Patent Number: 
Section: claims

1. A method of manufacturing a dual-cooled nuclear fuel rod assembly including inner and outer cladding tubes, pellets of nuclear fuel and a first and a second annular end plug, comprising the steps:providing a first annular end plug having inner and outer circumferences having respective diameters, a first end having an inner stepped joint undercut a distance into the inner circumference, the inner stepped joint intersecting a radial inner wall at its inner end and terminating at a radial end wall of the first end plug; and an outer stepped joint having an outer circumference with a diameter and extending between an inner radial wall intersected by the outer circumference and said radial end wall of said first end, said inner radial wall extending radially between the outer circumference of the outer stepped joint and the outer circumference of the first end plug;providing an inner cladding tube having inner and outer circumferences with respective diameters, and a first end area and second end area, and further having a wall thickness corresponding to the undercut distance of the inner stepped joint, said first end area terminating at a first radial end wall of the inner cladding tube, said second end area terminating at a second radial end wall of the inner cladding tube;providing an outer cladding tube having a first end area terminating at a first end radial wall and a second end area terminating at a second radial wall of the outer cladding tube, inner and outer circumferences with respective diameters, said inner circumference corresponding in diameter to the outer circumference diameter of said outer stepped joint of the first annular end plug;placing the first end of the inner cladding tube within said inner stepped joint of said first end plug so that the inner circumference of the first end plug is continuous with the inner circumference of the first end area of the inner cladding tube and so that a first end area portion of the inner cladding tube is within the inner stepped joint while leaving another first end area portion of the inner cladding tube exposed beyond the first end of the first end plug;performing a fillet weld between said another first end area portion of the inner cladding tube and the radial end wall of the first end plug where the inner stepped joint intersects the radial end wall to secure the first end area of the inner cladding tube to the radial end wall of the first end plug;placing the inner circumference of the first end of the outer cladding tube on the outer circumference of the outer stepped joint of the first end plug, and located so that the outer circumference of the first end of the outer cladding tube is continuous with the outer circumference of the first end plug, and so that the first radial end wall of the outer cladding tube abuts the inner radial wall of the outer stepped joint; andperforming a butt weld between the first radial end wall of the outer cladding tube and the inner radial wall of the outer stepped joint,placing the pellets between the inner and the outer cladding tubes from the second end areas of the cladding tubes;providing an annular second end plug with an outer circumference having a diameter; first and second ends terminating at radially extending end walls; an inner circumference having a diameter; a first end radial end wall; a second end radial end wall; a second outer stepped joint located on the second end adjacent the second end radial end wall; said outer stepped joint having a circumference with a diameter that is equal to the diameter of the inner circumference of the outer cladding tube at its second end area; the second outer stepped joint terminating at a second radial wall extending radially between the circumference of the outer stepped joint and the outer circumference of the second end plug;placing the second outer stepped joint of the second end plug within the second end area of the outer cladding tube so that the inner circumference of the second end area of the outer cladding tube fits closely over the circumference of the second outer stepped joint, with the second radial end wall of the outer cladding tube abutting the second radial wall of the second outer stepped joint, and with the outer circumference of the outer cladding tube continuous with the outer circumference of the second end plug, while placing the outer circumference of the second end area of the inner cladding tube within the inner circumference of the second end plug in close fitting relationship, and so that the second end wall of the inner cladding tube is adjacent the outer radial end wall of the second end plug; andperforming an edge weld between the second end of the inner cladding tube and the outer radial end wall of the second end plug, and a butt weld between the second radial end wall of the outer cladding tube and the second radial wall of the second outer stepped joint to secure the second ends of the inner and outer cladding tubes to the second end plug, with the pellets contained between the first and second end plugs and the inner and outer cladding tubes,wherein said second end area of the inner cladding tube extends distally beyond the second end area of the outer cladding tube when the inner and outer cladding tubes are welded to the first end plug. 2. The method according to claim 1, wherein the inner cladding tube is longer than the outer cladding tube. 3. The method according to claim 1, including providing a longitudinal and radially extending slit adjacent the inner end of the second end plug, said slit intersecting the inner circumference of the second end plug and the circumference of the second outer stepped joint before welding of the inner and outer cladding tubes to the second end plug. 4. The method according to claim 1, including forming the second end plug without an inner stepped joint corresponding to the inner stepped joint of the first end plug. 5. The method according to claim 1, including forming the diameter of the inner circumference of the second end plug substantially the same as the diameter of the outer circumference of the inner cladding tube through the second end area of the second end plug to the outer radial end wall of the second end plug.