Abstract: An apparatus for expanding an existing repair sleeve in a steam generator to permit a new repair sleeve to be installed. The apparatus includes a variable cross-section elongated cylindrical capsule adapted to be positioned within the existing repair sleeve and an explosive charge inside the capsule whereby detonation of the charge expands the existing repair sleeve to permit the new repair sleeve to be installed. The variable cross-section of the capsule permits the explosive force to be "tuned" so that the existing tube expands evenly along its entire length, including the hard to expand ends of the existing sleeve, without overexpanding the tube. In the preferred embodiment, the apparatus includes a conduit attached to one end of the capsule and at least one sleeve position indicator associated with the conduit. The sleeve position indicator associated with the conduit may be a predetermined index mark on the outer surface of the conduit, a hard stop located between the conduit and the capsule or both.