Patent Number: 
Section: description

FIGS. 5a and 5b show a one cell spacer grid for nuclear fuel assemblies in accordance with the primary embodiment of this invention. As shown in the drawings, the one cell spacer grid 10a is designed to place and support one elongated fuel rod 25. This spacer grid 10a comprises a plurality of grid strips 15 and 16, which are integrated into a four-walled cell having a square cross-section and being used for placing and supporting one fuel rod 25. The configuration and construction of one of the grid strips 15 and 16 is shown in FIG. 6. As shown in FIG. 6, the dipper-shaped coolant mixing vane 22 (hereinbelow, referred to simply as xe2x80x9cdipper vanexe2x80x9d), formed at each of the upper and lower ends of the grid strip 15 or 16, is designed to be specifically curved in a way such that each vane 22 is convex and concave and the upper and lower dipper vanes 22 are opposed to each other in the convex and concave direction. Due to such dipper vanes 22, the coolant is effectively changed in its flowing direction within the one cell grid 10a, thus forming an active swirling motion at positions around the top corners of the grid 10a.  In each of the dipper vanes 22, the concave portion performs a thermal hydraulic function capable of forming a swirling motion of the coolant, while the convex portion supports the fuel rod 25 within the cell of the grid 10. Since the upper and lower dipper vanes 22 are opposed to each other in the convex and concave direction as described above, the four upper support points, provided at the upper end of the grid 10a, and the four lower support points, provided at the lower end of the grid 10a, are alternately positioned along the boundary of the grid 10a. In the present invention, the upper and lower dipper vanes of each strip may have the same configuration as shown in FIGS. 5a and 5b or may have different configurations as shown in FIG. 7. FIG. 7 is a perspective view of one grid strip constituting the spacer grid in accordance with the first modification of the primary embodiment of this invention, with the dipper vanes being altered so as to reduce the pressure drop and to improve the mixing effect of coolant within a fuel assembly. A hole is formed on each of the dipper vanes 122 and 123 of FIG. 7 so as to allow the vane to be free from a pressure difference between the coolant flowing on the concave portion and the convex portion. FIG. 8 is a perspective view of a spacer grid 10, fabricated using the strips 15 and 16 of FIG. 6 and designed to be used in a nuclear fuel assembly having a 3xc3x973 array. FIG. 9 is a perspective view of a spacer grid 110 fabricated using the strips 115 and 116 of FIG. 7 to be used in a nuclear fuel assembly having a 3xc3x973 Array In the drawings, only one fuel rod 25, 125 is shown to be placed and supported within one cell of the grid 10, 110. Of course, it should be understood that the spacer grid 10, 110 of this invention may be designed to form a desired array, for example, a 14xc3x9714, 16xc3x9716, or 17xc3x9717 array. FIG. 10a is a front view of the grid strips constituting spacer grid according to the primary embodiment of this invention. FIG. 10b is a front view of grid strips, with the welded portions of the strips being altered from the structure of FIG. 10a in accordance with the second modification of the primary embodiment of this inventions. As shown in the drawings, the strips 15, 16, 315, 316, are generally classified into two types in accordance with the position of axial slots 226 and 326 extending from the ends of the strips to a depth. In order to fabricate a spacer grid using the strips 15, 16, 315, 316, the strips are assembled with each other by intersecting the strips at the slots 226, 326, thus forming a plurality of four-walled cells individually having four intersections. After the strips 15, 16, 315, 316 are assembled together as described above the strips are welded to each other. In the case of the strips 15, 16 of FIG. 10a, the assembled strips 15, 16 are welded together at the welding taps 227 formed at the ends of the slots 226. On the other hand, the assembled strips 315, 316 of FIG. 10b are welded together at the arcuate welding taps 327, 328 formed on the slots 326. When such arcuate welding taps 327, 328 are formed on the slots 326 as shown in FIG. 10b, it is possible to use the end portions of the intersections of the spacer grid as the dipper vanes, thus forming a stronger swirling motion of the coolant within the grid. In the present invention, the to process of welding the taps 227, 327, 328 is effectively performed through a TIG welding process or a laser beam welding process regardless of the positions of the taps 227, 327, 328 on the slots 226, 326. In the spacer grid 10 of this invention, each of the dipper vanes 22 supports a fuel rod 25 at its concave portion within the cell of the grid 10. Therefore, it is not necessary to cut away the grid strips 15, 16, 315, 316 at any portion for forming separate springs or dimples, and so the strips are free from a reduction in the effective sectional area. This finally increases the mechanical and structural strength of the strips. Since it is not necessary to cut away the grid strips 15, 16, 315, 316 at any portion for forming separate springs or dimples, the strips effectively prevent undesired lateral flow of coolant within the spacer grid. This allows the coolant to smoothly flow within the fuel assembly and allows the fuel rods 25 to be free from vibration. In one cell spacer grid 10a of this invention, the fuel rod 25 is supported by the dipper vanes 22 at four points at each of the upper and lower ends of the grid 10a. That is, the fuel rod supporting structure of the spacer grid 10a of this invention is improved, with the number of fuel rod supporting points within the grid 10a being increased in comparison with the conventional grids. The spacer grid 10a of this invention thus minimizes a fretting wear of the fuel rods 25 different from the conventional grid. Such a fuel rod supporting structure of this invention is more advantageous due the structure of spring-fuel rod-spring capable of absorbing external impact in double directions in comparison with the conventional structure of spring-fuel rod-dimple designed to absorb external impact in a single direction. In the spacer grid of this invention, the lower dipper vanes 22 control the amount and flowing direction of inflow coolant for one cell spacer grid 10a, while the upper dipper vanes 22 cooperate with the lower dipper vanes so as to form more active swirling motion of coolant within the fuel assembly. Such an active swirling motion of coolant within the fuel assembly is caused by the fact that the upper and lower dipper vanes 22 are opposed to each other in the convex and concave direction. Due to such shaped dipper vanes 22, it is possible to accomplish an axially twisted effect of one cell spacer grid 10a. This allows the coolant to maintain the desired active swirling motion within the total length of the fuel assembly. In the present invention, it should be understood that the upper and lower dipper vanes may be altered in shape and size as desired. The dipper vanes may be also formed with holes capable of accomplishing a uniform pressure distribution on the concave and convex portions while forming more active swirling motion of coolant within a fuel assembly. Such a swirling motion of the coolant within the fuel assembly improves the heat transferring efficiency from the elongated fuel rods 25 to the coolant and improves the thermal output power of a nuclear power plant. This is well known to those skilled in the art and further explanation is thus not deemed necessary. As described above the present invention provides a nuclear fuel spacer grid with dipper vanes. The spacer grid of this invention is designed to accomplish the effective operation of a nuclear fuel assembly and to minimize the fretting wear of the fuel rods, which is the important factor causing damage of the fuel assembly. The spacer grid of this invention improves the mechanical stability and safety of the fuel assembly, thus accomplishing the safety of a nuclear power plant even in case of the occurrence of an emergency. The spacer grid is also designed to allow coolant to smoothly flow within the fuel assembly and to result in a high coolant mixing effect. The spacer grid finally improves the thermal hydraulic performance of the nuclear power plant. 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.