Patent Number: 059178752
Section: summary

The present invention relates to apparatus for inspecting nuclear fuel assemblies and, in particular with apparatus which will rectify the tendency of a fuel assembly to incline from the perpendicular. Typically, a nuclear fuel assembly comprises a plurality of fuel rods and control rod guide thimbles held in parallel relationship by spacer grids arranged at spaced locations along the fuel rods and secured to the guide thimbles. Attached to opposite ends of the guide thimbles are top and bottom nozzles. When installed in the core of a nuclear reactor, the fuel assemblies are arranged in close proximity to one another between upper and lower core plates. Since the fuel assemblies in the reactor core are closely spaced, it is essential that each fuel assembly upon completion satisfies stringent quality control procedures, particularly with regard to dimensional accuracy. It is particularly important to ensure that the fuel assembly stands truly perpendicular when the bottom nozzle of an assembly is supported by a horizontal surface. A known technique for ensuring perpendicularity of a fuel assembly involves measuring the out of alignment of the assembly from the perpendicular using a theodilite or similar instrument and assessing how much material requires to be removed from the lower surfaces of the bottom nozzle. The fuel assembly is then placed in a vertical position and the required amount of material is removed by grinding one or more of the bottom nozzle surfaces. After the grinding operation, the fuel assembly is placed on a horizontal surface and remeasured. If necessary, further material is ground from the bottom nozzle until the desired verticality of the fuel assembly is attained. This technique for rectifying the tilt of a fuel assembly is not satisfactory in that it involves several time-consuming mechanical handling operations to move the assembly between the measuring and grinding positions. According to the invention there is provided apparatus for rectifying the tendency of a nuclear fuel assembly to incline from the perpendicular, the fuel assembly having interconnected upper and lower support members, the apparatus comprising an upper locating means for locating the upper support member and a base assembly adapted to support said lower support member whereby the fuel assembly extends perpendicularly between the upper locating means and said base assembly, elevating means incorporated in said base assembly, the elevating means being operable to engage the lower support member and raise the fuel assembly, the base assembly further including material removal means operable to remove material from a lower surface of the lower support member when raised by the elevating means whereby said lower surface is made to lie in a horizontal plane. Preferably the base assembly includes a support surface on which the lower surface of the lower support member rests, the elevating means being operable to raise the lower surface above the support surface. Advantageously the elevating means moves along an axis coaxial with a longitudinal central axis of the fuel assembly. The lower support surface may comprise a plurality of lower surfaces, the material removal means being operable to remove material from each of the lower surfaces, whereby each of the lower surfaces is made to lie in said horizontal plane. The elevating means preferably comprises a gimbal mounting arrangement which includes a gimbal member movably supported on a gimbal mount and arranged to engage the lower support member. Preferably the gimbal member has a spherically-shaped female bearing portion which receives a spherically-shaped male bearing portion provided on the gimbal mount. In a preferred embodiment the material removal means comprises a grinding wheel, the grinding wheel being mounted on a rotor member arranged for rotation about the axis along which the elevating means moves. Advantageously the grinding wheel is radially spaced from the axis about which the rotor member rotates, whereby the grinding wheel is able to remove material from each of the lower surfaces during one revolution of the rotor member. Preferably the base assembly includes a vertical post, the rotor member being arranged to rotate about the post, and wherein pressurised air bearings are provided between the post and the rotor member. The base assembly may include a stationary gear wheel arranged coaxially with respect to the axis along with the elevating means moves, the gear wheel having gear teeth provided around the periphery thereof, and a drive motor mounted on said rotor member, the drive motor having an output gear wheel arranged to mesh with the stationary gear wheel. Drive means are preferably provided for moving the grinding wheel along its axis of rotation. The drive means for moving the grinding wheel along its axis of rotation may comprise a stepper motor which moves the grinding wheel along said axis of rotation in incremental steps. Preferably the base assembly includes at least one locating pin and actuator means for extending and retracting the locating pin along a vertical axis, whereby the locating pin can be inserted into or retracted from a locating hole provided in said lower support member. The upper locating means preferably includes at least one upper locating pin and further actuating means for extending and retracting said upper locating pin along a vertical axis, whereby the upper locating pin can be inserted into or retracted from an upper locating hole provided in the upper support member. The upper locating pin may comprise a larger diameter portion of substantially the same diameter as that of the upper locating hole and a reduced diameter end portion, the further actuator means being operable to insert the larger diameter portion or the reduced diameter end portion into the upper locating hole. Advantageously the base assembly includes measuring means for determining the weight of the fuel assembly. The measuring means preferably comprises a load cell located beneath the elevating means.