This invention provides shielded access to the irradiated heat exchanger tube bundle of a nuclear steam generator contained within a cask after removal from the generator. This tube bundle must be reduced in size and confined for removal from the containment building through hatchways of generally limited size. Access to the enclosed bundle is at the flanged joints of adjacent sections forming the cask and which are vertically separated to permit an oxygen lance to be inserted therein for burning the tubes at that level. A rotatably supported annular seal ring covers the separation and shields against the escape of radiation or irradiated material during the burning process. The ring provides access ports through which roller supports are mounted on the cask and provide access of the cutting tool to the tubes. Rotation of the ring on the rollers permits the tool to be inserted completely around the cask. After the tubes within one section of the cask are severed, the tubes and that portion of the cask are removed and the ring raised to a next position where the procedure is repeated.