Patent Number: 062787586
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment according to the present invention will be hereinafter described by reference to the accompanying drawings. FIG. 1 is a partial top view of a support grid 40 for a fuel assembly according to the present invention, and a plurality of straps 50, 60 are assembled in a crossed manner as described later so as to define a plurality of grid cells 41 positioned in a rectangular arrangement. In other words, the disposition of the grid cells 41 is a square arrangement such as 14.times.14, 15.times.15 and 17.times.17. Then, fuel rods 9 are individually placed through these grid cells 41 as shown by the dash-and-two-dot Sines and resiliently supported as in conventional grids. Shape of the blanks for the straps 50, 60 are partially shown in FIGS. 2a and 2b, respectively. Describing the structure of the strap 50 by reference to FIG. 2a, slits 51 are formed along an upper side edge of the strap 50 at intervals of a length corresponding to a distance between opposite sides of the grid cell 44. The slits 51 are designed to receive the straps 60 as the other member and extend perpendicular to a longitudinal axis of the strap 50. At an open-end side of the slit 51, a welding tab 57 is protrudingly formed, and another welding tab 59 is formed at the opposite side edge. Mixing vanes 53, 55 are formed at the both sides of the opening of alternate slits 51. Furthermore, welding apertures 53a, 55b are formed in the base end on either side of the slits 51 and outer side ends 53b, 55b are shaped in a curved outline. At the bend line 54 shown by dash-and-two-dot lines, the mixing vanes 53, 55 are individually bent in opposite directions so as to place the outer side ends 53b, 55b close to fuel rods 9 with narrow gaps as shown in FIG. 1. The structure of the strap 60 to be joined to the strap 50 is depicted in FIG. 2b. Slits 61 similar to the slits 51 are formed at intervals along a lower side edge thereof. Further, welding tabs 67 are protrudingly formed at the side of open ends of the slits 61. As can be understood from FIG. 1, the slits 51, 61 are each positioned at the crossing portion between the straps 50, 60 and mixing vanes 63, 65 are protrudingly formed at the upper side edge of the strap 60 in alignment with slits 61 corresponding to the slits 51 without the mixing vanes 53, 55. The mixing vanes 63, 65 each have the same shape as that of the mixing vanes 53, 55 and have a weld aperture 63a, 65a and curved outer side ends 63b, 65b and are each to be bent in opposite directions at the bend line 64 during assembly. This state is shown in FIG. 1. The support grids 40 of the structure as shown in FIG. 1 are joined into a fuel assembly, which is in turn loaded in a nuclear reactor core. During operation of the nuclear reactor, the coolant flows upwards (from the lower portion of the illustration to the top portion in FIGS. 2a and 2b) between the fuel rod 9 and the straps 50, 60 and a portion thereof impinges onto the slanted mixing vanes 53, 55, 63, 65 and caused to swirl, thereby promoting mixing of the coolant. In the aforementioned structure, the bend lines 54, 64 parallel to the longitudinal axis are located above the welding apertures 53a, 55a, 63a, 65a and so the apertures do not show in the horizontal plane of projection as is clear in FIG. 1. In other words, the coolant impinging area of the mixing vanes 53, 55, 63, 65 are larger than in the conventional structure, thereby promoting and increasing agitation and mixing of the coolant. Next, another embodiment according to the present invention will be described making reference to FIG. 3 and FIGS. 4a and 4b. FIG. 3 is a partial top view of a support grid 140 for a fuel assembly and a plurality of straps 150, 160 are similarly assembled in a crossed manner so as to define a plurality of grid cells 141 in a square arrangement. Moreover, the fuel rods 9 are individually placed through and resiliently supported in these grid cells 141 as shown by dash-and-two-dot lines. In FIGS. 4a and 4b, the blank shapes of the straps 150, 160 are partially shown. Describing the structure of the strap 150 by reference to FIG. 4a, slits 151 are formed along the upper side edge and each of them extends vertically in the drawing. Welding tabs 157, 159 are formed in a similar pattern. Mixing vanes 153, 155 are formed at both sides of the opening of alternate slits 151 and welding apertures 153a, 155a are defined in the base end of the vanes 153, 155 at either side of the slits 151. A gap 152 between the mixing vanes 153, 155 is slightly larger than that in the structure shown in FIGS. 2a and 2b. Moreover, at the bend lines 154 shown by dash-and-two-dot lines, the mixing vanes 153, 155 are bent in opposite directions, respectively, so as to be adjacent to the fuel rods 9 as shown in FIG. 3. The structure of the straps 160 to be combined with the straps 150 is depicted in FIG. 4b. Slits 161 similar to the slits 151 are defined at intervals along the lower side edge of the strap 160. Moreover, welding tabs 167, 169 are protrudingly formed as in the strap 60. In alignment with the slits 161 corresponding to the slits 151 without the mixing vanes 153, 155, mixing vanes 163, 165 are protrudingly formed at the upper side edge of the strap 160. In addition, as apparent from a comparison of FIG. 4a with FIG. 4b, the mixing vanes 163, 165 are larger in length than the mixing vanes 153, 155. They are bent in opposite directions at bend lines 164 that are slanted to avoid the welding apertures 163a, 165a during assembly. This state is illustrated in FIG. 3. The straps 150, 160 of the aforementioned structure are assembled to become the support grid 140 after the mixing vanes 153, 155, 163, 165 are bent from their blank state. In this situation, the mixing vanes 163, 165 slightly overlap adjacent grid cells 141 as shown in FIG. 3 while the mixing vanes 153, 155 do not overlap adjacent grid cells because the gap 152 is relatively large and so the assembly of the straps is not obstructed by the mixing vanes 153, 155. The support grid 140 is also, in a way similar to one for the support grid 40, joined into a fuel assembly, which is loaded in a nuclear reactor core. During the operation of the nuclear reactor, the coolant flows upwards between the fuel rod 9 and the straps 150, 160 and a portion of the coolant impinges on the slanted or bent mixing vanes 153, 155, 163, 165 to be agitated, thereby promoting mixing. In the abovementioned structure, since the length of the mixing vanes 163, 165 is larger than conventional ones, the total area of the projected plane of the mixing vanes 153, 155, 163, 165 becomes larger thereby making the coolant impinging surface larger than that in the prior art, increasing the effects of agitation and mixing. As described above, according to the present invention, since the area of the slanted portion of the mixing vanes protrudingly formed at side edges of the straps constituting a support grid in a horizontal projected plane is increased by displacement of the bend line from which the slanted portion begins or the increased length of all the mixing vanes, the effects of agitation and mixing in the coolant can also be increased.