Nuclear fuel assembly having double enclosure top nozzle subassembly

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:

• (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.