Patent Number: 047986999
Section: summary

CROSS REFERENCE TO RELATED APPLICATION Reference is hereby made to the following co-pending U.S. patent application dealing with subject matter related to the present invention: "Control Rod End Plug with Stabilizing Configuration" by John F. Neilson et al, U.S. Ser. No. 634,729, filed July 26, 1984. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear reactors having rods, such as of the control or water displacer type, reciprocable into and out of a reactor core and, more particularly, is concerned with a wear sleeve composed of the same material as the guide thimble within which the rod reciprocably moves and attached to an end plug on the rod composed of material dissimilar to the guide thimble material such that the wear sleeve provides the contact interface between the rod end plug and the guide thimble. 2. Description of the Prior Art In a typical nuclear reactor, the reactor core is composed of a plurality of elongated fuel assemblies each of which contains a plurality of elongated fuel elements or rods. A liquid coolant is pumped upwardly through the core in order to extract heat generated in the core for the production of useful work. The heat output of the core is usually regulated by the movement of control rods containing neutron absorbing material such as B.sub.4 C, or by movement of water displacer rods such as the ones described in U.S. Pat. No. 4,432,934. In reactors of the pressurized-water type, each fuel assembly typically includes a plurality of cylindrical guide tubes or thimbles through which the cylindrical control rods or water displacer rods are reciprocably moved. Some of the coolant flow is usually diverted into the lower end of the guide thimble in order to cool the control rod. The control rod ordinarily generates heat in the nuclear transformation associated with its neutron absorbing function. During power operation of the reactor, most of the regulating control rods are maintained substantially withdrawn from the reactor core and thus disposed in withdrawn positions in which the lower end plug tips of the control rods are within the upper ends of the guide thimbles. While in such withdrawn positions, the control rods may experience significant vibration induced by coolant water flow within the guide thimbles which results in oscillatory contact of the rod end plug tips against the internal wall surfaces of the guide thimbles and wear on these surfaces. Continuous wear of the guide thimble walls can lead to perforation of the thimbles and significant weakening of the fuel assembly structure. Thus, there has arisen the need to significantly mitigate the affects of the control rod vibrations so as to bring guide thimble wear under control. Two approaches to solving this problem are disclosed in U.S. Pat. Nos. to Schukei et al (U.S. Pat. No. 4,292,132) and Verdone (U.S. Pat. No. 4,311,560). Both of these approaches have as a common objective the elimination of wear on the guide thimble wall by preventing vibratory contact of the control rod against its adjacent guide thimble wall. In the Verdone approach, a spring device is added to the lower end of the control rod which provides a uniform, resilient interference fit against the guide thimble wall and thereby prevents the rod tip from impacting the guide thimble wall. In the Schukel et al approach, the control rod has a hydraulic bearing formed at its lower tip which produces forces which counteract forces tending to drive the control rod tip against the guide thimble wall. In such manner, contact of the control rod against the guide thimble wall and resultant wear thereon are substantially avoided. While the approaches taken in these two patents operate reasonably well and achieve their objectives under the range of operating conditions for which they were designed, a need exists for an alternative approach to the wear problem which is simplier and less costly in its design and construction and is more reliable in its performance over the long term. SUMMARY OF THE INVENTION The present invention provides a wear sleeve on the lower end plug of the rod designed to satisfy the aforementioned needs. Unlike the prior art approaches which prevent vibratory contact with the guide thimble wall by interposing a device which either maintains continuous contact with the wall or prevents any contact at all from occurring, the present invention allows vibratory contact with the guide thimble wall, but provides the insert sleeve at the contact interface. Since the rate of wear is influenced to a large degree by the materials at the contacting interface, the sleeve in being composed of the same material as the guide thimble greatly reduces wear potential at the contact interface over that experienced heretofore between the dissimilar materials of the thimble and the rod lower end plug. The wear sleeve is simply and reliably attached to the rod end plug by means of a single circumferential countersink or protuberance swaged or mechanically rolled into the wear sleeve so as to extend into an annular groove circumferentially formed in the rod end plug. Accordingly, the present invention sets forth in a nuclear reactor including a plurality of upstanding guide thimbles, a plurality of control rods received in the guide thimbles and means supporting the control rods for movement relative to the thimbles between inserted and withdrawn positions, a wear sleeve disposed on an end plug attached to an end of each tubular cladding member of the control rod for providing a contact interface between the control rod and its respective guide thimble. On the one hand, the end plug and cladding member of the control rod are formed of the same material, such as zircaloy, while, on the other hand, the wear sleeve and guide thimble are formed of the same material, such as stainless steel. The end plug is rigidly attached and sealed to the end of the cladding member by a girth weld. Preferably, the wear sleeve is inserted about the end plug to a position where an inner end of the sleeve is spaced a short distance from the girth weld. The wear sleeve has generally the same outside diameter as the cladding member. Attachment of the sleeve is accomplished by an interlock connection in the form of a single circumferential countersink swaged mechanically rolled at a location intermediately between the ends of the sleeve into a circumferential groove formed near the inner end of the end plug. A single swaged countersink or protuberance is desirable in order to eliminate axial thermal mismatch between the dissimilar materials of the sleeve and the end plug. In the alternative, the sleeve is mechanically rolled so as to capture the sleeve on the end plug and provide a diametric gap therebetween such that some relative diametric movement is available in creating a dampening effect. An interlock connection is used, rather than a weld, because dissimilar metals cannot be welded together. These and other advantages and attainments of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.