Patent Number: 
Section: claims

1. A canister for dry storage of nuclear waste material, the canister comprising:a steel tubular first shell defining an internal cavity for storing nuclear waste material;a lid sealably attached to a first end of the first shell;an end closure attached to a second end of the first shell, the end closure comprising a steel base plate and an upturned annular closure flange disposed on a peripheral portion of the base plate;a circumferentially-extending butt joint formed between the annular closure flange and the second end of first shell;a circumferentially-extending butt weld formed at the butt joint which hermetically seals the annular closure flange to the second end of the first shell;a steel annular shell skirt adjoined and welded to a lower portion of the first shell proximate to its second end, the shell skirt extending circumferentially around the lower portion and having a height less than a full height of the first shell, wherein the shell skirt forms a hermetically sealed secondary pressure retention barrier over the butt weld; anda plurality of circumferentially spaced apart plug welds which secure the annular shell skirt directly to the base plate. 2. The canister according to claim 1, wherein the annular closure flange has an edge which is horizontally oriented at the butt joint to mate with the second end of the first shell. 3. The canister according to claim 1, wherein the lid is hermetically sealed to the first end of the first shell by a welded annular joint. 4. The canister according to claim 3, further comprising an annular closure ring welded to the lid and first end of the first shell which covers the welded joint of the lid. 5. The canister according to claim 1, wherein the annular closure flange is formed integrally with the first base plate as a unitary structural part thereof. 6. The canister according to claim 1, further comprising an annular concave surface depression formed in a top surface of the base plate adjacent the annular flange and butt joint. 7. The canister according to claim 1, wherein the first shell has a vertical orientation. 8. The canister according to claim 1, wherein the shell skirt covers the butt joint and is in direct contact with the butt weld. 9. The canister according to claim 1, wherein the first shell, annular shell skirt, base plate, and lid are formed of stainless steel. 10. The canister according to claim 1, wherein the annular shell skirt and first shell are each formed of a stainless steel which is compatible for welding the annular shell skirt to the first shell. 11. The canister according to claim 1, wherein the annular shell skirt comprises a top end welded to the first shell by a circumferential top weld, and a bottom end welded to the base plate by a circumferential bottom weld. 12. The canister according to claim 11, wherein the bottom end of the annular shell skirt is welded to a peripheral side of the base plate by the circumferential bottom weld. 13. The canister according to claim 11, wherein the circumferential top and bottom welds are each fillet welds. 14. The canister according to claim 1, wherein each plug weld extends completely through the annular shell skirt in a radial direction from an exterior surface of the skirt to the base plate. 15. The canister according to claim 1, wherein the annular shell skirt has a height less than half the full height of the first shell.