Patent Number: 047284838
Section: summary

CROSS REFERENCE TO RELATED APPLICATION Reference is hereby made to the following copending application dealing with related subject matter and assigned to the assignee of the present invention: "Position Sensing Apparatus" by George S. Jewell, assigned U.S. Ser. No. 678,520 and filed Dec. 5, 1984 now U.S. Pat. No. 4,583,297. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an apparatus used in an automated system for inspecting a fuel assembly for envelope, channel spacing and length and also for correcting error in the inspection fixture of the apparatus. 2. Description of the Prior Art In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend above and below the opposite ends of the fuel rods. The fuel rods which contain fissile material are grouped together in a closely-spaced array within each fuel assembly and the fuel assemblies, in turn, are mounted in side-by-side closely-spaced relationship with one another between the upper and lower core plates so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. In view of the densely-packed condition of the fuel rods and fuel assemblies in the core, the dimensional standards of envelope and length of each fuel assembly and the channel spacing between the adjacent fuel rods of each fuel assembly must be met within very close tolerances. Thus, at the completion of manufacture of each fuel assembly, quality control inspections are carried out to determine whether the fuel assembly meets the aforementioned dimensional standards. Currently, the fuel assembly quality inspection is performed at three separate stations: (1) envelope measurement; (2) channel spacing; and (3) length measurement. At the envelope measurement station, the out-of-straightness of the fuel assembly is quantified. The sides of a fuel assembly are normally not perfectly straight. The fuel assembly commonly exhibits a slight bow and twisting. Quantifying this behavior is performed by measuring the relative position of the grids to each other and inspecting for excessive displacements. The current method of envelope measurement uses twelve LVDT sensors mounted in a configuration of three sensors per side. A set of distance measurements is taken at each grid location to signify whether the grid is located either left/right or back/front of the center of the fuel assembly. At the channel spacing station, the distance between adjacent fuel rods within a fuel assembly is checked. Currently, an operator manually pulls a strain gauge probe through a channel and a computer translates the sensor output into distance measurements. At the length measurement station, the fuel assembly length is measured using a stick micrometer. The fuel assembly is set upright on a level table and an inspector measures the distance from the table surface to the bottom edge of the top nozzle. From the foregoing brief description of current practices, it will be readily understood that these stations are manual in nature, requiring an inspector to monitor equipment and process data. Consequently, a need has emerged to improve and automate the way in which fuel assembly inspection is carried out. SUMMARY OF THE INVENTION The present invention provides an apparatus for use in an integrated fuel assembly inspection system designed to satisfy the aforementioned needs. In contrast to the previous practices, the fuel assembly inspection apparatus of the present invention allows a fully integrated system wherein all inspections are performed in one station in a completely automated manner and all measurements are made using non-contact sensing techniques. Also, for increased accuracy of fuel assembly envelope measurement, the apparatus facilitates performance of correction for inspection fixture error on a real-time basis instead of on a sampled basis. Accordingly, the present invention is directed to fuel assembly inspection apparatus, comprising: (a) an elongated fixture mounted in a stationary upright position; (b) upper means mounted to an upper portion of the fixture and lower means mounted adjacent to a lower portion of the fixture for supporting a nuclear fuel assembly therebetween and extending along the fixture; (c) a bottom carriage having a central opening adapted to receive the fuel assembly therethrough when supported between the upper means and the lower means such that the bottom carriage will surround all sides of the fuel assembly, the bottom carriage being mounted to the fixture for generally vertical movement along the fixture and the fuel assembly; and (e) drive means for selectively moving the bottom carriage. In addition, means are disposed on the bottom carriage for measuring the fuel assembly envelope when the bottom carriage is moved to and stationed at selected axial positions along the fuel assembly. The envelope measuring means includes a single-axis positioning table disposed on each side of the bottom carriage adjacent a side of the fuel assembly, a proximity sensor mounted on each positioning table for movement along the adjacent side of the fuel assembly, and power means coupled to each sensor for stationing the sensor at a home position while the bottom carriage is moving along the fuel assembly and for sweeping the sensor relative to the side of the fuel assembly away from and back to the home position once the carriage is positioned at one of the selected axial positions along the fuel assembly. Further, means are disposed on the upper means and the bottom carriage for continuously monitoring fixture out-of-straightness and performing correction of the envelope measurement in response thereto. The monitoring and correction performing means includes a pair of X-Y axes lasers mounted on one of the upper means and the bottom carriage adjacent the fuel assembly, and a pair of matched X-Y photodetectors mounted on the other of the upper means and the bottom carriage adjacent the fuel assembly. The respective lasers provide straight line reference used to excite the corresponding photodetectors. The pairs of lasers and photodetectors facilitate measuring of both translational and rotational motion of the bottom carriage as the same moves up along the fuel assembly for facilitating adjustment of the envelope measurment for any fixture error at each of the axial positions along the fuel assembly. Still further, means are disposed on the bottom carriage for measuring channel spacing between fuel rods of the fuel assembly. The channel spacing measuring means includes a single-axis positioning table located on each of a pair of adjacent sides of the fuel assembly, a capacitive probe mounted on each of the tables for movement along the side of the fuel assembly, and motive means for driving the probe to specified channels locations along the fuel assembly side for taking channel spacing measurements once the bottom carriage is positioned at one of the selected axial positions along the fuel assembly. Finally, means are disposed on the bottom carriage and the fixture for measuring fuel assembly length when the bottom carriage has been moved between the bottom and top nozzles of the fuel assembly. The fuel assembly length measuring means includes a photoswitch mounted on each side of the bottom carriage adjacent a side of the fuel assembly and operable to detect an edge of the respective bottom and top nozzles of the fuel assembly, and means forming an optical scale mounted on the fixture and the bottom carriage for determining the position of the carriage as it moves along the fuel assembly when each photoswitch detects the respective edges of the bottom and top nozzles for deriving the length of the fuel assembly. 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.