Patent Number: 048448605
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The fuel element support grid of the invention, which includes intersecting integral fluid flow directing vanes "contained" within the strip width, is generally designated by the numeral 10. In FIG. 1, a fuel element or rod 12 is shown in position within one of the fuel cells for illustrative purposes. The support grid itself is made up of Type 1 strips 14 which show in FIG. 2 as being horizontal and parallel and Type 2 strips 16 which show in FIG. 2 as vertical and parallel. The strips 14 and 16 are stamped strips and are preferably of zircaloy, a common name for an alloy of zirconium with low percentages of tin, iron, chrome and nickel. Other well known grid materials ar Inconel and AM-350. Type 1 strip 14 is illustrated in FIGS. 4 and 5. The strip blank 14a, prior to stamping is seen in elevation in FIG. 3. Blank 14a includes slots 18 along its upstream marginal edge regularly spaced for receipt of Type 2 strips which will intersect with it in an "egg-crate" relationship. On the opposite edge of strip blank 14a are areas which will become major flow directing vanes 20a and minor flow directing vanes 22a. Separating each two major flow directing vanes 20a is a slot 24a each in alignment with a slot 18, and separating each two minor flow directing vanes 22a are V-cuts 26a, each in alignment with a slot 18. When the zircaloy strip blank 14a is stamped to create the Type 1 strips as seen in FIGS. 3 and 4, its portions 20a and 22a become the finally shaped major integral flow directing vane portions 20a and 22a, respectively, separated by slots and cuts 24a and 26a, as shown in FIGS. 4 and 5. FIGS. 6-8 correspond to FIGS. 3-5, except that FIG. 6 illustrates the Type 2 strip blank 16b and the integral fluid flow directing vane portions 20b and 22b are defined by V-cuts 26b and slots 28 along the downstream edge. The strips 16 cooperate with the strips 14 to provide the pairs of first and second intersecting and slottedly interlocking grid-forming strips. Each of the integral fluid flow directing vane portions 21a, 20b, 22a, and 22b have a weld material providing tab 30 thereon. Weld material providing tabs 30 also are located on either side of slots 18 and at the bottom edge of strip 16 opposite slots 28, on the lines of strip intersection. Turning again to FIG. 2, it will be seen that the fuel element support grid of the invention has geometry unlike the geometry of previous spacer grip designs because the strips on one side of the grid intersect at three points for each of the intersections formed by two strips 14 and 16. During operation, the spacer grid illustrated is oriented normally such that the grid side with the integral fluid flow directing vanes is downstream. If required for adequate circulation or further improvement to loading resistance, however, the geometry of the strips 14 and 16 could be such that they may be formed and assembled with integral fluid flow directing vane portions 20 and 22 at both the upstream and downstream extremities of the grid cells. The three point intersection of the flow directing vanes is accomplished by the illustrated shapes of the individual strips 14 and 16. Obviously, additional types of strips may be used, such as those shown in FIG. 9-19 as strips 14' and 16', described hereinafter, to produce a grid design with special fuel rod support features or with special cells to accommodate structural components of the fuel assembly. The strips 14 and 16 engage and are welded to a perimeter strip 40 which has no outward projections. It will be seen that certain portions of the flow directing vane portions 20 and 22 may be cropped to maintain the rectangular configuration within the perimeter. It is contemplated that only two of the three contact points could be welded at each intersection, as shown at points 32 in FIG. 2. Alternately, a third weld 34 can be provided for added strength. A third alternative exists for only using weld 34 to form the completed structure. In other words, the welds 32 of the illustrated embodiments would be located at the perimeter edge to perimeter edge mating intersections of the edges of the plurality of pairs of the integral fluid flow directing vane portions 20a, 20b, 22a and 22b remote from the areas of integral attachment of the vane portions 20a, 20b, 22a and 22b to their respective strips 14 and 16. The areas of integral attachment referred to for the weld 34 locations are at the base of cuts and slots 24a, 26a, 24b and 26b when the strips 14 and 16 are assembled. The welds 32 typically are formed from the material 30 by a tungsten inert gas welding operation and the welds 34 typically by electron beam or laser welding but other techniques may be used. It will be seen that in assembling the Type 1 and Type 2 strips into an intersecting relationship, alternate Type 1 strips are rotated 180.degree. and alternate Type 2 strips are rotated 180.degree.. This creates a pattern of generally sinusoidal diagonal curves on the downstream side of the grid in the straight strip embodiment. In strength tests of models of selected internal grid portions designed to simulate both a conventional fuel element support grid and a fuel element support grid of the invention with its "contained" integral fluid flow directing vanes, a strength improvement of at least 63% was found in the new structure over the conventional grid structure of the same strip height and the same strip thickness. This 63% strength improvement figure was obtained for samples with the mixing vanes on one side of the grid only, and for two of the three alternative weld configurations. Fuel element support springs and arches were not included in any of the samples. The testing of the simulated grids suggested that the presence of springs and arches on the grid structure would be no more deleterious than in a standard grid structure. Moreover, as seen in FIGS. 9-19, advanced design wavy grids with bent strips performing the arch function are compatible with the new vane structure. The strength improvement was found in tests involving ten samples of grids similar to that depicted in FIG. 1. All samples had weld nuggets at the strip intersections on the upstream side or the side of the grid remote from the integral fluid flow directing vanes. The welds were tungsten inert gas welds. Samples A and B had welds as seen in FIG. 2 at locations 32 provided to attach the vanes to each other, thereby attaching the first and second intersecting and slottedly interlocking strips of each pair of Type 1 and Type 2 strips together. Samples A and B were loaded in a direction parallel to the Type 2 strips. Samples C and D were the same as Samples A and B but the load direction was applied at right angles. Sample E was like Samples A and B but included two cells spaced from each other containing thimbles. Sample F was the same as Sample E with the load applied at right angles. Samples E and F had no vane welds in the grid cells containing the thimbles (location 32 in FIG. 1). Samples G and H had electron beam welds at the point of intersection of the integral fluid flow directing vanes adjacent their areas of integral connection with the strips (location 34 in FIG. 1) but were otherwise the same as Samples A and B. Samples I and J were the same as Samples G and H with the load applied at right angles. In the case of Sample J, one electron beam weld on the downstream side was missing. All grids were the same size, to the extent possible. TABLE I ______________________________________ Samples Load Capability (lb.) ______________________________________ A 1120 B 1300 C 1270 D 1230 E 1200 F 1100 G 1210 H 1337 I 1490 J 1337 (Conventional Grids) (654,697,642,689,680) ______________________________________ An obvious strength improvement over conventional grids was obtained with the grid of the present invention and its integral fluid flow directing vanes. While the third possible weld configuration was not tested (welds only at locations 34 in FIG. 1), a strength improvement would also be expected with this type grid. In the embodiment illustrated in FIGS. 9-19, the grid members include bends 42 at intermediate points along sides of the compartments formed by the strips. The bends 42 provide integral arch-acting strip portions for fuel rod support, thus eliminating the need for prior art flow-blocking fuel support arches projecting out of the strip plane or profile. Such a structure obviously minimizes the support structure interference with flow since the strip cross-section provides the strip support function and the arch stiff contact function within the same stream engaging profile as the strip alone presents. Thus there is no necessity for discrete arches projecting from the body of the strip. This advantage is present and still permits use of the cantilever shape of springs 52 which blocks less flow area than axially-oriented spring types used by others. Competing grids with a pressure drop effect as low as permitted by this wavy strip design which include, for example, separate arches, will not have a sufficient mechanical strength to accommodate the design loads associated with seismic or lossof-coolant accidents. In those design cases, the strip thickness must be increased and either the fuel support pressure drop will increase or other fuel components besides grids must be modified (and weakened) in order to maintain a pressure drop as low as permitted by this new wavy strip design. Moreover, if thinner strip material is used in an attempt to match the low pressure drop permitted by this wavy or bent strip embodiment, the effect of normal hydrogen embrittlement and surface oxidation will be greater because of the thinner material. This can represent a limit on the residence time permitted for the fuel. Comparing the embodiment illustrated in FIG. 9 to that of FIGS. 1-8, it can be seen that every second intersection of wavy strips 14' and 16' has been modified to utilize the geometry of the FIGS. 1-8 intersecting vane portions as shown at major vane portions 20' and minor vane portions 22'. Welds 32' and 34 along the perimeter edge to perimeter edge mating intersections of the FIG. 9 embodiment correspond to the welds 32 and 34. The geometry of strips contacting along contoured surfaces as taught in the FIG. 1-8 embodiment is preserved and still permits the wavy or bent strip advantages. Individual strip geometries which comprise the grid shown in FIG. 9 are illustrated in FIGS. 10-17. The strip type illustrated in FIGS. 10 and 11 is designated 44 in FIG. 9. The strip type illustrated in FIGS. 12 and 13 is designated 46 in FIG. 9. The strip type illustrated in FIGS. 14 and 15 is designated 48 in FIG. 9. The strip type illustrated in FIGS. 16 and 17 is designated 50 in FIG. 9. Because the grid utilizing wavy strips has different performance characteristics under loading, it is contemplated that despite the reduced number of reinforced intersections in the embodiment of FIGS. 9-19, the strength improvement to be gained by the new geometry may be even more significant than that in the straight strip embodiment of FIGS. 1-8. Hydraulically, the increased blockage area and taller strip height associated with the new design represent penalties in pressure drop. On the other hand, it is believed that the reinforcement provided by the new design will enable a reduction in strip thickness or weld nugget size that will more than compensate for these penalties. Therefore, the overall grid pressure drop with the new design would be lower than for an equivalent strength grid. In the case of the wavy strip design of FIG. 9-19, features of the grid strips that existed in the design of FIGS. 1-8 are utilized to improve the downstream DNB benefit. As shown in FIGS. 18 and 19, every subchannel in the fuel rod array has both redirected flow and swirl established by the new feature. This should improve not only the DNB performance, but also corrosion behavior since there will be a more uniform temperature distribution within the rod array. The wavy strip grid design of FIGS. 9-19 should produce reduced pressure drop, equivalent strength, and improved thermal performance and corrosion behavior.