottom wall 22, and fixed at their lower end, for example by screws, to the lower end piece 18. The screws may comprise a thin skirt for deformation into a cavity of end piece 18 for preventing them from rotating. The sliding fit of the guide tubes in the end pieces ensures mutual guidance of the two subassemblies and participates in the mechanical strength of the assembly. Referring to FIG. 4 (where the elements corresponding to those of FIG. 1 are designated by the same reference number) a modified embodiment includes an upper end piece designed so that the springs 30 are disposed between two plates. For that, the end piece 16 consists of a bottom wall 22, having passages (not shown) for coolant flow, extended upwardly by a frame with gripping flanges 32 and downwardly by a skirt 40 having an abutment flange 42. The upper part of each guide tube 24 slides within the bottom wall 22 whereas the upper end of each guide tube 20 is securely connected to the bottom wall. A plate 28 with coolant flow apertures is slidably mounted in skirt 40. The guide tubes 24 are fixed to plate 28, but the exterior of each guide tube 24 which slides in bottom wall 22 is beyond the connection zone 26. The guide tubes 20 have a sliding fit in plate 28. Springs 30 are disposed concentrically to the end parts of the guide tubes 24 and bias the two substructures into the abutment position as shown in FIG. 4. When the upwardly flowing coolant exerts on the substructures an upwardly directed force, the major part of this force is absorbed, as in FIG. 1, by direct contact of the end piece 16 with the upper core plate 38, whereas spring 30 exerts on plate 28 a sufficient force for maintaining the lower end piece 18 in contact with the core support plate 36. In the modified embodiment shown in FIG. 5, where the elements already described again have the same reference numbers, springs 30 which tend to spread the end pieces apart are disposed in the lower part of the assembly. The first substructure includes the grids 12, the guide tubes 20 fixed to the bottom wall 22 of the upper end piece 16 and an apertured plate or grid 44 slidably mounted on the guide tubes 24. The second substructure includes the guide tubes 24 fixed to the lower end piece 18 and slidable in the upper end piece 16. Stop means may be provided for limiting the extent of movement of parts 16 and 18 away from each other under the action of springs 30 and/or during handling of the fuel assemblies suspended by the upper end piece 16. As shown in FIG. 5, the stop means are formed by enlarged sockets 46 permanently fixed to the guide tubes 24 and arranged for contact with plate 44. Springs 30 are disposed about the portion of the guide tubes 24 situated between plate 44 and the lower end piece 18. The embodiment which has just been described has the advantage that the upper end piece 16 has no spring, which facilitates movement of the control rod clusters used for controlling the core reactivity and facilitates handling of the assembly. Finally, the assembly shown schematically in FIG. 6 is of the floating grid type, in which some of the grids are not connected to the guide tubes of the first substructure. Referring to FIG. 6, the assembly has a general construction similar to that shown in FIG. 1. But some of the grids 12a are slidably mounted on the fuel rods 20 and 24. Some of the fuel rods, such as those shown at 46 are provided with resilient means additional to the springs 30 and increasing the force biasing the end pieces 16 and 18 away from each other. The number of such rods 46 will depend on the additional force to be exerted. Each of the fuel rods 46 has a lower end plug 48 bearing on the lower end piece 18. At its upper end, each such rod carries resilient means which will now be described (the arrangement being reversed if required). Referring to FIG. 8 which is a view of the part of the fuel rod shown in a dash dot line in FIG. 7 at an enlarged scale, such resilient means include a tubular push rod 50 whose frusto-conical end part engages in a hole 52 in the bottom wall of the upper end piece 16. Push rod 50 is slidably received on a bolt 54 carried by end plug 56 of rod 46. A helical spring 58 is disposed within the push rod 50 between the end part of the push rod and a spacer 60 retained by an internal shoulder of push rod 50. The spacer 60 prevents loss of the spring 58 before the push rod is mounted on the bolt 54. Each spring 58 transmits from one end piece to the other a force participating in the hold down function and completing the action of the springs 30. Each spring 58 may for example exert a precompression force of 1 decanewton. If each fuel rod of a typical fuel assembly, whose rods are distributed at the 17.times.17 nodes of a square lattice is provided with such resilient means, the total force may reach about 250 daN, i.e. about 25% of the force exerted by a conventional hold down device. Such a contribution allows transmission of part of the hydraulic forces due not only to the bundle of fuel rods, but also to the floating grids 12a. The arrangement which has just been described is applicable not only to "floating grid" assemblies, but also to assemblies in which all grids are fixed to guide tubes.