Patent Number: 047028831
Section: claims

1. In a nuclear fuel assembly having at least one control rod guide thimble and a top nozzle, said guide thimble including an upper extension member, said top nozzle including an upper hold-down plate having a passageway slidably receiving an upper end portion of said extension member, an improved structure for removably attaching said upper hold-down plate on said guide thimble upper extension member, comprising: (a) means defining a recess on said upper end portion of said extension member;  (b) a stop member having upper and lower portions, said stop member lower portion adapted to connect onto and disconnect from said extension member upper end portion, said stop member upper portion having an outside diameter greater than that of said stop member lower portion and extending above said extension member upper end portion when said stop member lower portion is connected onto the same, said stop member also having a ledge formed thereon at a transition between its upper and lower portions which defines an upper limit of said recess when said stop member is connected onto said extension member upper end portion; and  (c) an element mounted in said upper hold-down plate and extending therefrom into said passageway of said plate and said recess of said extension member upper end portion, said element being positioned to slide upwardly along said recess until making engagement with said ledge on said stop member when said stop member is connected onto said extension member upper end portion for limiting upward movement of said upper hold-down plate along said guide thimble, said element being positioned to slide upwardly along and past said recess when said stop member has been disconnected from said extension member upper end portion for allowing removal of said upper hold-down plate from said guide thimble.  said upper end portion of said extension member has means defining a threaded section on the interior thereof;  said stop member lower portion has a complementary threaded section defined on the exterior thereof adapted to thread into said threaded section of said extension member upper end portion for releasably connecting said stop member into said guide thimble extension member.  (a) means defining an axially extending recess on the exterior of said upper end portion of each of said guide thimble extension members, said recess terminating at an upper terminal edge of said extension member upper end portion;  (b) means defining a threaded section on the interior of said upper end portion of each of said guide thimble extension members;  (c) a plurality of stop members each having upper and lower portions, said each stop member lower portion having a threaded section defined on the exterior thereof adapted to thread into and unthread from said threaded section of one of said extension member upper end portions, said stop member upper portion having an outside diameter greater than that of said stop member lower portion, said stop member also having an overhanging ledge formed thereon at a transition between its upper and lower portions which defines an upper end of said recess on said one extension member upper end portion when said stop member is threaded therein; and  (d) a plurality of elements mounted in said upper hold-down plate and each extending therefrom into one of said passageways of said plate and said recess of one of said extension member upper end portions, said each element being positioned to slide upwardly along said corresponding recess until making engagement with said ledge on one of said stop members when said stop member is threaded into said corresponding extension member upper end portion for limiting upward movement of said upper hold-down plate along said guide thimble, said each element being positioned to slide upwardly along and past said corresponding recess when said one stop member has been unthreaded from said corresponding extension member upper end portion for allowing removal of said upper hold-down plate from said guide thimble. 2. The improved attaching structure as recited in claim 1, wherein said stop member upper portion has an outside diameter substantially the same as the outside diameter of said upper end portion of said guide thimble extension member which adapts said stop member upper portion to be slidably receivable in said upper hold-down plate passageway. 3. The improved attaching structure as recited in claim 1, wherein: 4. The improved attaching structure as recited in claim 1, wherein said recess terminates at an upper terminal edge of said guide thimble extension member upper end portion. 5. In a nuclear fuel assembly having a plurality of control rod guide thimbles and a top nozzle, said guide thimbles each including an upper extension member, said top nozzle including an upper hold-down plate having a plurality of passageways slidably receiving upper end portions of said respective extension members of said guide thimbles, an improved structure for removably attaching said upper hold-down plate on said guide thimble upper extension members, comprising: 6. The improved attaching structure as recited in claim 5, wherein said upper portion of each of said stop members has an outside diameter substantially the same as the outside diameter of said upper end portion of each of said guide thimble extension members which adapts said each stop member upper portion to be slidably receivable in one of said upper hold-down plate passageways.