Patent Number: 048200581
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: "Wear Sleeve for a Control Rod End Plug" by S. Cerni et al, U.S. Ser. No. 634,725, 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 control rod end plug having an asymmetrical configuration which causes coolant axially flowing along the control rod to impart a lateral stabilizing force against the control rod which presses the control rod at its end plug against the wall of a guide thimble within which the rod reciprocably moves and thereby prevent lateral vibration of the rod. 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 those 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 During power operation of the reactor, most of the regulating control rods are maintained substantially withdrawn from the reactor core and thus disposed in withdrawn positions in which the lower end plug tips of the control rods are within the upper ends of the guide thimbles. While in such withdrawn positions, the control rods may experience significant vibration induced by coolant water flow within the guide thimbles which results in oscillatory contact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. Patents 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 Schukei 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 an asymmetrical configuration on the lower end plug of the control rod designed to satisfy the aforementioned needs. Unlike the prior art approaches which prevent vibratory contact with the guide thimble wall by interposing some added device which either maintains continuous contact with the wall or prevents any contact at all from occurring, the present invention merely reshapes one of the basic parts of the control rod; its end plug. Underlying the present invention is the recognition that one of the causes of coolant flow-induced vibration is vortex shedding around the tip of the end plug of the prior design. The prior end plug design, which has a symmetrical configuration about the axial centerline of the control rod, is typical of geometries which tend to promote vortex shedding and flow-induced vibration. By using an asymmetric configuration at the end plug tip, vortex shedding type of flow-induced vibration is greatly reduced. Furthermore, the present invention recognizes that lateral vibration of the control rod due to axial flow of coolant can be prevented by imposing a small, steady-state lateral force on the rod. This force can be produced by the same asymmetrical configuration which reduces vortex shedding. Such end plug shape causes non-symmetric flow velocities around the end plug tip. The magnitude of the lateral force is a function of the coolant axial flow rate and the shape of the end plug tip. Several end plug configurations will achieve these desired results of reduction of vortex shedding type of flow-induced vibration and creation of a lateral, steady-state force on the control rod. 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, an end plug having an asymmetrical configuration attached to an end of each control rod which produces, in response to axial flow of coolant along the control rod and within its respective guide thimble, a lateral steady-state force on the control rod which presses the control rod end plug against a wall of the guide thimble so as to substantially prevent lateral vibration of the control rod due to the axial flow of coolant. Several different asymmetrical designs can be used to achieve non-symmetrical flow velocities around the tip of the end plug which produce the lateral force. In one design, a flat is formed, such as by machining, on one side of an otherwise axially symmetrical tapered outer surface of the end plug. Other designs have either a concave surface formed on one side of the tapered outer surface of the end plug, a tapered configuration which is offset to one side of the axis of the control rod, or a pair of flats on opposite sides of the tapered outer surface which form different angles with the axis of the control rod. 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.