Patent Number: 054405998
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The fuel rods support grid according to the invention is generally designated by the numeral 10 and includes a first set of parallel strips 12 which are slottedly interlocked with a second set of parallel strips 14 which are arranged at right angles to the first set. The intersections defined between each of the first and second sets of strips are welded to provide a permanent interconnection. As seen in FIG. 2, the strips have integral tabs 18 which extend upwardly from their upper or downstream edges. These tabs are each so shaped that they can be bent along a pair of bend lines 20 such as are shown in FIG. 2 to form a pair of "side-supported" vanes 22. As will be appreciated, the tabs 18 are coplanar with the strips 12 so that the bend lines extend generally in the direction of coolant flow through the grid 10. The vanes 22 in accordance with the first embodiment are bent by predetermined amounts in opposite directions as indicated in FIG. 1. That is, one vane is bent "outwardly" so as to extend in the direction of the viewer, while the other is bent "inwardly" and away from the viewer. A flat upper edge portion 24 is defined at the apex of the tab 18 as it appears after the vanes are bent at the predetermined angles with respect to the tab 18. In the embodiment illustrated in FIGS. 3 and 4, the grid 10' has integral side-supported vanes 22' which are contiguous and are side-supported by the integral tabs 18' formed on the strips 12'. In this instance also, the vanes are formed by bending the tab along bend lines 20'. In this embodiment, the bend lines converge at a point and in a manner which eliminates the upper edge portion 24. The contiguous side-supported vanes 22' which result from the bending of the tab can be seen in the plan view of FIG. 3. As shown in FIGS. 1 and 3, the flow directing vanes 22 and 22' are such as to create coolant flow components which leave the mixing vanes generally in the direction indicated by the small arrows. In a third embodiment, shown in FIG. 5, the strip 12" is provided with a predetermined number of integral tabs 18" each of which is bent along bend lines 20" to form pairs of side-supported elongated vanes 22". In this instance, each pair of vanes are separated by an edge portion 24". The base width "X" of each support tab 18" is selected to endow it and the vanes which are integral therewith with a desired amount of mechanical support and rigidity which is sufficient to enable the required mixing and longevity requirements to be met. The dimension "X" is constrained by factors such as the fuel rod pitch and the number of mixing vanes per intersection. Before bending, the side-supported mixing vanes and the strips which are used in the first, second, or third embodiment, for example, can be fixedly interconnected using a weld nugget technique. To facilitate this type of welding, cut-outs 26 can be formed in each strip 12 to accommodate the nuggets which are created. FIG. 6 illustrates a fourth embodiment which features contiguous mixing vanes 22"' formed by bending the sides of the integral support tab 18"' along bend lines 20"'. Similar to the above embodiments, the bend lines 20"' are coplanar with the strips 12 and converge generally in the direction of coolant flow through the grid. Thus, the mixing vanes according to the present invention are stronger than existing vanes, result in lower pressure drop, and produce equal or superior critical heat flux performance relative to the existing spacer grid designs. Moreover, grids made according to the principles of the invention utilizing the side-supported mixing vanes have bend angles which are easier to maintain under operating and fuel reconstitution conditions. FIG. 7 shows the preferred embodiment of the present invention. In this arrangement, the spacer grid assembly 500 is comprised of a set of strips 501 which are slotted on the top side, interlocked with the set of strips 502 which are slotted on their bottom side, and welded together at the intersection joints. The strips 501 slotted on the top do not have any vanes, only rod support features, and, as such, can be used in the fabrication of unvaned spacer grid assemblies as well as vaned spacer grid assemblies. The strips 502 slotted on the bottom have vanes on the top side that are located at the desired intersection joints. The assembling of the interlocking strips is greatly simplified compared to the prior art because the interleaving of the "slots-top" strips 501 (no vanes) and the "slots-bottom" strips 502 (vaned) does not require the vanes to pass by any rod support features. The flow directing vanes 22' provided according to the principals of the invention are formed by bending portions of a tab 18' which are integral with the grid strip (see FIG. 4). This results in the each vane 22' being supported along its side rather than along its base as in the case of the prior art arrangements. The angle of the bend axis can be varied to achieve the desired flow characteristics with typical angles being in the 20.degree. to 40.degree. range. The angle to which the vane is bent can also be varied, the preferred embodiment is 90.degree. but angles of more or less than 90.degree. can be used. The bending of the vane 22' along its side rather than its base stiffens the vane so it is much less vulnerable to damage due to a change in the bend angle, and provides significantly more support for loads imposed on the vane by the fuel rod end cap during initial rod insertion or reconstitution. Also with regards to rod insertion, the slope of the upper edge of the vane is such that it would guide a fuel rod end cap into its intended cell. The orientation of the vane 22' after bending has the narrow portion of the vane toward the bottom (near the top of the main grid strips) and the wide portion of the vane toward the top. Since the wider portion of the vane blocks or redirects more flow than the thinner portion, and since it is further removed axially from the main body of the spacer grid assembly, it helps to decouple the hydraulic effect of the vane from that of the grid, thereby reducing the pressure drop and increasing the free-stream flow deflected by the vane 504 compared to a vane aligned in the prior art orientation. The portion of the integral tab 18' that remains in the plane of the strip not only serves to reinforce the vane but also to minimize lateral leakage of the flow that has been redirected by the vane attached to the tab. The "sidewall" forces more of the flow to continue up the underside of the vane and exit at the end of the vane 22', as intended. High pressure water tests with electrically heated rods have been performed in 6.times.6 rod arrays with side-supported mixing vanes to assess the performance of the side-supported mixing vanes relative to grids without vanes. The power required to cause Departure from Nucleate Boiling was measured in these tests. A comparison of the performance of the side-supported vaned grids relative to grids without vanes is provided in FIG. 11. More specifically, FIG. 11 presents a plot of the ratio of test section power to cause Departure from Nucleate Boiling to the test section power predicted to cause Departure from Nucleate Boiling in the absence of the mixing vanes. This ratio is plotted vs. the temperature of water at the inlet to the test section. The side-supported vanes clearly improve Departure from Nucleate Boiling performance since the ratio is everywhere greater than 1. It will be noted that while a typical configuration of the preferred embodiment is shown in FIG. 7 with the pre-bend tab geometry of FIG. 7 shown in FIG. 4, it is within the scope of the invention to use alternative vane geometries to tailor the desired flow characteristics. Two possible examples are shown in FIGS. 2 and 6. By alternating the direction of bending of the vanes, various swirl patterns within an array of fuel rods 508 can be achieved (see FIGS. 8, 9 and 10). In view of the above disclosure, the various possible changes and modifications which are possible without departing from the scope of the present invention will be self evident to those skilled in the art of spacer grid designs.