Patent Number: 051805499
Section: claims

1. A double enclosure top nozzle subassembly for a nuclear fuel assembly, said top nozzle subassembly comprising: (a) an upper structure including a top plate and an outer sidewall enclosure rigidly connected to and depending from an outer peripheral edge of said top plate;  (b) a lower structure including a lower adaptor plate and an inner sidewall enclosure rigidly connected to and upstanding from an outer peripheral edge of said lower plate, said lower adapter plate being disposed below said top plate, said inner sidewall enclosure being disposed within said outer sidewall enclosure, said inner and outer sidewall enclosures being movable in sliding contacting relationship relative to one another so as to permit movement of said top plate toward and away from said lower adapter plate;  (c) interengaging means on respective upper and lower peripheral edges of said inner and outer sidewall enclosures for defining stops which limit the movement of said top plate and lower adaptor plate away from each other so as to retain said outer and inner sidewall enclosures in said sliding contacting relationship with one another and prevent movement of said lower peripheral edge of said outer sidewall enclosure below said lower adaptor plate, said interengaging means on said respective upper and lower peripheral edges of said inner and outer sidewall enclosures also defining sliding surfaces between said inner and outer sidewall enclosures being maintained in continuous contact with one another as said top plate and lower adapter plate move toward and away from one another; and  (d) a plurality of resiliently-yieldable biasing devices disposed within said inner and outer sidewall enclosures and extending between and engaging said top plate and said lower adapter plate, said devices being movable between compressed and expanded states in response respectively to application and removal of a hold-down force on said upper structure in the direction of said lower structure for respectively permitting and causing movement of said inner and outer sidewall enclosures relative to one another and said top plate toward and away from said lower adapter plate and thereby said top nozzle subassembly between compressed and expanded conditions.  an inwardly-projecting flange on said lower edge of the outer sidewall enclosure defining a contact surface engaged with an exterior surface of said inner sidewall enclosure; and  an outwardly-projecting flange on said upper edge of said inner sidewall enclosure defining a contact surface engaged with an interior surface of said outer sidewall enclosure, said flanges overlapping with one another to define said stops.  said lower adapter plate has a depression formed in a topside of said lower adapter plate along each of said sides and approximately midway between said corners thereof;  said top plate has a pair of guide grooves defined in the underside of said top plate along each side and adjacent said corners thereof; and  each of said leaf springs has a generally U-shaped configuration composed of a lower bight portion and upper end portions connected to and extending upwardly from said lower bight portion, said leaf spring being seated at said lower bight portion within one of said depressions of said lower adapter plate and at said opposite upper end portions within said guide grooves of said top plate.  (a) an upper structure including a top plate and an outer sidewall enclosure rigidly connected to and depending from an outer peripheral edge of the top plate;  (b) a lower structure including a lower adapter plate and an inner sidewall enclosure rigidly connected to and upstanding from an outer peripheral edge of said lower adapter plate, said lower adapter plate being disposed below said top plate, said inner sidewall enclosure being disposed below said top plate, said sidewall enclosure, said inner and outer sidewall enclosures being movable in sliding contacting relationship relative to one another so as to permit movement said top plate toward and away from said lower adapter plate;  (c) interengaging means on respective upper and lower peripheral edges of said inner and outer sidewall enclosures for defining stops which limit the movement of said top plate and lower adapter plate away from each other so as to retain said outer and inner sidewall enclosures in said sliding contacting relationship with one another and prevent movement of said lower peripheral edge of said outer sidewall enclosure to below said lower adapter plate, said interengaging means on said respective upper and lower peripheral edges of said inner and outer sidewall enclosures also defining sliding surfaces between said inner and outer sidewall enclosures being maintained in continuous contact with one another as said top plate and lower adapter plate move toward and away from one another; and  (d) a plurality of resiliently-yieldable biasing devices disposed within said inner and outer sidewall enclosures and extending between and engaging said top plate and the lower adapter plate, said devices being movable between compressed and expanded states in response respectively to application and removal of a hold-down force on said upper structure in the direction of said lower structure for respectively permitting and causing movement of said inner and outer sidewall enclosures relative to one another so as to move said top plate toward and away from said lower adapter plate and thereby said top nozzle subassembly between compressed and expanded conditions.  an inwardly-projecting flange on said lower edge of the outer sidewall enclosure defining a contact surface engaged with an exterior surface of said inner sidewall enclosure; and  an outwardly-projecting flange is an inwardly-projecting flange on said upper edge of said inner sidewall enclosure defining a contact surface engaged with an interior surface of said outer sidewall enclosure, said flanges overlapping with one another to define said stops.  said lower adapter plate has a depression formed in a topside of said lower adapter plate along each of said sides and approximately midway between said corners thereof;  said top plate has a pair of guide grooves defined in the underside of said top plate along each side and adjacent said corners thereof; and  each of said leaf springs has a generally U-shaped configuration composed of a lower bight portion and upper end portions connected to and extending upwardly from said lower bight portion, said leaf spring being seated at said lower bight portion within one of said depressions of said lower adapter plate and at said opposite upper end portions within said guide grooves of said top plate. 2. The top nozzle subassembly as recited in claim 1, wherein said interengaging means includes: 3. The top nozzle subassembly as recited in claim 1, wherein said top plate of said upper structure includes an annular body. 4. The top nozzle subassembly as recited in claim 3, where said outer peripheral edge of said top plate is defined by a rim projecting outwardly from an upper peripheral edge of said annular body so as to define an annular cavity surrounding said annular body for receiving an upper peripheral edge of said outer sidewall enclosure. 5. The top nozzle subassembly as recited in claim 3, wherein said top plate includes a plurality of indentations defined along the periphery and underside of said annular body in spaced relation to one another, said indentations facing outwardly of said body and downwardly therefrom, the portions of said annular body between said indentations forming downwardly protruding tabs along the periphery of said body for attaching said top plate to an upper peripheral edge of said outer sidewall enclosure. 6. The top nozzle subassembly as recited in claim 5, wherein said top plate further includes a plurality of pins inserted through said upper peripheral edge of said outer sidewall enclosure and said tabs of said annular body for securing said outer sidewall enclosure and said annular body together. 7. The top nozzle subassembly as recited in claim 5, wherein said inner sidewall enclosure of said lower structure has a plurality of upper edge portions spaced apart by notches defined between said upper edge portions, said upper edge portions and said notches being capable of mating respectively with said spaced indentations and tabs in said annular body of said top plate of said upper structure when said top nozzle subassembly is in said compressed condition. 8. The top nozzle subassembly as recited in claim 1, wherein said top plate of said upper structure and said lower adapter plate of said lower structure are generally rectangular in configuration having four sides defining four corners. 9. The top nozzle subassembly as recited in claim 8, wherein said biasing devices are a plurality of leaf springs disposed between said top plate and said lower adapter plate and movable between expanded and compressed states. 10. The top nozzle subassembly as recited in claim 9, wherein: 11. The top nozzle subassembly as recited in claim 1, wherein each of said outer and inner sidewall enclosures of said upper and lower structures is composed of generally planar pairs of vertical wall portions rigidly interconnected together at their opposite vertical edges. 12. In a nuclear fuel assembly including a bottom nozzle, a plurality of guide thimbles having upper and lower ends and being attached at said lower ends to said bottom nozzle and extending upwardly therefrom, an array of upstanding fuel rods extending along and spaced from said guide thimbles and spaced at their lower ends above said bottom nozzle, and a plurality of support grids axially spaced along and connected to said guide thimbles for supporting said array of upstanding fuel rods, a double enclosure top nozzle subassembly which permits increased fuel rod thermal growth and burnup, said top nozzle subassembly comprising: 13. The top nozzle subassembly as recited in claim 12, wherein said interengaging means includes: 14. The top nozzle subassembly as recited in claim 12, wherein said top plate of said upper structure includes an annular body having an inner peripheral edge defining a large central opening and aligned above said outer perimeter of said guide thimbles. 15. The top nozzle subassembly as recited in claim 16, where said outer peripheral edge of said top plate is defined by a rim projecting outwardly from an upper peripheral edge of said annular body so as to define an annular cavity surrounding said annular body for receiving an upper peripheral edge of said outer sidewall enclosure. 16. The top nozzle subassembly as recited in claim 14, wherein said top plate includes a plurality of indentations defined along the periphery and underside of said annular body in spaced relation to one another, said indentations facing outwardly of said body and downwardly therefrom, the portions of said annular body between said indentations forming downwardly protruding tabs along the periphery of said body for attaching said top plate to an upper peripheral edge of said outer sidewall enclosure. 17. The top nozzle subassembly as recited in claim 16, wherein said top plate further includes a plurality of pins inserted through said upper peripheral edge of said outer sidewall enclosure and said tabs of said annular body for securing said outer sidewall enclosure and said annular body together. 18. The top nozzle subassembly as recited in claim 16, wherein said inner sidewall enclosure of said lower structure has a plurality of upper edge portions spaced apart by notches defined between said upper edge portions, said upper edge portions and said notches being capable of mating respectively with said spaced indentations and tabs in said annular body of said top plate of said upper structure when said top nozzle subassembly is in said compressed condition. 19. The top nozzle subassembly as recited in claim 12, wherein said top plate of said upper structure and said lower adapter plate of said lower structure are generally rectangular in configuration having four sides defining four corners. 20. The top nozzle subassembly as recited in claim 19, wherein said biasing devices are a plurality of leaf springs disposed between said top plate and said lower adapter plate and movable between expanded and compressed states. 21. The top nozzle subassembly as recited in claim 20, wherein: 22. The top nozzle subassembly as recited in claim 12, wherein each of said outer and inner sidewall enclosures of said upper and lower structures is composed of generally planar pairs of vertical wall portions rigidly interconnected together at their opposite vertical edges.