Patent Number: 048658006
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: "Apparatus for Integrated Fuel Assembly Inspection System" by Hassan J. Ahmed et al, assigned U.S. Ser. No. 855,266 and filed Apr. 24, 1986, now U.S. Pat. No. 4,728,483. 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 and method for inspecting the dimensional characteristics of fuel assembly grids. 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 a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the opposite ends of the fuel rods. The grids as well known in the art are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. Conventional designs of grids include a multiplicity of interleaved inner straps having an egg-crate configuration designed to form cells which individually accept the fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and relatively rigid protrusions (called dimples) formed into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell. Additionally, outer straps are attached together and peripherally enclose the inner straps to impart strength and rigidity to the grid. The outer straps conventionally have springs integrally formed into the metal thereof which project into respective ones of the cells disposed along the perimeter of the grid. The manufacture of a fuel assembly grid is an intricate operation, requiring not only the assembling of the straps in interleaved fashion to form the grid but also their retention together in precise positions relative to one another during subsequent welding thereof. The newly-manufactured grid must meet high standards in terms of its cell size (that is, the distance between opposite springs and dimples within each cell), envelope, squareness and dimple perpendicularity for it to be able to properly perform its function in the fuel assembly. Thus, the grid must be meticulously inspected to ensure that such standards are met. Heretofore, grid inspection has required the performance of a series of time-consuming, essentially manual, procedures. Grid inspection, as practiced heretofore, has constituted an impedient to improvement of overall productivity of fuel assembly manufacture. Consequently, a need has emerged to improve and automate the way in which fuel assembly grid inspection is performed. SUMMARY OF THE INVENTION The present invention provides a fuel assembly grid inspection apparatus and method designed to satisfy the aforementioned needs. In contrast to the previous manual procedures, the apparatus and method of the present invention employ a precision measurement device in conjunction with an automated universal grid fixture. The measurement device measures the grid by viewing it, rather than by contacting or touching it as in the case of one prior art measurement machine disclosed in U.S. Pat. No. 4,007,544 to Kirby et al, being assigned to the assignee of the present invention. The measurement device and the automated universal grid fixture of the present invention together, under software control, can accomplish inspection of the dimensional characteristics of a wide variety of fuel assembly grid designs in terms of dimple perpendicularity in X-Y directions, cell size in X-Y directions, vane position, squareness and envelope. Specialized application software is utilized wherein there is an individual program for each different grid design inspected. Accordingly, the present invention is directed to a fuel assembly grid inspection apparatus, comprising the combination of: (a) a precision noncontact measurement device having illuminating means and viewing means defining an inspection field of view, the viewing means being adapted to view and record an image of the fuel assembly grid located in the field of view to provide information about the grid from which measurements can be calculated, and (b) a fixture adapted to support the grid within the inspection field of view such that portions of the fixture which project into the field of view to support the grid are substantially transparent to the viewing means of the measurement device. More particularly, the measurement device of the apparatus includes a stationary base and an inspection platform movable in the Y direction with respect to the stationary base. The viewing means of the device is disposed in spaced relation above the inspection platform and movable in X and Z directions with respect to the stationary base. The fixture of the apparatus includes a mounting base having first and second pairs of opposing portions bounding the perimeter of the inspection field of view. The mounting base is supported by the inspection platform of the measurement device and movable therewith. Also, the fixture includes first and second pairs of upright grid supports, and guide means mounted along the opposing mounting base portions of the first pair thereof. Each of the pairs of grid supports is mounted to the guide means in spaced relation to one another for adjustable slidable movement therealong. Each of the grid supports has thereon one of the substantially transparent grid engaging portions which project into the field of view to support the grid. Still further, the fixture includes a pair of extendable and retractable actuators. One actuator is mounted on one of the opposing mounting base portions of the first pair thereof and the other is mounted on one of the opposing mounting base portions of the second pair thereof. The actuators are actuatable to locate the grid in a desired position with respect to the inspection field of view by causing movement of the grid in X and Y directions. Also, a plurality of sensors are provided on the fixture. One sensor is mounted on the other of the opposing mounting base portions of the first pair thereof and the other being mounted on the other of the opposing mounting base portions of the second pair thereof. The sensors are responsive to contact by the grid when the latter has been moved to its desired position with respect to the inspection field of view. Also, the present invention is directed to a fuel assembly grid inspection method, comprising the steps of: (a) defining an illuminated inspection field of view; (b) supporting in unobstructed relationship within the field of view a fuel assembly grid of a design having known standard measurements by using a fixture whose portions which project into the field of view to support the grid therein are substantially transparent to the field of view; (c) viewing the grid within the field of view; and (d) recording an image thereof to provide information about the grid from which actual measurements can be calculated and compared to the known standard measurements for the particular grid design. More particularly, the supporting step includes providing a fixture universally adapted for supporting within the inspection field of view any one of a plurality of fuel assembly grids of different designs having different known standard measurements. Additionally, the viewing step includes maintaining a video camera and lens system pointed toward the field of view and the grid supported therein, and moving at least one of the grid and the video camera and lens system relative to the other for viewing the grid. Still further, the present invention is directed to a fuel assembly grid inspection method, comprising in combination the steps of: (a) defining an illuminated inspection field of view; (b) supporting within the field of view a fuel assembly grid having at least a pair of fuel rod contacting dimples disposed in each cell of a plurality thereof defined in the grid which are adapted to receive fuel rods therethrough; (c) inspecting the pair of dimples for perpendicularity with respect to one another by viewing the dimples at separate instances from the same location within the field of view; and (d) recording a separate image of each dimple to provide information from which actual measurements of any offset in X and Y directions of one dimple with the other can be calculated. Yet further, the present invention is directed to a fuel assembly grid inspection method, comprising in combination the steps of: (a) defining an illuminated inspection field of view; (b) positioning a fuel assembly grid support fixture about the perimeter of the field of view such that portions of the fixture which project into the field of view to support a grid therein are substantially transparent to the field of view; (c) supporting on the fixture within the field of view a fuel assembly grid of a design having known standard measurements; (d) sensing the position of the fixture for providing information to determine the correctness thereof before proceeding with inspection of the grid; and (e) inspecting the grid within the field of view. 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.