A method of replacing a damaged thermal sleeve in a reactor vessel head adapter that connects a control rod drive mechanism to a reactor vessel head, the method comprising the steps of: accessing the damaged thermal sleeve from an underside of the reactor vessel head; removing the damaged thermal sleeve; obtaining a replacement thermal sleeve having: an elongated tubular body disposed about a central longitudinal axis and extending between a bottom end and a top end; a flanged region disposed at or about the top end which extends radially outward from the tubular body; and characterised by a plurality of slots defined in the tubular body, each slot extending through and from the top end toward the bottom end along a predetermined distance, each slot having a width which is sufficient to narrow a maximum outside diameter of the flanged region from a first diameter (D 1) to a second diameter in order to clear any obstruction in an opening of a tubular member in which the thermal sleeve is to be inserted; altering a maximum outside diameter of the flanged region on the replacement thermal sleeve to an extent to clear any obstruction in the opening in the reactor head adapter that will be encountered in inserting the thermal sleeve to a location within the opening originally occupied by the damaged thermal sleeve; inserting the replacement thermal sleeve through the opening from the underside of the reactor vessel head; and expanding the maximum outside diameter of the flanged region into a recess in the opening of the reactor head adapter. accessing the damaged thermal sleeve from an underside of the reactor vessel head; removing the damaged thermal sleeve; obtaining a replacement thermal sleeve having: an elongated tubular body disposed about a central longitudinal axis and extending between a bottom end and a top end; a flanged region disposed at or about the top end which extends radially outward from the tubular body; and characterised by a plurality of slots defined in the tubular body, each slot extending through and from the top end toward the bottom end along a predetermined distance, each slot having a width which is sufficient to narrow a maximum outside diameter of the flanged region from a first diameter (D 1) to a second diameter in order to clear any obstruction in an opening of a tubular member in which the thermal sleeve is to be inserted; an elongated tubular body disposed about a central longitudinal axis and extending between a bottom end and a top end; a flanged region disposed at or about the top end which extends radially outward from the tubular body; and characterised by a plurality of slots defined in the tubular body, each slot extending through and from the top end toward the bottom end along a predetermined distance, each slot having a width which is sufficient to narrow a maximum outside diameter of the flanged region from a first diameter (D 1) to a second diameter in order to clear any obstruction in an opening of a tubular member in which the thermal sleeve is to be inserted; altering a maximum outside diameter of the flanged region on the replacement thermal sleeve to an extent to clear any obstruction in the opening in the reactor head adapter that will be encountered in inserting the thermal sleeve to a location within the opening originally occupied by the damaged thermal sleeve; inserting the replacement thermal sleeve through the opening from the underside of the reactor vessel head; and expanding the maximum outside diameter of the flanged region into a recess in the opening of the reactor head adapter.