Patent Number: 053316797
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a fuel spacer for a fuel assembly and, more particularly, an independent cell type fuel spacer for the fuel assembly. A known fuel spacer for a fuel assembly incorporated in a boiling water type reactor has a structure, as for example, is disclosed in the Japanese Patent Laid-open (KOKAI) No. 59-65287 (65287/1984) and shown in FIG. 15. Referring to FIG. 15, the fuel spacer is composed of a plurality of tubular ferrules 41 in each of which a fuel rod is charged and these tubular ferrules 41 are arranged in a lattice structure, in which the adjoining ones are joined together by welding means, for example. A water rod is also charged in the tubular ferrule 41. As shown in FIG. 15, the ferrule 41 has inward projections 13b formed to its cylindrical wall, and a cutout 15 is formed to the ferrule wall and a pawl portion 16 is also formed so as to project in the cutout 15. Also known are cylindrical ferrules 42 each having an octagonal cross section as shown in FIG. 16, in which each ferrule 42 has inward projections 43 formed to the cylindrical wall thereof and reference numeral 44 denotes a continuous loop spring. Furthermore, there has been also studied a fuel spacer to be utilized for a fuel assembly in which a water rod has an outer diameter larger than an inner diameter of a cylindrical ferrule, such as that disclosed in the Japanese Patent Laid-open (KOKAI) No. 61-198096 (198096/1986). Recently, there has been developed an analysis of a mechanism between a transition boiling generation and a shape of a fuel spacer of a fuel assembly. Namely, it has been found out that transition boiling is likely to be generated at a portion near the lower ends of a first or second fuel spacer from the upper side of the fuel assembly in an installed state at a time when a power of the fuel assembly approaches its critical power output (at present, seven or eight fuel spacers are disposed axially along the fuel assembly). The fuel spacer acts to deposit liquid drops of a coolant to a surface of a fuel rod on a downstream side of the fuel spacer by agitating two phase flows of steam and liquid, thereby making the thickness of a liquid film thicker. As a result, the critical power output increases, contributing in the increasing of thermal margin of the fuel, increasing the power density of a reactor and increasing the power generation capacity. Accordingly, it has been considered to increase the number of the fuel spacers to be arranged. However, increasing of the number of the fuel spacers results in the increasing of fuel axial pressure loss. Because fuel spacers each have a relatively large local pressure loss due to rapid constriction or expansion of flow sectional area, and fuel spacer local pressure loss occupys a large portion of axial pressure loss. It provides a problem in stability of the fuel assembly and increasing the necessary lift of a pump for circulating a coolant in a core, which also results in an economical disadvantage. SUMMARY OF THE INVENTION An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art described above and to provide a fuel spacer for a fuel assembly capable of achieving a reduced pressure loss of the fuel spacer and improving the limit power output of the fuel assembly. This and other objects can be achieved according to the present invention by providing a fuel spacer assembly for a fuel assembly comprising: a plurality of tubular ferrules each forming a fuel rod insertion passage in which a fuel rod is inserted; PA1 a support means in the shape of belt for supporting a periphery of the tubular ferrules bundled in a lattice arrangement; and PA1 a spring means for axially supporting the fuel rods disposed in the ferrules, PA1 wherein each of the tubular ferrules has a cylindrical wall to which an inward projection is formed to support the fuel rod, adjoining ferrules are joined together horizontally, each of the ferrules having at least one end to which a plurality of cutout portions are formed circumferentially of the end portion, and flat portions being formed in some of cutout portions, and the adjoining ferrules being spot welded to each other at flat portions thereof. Four cutout portions are preferably formed to at least one end of the ferrule and each cutout portion may take various shapes such as substantially rectangular, trapezoidal or triangular shapes, or in combination thereof. The cutout portions may be formed to both axial ends of each of the ferrules so as to have the same shape or different shapes. The petal portions formed to an end portion of the ferrule on a downstream side are twisted outward with respect to a coolant flow between fuel rods to provide an opened end of the ferrule revolution flow. According to the preferred embodiments of the present invention, as described above, the tubular ferrule of the fuel spacer provides the cross sectional change in a trapezoid shape with respect to the coolant flow direction in the fuel assembly instead of a rectangular shape as made in the conventional art. Accordingly, the local pressure loss caused by the rapid constriction or expansion of the coolant flow passage at the location of the fuel spacer can be made small, and the liquid films formed on the surfaces of the fuel rods supported by the fuel spacer cannot be peeled off. Consequently, the pressure loss of the fuel spacer is made small in comparison with the usage of the conventional ferrules, and the critical power output of the fuel assembly can be also improved. Further natures and features of the present invention will be made more clear hereinlater, through description with reference to the accompanying drawings.