Patent Number: 047479965
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and method for adjusting the level of a nuclear fuel rod in a nuclear reactor fuel assembly. 2. Description of the Prior Art Conventional fuel assemblies of the type used in commercial nuclear reactors for generating electric power typically comprise a plurality of fuel rods, with each rod including a stack of fuel elements mounted inside a metal tube which is sealed by end plugs. The fuel rods are held in a laterally spaced apart generally rectangular arrangement by a supporting "skeleton" which consists of a number of grids which are spaced at lengthwise intervals along the length of the assembly. The fuel rods are individually mounted to the grids by means of spring mountings, which permit limited axial movement of each rod. Control rod guide thimbles extend parallel to the rods and serve to interconnect the grids, and the guide thimbles extend axially beyond the ends of the rods for mounting top and bottom nozzles. It is important for the proper functioning of the fuel rods that all of the rods be positioned in the assembly at the same level, and so that the ends of all of the rods lie in the same plane at each end of the assembly. Thus each end of each rod should be spaced from the adjacent nozzle a predetermined gap distance. However, when the fuel rods are axially loaded into the assembly, it is difficult to insure that all rods are positioned at the proper level, and it is common for certain ones of the rods to be loaded at an improper level. When this occurs, the nozzle at one end of the assembly must be disassembled so as to permit the misaligned fuel rods to be manipulated and the gap distance to be corrected. As will be apparent, the required disassembly and subsequent reassembly of the nozzle is a time consuming and thus expensive operation. The prior U.S. patent to Doss et al, No. 4,265,010, discloses a tool for adjusting the level of a fuel rod in a fuel assembly, and which is designed to avoid the necessity of disassembling the nozzle to correct the level of misaligned rods. In particular, the tool includes one end which is insertable between the nozzle and the exposed ends of the fuel rods, and the insertable end includes two plates which may be laterally separated by actuating a threaded interconnection between the plates, to thereby move the misaligned fuel rod with respect to the nozzle. While this tool represents a significant improvement in the art, the tool must be carefully used to insure that the final spacing is correct, and also to insure that no lateral forces are imparted to the rod which could adversely effect its lateral separation from the adjacent rods in the assembly. It is accordingly an object of the present invention to provide an apparatus and method for adjusting the elevation of a fuel rod in a nuclear reactor fuel assembly, and which is adapted to efficiently adjust the level of individual rods without requiring the disassembly of an end nozzle. It is also an object of the present invention to provide an apparatus and method of the described type which is adapted to adjust the gap between a misaligned rod and the nozzle to a predetermined maximum distance, which corresponds to the proper gap distance. It is still another object of the present invention to provide an apparatus and method of the described type which does not exert an unacceptable lateral force on the fuel rod during axial movement thereof. SUMMARY OF THE INVENTION These and other objects and advantages of the present invention are achieved in the embodiment illustrated herein by the provision of an apparatus and method which includes a tool comprising an expander having two elongate generally parallel and laterally spaced apart arms extending in a longitudinal direction, and with the two arms including first laterally aligned ends which are fixedly interconnected to each other and opposite laterally aligned free ends. At least one of the arms is readily flexible and so as to permit lateral movement of the free ends toward and away from each other. The tool also includes an actuator operatively mounted to the expander so as to be disposed between the two arms and so as to be movable with respect to the expander. Cooperating surface means is mounted to the arms of the expander and to the actuator for laterally separating the free ends of the arms to a predetermined maximum distance upon movement of the actuator in a predetermined direction with respect to the arms. In a preferred embodiment, the cooperating surface means comprises an opposing pair of inclined surfaces mounted to the arms, and oppositely facing diverging surfaces mounted to the actuator. Also, the cooperating surface means further includes an opposing pair of generally parallel surfaces mounted to the arms adjacent the laterally closest ends of the pair of inclined surfaces, and the actuator includes a pair of parallel outer surfaces mounted adjacent the laterally most separated ends of the diverging surfaces. Thus the arms may be spaced apart by said predetermined maximum distance when the pair of parallel outer surfaces of the actuator is positioned between the generally parallel surfaces of the arms. The method of the present invention includes the steps of positioning the above described tool between a nozzle of the fuel assembly and the end of a fuel rod located at an incorrect level which is unduly close to the nozzle, and with the free ends of the arms disposed in an initial position wherein the free ends are relatively closely spaced and disposed directly between the nozzle and the end of such fuel rod. The actuator is then longitudinally moved with respect to the arms so as to laterally separate the arms to an expanded position wherein the free ends are separated said predetermined maximum distance. Thereafter, the actuator is moved in the opposite direction and the tool is removed from the fuel assembly. The method preferably also includes the step of maintaining the free ends of the arms substantially parallel to each other during movement from the initial position to the expanded position so as to avoid lateral forces being imparted to the misaligned fuel rod. This may be accomplished by manually pressing the arms toward each other so that the arms flex and the free ends assume a parallel relationship during such movement.