Radioactive material storage vessel

A body portion of a vessel comprises an outer cylinder having a cylindrical shape and an inner cylinder having a hollow square cylindrical shape disposed coaxially inside of the outer cylinder. Plates are integrally disposed to four corners of the inner cylinder protruding toward the outer cylinder respectively. The top ends of the plates are joined to the outer cylinder respectively. The inner circumference of the inner cylinder constitutes a square hole passing through the cylindrical body from the lower end to the upper end. A hole passing through the vessel from the lower end to the upper end is constituted by the square hole of the cylindrical body and a square hole of a lid. Spent fuel rods are inserted to four containing portions formed by the outer cylinder, the inner cylinder and the plates in the cylindrical body. Cooling air flows along the outer circumference of the vessel contained in a containing pipe, as well as it flows into the hole. With such a constitution, since cooling air absorbs after-heat released from both of the outer circumference and the hole of the vessel simultaneously, cooling efficiency is improved. (I.N.)