Patent Number: 055770819
Section: summary

BACKGROUND OF THE INVENTION 1. Technical Field of the Invention The present invention relates to a method of forming grids for nuclear fuel assemblies, as well as to the grids formed by the application of the same method. 2. Background Art In the conventional method of manufacturing nuclear fuel assembly grids, straps 1, which are formed, for example, of nickel alloy so as to have slits therein, are, as shown in FIGS. 7A to 7C, assembled into a grid form, and the intersections 2 of the straps 1 are then brazed. In this method, nickel is usually plated on the surfaces of the straps 1 in advance in order to ensure high-quality brazing. More specifically, FIG. 7A depicts a part of the assembled straps 1, showing a brazing filler metal (hereinafter referred to as "filler metal") being adherently placed on top of the intersections 2 of the straps 1. When the straps 1 are heated in a vacuum type brazing furnace (hereinafter referred to as "vacuum furnace"), the filler metal melts and flows down along the intersections 2, so that the intersections 2 are presumed to be brazed uniformly over their entire lengths. FIG. 7B depicts the result of brazing in the case where the brazing is effected on the straps having nickel-plated surfaces, whereas FIGS. 7C and 7D depict the result of brazing in the cases where the brazing is carried out on the non-Ni plated straps. In either of the straps shown in FIGS. 7C and 7D, good brazing results have not been obtained due to insufficient or ununiform flow of the filler metal flowing down from the top of the intersections 2 therealong. In FIG. 7C, unbrazed portions are shown remaining, whereas in FIG. 7D, the width and thickness of the brazed parts have become excessive. Thus, it has hitherto been conventional to carry out nickel plating on the straps 1 in order to ensure excellent brazing quality. However, such a plating process is unavoidably associated with an increased cost and environmental pollution. Therefore, there has been the need for the development of a novel brazing technique which requires no plating treatment but achieves excellent brazing quality. SUMMARY OF THE INVENTION It is therefore the primary object of the present invention to provide a method of forming a nuclear fuel assembly grid which requires no plating treatment but ensures improved brazing quality. Another object of the invention is to provide a nuclear fuel assembly grid which is obtained by the aforesaid method. According to a first aspect of the invention, there is provided a method of forming a nuclear fuel assembly grid, which includes the steps of (a) preparing a plurality of formed alloy straps each having slits, (b) arranging the straps into a grid form by intersecting the straps with each other through the slits, and (c) brazing intersections of the associated straps, the method further comprising the step of: (d) subjecting those portions to be brazed to a pretreatment prior to the arranging step (b), the pretreatment including applying a paste, comprising a mixture of a filler metal and a vehicle, to the portions to be brazed to form a thin film thereon. In the foregoing, the pretreatment step (d) may further include drying the paste film, heating the straps in a vacuum furnace to melt the filler metal in the paste, and cooling the straps; and the brazing step (c) may include placing a filler metal daub on top of the intersections of the straps, and heating the straps in a vacuum furnace to effect the brazing of the straps. Alternatively, the method may further comprise the steps of drying the paste film, heating the straps in a vacuum furnace to melt the filler metal in the paste, and cooling the straps between the arranging step (b) and the brazing step (c); and the brazing step (c) may include placing a filler metal daub on top of the intersections of the straps, and heating the straps in a vacuum furnace to effect the brazing of the straps. Furthermore, the brazing step (c) may include placing a filler metal daub on top of the intersections of the straps, and heating the straps in a vacuum furnace to effect the brazing of the straps; and the method may further comprise the steps of drying the paste film after the pretreatment step (d), subsequently effecting the brazing step (c), and heating the straps in a vacuum furnace to effect the brazing of the straps. In this modification, the step of drying the paste film may be omitted. Moreover, the filler metal may contain Ni powder, whereas the vehicle may contain a synthetic resin and an organic solvent. In addition, the paste may further contain an additional substance selected from the group consisting of water, an organic solvent such as methyl alcohol and a liquid surface active agent such as polyethylene glycol fatty acid ester. According to another aspect of the invention, there is provided a nuclear fuel assembly grid formed using the aforesaid method. In the method of the present invention, since the pretreatment is carried out on the surfaces of the straps, excellent solderability can be ensured even though the plating is omitted. Therefore, the nuclear fuel assembly grids of excellent quality can be obtained at reduced cost without causing any environmental problems.