Patent Number: 052079805
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear fuel assemblies and, more particularly, is concerned with top nozzle-mounted replacement guide pin assemblies for alignment of fuel assemblies with an upper core plate. 2. Description of the Prior Art A pressurized water nuclear reactor (PWR) typically includes a nuclear reactor core composed of a large number of nuclear fuel assemblies supported between vertically spaced upper and lower core plates. The upper and lower core plates are supported by a core support barrel which surrounds the reactor core. The fuel assemblies are arranged in vertical orientations and in side-by-side relationship. A typical fuel assembly includes a plurality of fuel rods held in a stationary array by a skeleton of the fuel assembly. The fuel assembly skeleton is composed of a plurality of control rod guide thimbles disposed between and extending parallel to the fuel rods, a plurality of grids mounted to and axially spaced along the guide thimbles and supporting the array of fuel rods, and top and bottom nozzles disposed adjacent to and attached at opposite ends of the guide thimbles. The top nozzle of the fuel assembly is an enclosure or housing formed by a transversely extending lower adapter plate and an upper annular flange with an upstanding sidewall extending between and integrally interconnecting the adapter plate and flange at their respective peripheries. The lower adapter plate is provided with one plurality of holes to permit the flow of coolant upward through the top nozzle and another plurality of holes to receive the upper ends of the guide thimbles which, in turn, are attached to the lower adapter plate. The upper annular flange defines a central top opening of the top nozzle within which may be disposed a rod cluster control assembly (RCCA). The RCCA contains a plurality of control rods which can be inserted into and withdrawn from the guide thimbles of the fuel assembly through the set of holes in the lower adapter plate receiving the upper ends of the guide thimbles. The upper annular flange also has two holes disposed in two of the diagonal corners of the top nozzle which mate with guide pins mounted in the upper core plate. The guide pins disposed between the upper core plate and top nozzle of each fuel assembly provide proper alignment and engagement of the fuel assembly with the upper core plate so that the guide thimbles of the fuel assembly will extend vertically in alignment with the control rods for receiving the control rods from above. In the event a guide pin should incur damage affecting the proper alignment of the fuel assembly, this may adversely impact the capability to move the control rods into the guide thimbles of the fuel assembly. Thus, it would be highly desirable to be able to take steps to replace the damaged guide pin. However, attempts to perform in-situ guide pin repair and replacement have not been pursued in view of the following obstacles which are a result of prior art guide pin designs. First, all procedures must be performed remotely underwater due to unacceptably high levels of radiation. Second, duplication of original equipment guide pin shank preload via shrink fitting is practically impossible to achieve since it was carried out by submersion of the pin shank into liquid nitrogen. Third, access to accommodate guide pin nut removal is difficult since the nut is recessed in the top side of the upper core plate. Fourth, extensive tooling design and development is necessary to qualify tooling and repair procedures. Consequently, there is a need for improvements in the design of the guide pins for avoiding the abovedescribed drawbacks. SUMMARY OF THE INVENTION The present invention provides a top nozzle-mounted replacement guide pin assemblies designed to satisfy the aforementioned needs. The replacement guide pin assembly of the present invention emulates the alignment and function of the original equipment guide pin while minimizing the amount of repair performed on the reactor internals upper core plate. Since the replacement guide pin assembly of the present invention is installed in the fuel assembly top nozzle, the design, tooling, financial and scheduling concerns associated with installing replacement guide pin assemblies in the upper core plate are alleviated. Accordingly, the present invention is directed to a guide pin assembly for aligning a nuclear fuel assembly with an upper core plate of a nuclear reactor core. The guide pin assembly comprises: (a) an elongated guide pin body having a base portion being insertable within a hole in the top nozzle, the guide pin body having a longitudinal axis; (b) an expandable body insertable within the top nozzle hole with the base portion of the guide pin body and capable of expanding radially outwardly relative to the longitudinal axis of the guide pin body to provide an interference fit with the top nozzle; and (c) means insertable within the top nozzle hole with the expandable body and the base portion of the guide pin body for threading with the base portion of the guide pin body to produce a predetermined displacement of the threading means relative to the guide pin body along the longitudinal axis thereof sufficient to impart a radially and outwardly directed force on the expandable body to produce expanding thereof within the hole of the top nozzle into the interference fit with the top nozzle and thereby secure the guide pin body to the top nozzle. More particularly, two embodiments of the guide pin assembly are disclosed being mountable to the top nozzle of the nuclear fuel assembly. The first embodiment of the guide pin assembly comprises: (a) an elongated guide pin body having a longitudinal axis, the guide pin body also having a lower expandable base being insertable within a hole in the top nozzle and capable of expanding radially outwardly relative to the longitudinal axis to provide an interference fit with the top nozzle; (b) means insertable within the hole in the top nozzle and interfitted with the guide pin body for imparting a radially and outwardly directed force on the lower expandable base to expand the base within the hole of the top nozzle into the interference fit with the top nozzle and thereby secure the guide pin body to the top nozzle in response to a predetermined displacement of the imparting means relative to the guide pin body along the longitudinal axis thereof; and (c) means insertable within the hole in the top nozzle and interfitted with the imparting means and the guide pin body for threading into the guide pin body to cause the predetermined displacement of the imparting means. The second embodiment of the guide pin assembly comprises: (a) an elongated guide pin body having a lower attachment base being insertable within a hole in the top nozzle, the guide pin body having a longitudinal axis; (b) an expandable insert insertable within the hole in the top nozzle and interfitted about the lower attachment base of the guide pin body and capable of expanding radially outwardly relative to the longitudinal axis of the guide pin body to provide an interference fit with the top nozzle; and (c) means insertable within the hole in the top nozzle and interfitted with the guide pin body and the expandable insert for threading with the lower attachment base of the guide pin body to produce a predetermined displacement of the threading means relative to the guide pin body along the longitudinal axis thereof sufficient to impart a radially and outwardly directed force on the expandable insert to produce expanding thereof within the hole of the top nozzle into the interference fit with the top nozzle and thereby secure the guide pin body to the top nozzle. These and other features and advantages 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.