Patent Number: 
Section: description

FIG. 5 is a perspective view of a double strip mixing grid for nuclear reactor fuel assemblies having a 5xc3x975 array in accordance with the primary embodiment of the present invention. FIG. 6 is a plan view of the double strip mixing grid of FIG. 5, with fuel rods removed from the grid. FIG. 7 is a plan view of the double strip mixing grid of FIG. 5, showing the operational function of mixing blades of the grid. FIG. 8 is a plan view of the double strip mixing grid of FIG. 5, showing the coolant currents formed by the mixing blades of the grid. FIG. 9 is a perspective view of a double strip mixing grid for nuclear reactor fuel assemblies having a 5xc3x975 array in accordance with the second embodiment of the present invention. FIG. 10 is a plan view of the double strip mixing grid of FIG. 9, with only one fuel rod set within the central cell of the rid. FIG. 11 is a plan view, showing the construction of one intersection of the grid of FIG. 10. FIG. 12 is a top perspective view of the intersection of FIG. 11. FIG. 13 is a bottom perspective view of the intersection of FIG. 11. FIG. 14 is a perspective view of a nozzle sheet of each, double strip according to this invention. FIG. 15 is a perspective view of a bladed sheet of the double strip according to this invention. FIG. 16 is a perspective view of a double strip having a coolant channel according to this invention, with the coolant channel, being cut along its central axis. As shown in the drawings, the double strip mixing grid 310 or 410 according to the preferred embodiments of this invention is used for placing and supporting a plurality of elongated nuclear fuel rods 325 within a nuclear reactor fuel assembly, and comprises two sets of intersecting double inner strips, which are arranged while intersecting each other at right angles prior to being encircled with four perimeter strips, thus forming an egg-crate pattern. Each of the two sets of grid strips is fabricated by integrating two thin sheets together into a single structure while defining a plurality of coolant channels in each of the two strips. In such a case, the two thin sheets of each of the strips are preferably continuously welded together at their junctions. In the double strip mixing grid 310 or 410 of this invention, the outlet of each coolant channel is formed by cutting a predetermined portion of one sheet of each of the two strips at the top edges of the strips. In addition, the cross-sectional area of each of said coolant channels is gradually enlarged in a direction from the inlet to the outlet of the channel, or varies such that it is maximized at a middle portion of the channel supporting a fuel rod within each, four-walled cell. In the double strip mixing grid 310 or 410 of this invention, each of the intersecting strips has a thickness preferably ranging from 0.25 mm to 0.40 mm, while each of the coolant channels has a width preferably ranging from 7 mm to 10 mm. In the double strip mixing grid 310 of FIG. 5, the two intersecting double strips are each fabricated by integrating two stamped thin sheets together into a single structure while forming a plurality of regularly spaced coolant channels between the two sheets. In the-present invention, each of the thin sheets of the double strip is preferably made of zircaloy, or the alloy of tin, iron, chrome and zirconium. However, it should be understood that the sheets of the strips may be preferably made of inconel that has been typically used as a material of such grid strips in the prior art. In the mixing grid 310 of FIG. 5, four first grid strips, each having a plurality of swirling flow blades 330 and a plurality of lateral-flow blades 331, regularly intersect four second grid strips, having the same construction as that of the first strips, at right angles prior to being encircled with four perimeter strips, thus forming a double strip mixing grid having a 5xc3x975 array with twenty five cells. In FIG. 5, the perimeter strips are not shown, and so the sixteen outside cells defined by the two sets of intersecting inner strips and the four perimeter strips are not-completely formed. Only nine fuel rods 325 are set in the nine inside cells formed by the intersecting inner strips. In the double strip mixing grid 310 of FIG. 5, the coolants flowing in the coolant channels of the strips are mixed with the coolants flowing outside the channels at positions around the mixing blades provided at the nozzles of the channels, thus forming swirling flow currents at positions around the swirling flow blades 330 and lateral flow currents at positions around the lateral flow blades 331. The coolants within the nuclear fuel assembly are thus actively and effectively mixed together. In such a case, the swirling flow currents are formed at positions around the intersections of the strips included in the grid, while the lateral flow currents are created between the four-walled cells of the grid. In such a case, the lateral flow currents of coolants flow toward the swirling flow blades 330, and so the lateral flow currents promote the formation of a swirling flow of coolants, in addition to forcing the coolants to be actively and effectively mixed together. FIG. 6 is a top plan view, showing the configuration of the double strip mixing grid 310 of FIG. 5 having both the swirling flow blades 330 and the lateral flow blades 331. The arrangement of both the swirling flow blades 330 and the lateral flow blades 331 on the intersecting double strips of the grid is shown in FIG. 6 in more detail. In the double strip mixing grid of FIG. 6, a plurality of first inner double strips 315, having only the swirling flow blades, intersect a plurality of second inner double strips 316, having both the swirling flow blades and the lateral flow blades, at right angles to form a desired egg-crate pattern. In the present invention, it is possible to fabricate a desired double strip mixing grid 310 of FIGS. 5 and 6 by intersecting the two types of double strips 315 and 316 together at right angles prior to encircling the intersected strip structure with the four perimeter strips. FIGS. 7 and 8 show the blade shape of the double strip mixing grid 310 of FIG. 6, and the coolant currents 340 formed by the swirling flow blades 330 and the coolant current 341 formed by the lateral flow blades 331 of the grid 310. It is possible for those skilled in the art to more clearly understand the style of the coolant currents formed by the two types of blades 330 and 331, in addition to understanding the forming positions of the coolant currents within the grid in more detail. FIG. 9 shows a double strip mixing grid 410 for nuclear reactor fuel assemblies having a 5xc3x975 array in accordance with the second embodiment of the present invention, with the double strips of the grid 410 having only the swirling flow blades 330 without having the lateral flow blades 331, different from the double strip mixing grid 310 of the primary embodiment. In this drawing, only one fuel rod is set in the central cell of the mixing grid 410 for ease of description. In the present invention, it is possible to fabricate a double strip mixing grid 410 of the second embodiment using two types of strips 318 and 319. However, it should be understood that the grid 410 of the second embodiment can be formed using one of either type of strips 318 or 319 without affecting the functioning of this invention. FIG. 10 is a plan view of the double strip mixing grid 410 of FIG. 9, showing both the shape of the mixing blades including the swirling flow blades 330 and the shape of the intersecting double strips in more detail. FIG. 11 is a plan view, showing the construction of one intersection of the double strip mixing grid 410 of FIG. 10. The shape of the swirling flow blades 330 formed on the grid 410 of the second embodiment is shown in more detail in FIG. 11. In the present invention, the two sets of inner strips are intersected together at right angles, and are welded together at their intersections through a TIG (Tungsten Inert Gas) welding process or a laser beam welding process, thus forming the welded intersections 317 as shown in FIG. 11 and a desired number of four-walled cells for the fuel rods. FIGS. 12 and 13 are top and bottom perspective views of the intersection of FIG. 11, respectively. In a nuclear fuel assembly fabricated with the double strip mixing grids of this invention, coolants flow from the bottom edge of each grid to the top edge of each grid while passing through the inside and outside of the coolant channels defined in the two sheets of each of the intersecting double strips, and are mixed together at positions around the nozzles of the coolant channels, provided along the top edge of the grid and having the swirling flow blades, thus forming desired swirling motion of coolants. Such a swirling motion of coolants is shown by the arrows designated by the reference numeral 340 in FIG. 12. In order to prevent the fuel rod positioning springs of the intersecting double strips included in the mixing grid of this invention from being excessively increased in their strength, a vertical slot 350 is formed on each sheet of the strips at a position around each coolant channel. When each thin sheet of the intersecting double strips, having a thickness of 0.35 mm, is provided with a slot 350, having a width of 0.5 mm and a length of 16 mm, at each of the spring portions, it is possible to decrease the stiffness of the positioning springs by ⅓ times, and increase the elastic range of the springs two times in comparison with a grid not having such slots. FIG. 14 is a perspective view of a nozzle sheet of each double strip included in the mixing grid of this invention. FIG. 15 is a perspective view of a bladed sheet of the double strip included in the mixing grid of this invention. When the two sheets of FIGS. 14 and 15 are integrated together into a single structure, it is possible to fabricate a double strip having a plurality of coolant channels of FIGS. 11, 12 and 13. In In the present invention; a plurality of first portions having coolant nozzles 361 and a plurality of second portions having swirling flow blades 330 are alternately arranged along each sheet of the double strips of the double strip mixing grid of this invention. The two sheets may be integrated into a first double strip 318 or a second double strip 319 of FIGS. 11, 12 and 13 by arranging the nozzles 361 and the blades 330 at alternating positions. A plurality of first and second double strips 318 and 319 intersect each other at right angles prior to being welded at their intersections, thus forming a desired double strip mixing grid of FIG. 8. FIG. 16 is a perspective view of a double strip, fabricated by integrating the two sheets of FIGS. 14 and 15 together to form a plurality of regularly spaced coolant channels between the two sheets. In the drawing of FIG. 16, the coolant channel is cut along its central axis. As shown in FIG. 16, coolant flows into the channel through the inlet 360 and flows out of the channel from the outlet 361 during an operation of the nuclear reactor. In such a case, the coolant current from the outlet 361 restricts a formation of vortexes in the coolant, which flows outside the channel and collides against the mixing blades to make such vortexes if the grid does not form such a coolant current discharged from the outlet 361. The coolant current from the outlet 361 also makes a smooth flow of coolant currents formed by the mixing blades. In the double strip mixing grid of this invention, the coolant mixing blades are provided along the top edges of the intersecting double strips having coolant channels. Therefore, it is possible to fabricate a double strip mixing grid 410 having only the swirling flow blades, a double strip mixing grid 310 having both the swirling flow blades and the lateral flow blades, or a double strip mixing grid having only the lateral mixing blades. As described above, the present invention provides a double strip mixing grid for nuclear reactor fuel assemblies. In the present invention, a plurality of inner double strips, each fabricated by integrating two thin sheets together into a single structure having a plurality of coolant channels, are intersected at right angles to form a desired mixing grid. Due to the coolant channels, the mixing grid of this invention effectively mixes the low temperature coolant with the high temperature coolant within a nuclear fuel assembly during an operation of a nuclear reactor, thus improving the thermal efficiency of the nuclear fuel assemblies. This mixing grid also effectively prevents the coolant from being partially overheated, and so it is possible to improve the soundness of nuclear reactors. In the double strip mixing grid of this invention, the coolant currents discharged from the nozzles of the channels restrict a formation of vortexes in the coolant, which flows outside the channels and collides against the mixing blades of the top edges of the strips to make such vortexes if the grid does not form such coolant currents discharged from the nozzles of the channels. Therefore, the mixing grid of this invention further enhances its coolant mixing function in comparison with conventional mixing grids only having the mixing blades without such channels. This mixing grid thus further improves the thermal efficiency of the nuclear fuel assemblies. In addition, the double strip mixing grid of this invention is designed to elastically support an elongated fuel rod by the sheets of the double strips collaterally acting as positioning springs. Therefore, each fuel rod set within a cell of the grid of this invention is elastically supported by four positioning springs. This means that the double strip mixing grid of this invention effectively supports a displacement of each fuel rod by two positioning springs in the same manner as that of a conventional grid having both positioning springs and dimples. The mixing grid of this invention thus stably supports each fuel rod during an operation of a nuclear reactor, different from a conventional grid supporting the fuel rod by one positioning spring. In addition, the double strips of the mixing grid of this invention is provided with a vertical slot at a position, where the strip comes into contact with a fuel rod while supporting the fuel rod. The elastic range of the positioning springs of the mixing grid according to this invention is preferably enlarged. Due to the slots, each positioning spring of the double strip is desirably and elastically opened at a position around each slot to support a fuel rod at two support surfaces when the spring supports the fuel rod. Therefore, each positioning spring provides two support surfaces for the fuel rod, thus enlarging the fuel rod contact area of the grid and effectively protecting the fuel rod from a fretting corrosion. In addition, the intersecting inner strips of the double strip mixing grid of this invention may be preferably and continuously welded together at their intersections through a continuous welding process, in addition to a conventionally performed alternate spot welding process. Therefore, it is possible to improve the mechanical strength of the mixing grid for nuclear fuel assemblies. This finally improves the mechanical strength of the nuclear fuel assemblies. 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.