Patent Number: 051749475
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to nuclear fuel pellet inspection and, more particularly, is concerned with a nuclear fuel pellet turning apparatus and method for facilitating surface inspection of pellets. 2. Description of the Prior Art In a typical nuclear reactor, the reactor core includes a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between and connected at opposite ends to the nozzles and a plurality of transverse support grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a multiplicity of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material in the form of a plurality of generally cylindrical nuclear fuel pellets maintained in a row or stack thereof in the rod. The fuel rods are grouped together in an array which is organized 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. A liquid, coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work. During the manufacture of the fuel rods for the fuel assemblies, visual inspection of the nuclear fuel pellets is a key operation for the production of quality fuel assemblies. The entire cylindrical surface of the pellet is checked for dimensional quality, cracks and other defects, including chipped edges. The practice heretofore for inspecting pellet surfaces has been to provide side-by-side rows of aligned pellets on a first pellet support tray. After an operator visually inspects the exposed portions of the external cylindrical surfaces of the pellets, a second empty pellet support tray is placed upside down over the pellets loaded on the first support tray. With the two trays held together, they are manually inverted so as to transfer the pellets en masse from the first support tray to the second support tray. The operator again visually inspects the newly-exposed portions of the external cylindrical surfaces of the pellets to complete the inspection of the pellets. This conventional practice has several drawbacks. First, the weight of the pellets and the two trays is oftentimes excessive and unacceptable from the standpoint of the capabilities of the typical operators. Second, the trays are often warped slightly, which could permit pellets to fall out during the turn-over or inverting action. Third, and more importantly, the portion of the cylindrical surface of the pellet than occurs at the horizontal midplane of the pellet cannot be effectively inspected due to the geometry of the pellet array and the overhead position of the light source. This questionable region amounts to approximately ten percent of the cylindrical surface of the pellet. Consequently, a need exists for improvement of the manner in which nuclear fuel pellet surface, inspection is carried out so as to eliminate the problems associated with the prior art method and still further improve the quality of fuel fuel pellet inspection. SUMMARY OF THE INVENTION The present invention provides a pellet turning apparatus and method designed to satisfy the aforementioned needs. The pellet turning apparatus and method of the present invention improves the quality of inspection by reducing the amount of effort required by the operator and by enabling the ability of the operator to see the entire surface of the pellet. The requirement to place an empty tray over the loaded tray and to invert the two trays and pellets is eliminated, relieving the operator of a difficult and heavy task. A substantially less cumbersome and time-consuming way is introduced for transferring the pellets to and from the inspection tray. Also, rolling or turning the pellets in situ, or while remaining in their original place, is introduced, permitting total circumferential sighting and inspection of the external surfaces of the pellets. Accordingly, the present invention is directed to a pellet turning apparatus for facilitating surface inspection of nuclear fuel pellets. The pellet turning apparatus includes: (a) a pellet turner assembly; and (b) a pellet supply tray interface assembly. The pellet turner assembly includes a pellet turner deck and a swivel mechanism for supporting the deck on a stand. The pellet turning apparatus also includes a pellet rake for use by an operator in loading and unloading pellets onto and from the pellet turner deck. More particularly, the pellet turner deck is composed of a frame mounted on the swivel mechanism, a plurality of elongated laterally-spaced pellet channel-defining members extending between opposite ends of the frame, and means for supporting the pellet channel-defining members at their opposite ends in spaced relation above the base frame. The pellet turner deck also includes a pellet rolling plate supported on the frame below the pellet channel-defining members for supporting pellets in rows between the channel-defining members and an actuating mechanism coupled to the pellet rolling plate. The actuating mechanism is operable to cause oscillatory movement of the plate relative to the channel-defining members for producing in situ simultaneous turning of the pellets within the channels to permit visual inspection of the entire circumferential surfaces of the pellets. Also, the swivel mechanism supporting the pellet turner deck, in turn, is operable for swiveling and tilting the pellet turner deck to thereby permit an operator to easily visually inspect the opposite edges of the pellets for the presence of cracks and chips. Further, the pellet supply tray interface assembly is mounted at a pellet loading end of the pellet turner deck. The interface assembly is operable for disposing a pellet transfer end of the pellet supply tray at a desired elevation relative to the pellet loading end of the pellet turner deck. The interface assembly includes a tray elevating mechanism mounted to the pellet loading end of the pellet turner deck. The tray elevating mechanism has at least one and preferably a plurality of engaging elements for coupling with the pellet transfer end of the pellet supply tray. An actuating cam mechanism is coupled to the tray elevating mechanism for moving the elevating mechanism to raise or lower the engaging elements of the elevating mechanism and thereby raise or lower the pellet transfer end of the supply tray respectively above or below the pellet loading end of the pellet turner deck. The present invention is also directed to a pellet turning method for facilitating surface inspection of nuclear fuel pellets. The pellet turning method includes the steps of: (a) supporting a plurality of nuclear fuel pellets in rows in laterally-spaced channels extending between opposite ends of a pellet turner deck; and (b) producing simultaneous turning of the pellets in situ within the channels to permit visual inspection of the entire circumferential surfaces of the pellets. The method also includes the steps of producing simultaneous tilting of the pellet deck to permit visual inspection of the opposite edges of the pellets for the presence of cracks and chips, and raking pellets along the channels for loading and unloading the pellets to and from the pellet turning deck. More particularly, the turning of the pellets is produced by moving in oscillatory fashion and in a direction extending in transverse relation to the channels a flat plate located below the channels and supporting the pellets in rows within the channels. For facilitating transfer of pellets between the supply tray and the pellet turner deck, a pellet transfer end of a pellet supply tray is either raised or lowered to a desired elevation above or below a pellet loading end of the pellet turner deck. These and other features and advantages 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.