Patent Number: 055770819
Section: description

DETAILED DESCRIPTION OF THE EMBODIMENTS The present invention will be described in detail with reference to the accompanying drawings. For the manufacture of a nuclear fuel assembly grid in accordance with the method of the present invention, formed straps 1 having slits 4, as depicted in FIGS. 1A and 1B, are employed, as is the case with the conventional method. As depicted in FIG. 2, these straps 1 are assembled into a grid form by inserting one strap into the slit of the associated strap, and the intersections 2, at which the assembled straps perpendicularly intersect each other, are subjected to brazing, as indicated by reference numeral 15 in FIG. 2. Hereinbelow, a first embodiment of the grid-forming method of the present invention will be explained with reference to a flow chart shown in FIG. 3. First, the surfaces of the formed strap 1 are cleaned by means of an ultrasonic cleaner (or an etching device, when appropriate) to remove the adhering oils or the like, and the straps 1 thus cleaned are placed on a screen printing machine (Step 1, abbreviated to S1 in the drawings and the same shall apply hereinafter). In parallel with the above operation, a vehicle is added to a filler metal powder (e.g. above 50% nickel content), and surface active agents, alcohols or the like are further added thereto and mixed to obtain a paste having adjusted viscosity and solid content (Step 2). Subsequently, as shown in FIGS. 1A and 1B, the paste is thinly applied to the elongated portions along the slit 4 as well as the portions extending therefrom in a direction in which the slit 4 extends (Step 3). Such an application of the paste is carried out using a known screen printing technique. When the paste is dried (Step 4), the straps 1 are accommodated in a vacuum furnace and heated therein, so that a preliminary brazing is effected with the filler metal component in the paste (Step 5). The resulting straps 1 are then cooled in the furnace (Step 6), and after having been taken out from the furnace, the conditions of the preliminary brazing are checked (Step 7). Thereafter, the straps 1 are assembled into a grid form to provide a temporarily assembled grid (Step 8). Subsequently, the filler metal daub is adherently placed on the top of the intersections 2 of the temporarily assembled straps 1. The temporarily assembled straps are then introduced into the vacuum furnace and heated therein. Thus, the filler metal is melted to spread on the preliminarily brazed portions due to the surface affinity, resulting in a uniform brazing of the strap intersections 2 (Step 10). In the foregoing process, a nickel foil may be cut into strips and disposed adjacent to the straps 1 so as to serve as an oxygen adsorbing getter. Thus, the formation of the grid is completed (Step 11). FIG. 4 shows a flow chart of the steps of the method of forming the grid in accordance with a modified embodiment of the invention. In a manner similar to that of the method illustrated in FIG. 3, the surfaces of the straps 1 are cleaned by means of an ultrasonic cleaner (or an etching device, when appropriate) to remove the adhering oils or the like, and the straps 1 thus cleaned are placed on a screen printing machine (Step 21). In parallel with the above operation, a vehicle is added to a filler metal powder (e.g. above 50% nickel content), and surface active agents, alcohols or the like are further added thereto and mixed to obtain a paste having adjusted viscosity and solid content (Step 22). Subsequently, as shown in FIGS. 1A and 1B, the paste is thinly applied to the elongated portions along the slit 4 as well as the portions extending therefrom in a direction in which the slit 4 extends (Step 23). Then, before the paste is dried, the straps 1 are assembled into a grid form (Step 24). When the paste is dried (Step 25), the straps 1 are accommodated in a vacuum furnace and heated therein, so that a preliminary brazing is effected with the filler metal component in the paste (Step 26). The resulting straps are then cooled in the furnace (Step 27), and after having been taken out from the furnace, the conditions of the preliminary brazing are checked (Step 28). Thereafter, the filler metal daub is adherently placed on the top of the intersections 2 of the straps 1 thus temporarily assembled (Step 29). Then, the temporarily assembled straps 1 are introduced into a vacuum furnace and heated therein. The filler metal is thus melted to spread on the preliminarily brazed portions due to the surface affinity, resulting in a uniform brazing of the strap intersections 2 (Step 30). Thus, the formation of the grid is completed (Step 31). FIG. 5 shows a flow chart of the steps of the method of forming the grid in accordance with another modified embodiment of the invention. In a manner similar to that of the method illustrated in FIG. 4, the surfaces of the straps 1 are cleaned by means of an ultrasonic cleaner (or an etching device, when appropriate) to remove the adhering oils or the like, and the straps 1 thus cleaned are placed on a screen printing machine (Step 41). In parallel with the above operation, a vehicle is added to a filler metal powder (e.g. above 50% nickel content), and surface active agents, alcohols or the like are further added thereto and mixed to obtain a paste having adjusted viscosity and solid content (Step 42). Subsequently, as shown in FIGS. 1A and 1B, the paste is thinly applied to the elongated portions along the slit 4 as well as the portions extending therefrom in a direction in which the slit 4 extends (Step 43). Then, before the paste is dried, the straps 1 are assembled into a grid form (Step 44). When the paste is dried (Step 45), the filler metal daub is adherently placed on the top of the intersections 2 of the straps 1 (Step 46). Thereafter, the straps 1 thus temporarily assembled are accommodated in a vacuum furnace and heated therein. Thus, the filler metal component in the paste as well as the filler metal adhered to the top of the intersections 2 of the straps 1 are melted to spread on the preliminarily brazed portions due to the surface affinity, resulting in a uniform brazing of the strap intersections 2 (Step 47). The formation of the grid is thus completed (Step 48). FIG. 6 shows a flow chart of the steps of the method of forming the grid in accordance with a further modified embodiment of the invention. In a manner similar to that of the method illustrated in FIG. 4, the surfaces of the straps 1 are cleaned by means of an ultrasonic cleaner (or an etching device, when appropriate) to remove the adhering oils or the like, and the straps 1 are placed on a screen printing machine (Step 51). In parallel with the above operation, a vehicle is added to a filler metal powder (e.g. above 50% nickel content), and surface active agents, alcohols or the like are added thereto and mixed to obtain a paste having adjusted viscosity and solid content (Step 52). Subsequently, as shown in FIG. 1, the paste is thinly applied to the elongated portions along the slit 4 as well as the portions extending therefrom in a direction in which the slit 4 extends (Step 53). Then, before the paste is dried, the straps 1 are assembled into a grid form (Step 54). Thereafter, the filler metal daub is adherently placed on the top of the intersections 2 of the straps 1 thus temporarily assembled (Step 55). Subsequently, the temporarily assembled straps 1 are accommodated in a vacuum furnace and heated therein. Thus, the filler metal adhering to the top of the intersections 2 of the straps 1 as well as the filler metal component in the paste are melted to spread on the preliminarily brazed portions due to the surface affinity, resulting in a uniform brazing of the strap intersections 2 (Step 56), and the formation of the grid is thus completed (Step 57). In the methods as described above, the paste containing a filler metal therein is applied to the straps, and the filler metal is further placed on the paste to carry out the brazing, the filler metal is made to flow on the paste uniformly and smoothly without forming any voids. Accordingly, the prior art problems such as high cost, pollution or the like can be avoided, and the nuclear fuel assembly grids having excellent brazing qualities can be obtained. Finally, the present application claims the priority of Japanese Patent Application No. 6-169851, filed Jul. 21, 1994, which is incorporated herein by reference.