Patent Number: 043269214
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIG. 1 a conventional design of fuel assembly 10 including a square array of fuel rods 12 held in spaced relationship with each other by egg-crate grids 14 and a retention grid 16 at its base. The assembly may comprise a 14.times.14 square array of fuel rods of the type shown, for example, and extend for about 13 feet in length. The assembly includes control rod guide thimbles 18 which are interspersed among the fuel rods and appropriately secured to the grids by mechanical expansion of the material or welding in a well-known manner. The upper end of each guide thimble is fixed to an extension 20, as by screw threads, and the extension itself terminates in an opening 22 formed in upper core plate 24. An upper end plate 26 is immovably fixed by a locking arrangement 28 to the extension of a centrally disposed guide thimble 30, and to the extensions 20 on guide thimbles 18. Hold-down plate 34 is movably mounted on central extension 30, as shown, and on extensions 20, and is biased into contact with the underside of core plate 24 by springs 36. This arrangement accommodates hydraulic lifting forces acting on the assembly together with those forces resulting from thermal growth of the assembly during reactor operation. To control the fission process in each fuel assembly, control rods 38 driven from a common drive mechanism (not shown) are arranged for gradual insertion and withdrawal in the guide thimbles in a manner also well known in the art. At startup, the control rods are inserted to the minimum extent, i.e., about 12" into the upper end of the guide thimbles. During reactor operation, coolant circulated upwardly through the fuel assemblies, also flows through the guide thimbles which are open at both ends. Although a control rod is always positioned at some level in its corresponding guide thimble, the hydraulic forces imposed on the control rod by the coolant as it enters the space between the control rod and guide thimble inner walls and flows out the upper end, induce vibrations in the control rod of sufficient magnitude to cause wear of the guide thimble walls. As indicated in the foregoing discussion, wear of the guide thimbles occurs at the point where the tip of a control rod interfaces with the guide thimble inner walls. As reactor operation takes place, the control rods are withdrawn to about twelve inches from the top of the assembly and remain there during the entire cycle, and are inserted at shutdown. Hydraulically induced vibrations over this time period cause wear along a substantial length of the guide thimble inner walls. FIGS. 2-4 illustrate an improved guide thimble designed to alleviate guide thimble wear during reactor operation. As shown, the guide thimble may be a Zircaloy or stainless steel tube 18 attached to grid 14 through the intermediary of a stainless steel sleeve 40. The sleeve and guide thimble material are expanded outwardly at 41 by a special tool both above and below the straps of each grid 14 to provide a relatively rigid skeleton structure for the fuel assembly and to prevent axial movement of the grids on the guide thimbles. Extension 20 is attached to the upper end of the guide thimble in much the same way by swaging the thimble material into grooves 42 machined in the extension inner walls. To eliminate or substantially reduce wear between each control rod 38 and guide thimble inner walls, a series of local deformations or insets 44 are made in the guide thimble walls. They preferably are formed at 90.degree. intervals around the guide thimble and are spaced along the guide thimble length at about six inch intervals. The insets or indentations made in the guide thimble walls are each about 1.32 inch long and about 0.32 inch wide and project inwardly from the guide thimble walls about 0.030 inch. With this radial reduction in space, the clearance between the control rod and guide thimble walls has been changed from 0.087 inch to 0.027 inch. This reduction in the local diametrical clearance serves to reduce the normal force caused by the lateral vibrating motion of the control rod on the guide thimble thus minimizing the opportunity for wear to take place. A major advantage derived from utilizing insets, and particuarly insets of the design disclosed herein is that they promote control rod wear, although minimal, against the insets in a line contact mode rather than point contact heretofore experienced in prior designs. The invention therefore minimizes wear by reducing the load or magnitude of forces available to act against the insets, and by encouraging line contact between the coacting parts. If necessary or desirable, additional insets can be remotely formed on the guide thimbles after irradiation in a reactor. In the modification of FIG. 4, in lieu of forming insets directly in the guide thimble material, a stainless steel, or other material, tube or liner 46 may be inserted in the end of the guide thimble and for about 12-14 percent of its length. The liner is swaged into grooves 42 in the extensions. To provide line contact with the control rod, the liner material is deformed or indented inwardly in the same manner as that described in relation to FIGS. 2 and 3. The insets may be chrome plated to enhance the wear properties if deemed desirable or necessary. It will be apparent that many modifications and variations are possible in light of the above teachings. It therefore is to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.