Abstract:
An improved support grid of a nuclear reactor fuel assembly is described. The support grid has a plurality of first and second straps which are assembled so as to form an eggcrate structure defining grid cells in a square arrangement. The support grid includes a plurality of mixing vanes integrally formed on a side edge of the first and second straps adjacent to an area where the straps cross. The mixing vanes slanted to be adjacent to a fuel rod are each shaped to show no welding apertures formed therein in their horizontal planes of projection thereby maximizing the coolant impinging area to agitate and swirl a coolant flow.

Description:
FIELD OF INVENTION 
     The present invention relates to a nuclear reactor fuel assembly, and more specifically to a support grid structure for supporting fuel rods thereof. 
     BACKGROUND OF THE INVENTION 
     A typical structure of a fuel assembly for a pressurized water reactor is shown in FIG.  5 . Describing it briefly, an upper and lower nozzle  1 ,  3  having a plurality of coolant flow holes machined therein are connected with a plurality of hollow guide tubes  5  which are parallel to one another. Support grids  7  having grid cells positioned in a rectangular arrangement which individually receive the hollow guide tubes  5  therethrough and which are fixed to the guide tubes  5  support the fuel rods  9  by placing them individually through the remainder of the grid cells. A fuel assembly  10  is thus constructed and in order to more clearly show the structure, a portion of the fuel assembly  10  from which the surroundings about one support grid  7  are removed is shown in a partial perspective view in FIG.  6 . As readily understandable from FIG. 6, the disposition of the fuel rods  9  is in a square arrangement with equal numbers of columns and rows and it is constructed showing some of the fuel rods  9  at specified locations being replaced with the hollow guide tubes  5 . 
     The support grid  7  defining grid cells which individually receive, as described before, the fuel rods  9  and the hollow guide tubes  5  disposed in a square arrangement, is essentially constructed by combining two kinds of thin metal straps as shown in FIGS. 7 a  and  7   b , namely, straps  20 ,  30 , with each other in a perpendicular relationship. The support grid  7  defines grid cells located in a 14×14 arrangement, and the straps  20 ,  30  each essentially have an identical configuration and differ from each other in the positions at which slits  21 ,  31  for receiving another strap corresponding thereto are formed. In other words, slits  21  in the strap  20  are positioned on an upper side (downstream in the coolant flow), while slits  31  in the strap  30  are positioned on a lower side (upstream in the coolant flow). Mixing vanes  23 ,  25 ,  33 ,  35  are integrally formed at a downstream edge of the straps  20 ,  30  in alignment with the slits  21 ,  31 . Furthermore, tabs  27 ,  29 ,  37 ,  39  for welding the straps  20 ,  30  which are assembled by using slits  21 ,  31  are provided. The positional relationship among the slits  21 ,  31 , the mixing vanes  23 ,  25 ,  33 ,  35  and the tabs  27 ,  29 ,  37 ,  39  that are described above is schematically shown in an enlarged manner in FIGS. 5 a  and  8   b.    
     The above described mixing vanes  23 ,  25  and  33 ,  35  are bent and slanted in opposite directions, respectively, as shown in FIG. 9, when the straps  20 ,  30  are assembled. A partial top view of the support grid  7  corresponding to FIG. 9 is shown in FIG.  10 . As seen from FIG. 10, the outer extremities of the mixing vanes  23 ,  25 ,  33 ,  35  are close to the fuel rods  9  depicted by dash-and-two-dot lines, but not close enough to come into contact with the fuel rods  9 . In these structures, since a coolant stream which flows through the support grid  7  impinges on the mixing vanes  23 ,  25 ,  33 ,  55 , the coolant stream is agitated to be stirred and make the temperature distribution therein uniform. 
     SUMMARY OF THE INVENTION 
     However, in the conventional support grid  7 , as clearly shown in FIGS. 8 and 10 in particular, welding apertures  26 ,  36  are formed by an inner side of the mixing vanes  23 ,  25 ,  33 ,  35 , respectively. Since the coolant stream passes through these welding apertures  26 ,  36  without impingement or interference, no agitation of the coolant passing through the welding apertures can be expected and the agitating and mixing function of all the mixing vanes is therefore not sufficient to achieve the desired effects. 
     Accordingly, an object of the present invention is to provide a support grid with mixing vanes for a fuel assembly which do not increase a pressure drop in the coolant flow and which further provides improved agitating and mixing functions. 
     In order to accomplish the object described above, according to the present invention, in a support grid of a nuclear reactor fuel assembly wherein the support grid has a plurality of first straps which are made of thin metal band plate, and a plurality of second straps which are made of thin metal band plate, the first and second straps are each provided with slits extending widthwise for receiving the other of the straps and assembled so as to receive opposite straps in the slits of each strap to cross each other and thereby form an eggcrate structure, mixing vanes integrally formed on a side edge of the first and second straps adjacent to a crossed area and in alignment with a slit, each of the mixing vanes being slanted so as to be adjacent to a nuclear reactor fuel rod to be placed through a grid cell of the eggcrate structure, the slanted portion of the mixing vane being shaped so as to maximize an area of the slanted portion of the mixing vane projected onto a plane perpendicular to coolant flow direction. Preferably, the mixing vanes are shaped such that they have a welding aperture formed in a base portion at the side of the slit, and a bend line extending parallel to a longitudinal axis of the strap is located closer to a distal end side than the welding aperture and a curved outer edge of the mixing vanes is located at the distal end side of the bend line. Furthermore, in place of the above, the shape of the mixing vane can be formed such that the bend line at which the slanted surface of the mixing vane begins is slanted with respect to the longitudinal axis of the strap so as to avoid the welding aperture and the length of the mixing vane of the first strap is larger than the length of the mixing vane of the second strap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a partial plan view showing a preferred embodiment according to the present invention; 
     FIG. 2 a  is a partial side view showing a blank shape for a member used in the preferred embodiment according to the present invention; 
     FIG. 2 b  is a partial side view showing a blank shape for another member used in the preferred embodiment; 
     FIG. 3 is a partial top view showing another preferred embodiment; 
     FIG. 4 a  is a partial side view showing a shape of a blank for a member used in said other embodiment; 
     FIG. 4 b  is a partial side view showing a shape of a blank for another member used in said other embodiment; 
     FIG. 5 is a shortened elevational view of a fuel assembly in which a support grid according to the present invention is incorporated; 
     FIG. 6 is a partial perspective view showing a portion cut out from the fuel assembly in FIG. 5; 
     FIG. 7 a  is a side view showing a shape of a blank for a strap of a conventional support grid; 
     FIG. 7 b  is a side view showing a shape of a blank for another strap of the conventional support grid; 
     FIG. 8 a  is a schematic view showing a portion of the strap in FIG. 7 a  in an enlarged fashion; 
     FIG. 8 b  is a schematic view showing a portion of the strap in FIG. 7 b  in an enlarged fashion; 
     FIG. 9 is a partial perspective view of the conventional support grid; and 
     FIG. 10 is a partial top view of the conventional support grid. 
    
    
     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 14×14, 15×15 and 17×17. 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. 2 a  and  2   b , respectively. Describing the structure of the strap  50  by reference to FIG. 2 a , 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  53   a ,  55   b  are formed in the base end on either side of the slits  51  and outer side ends  53   b ,  55   b  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  53   b ,  55   b  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. 2 b . 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  63   a ,  65   a  and curved outer side ends  63   b ,  65   b  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. 2 a  and  2   b ) 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  53   a ,  55   a ,  63   a ,  65   a  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. 4 a  and  4   b . 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. 4 a  and  4   b , the blank shapes of the straps  150 ,  160  are partially shown. Describing the structure of the strap  150  by reference to FIG. 4 a , 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  153   a ,  155   a  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. 2 a  and  2   b . 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. 4 b . 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. 4 a  with FIG. 4 b , 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  163   a ,  165   a  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.