Abstract: Method for expanding tubular members, such as U-shaped heat transfer tubes having ends thereof located in the confined space adjacent the curved sides of the bowl-shaped lower plenum of a typical nuclear steam generator. The apparatus includes an elongate mandrel having a flow channel therethrough in communication with a resilient tubular bladder surrounding the mandrel. The bladder is flexible about its longitudinal axis due to the ribbed construction of the wall thereof. The mandrel includes a plurality of segments, adjacent ones of the segments interconnected by a ball-and-socket joint therebetween, so that the segments swivel about respective ones of the ball-and-socket joints. Thus, the mandrel and the expandable bladder connected thereto are flexible rather than rigid in order to be easily inserted into the tube ends located adjacent the curved sides of the bowl-shaped plenum and in order to easily traverse the upper U-bend portion of the tube.