Patent Number: 061309273
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 5 is a perspective view of a spacer grid for nuclear fuel assemblies in accordance with the primary embodiment of this invention. As shown in the drawing, the grid 10 of this invention comprises two sets of intersecting grid strips 15 and 16 which are arranged in sets at right angles to each other prior to being encircled by four perimeter strips 30, thus forming an egg-crate pattern. Each of the first and second strips 15 and 16 is made up of two narrow sheets which are formed through a stamping process, thus defining a plurality of coolant channels 29 on each grid strip. Each sheet of the above strips 15 and 16 is most preferably made of zircaloy, the alloy of tin, iron, chrome and zirconium. However, it should be understood that said strips may be preferably made of inconel which has been typically used as a material of such grid strips. In the embodiment of FIG. 5, four first grid strips 15 regularly intersect four second strips 16 at right angles prior to being encircled by the four perimeter strips 30, thus forming a grid 10 having a 5.times.5 array with twenty five cells. In this embodiment, the coolant channels 29 are designed to have a generally rectangular cross-section as best seen in FIG. 6. For ease of description, only one fuel rod 25 is shown as placed and supported within an associated cell in FIG. 5. The above channels 29 are specifically designed to forcibly circulate coolant from the high temperature regions 22 about the fuel rods 25 to the low temperature regions 23, thus causing the high temperature coolant to be effectively deflected prior to being mixed with the low temperature coolant. A uniform temperature distribution is thus formed within the fuel assembly. FIG. 6 is a partial perspective view, showing the configuration of the coolant channel 29. As shown in the drawing, the channel 29 has an upright Y-shaped configuration with one inlet being formed around the high temperature region 22 and two outlets or nozzles 21 being formed around the lower temperature regions 23. The cross-sectional area of the inlet is larger than the summed cross-sectional area of the two outlets or nozzles 21. In addition, the cross-sectional area of the channel 29 gradually varies along the axis of the channel 29 in a way such that the middle portion "A" of the channel 29 has the largest cross-sectional area which is almost twice as large as that of the inlet. FIG. 7a is a partial plan view, showing the top of the grid of FIG. 5 with four coolant channels 29 being formed around each intersection 17 of the two grid strips 15 and 16. FIG. 7b is a partial perspective view of FIG. 7a. The first and second grid strips 15 and 16 and the perimeter strip 30 are shown in FIGS. 8, 9 and 10, respectively. FIG. 8 shows the configuration of the first strip 15, while FIG. 9 shows the second strip 16 arranged in sets at right angles to the first strip 15. The two sets of grid strips 15 and 16 are different from each other in the position of notches 26 and 326 at which the strips 15 and 16 intersect each other at right angles to form a grid 10. In order to fabricate the grid 10, two narrow sheets are integrated into each grid strip 15, 16 in a way such that the channels 29 are defined on the resulting grid strip. After forming a plurality of grid strips 15 and 16 in the same manner as described above, the strips 15 and 16 are arranged in sets at right angles to each other, thus forming a plurality of four-walled cells. In such a case, the number of grid strips 15 and 16 to be used for fabricating a grid is twice as many as the number of notches 26, 326 of each strip since the grid has to have a square array. At the intersections of the two sets of grid strips 15 and 16, the taps 27 of the first strips 15 engage with the taps 328 of the second strip 16, respectively. The two sets of strips 15 and 16 are welded together at said taps 27 and 328 through a TIG welding process or a laser beam welding process, thus forming the welded intersections 17 and 317 with a plurality of four-walled cells for the fuel rods as shown in FIGS. 7a and 7b. The perimeter strips 30, free from notches or taps different from the above-mentioned intersecting grid strips 15 and 16, are individually made up of two narrow sheets. In each of the four perimeter strips 30, the inside sheet 31 is deformed in the same manner as that described for the sheets of the intersecting grid strips 15 and 16, thus forming a plurality of coolant channels. Meanwhile, the outside sheet 32 of each perimeter strip 30 is not deformed, but has a flat, narrow configuration. The top and bottom edges of the inside sheet 31 of each perimeter strip 30 are cut away into the same configuration as those of the outside sheet, and so the top and bottom edges of the two sheets 31 and 32 have the same curved configuration. The curved configuration of both edges of the perimeter strips 30 is best seen in FIG. 5 and FIG. 10. The four perimeter strips 30, as best shown in FIG. 10, individually comprising the two sheets 31 and 32 with coolant channels, encircle the intersecting grid strips 15 and 16. The perimeter strips 30 are, thereafter, welded together, thus forming a grid for used in a nuclear fuel assembly. When producing each of the grid and perimeter strips, it is preferable to integrate two narrow sheets into a strip through an electric resistance spot welding process. In addition, the grid is preferably fabricated by intersecting the two sets of grid strips at the notches and welding the intersecting grid strips to each other at said notches. FIG. 11 is a perspective view of a spacer grid in accordance with the second embodiment of this invention. FIG. 12 is a partial perspective view, showing the configuration of a coolant channel formed on a grid strip included in the grid of FIG. 11. In this embodiment, the coolant channels 329 of the grid 310 have an elliptical cross-section. As shown in FIGS. 11 and 12, the above elliptical channels 329 are specifically designed to have a reversed Y-shaped configuration with two inlets being formed around the low temperature regions 323 and one outlet or nozzle 321 being formed around the high temperature region 322. Therefore, the channels 329 of this embodiment forcibly circulate coolant from the low temperature regions 323 to the high temperature regions 322 about the fuel rods 325 in a manner reversed to that described for the channels 29 of the primary embodiment. At any rate, the channels 329 cause the low temperature coolant to be effectively mixed with the high temperature coolant, thus forming a uniform temperature distribution the fuel assembly. The cross-sectional area of each channel 329 gradually varies along the axis of the channel 329 in a way such that the middle portion "B" of the channel 329 has the largest cross-sectional area which is almost twice as large as the summed cross-sectional area of the two inlets. That is, each of the channels 329 is gradually increased in the cross-sectional area within the region from the inlets to the middle portion "B", thus forming a diffuser. Each channel 329 is, thereafter, gradually reduced in the cross-sectional area within the remaining region from the middle portion "B" to the outlet 321, thus forming a nozzle. The cross-sectional area of the outlet 321 is smaller than the summed cross-sectional area of the two inlets. In the present invention, the coolant channels of the grid may be selectively changed in the configuration without affecting the functioning of this invention between four designs: an upright Y-shaped configuration with a generally rectangular cross-section, an upright Y-shaped configuration with a generally elliptical cross-section, a reversed Y-shaped configuration with a generally rectangular cross-section, and a reversed Y-shaped configuration with a generally elliptical cross-section. FIG. 13a is a partial plane view, showing the top of the grid 310 of FIG. 11 with four coolant channels being formed around each intersection of the grid strips. FIG. 13b is a partial perspective view of the grid of FIG. 13a. As shown in FIGS. 12, 13a and 13b, the configuration of each coolant channel 329 of the grid strips 315 and 316 is different from that of the strips 15 and 16 of the primary embodiment. However, the coolant channel 329 is designed to have a diffuser within the region from the inlets to the middle portion "B" and a nozzle within the remaining region from the middle portion "B" to the outlet 321, thus allowing the coolant to be discharged from the outlet 321 at a high speed as expected from a conventional nozzle. In the present invention, it is most important to design the width dimension of the largest cross-sectional portion of a coolant channel 329 regardless of the rectangular or elliptical cross-section of said channel 329 since the largest cross-sectional portion acts as a positioning means for elastically and movably placing and supporting the fuel rods 325 in addition to the original function as a part of the coolant channel. In the channel-shaped dimples as shown by the character "A" of FIG. 6 or "B" of FIG. 12, each positioning dimple is so wide as to be substantially rigid in comparison with conventional cantilever springs, thus having a less flexibility failing to effectively and elastically support an elongated fuel rod. As well known to those skilled in the art, the displacement of a loaded beam is in proportion to the cube of an interval between the supported points of the beam and in proportion to a reciprocal of the cube of the beam's thickness. Therefore, in order to maximize the spring action of the channel-shaped positioning dimples of this invention, it is very important to set the dimension of both the width of the middle portion of the channel and the thickness of each sheet of the intersecting grid strip. When setting the thickness of each sheet to about 0.15 mm-0.3 mm and the width of the middle portion of each channel to about 6 mm to 9 mm, the channel-shaped dimples of the this invention accomplishes a desired operational function as expected from conventional positioning springs used for placing and supporting fuel rods within a fuel assembly. As described above, the present invention provides a grid with nozzle-type coolant deflecting channels for use in nuclear fuel assemblies. In the grid of this invention, two types of grid strips are arranged in sets at right angles to each other prior to being encircled by four perimeter strips, thus forming a grid with a plurality of four-walled cells. Each of the intersecting grid strips comprises two narrow sheets which are deformed at a plurality of portions to provide coolant channels. The coolant channels are designed to forcibly deflect the coolant within a fuel assembly, thus mixing low temperature coolant with high temperature coolant and forming a uniform temperature distribution within the fuel assembly. The grid of this invention thus improves the thermal efficiency of a nuclear reactor. The above grid also effectively prevents the coolant from being partially overheated and this improves the soundness of the reactors. The coolant channels formed on the grid of this invention also act as positioning dimples for elongated fuel rods. Furthermore, the coolant channels are so designed as to have a varying cross-sectional area with the largest cross-sectional area being positioned at each dimple, and so the coolant provides the dimples with an additional spring action due to the highest coolant pressure at said dimples. The channels of this invention are more effectively used as positioning dimples for elongated fuel rods. Due to the specifically designed channels, the grid of this invention effectively and violently mixes the low temperature coolant with high temperature coolant, thus improving thermal efficiency of a reactor without needing any swirling motion of coolant. Therefore, the grid of this invention allows the elongated fuel rods to be effectively protected from fretting wear caused by such a swirling motion of coolant. The summed thickness of two sheets of each grid strip according to this invention is less than that of a conventional grid strip and this conserves the material of the intersecting grid strips. In addition, it is not necessary to cut away the grid strips to form the channel-shaped dimples of this invention, the effective cross-sectional area of the strips is not practically reduced and so the buckling strength of the strips resisting of any lateral load is not reduced. Such channel-shaped dimples, which are provided on the intersecting grid strips without being cut away different from the prior art, basically prevent any lateral circulation of coolant within a fuel assembly. The dimples of this invention thus prevent any vibration of the fuel rods due to a laterally directed force caused by such a lateral circulation of the coolant. It is thus possible to more effectively protect the elongated fuel rods from fretting wear. 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.