An integral reusable locking arrangement for a top nozzle of a reconstitutable fuel assembly includes a thin-walled tubular section on the upper end portion of each guide thimble of the fuel assembly above an externally threaded section thereon and an axial section on ecah alignment sleeve of the top nozzle above its lower internally threaded section. The tubular section has an annular circumferential protrusion defined thereon having an external diametric size which is greater than that of the remainder of the tubular section. The axial section on the sleeve has an internal diametric size which is greater than that of the tubular section but less than that of the protrusion. Therefore, rotational movement of the sleeve relative to the guide thimble from its raised toward its lowered position to thread its lower internally threaded section on the externally threaded section of the guide thimble causes interference contact of the axial section with the tubular section protrusion so as to produce a locking force. The locking force which must be overcome in order to unthread the internally threaded section of the sleeve from the externally threaded section of the guide thimble and detach the top nozzle from the guide thimble. The locking force takes the form of a constant torsional drag produced between the tubular section protrusion and the axial section interior surface as the sleeve is rotatably moved relative to the guide thimble between its lowered and raised positions. The circumferential protrusion on the tubular section is axilly displaced from the axial section when the internally threaded section on the sleeve is initially rotatably moved into threaded engagement with the externally threaded section on the guide thimble, whereby the mechanical advantage produced by threading the internally threaded section of the sleeve on the externally threaded section of the guide thimble can be used to overcome the torsional drag and force the sleeve onto the guide thimble.