Abstract: The hold-down plate of the top nozzle of a fuel assembly is removably attached on the upper extension members of the fuel assembly guide thimbles by an improved structure associated with passageways through the hold-down plate which receive upper end portions of the guide thimble upper extension members. The improved attaching structure includes recesses on the upper end portions of the guide thimble extension members with the recesses terminating at upper terminal edges thereof, elements mounted in the upper hold-down plate and extending therefrom into the passageways of the plate and the recesses of the extension member upper end portions, and a plurality of stop members connectable on the upper end portions of the guide thimble extension members and having overhanging ledges respectively formed thereon which define upper limits of the respective recesses on the extension member upper end portions.