Patent Number: 051125714
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention will now be described with reference to FIGS. 1 to 10. As shown in FIGS. 9 and 10, in a fuel assembly for use in a BWR according to present invention, a plurality of fuel rods 1, each consisting of fuel pellets 22 charged into a cell 21 having end plugs 23 welded onto the upper and lower ends thereof, are arranged in arrays, for instance, in eight rows and eight columns, in a channel box 7, with the upper and lower ends of the fuel rods 1 being fixed by tie-plates 8 and 9. A handle 10 is mounted on an upper portion of the assembly. In order to keep the thin, elongated fuel rods 1 correctly spaced from one another, spacer elements of a fuel spacer 2 are disposed in several stages which are vertically separated. As shown in FIG. 7, a round-cell-type fuel spacer 2 has a plurality of cylindrical cells 11 arranged in a grating-like manner. Adjacent cylindrical cells 11 are joined together by welding. As shown in FIG. 1, a fuel rod 1 inserted into a cylindrical cell 11 is supported by two projections 13 and a loop-shaped spring 12, which are provided within the cell 11. The cylindrical cell 11 has vanes 3 for generating swirls, as shown in FIGS. 1 and 2. Each vane 3 is formed by forming a cut in a part of the side wall of the cell 11 so as to separate a portion of the side wall from the cut, and bending the separated portion. The vane 3 thus formed is bent from the cut and projects obliquely outward. As shown in FIG. 7, the vanes 3 are disposed while projecting into the corresponding inter-rod space 5 surrounded by adjacent cylindrical cells 11, the space being substantially rhombic. The number of vanes 3 provided on one cell 11 varies in accordance with the position of the cell 11 in the cell matrix. As shown in FIG. 7, the corner cells each have one (the smallest number) vane, while the cells in inward arrays each have four (the greatest number) vanes. In the spacer 2 having the above-described construction, the vanes 3 cause the generation of swirls of coolant in the spaces between the fuel rods 1, which in turn causes an increased mount of liquid drops to adhere to the fuel rods 1, thereby increasing the thickness of the liquid films on the fuel rods 1. As a result, the transfer of heat from the fuel rods 1 to the liquid films formed by the coolant is promoted. This enables an increase in the critical output, hence, in the allowable power level. The cuts for forming the vanes 3 can be easily formed simultaneously with the formation of the spring portions for holding the fuel rods 1 in place, which are formed by embossing. The formation of the vanes 3, therefore, does not involve the drawback of the prior art where the formation of an oblique groove extending from a lower position to an upper position of the cell is difficult and necessitates an additional process. Another advantage is that, in contrast with the prior art where a guide groove is formed by bending at a sharp angle a portion of the cell cylinder which extends from a lower position to an upper position thereof, a vane 3 is formed by cutting a relatively small part of the side wall of the cell; this assures sufficient strength in the side wall. Furthermore, the degree of pressure loss in the spaces 5 between the fuel rods 1 is low. Still further, the flow of a liquid film between a cell and the fuel rod, that is, the endo-cell flow of liquid film, can be substantially free from disturbance. Although in the embodiment shown in FIG. 2, each vane 3 is formed by cutting an inverted-L-shaped slit in the associated side wall of the cell, the cut will not necessarily have this shape so long as the vanes 3 have tip portions which obliquely project into the corresponding spaces 5 between the fuel rods 1. For instance, each vane may be formed by cutting, for instance, a U-shaped slit. FIGS. 3 and 4 show another embodiment of the fuel spacer according to the present invention. A fuel spacer 2A according to this embodiment has cylindrical cells 11A receiving fuel rods 1. Each cell llA has vanes 3A which are each formed by cutting a vertically extending slit from the upper end of the cell, and bending the separated portion of the side wall of the cell llA in such a manner that it projects outward. If this construction is adopted, it is possible, similarly to the case of the fuel spacer 2, to promote the transfer of heat from the fuel rods to liquid films, thereby raising the allowable power level of the fuel rods 1. Another advantage is that the process of forming the vanes 3A is simpler than that required in the embodiment shown in FIGS. 1 and 2. When a fuel spacer of the same type as the above-described fuel spacer 2 or 2A having vanes of the same type as the vanes 3 or 3A is used in a fuel assembly, the amount of liquid drops flowing in the vapor through the inter-rod spaces 5 decreases in effect as the coolant flows toward the upper ends of the fuel rods 1. This means that the increment in the pressure loss, which the provision of the vanes 3 or 3A entails, rapidly decreases toward the upper ends, till the vicinity of the upper ends of the fuel rods 1 is substantially free from such an increment in the pressure loss. Conversely, in this area, it is of more importance that stronger swirls be generated so as to secure a sufficient amount of liquid films. A fuel assembly capable of meeting this requirement will be described as another embodiment. A fuel assembly of this embodiment has a fuel spacer 2B, of which essential parts are shown in FIG. 5. The fuel spacer 2B has cylinders 4, each being shown in FIG. 6. Specifically, in order to provide vanes, none of the cylindrical cells 11B of the spacers 2B are subjected to direct machining, but thin-walled cylinders 4, each having vanes 3B provided therein, are disposed in the spaces 5 between fuel rods 1 while being fixed to adjacent cylindrical cells 11B. According to this embodiment, since, similarly to the foregoing embodiments, swirls are generated in the cylinders 4, the transfer of heat from the fuel rods 1 to the liquid films is promoted, and the allowable power level of the fuel rods 1 is thus raised. Another advantage is that since the vanes 3B are provided within the cylinders 4, it is possible to design the dimensions and the configuration of the vanes more freely than the foregoing embodiments where portions of the side walls of the cylindrical cells are formed as vanes, and to secure the generation of stronger swirls. The section of the cylinders 4 are not necessarily circular; so long as the degree of pressure loss is as low as possible and the cylinders can be sufficiently firmly fixed to the side walls of the cylindrical cells 11B, the section of the cylinders may be angular. In order to generate still stronger swirls in the vicinity of the upper ends of the fuel rods 1, it is possible to arrange a fuel spacer of a different type in the vicinity of the upper ends of the fuel rods 1, the fuel spacer having spiral vanes 6, each being shown in FIG. 8, instead of the cylinders 4 shown in FIG. 5. When the merit of swirls generated by vanes is balanced against the increment in the degree of pressure loss entailed by the provision of the vanes, it is understood that the vanes may not necessarily be of one type throughout various areas arranged longitudinally of the fuel rods 1. Specifically, it is preferable to dispose an ordinary fuel spacer element having no vanes at a lower stage on the fuel rods where the pressure loss is a factor of importance requiring consideration, while a fuel spacer element having vanes is disposed at an upper stage on the fuel rods where the generation of swirls is a more important factor. It is also possible to dispose an ordinary fuel spacer element having no vanes at a lower stage on the fuel rods, while a vaned fuel spacer element of the type 2 or 2A is disposed at an intermediate stage on the fuel rods, and a vaned fuel spacer element which is either of the type 2B or of the type provided with the spiral vanes (shown in FIG. 8) is disposed at an upper stage of the fuel rods. Within the section of the fuel spacer shown in FIG. 7, the fuel rods in the arrays closest to the outer periphery of the fuel spacer as well as those in the arrays second to the closest are exposed to a thermally severe condition when compared to others. Therefore, vanes may be disposed only in the spaces surrounded by the fuel rods in these arrays. According to the present invention, vanes disposed in the spaces between the fuel rods generate swirls. Therefore, the transfer of heat from the fuel rods serving as the heat source to the coolant is promoted, thereby raising the allowable power level of the fuel assembly. Also, the void ratio is lowered, thereby increasing the reactivity. The fuel spacer according to the present invention, which achieves these advantages, is such that either vanes are formed by simply bending portions of the side walls of the cells of the spacer, or the spacer itself is not subjected to any direct machining and separate members, i.e., cylinders having built-in vanes or spiral vanes, are fixed to the spacer. Therefore, in contrast with the prior art where guide grooves are obliquely formed from lower positions to upper positions of the cells of the spacer, it is possible to assure a sufficient strength for performing the fundamental function of a fuel spacer which is to maintain fuel rods in their correct position. In the type where vanes are formed by directly subjecting the spacer cells to machining, vanes are formed simultaneously with the embossing of the spacer cells and by cutting parts of the side walls of the spacer cells and bending portions of the side walls from the cuts. Therefore, the manufacture of the spacer remains simple.