Patent Number: 049869590
Section: summary

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 expandable top nozzle subassembly allowing improved utilization of space for accommodating greater growth and burnup of fuel rods of the fuel assembly. 2. Description of the Prior Art In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies which receive support and alignment from upper and lower transversely extending core support plates. The upper and lower core support plates are directly or indirectly attached to a support barrel which surrounds the entire core and extends between the ends thereof. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array by a plurality of grids spaced along the fuel assembly length and attached to the control rod guide thimbles. The guide thimbles extend slightly above and below the ends of the fuel rods. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles to thereby form an integral fuel assembly. The fuel assemblies are arranged vertically resting on the lower core support plate. To facilitate handling and installation, the fuel assemblies are generally not secured to the lower core support plate. Temperatures at various times within the core may vary greatly, such as, from cold shutdown to normal operating conditions. Also, different materials exhibit different irradiation and thermal growth characteristics. Since the materials of fuel assembly components are generally different than those used in the core support barrel and undergo greater thermal expansion, the resulting increase in length of the fuel assemblies in the axial or vertical direction must be accommodated. For this reason, the fuel assemblies are not usually attached to the upper and lower core plates but rather are supported in a manner which permits some relative motion therebetween. The axial thermal expansion differential between the fuel assemblies and the core support barrel has been accommodated by insuring that the axial spacing between the upper and lower core support plates is somewhat greater than the axial length of the fuel assemblies. Normally, this is accomplished by providing an axial gap between the top of the fuel assemblies and the upper core support plate. However, the presence of the gap can result in upward lifting of the fuel assemblies due to the hydraulic forces produced on the fuel assemblies in the upward direction by coolant flow. Thus, fuel assemblies have also employed hold-down devices with the top nozzles to prevent the force of upward coolant flow from lifting the fuel assemblies into damaging contact with the upper core support plate, while at the same time allowing for changes in fuel assembly length due to core-induced thermal expansion and the like. Representative to the prior art fuel assemblies with hold-down devices are those disclosed in U.S. Pat. No. Re. 31,583 to Klumb, U.S. Pat. No. 4,078,967 to Anthony, U.S. Pat. No. 4,534,933 to Gjertsen et al, U.S. Pat. Nos. 4,620,960 and 4,670,213 to Wilson et al. As mentioned previously, the guide thimbles of fuel assemblies extend slightly above and below the ends of the fuel rods. Thus, the top and bottom nozzles of fuel assemblies secured at opposite ends of the guide thimbles likewise are spaced above and below the fuel rod ends. This space between the opposite ends of the fuel rods and adjacent portions of the top and bottom nozzles accommodates increase in length of the fuel rods due to growth as fuel rod burnup occurs during normal reactor operation. With improvements in various aspects of fuel assembly design, it has become feasible to increase the allowable burnup of the fuel rods. This increase in burnup is desirable because it decreases the frequency of plant shutdowns and the buildup of spent fuel. However, to permit the fuel rods to operate to a higher burnup, an increase of approximately 0.5 inch minimum in fuel rod length is necessary due to extra growth. This necessitates an increase in the space between the adapter plates of the top and bottom nozzles to accommodate this additional fuel rod growth. At the same time, there still must be enough space between the top plate and adapter plate of the top nozzle to allow the handling equipment of the core to get between the plates and latch onto the underside of the top plate of the top nozzle. Currently, there is not enough room between the adapter plates of the top and bottom nozzles to permit the additional 0.5 inch growth in fuel rod length. Consequently, a need exist for a way to accommodate extra fuel rod thermal growth without impairing the handling capability of the core equipment currently in use. SUMMARY OF THE INVENTION The present invention provides an expandable top nozzle subassembly designed to satisfy the aforementioned needs. The expandable top nozzle subassembly of the present invention enables a fuel assembly to accommodate fuel rod growth in excess of one inch, allowing a reactor to operate at a higher burnup rate. This length extension will allow the fuel rods to be pushed to burnups greatly in excess of those used today. Additionally, where extremely high burnups are not required, the mechanical duty of the fuel rods can be reduced significantly. The present invention makes use of "dead space" existing in the conventional top nozzle between its top plate and bottom adapter plate. This space is characterized as "dead" since it is only used during installation and removal of the fuel assembly and not during operation of the core. The expandable (and compressable) top nozzle subassembly of the present invention thus provides for the additional fuel rod growth space needed, while continuing to allow the use of current handling systems and thus eliminating potential costs to customers in design changes. Accordingly, the present invention is directed to an expandable top nozzle subassembly for a nuclear fuel assembly which permits increased fuel rod growth and burnup. The top nozzle subassembly comprises: (a) an upper structure including a top plate and a sidewall enclosure rigidly connected to and depending from an outer peripheral edge of the top plate; (b) a lower adapter plate disposed below the top plate of the upper structure and within the enclosure thereof, the adapter plate and sidewall enclosure being slidably movable relative to one another so as to move the top plate toward and away from the lower adapter plate; (c) interengaging means on a lower edge of the enclosure and on an outer peripheral edge of the adapter plate for capturing and retaining the adapter plate within the enclosure upon movement of the enclosure relative to the adapter plate which moves the top plate away from the adapter plate; and (d) a plurality of resiliently-yieldable biasing devices disposed in the enclosure and extending between and engaging the top plate and the adapter plate, the devices being movable between compressed and expanded states in response respectively to application and removal of a hold-down force on the upper structure in the direction of the adapter plate for respectively permitting and causing movement of the enclosure relative to the adapter plate so as to move the top plate toward and away from the adapter plate and thereby the top nozzle subassembly between compressed and expanded conditions. More particularly, the top plate has a plurality of corner portions and at least one recess formed in a lower surface of the top plate at each corner portion. The adapter plate has a plurality of corner portions and at least one recess formed in an upper surface of the adapter plate at each corner portion. Each recess of the adapter plate is aligned below one of the recesses of the top plate. The biasing devices are a plurality of coil springs movable between expanded and compressed states. Each coil spring is disposed between the top plate and lower adapter plate and seated at its opposite upper and lower ends in respectively aligned pairs of the recesses of the top plate and adapter plate. Further, the sidewall enclosure of the upper structure is composed of generally planar vertical wall portions rigidly interconnected together at their opposite vertical edges. The interengaging means on the lower edge of the sidewall enclosure is a retaining structure which projects inwardly from the sidewall enclosure and is composed of a series of spaced fingers. The interengaging means on the outer peripheral edge of the adapter plate is an undercut seat structure having a cross-sectional configuration which interfits in overlying relation with the cross-sectional configuration of the inwardly-projecting retaining structure of the sidewall enclosure so as to capture and prevent movement of the adapter plate past the lower edge of the enclosure. 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.