The invention refers to a fuel assembly comprising a lower end structure, an upper end structure including a top nozzle (5), a plurality of fuel rods and a plurality of guide thimbles (3). The top nozzle includes a passageway and an annular groove (10) in said passageway. A sleeve (11) is provided for attaching the guide thimble (3) to the top nozzle (5). The sleeve has at least three slots (12) and includes at least three bulges (13). Each bulge (13) has two ends and extends between two of the slots (12). At least one of the ends of the bulge (13) extends to a position at a distance (d) from the respective slot (12). The invention also refers to a guide thimble device (9) for use in a fuel assembly.