GB1107858A - Improvements in and relating to nuclear fuel elements - Google Patents

Abstract

1,107,858. Fuel elements. BABCOCK & WILCOX CO. 23 July, 1965 [24 July, 1964], No. 31599/65. Heading G6C. A plate-type fuel element is manufactured from a number of fuel plates in each of which fuel is enclosed within cladding by forming a stack from the fuel plates alternating with spacing members, pressing the elements of the stack together, depositing weld metal, whilst the stack is under pressure, on, to extend between, the plates and, after the weld metal has been deposited, removing the spacing members. This method is especially applicable to plates clad in Al or Al-based alloy since the heat of the welding operation has, using methods hitherto known, caused such warpage and distortion of the fuel plates that it was impossible to achieve the required dimensional tolerances. Copper spacer plates may be used which, after welding, are removed by dissolving in nitric acid. The fuel plates each have a fissile material-bearing insert surrounded on all sides by Al or Al-based alloy cladding and may be provided at each end with both an alignment hole and a cut-out window section. The alignment holes are used during the assembly of the fuel element to assume the necessary precise alignment of the fuel plates, these being stacked upon pins on a lower jig-block of a jig. The windows are utilized to provide extensions on either side of the plates beyond those portions which contain the fuel inserts by cutting off the ends of the plates in the finished fuel element about lines passing transversely through the windows. The fuel and spacer plates are stacked upon a lower jigblock and an upper jig-block is placed on top of the stack, a perpendicular compressive force then being applied hydraulically to the assembly and subsequently maintained during the welding process by means of threaded connections on the upper ends of the pins of the lower jigblock. The spacer plates are of slightly less width than the fuel plates, the jig-blocks being of the same width as the spacer plates. Whilst the composite stack of fuel and spacer plates is under compression, a welding means is reciprocally driven transversely of the edges of the stacked fuel plates while at the same time it is moved longitudinally so as to produce overlapping transverse welds integrally joining the projecting edges of the plates together. The depth of the weld penetration is so controlled as to prevent fusion of the weld metal with the spacer plates. After welding and while still compressed, the entire assembly is annealed to assure dimensional stability after the jig-blocks have been removed. Following the removal of the spacer plates, the outer faces of the deposited weld metal are machine-finished to the desired fuel element dimensions. Fuel elements having curved or arcuate fuel plates can be made by this method, it only being necessary for jig-blocks and spacer plates conforming to the contours of the fuel plates to be used. In an alternative method, the longitudinal edges of the fuel plates are bent almost to a right-angle from the main surfaces of the plates, with the amounts of the edges being bent equal to the; spacing between adjacent fuel plates. In this manner, the inner side walls of the coolant flow channels will be smooth since the depth of the weld metal penetration is limited to the outside surfaces of the bent legs of the fuel plates.