Abstract: To enable the benefits to be obtained simultaneously of using cast iron as a major constituent and of using helical prestressing, the cylindrical wall structure of a pressure vessel (which may be for a nuclear reactor) is made of truncated wedge shaped cast-iron slabs (the wedge faces of the slabs lying in radial planes of the wall structure and mutually adjacent slabs abutting one another in those planes, with a plurality of such slabs forming an annular assembly and a plurality of such assemblies stacked coaxially to form the cylindrical wall structure), the slabs having cast-in through passages, disposed obiquely, both right and left-handed in alternate layers at progressively increasing radial distance with respect to the cylindrical wall structrue, and the passages of one slab in alignment with the ends of passages of adjacent slabs in the same annular assembly and in the annular assemblies stacked adjacent thereto to provide in the assembled cylindrical wall structure a system of right- and left-handed