Patent Number: 
Section: description

As shown in these drawings, the spacer grid 100 used for supporting fuel rods within a nuclear fuel assembly in accordance with the present invention is fabricated by interlacing a plurality of first and second inner straps 10 and 20 at right angles at the slits 12 and 22, thus forming a desired spacer grid 100 having an egg-crate pattern. The interlaced first and second inner straps 10 and 20 are welded together at the top and bottom of each interlaced cross point, thus being integrated into a single structure. Each of the first and second inner straps 10 and 20 is integrated at their upper edge with a plurality of double deflected vanes 30. Each of the double deflected vanes 30 comprises a swirl flow inducing vane 31 integrally extending upward from the upper edge of an associated inner strap, while being deflected toward fuel rod, and a main vane 32 formed by bending the middle of the double deflected vane along the predetermined inclined bending line. As shown in FIGS. 6 and 7, the first and second inner straps 10 and 20 of the spacer grid 100 each has a rectangular body 11 or 21, which has a thin, straight cross-section. Each of the first and second inner straps 10 and 20 also has a plurality of vertical slits 12 or 22, which extend from the upper or lower edge of an associated inner strap to its middle, while being regularly spaced apart from each other. A pair of welding tabs 50 is formed on the edge of each inner strap 10 or 20 at the sides of each vertical slit 12 or 22, and melts during an intersection welding process to form welding nuggets. In each of the inner straps 10 and 20, the slits 12 or 22 are spaced at an interval equal to the pitch of the fuel rods. As shown in FIG. 5, The double deflected vanes 30 are integrated with the upper edge of each of the first and second inner straps 10 and 20 and are formed either side of the vertical slits 12 and 22, such that a pair of vanes 30 around each of the slits 12 and 22 are rotationally symmetrical with each other. The double deflected vanes 30 each comprise a swirl flow inducing vane 31 and a main vane 32. The swirl flow inducing vane 31 is integrated with the upper edge of an associated strap 10 or 20, and is deflected in a direction toward an associated fuel rod 200. The main vane 32 is formed by bending the middle of the double deflected vane along the predetermined inclined bending line. Each of the vanes 30 is thus deflected twice to have double deflection angles. The swirl flow inducing vane 31 is an asymmetric triangular shape which has an inclined upper edge, an inclined bending line and a base portion 311 and is deflected toward an associated fuel rod 200 at a predetermined acute angle relative to the plane of an associated inner strap 10 or 20, as best seen in FIG. 5. The swirl flow inducing vane 31 simultaneously generates negative pressure at the back of the vane and high pressure at the front of the vane. This high and low pressure distribution makes it easy to cause axial flow of the coolant to lateral flow of coolant, guides the axial coolant flow to the main vane, and increases the total swirl flow of coolant inside the spacer grid. The width of base portion 311 of the swirl flow inducing vane 31 is substantially equal to the distance between the welding nuggets formed at each intersection of the inner straps 10 and 20. The base portion 311 is increased in its length in comparison with that of a conventional spacer grid, and so it is possible to prevent an unexpected bending of the vanes due to the contact of the vanes with fuel rods during an insertion of the fuel rods into a cell opening. The vane has a polygonal shape with a wide base portion and a narrow top portion, to make a smooth variation of cross sectional of the coolant passage at upper part of strap, thereby reducing the pressure loss of the coolant flow through the spacer grid upper portion and improving hydraulic performance of the spacer grid. The main vane 32, which is formed by bending the middle of the double deflected vane along the predetermined inclined bending line, is deflected in the direction toward an associated fuel rod 200 relative to the plane of the swirl flow inducing vane 31 and has a curved edge 321 which is shaped to surround the associated fuel rod 200 and preferably maintains a constant distance from the surface of the fuel rod 200, as shown in FIGS. 5 and 7, is to generate a swirl flow of coolant in the fuel assembly. As shown in FIGS. 3 and 4, a pair of double deflected vanes 30 are positioned within each of the square cells such that the vanes 30 face each other and are deflected toward the fuel rod inside the square cell so as to generate swirl flow about the axis of the fuel rod 200 in a cell. As described above, the present invention provides a spacer grid with a plurality of double deflected vanes for nuclear fuel assemblies, which generate a strong swirl flow around the fuel rod, preventing the vane from bending upon rod insertion and causing low pressure drop in the outlet of the spacer grid. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.