Patent Number: 047598972
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to apparatus for determining external dimensional relationships of three-dimensional objects for ascertaining distortion thereof. In particular, the invention relates to apparatus for determining such dimensional relationships and distortion data in irradiated nuclear fuel assemblies. A nuclear fuel assembly typically comprises an array of elongated fuel rods and control rods engaged with and held in position by a plurality of grids longitudinally spaced-apart between top and bottom nozzles. In a typical reactor core, there may be between 160 and 190 such fuel assemblies, each fuel assembly including approximately 200 fuel rods, and each being held in place by retaining springs. Prior to use, each fuel assembly is substantially straight, the fuel pins and associated control rods for the fuel assembly all being parallel to each other and perpendicular to the top and bottom nozzles and to the grids. However, each reactor cycle results in a certain amount of distortion of the fuel assembly. This distortion includes "bow", "twist" and "tilt". "Bow" is defined as the deviation of the centroid of each grid from a straight line connecting the centroids of the top and bottom nozzles. "Twist" is defined as the angular rotation of each grid with respect to a zero reference plane defined by one face of the top nozzle. "Tilt" is defined as the distance between the centroid of the bottom nozzle and a vertical line passing through the centroid of the top nozzle. After each refueling cycle, fuel assemblies are moved to different locations in the reactor core, new fuel assemblies being added along the outside or periphery of the core and being gradually moved inwardly toward the center. After about three refueling cycles, a fuel assembly will reach the center of the core and become spent, these spent assemblies being removed during the next refueling. Each fuel assembly has locating holes in the bottom nozzle for receiving locating pins in the bottom core plate, accurately to position the fuel assembly in the reactor core. The above-described distortion can prevent accurate alignment of the bottom nozzle with these locating pins during repositioning of the fuel assemblies. Furthermore, because of newer fuel assembly design utilizing thinner walls and higher spring forces for retaining the fuel assembly in place, bow is becoming even more of a problem than heretofore. Deformation of as little as one-half inch can prevent accurate resetting of a fuel assembly in a new location in the reactor core. 2. Description of the Prior Art Previously, bow has been measured by hanging a measurement plate with horizontal rulings in front of the fuel assembly and reading the bow against the ruled markings, and thereby estimating the amount of bow in the fuel assembly. Alternatively, a closed circuit TV camera may be moved along the periphery of the fuel assembly, the camera movement being monitored to provide an estimate of a fuel assembly bow. Both of these prior techniques are time consuming and relatively inaccurate. Heretofore, no means has been available for making adequate tilt and twist measurements, and such measurements have generally not been attempted. SUMMARY OF THE INVENTION It is a general object of the present invention to provide an improved system for determining external dimensional relationships of nuclear fuel assemblies, which system avoids the disadvantages of prior techniques while affording additional structural and operating advantages. An important object of the present invention is the provision of a system of the type set forth which permits measurement of tilt and twist as well as bow of nuclear fuel assemblies. In connection with the foregoing object, it is another object of the present invention to provide a system of the type set forth which provides accurate and precise measurements. Still another object of the invention is the provision of a system of the type set forth which is capable of remotely-controlled underwater operation. Yet another object of the invention is the provision of a system of the type set forth which is relatively light-weight and portable. It is still a further object of the invention to provide a measurement system of the type set forth which can effect measurements on a fuel assembly when it is in an essentially free-hanging condition on a handling tool, without any need for releasing and reengaging the fuel assembly with respect to the handling tool. Yet another object of the invention is the provision of a measurement system of the type set forth which permits all measurements to be made with respect to references determined relative to the top nozzle of the fuel assembly, thereby obviating positioning of the fuel assembly at a precise location. These and other objects of the invention are attained by providing apparatus for determining external dimensional relationships of a nuclear fuel assembly including an array of elongated fuel rods engaged in transverse grids and extending between top and bottom nozzles, the apparatus comprising: support means, positioning means on the support means for fixedly positioning the fuel assembly in a predetermined orientation to establish a portion of the fuel assembly as a reference, carriage means mounted on the support means for movement longitudinally with respect to the fuel assembly substantially the entire length thereof when it is disposed in the predetermined orientation, measuring means on the carriage means, and drive means on the carriage means for moving the measuring means between a retracted condition and a measuring condition disposed in measuring engagement with the fuel assembly to measure dimensional relationships thereof relative to said reference thereon. The invention consists of certain novel features of a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.