Patent Number: 047986999
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also, in the following description, it is to be understood that such terms as "forward", "rearward", "left", "right", "upwardly", "downwardly", and the like, are words of convenience and are not to be construed as limiting terms. In General Referring now to the drawings, and particularly to FIG. 1, there is shown an overall combination of a fuel assembly, generally designated by the numeral 10, an upper core support plate 12 disposed above and extending across the top of fuel assembly 10, and a spider assembly 14 disposed above the upper core support plate. Each of these components will be described and discussed separately. The fuel assembly 10, being shown in a vertically foreshortened form in FIG. 1, basically includes a lower end structure or bottom nozzle 16 for supporting the assembly on a lower core plate (not shown) in the core region of a reactor (not shown) and a number of longitudinally extending guide tubes or thimbles 18 which project upwardly from the bottom nozzle 16. The assembly 10 further includes a plurality of transverse grids 20 axially spaced along the guide thimbles 18 and an organized array of elongated fuel rods 22 transversely spaced and supported by the grids 20. Finally, the assembly 10 has an instrumentation tube 24 located in the center thereof and an upper end structure or top nozzle 26 attached to the upper ends of the guide thimbles 18. With such arrangement of parts, the fuel assembly 10 forms an integral unit capable of being conventionally handled without damaging the assembly parts. Since the fuel assembly 10 does not form a part of the present invention and is merely for illustrational purposes, any further description thereof is unnecessary and thus will not be given. For a more detailed description of the fuel assembly 10, reference should be made to the pending patent application of Robert K. Gjertsen et al, entitled "Nuclear Reactor Fuel Assembly with Improved Top Nozzle and Hold Down Means"; filed Oct. 17, 1983; and assigned U.S. Ser. No. 542,625. The upper core support plate 12, being conventional, extends across the top of the fuel assembly 10 as well as across the top of other identical fuel assemblies (not shown) arranged within the core. For the sake of brevity, it should suffice to say that the core plate 12 has a multiplicity of flow openings 28 (only one of which is seen in FIG. 1) to allow coolant to pass upwardly through the core, and that at least some of these openings are aligned over the guide thimbles 18 such that control rods 30 can pass down through the core plate 12 and be inserted into the guide thimbles 18 of the fuel assembly 10. Connected to the upper ends of the control rods 30 is the spider assembly 14 which supports the rods for vertical movement within the guide thimbles 18 by a conventional drive mechanism (not shown). In the illustrated embodiment, the spider assembly 14 is disposed above the core plate 12 and is restably supported thereon when the control rods 30 are fully inserted in the guide thimbles 18 as seen in FIG. 1. In other arrangements, the spider assembly is located between the bottom of the upper core plate and the top of the fuel assembly. As best seen in FIGS. 2 and 3, the spider assembly 14 basically includes a central hub 32, a plurality of vanes 34 radially extending outwardly from the hub 32, and a plurality of fingers 36 associated with the vanes 34 for connection with the upper ends of the control rods 30. The central hub 32 is preferably in the form of an elongated cylindrical tube having on its upper end an internally threaded segment 40 for connection with the drive mechanism (not shown) which vertically raises and lowers the spider assembly 14 and the control rods 30 therewith in a conventional manner. The tubular hub 32 houses a common load absorbing mechanism which includes a coil spring 42 held in a state of compression and a nipple 44 which seats in a shallow cavity (not shown) provided in the top surface of the core plate 12 to assist in proper alignment of the control rods 30 within the core plate openings 28 and the guide thimbles 18. As is well known, the primary purpose of such a load absorbing mechanism is to prevent shock loading of the core plate 12, as well as the fuel assembly 10, as the spider assembly 14 abuts the top of the core plate 12 when the control rods 30 are fully inserted in the guide thimbles 18. As seen in FIG. 3, each control rod 30 is supported by one of the elongated fingers 36 of the spider assembly 14. The lower end 46 of each finger 36 is drilled and internally threaded for connection with the upper end 48 of one control rod 30. Each control rod 30 includes an elongated tubular cladding member 50 and an end plug 52 attached to the lower end 54 (see FIG. 4) of the cladding member. In some control rod designs, a plurality of pellets 56 of neutron absorbing material are arranged in an end-to-end stack within the cladding member 50. In other control rod designs, the pellets are of a material which does not absorb neutrons (water displacer rods) and the control of the reactor is achieved by the displacment of the water moderator as described in the above-mentioned U.S. Pat. No. 4,432,934. As mentioned earlier, the power level of the reactor is usually regulated by the insertion and withdrawal of the control rods 30 into and from the guide thimbles 18. The control rods 30 are fully inserted during reactor shutdown, and some are withdrawn when the reactor is operating at full power. However, even in their withdrawn positions, the control rods 30 still extend into the upper end of the guide thimbles 18 a short distance, such as six inches or so. When the control rods 30 are fully inserted into the guide thimbles 18, and thus within the reactor core (not shown), they will generate heat. Provision is made for cooling the control rods to prevent the pellets therein from melting. Typically, the lower portions of the guide thimbles have openings (not shown) whereby some of the pumped coolant entering the bottom of the fuel assembly 10 is diverted into the thimbles 18 and flows upwardly therein over the control rods 30. As previously mentioned, particularly when the control rods 30 are in their withdrawn positions, the flow of water upwardly through the thimbles 18 past the partially inserted control rods induces vibratory motion in the lower ends of the rods which, absent the present invention, produces vibratory contact of their end plugs 52 with the internal wall of the thimbles 18. Wear Sleeve on Control Rod End Plug Heretofore, the vibratory contact has taken place at the interface of dissimilar materials and excessive wear has occurred as a result. For instance, in certain reactors stainless steel control rod lower tips contact a zircaloy thimble wall, producing excessive wear of the zircaloy wall. In other reactors the thimbles are made of stainless steel and the control rods of zircaloy. Irrespective of which material is used to fabricate which part, it is vibratory contact of two dissimilar materials which causes excessive wear. Referring now to FIGS. 4 and 5, the present invention provides a wear sleeve 58 at the contact interface of the control rod 30 with the guide thimble 18 which is composed of the same material as the guide thimble. In the illustrated embodiment of the fuel and spider assemblies 10,14, the guide thimble 18 is composed of stainless steel, while the cladding member 50 and end plug 52 of the control rod 30 are formed of zircaloy. Therefore, the wear sleeve 58 is composed of stainless steel. As in the past, the end plug 52 is rigidly attached and sealed to the lower end 54 of the cladding member 50 by a girth weld 60. Now, the weld 60 is formed at the circumferential location of contact between the lower edge 62 of the cladding member 50 and a shoulder 64 on the end plug 52 formed at the merger of a narrow annular segment 66 and an inner end 68 of the plug. The outside diameter of the plug segment 66 is substantially the same as the outside diameter of the cladding member 50. As seen in FIG. 4, the wear sleeve 58 is inserted about the end plug 52 to a position where an inner end 70 of the sleeve abuts another shoulder 72 on the end plug 52 which is located a short distance from the girth weld 60. The shoulder 72 is formed at the merger of the annular segment 66 and an elongated body 74 of the end plug which has an outside diameter generally the same as that of the inner end 68 of the plug, that being slightly less than the inside diameter of the cladding member 50. In such position, the wear sleeve 58, having a cross sectional shape concentric with that of said end plug body 74, encircles the elongated plug body, as seen in FIG. 5, and extends along it through approximately one-third of the length of the body toward an outer tapered end 76 of the plug 52, as seen in FIG. 4. The wear sleeve 58 has generally the same outside diameter as the cladding member 50. Attachment of the sleeve 58 to the end plug 52 is accomplished by an interlock connection, generally designated 78, composed of a single circumferential countersink or protuberance 80 swaged or mechanically roll formed at a location intermediately between the inner end 70 and an outer end 82 of the sleeve 58. The protuberance 80 extends into a circumferential groove 84 machined or otherwise suitably formed in the plug body 74 at a location spaced outwardly from the shoulder 72 on the annular segment 66 of the plug 58. A single swaged protuberance 80 is desirable in order to eliminate axial thermal mismatch between the dissimilar materials of the sleeve 58 and the plug 52. Also, an interlock connection is used, rather than a weld, because of the difficulty of welding dissimilar materials together. In the preferred embodiment, the sleeve 58 snugly contacts the end plug 52: however, the sleeve 58 may only loosely contact the end plug in thereby providing a small diametric gap 85 between the inner wall of the sleeve 58 and the outer surface of the end plug 52, as seen in FIG. 6. Such gap would allow some relative diametric movement beween the wear sleeve and the end plug in thereby providing a dampening mechanism if vibration occurred. With such loose bit arrangement, when the end plug impacts the thimble, water is "squeezed" out as the sleeve moves relative to the end plug. This action would occur for every impact thereby providing a dampening effect which should reduce the impact force and, hence, wear. The vibratory contact between the guide thimble 18 and the control rod 30 now takes place at the interface provided by the exterior surface 86 of the wear sleeve 58 mounted on the end plug 52. Since in the illustrated embodiment, a stainless steel-to-stainless steel wear couple is provided at the interface, the degree of wear should be minimized over that experience heretofore. It is thought that the wear sleeve of the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrangement thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely a preferred or exemplary embodiment thereof.