Abstract: An inner tube (1) and an outer tube (6) are provided of suitable shape and size for the inner tube to be received coaxially inside the outer tube, thereby causing the outside face of the inner tube to face the inside face of the outer tube. A neutron-absorbing screen (2) is fixed to one of said faces. The inner tube is inserted into the outer tube, and the inner tube is radially expanded into the outer tube while leaving the outer tube externally free to expand, and thereby plastically deforming both tubes and obtaining a solid single block assembly of desired shape and size for the container. The container may be used for storing radioactive material.