Patent Number: 048790907
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, as previously noted, FIGS. 1 and 2 are schematic views of vanes. FIG. 1 is a prior art type of vane 1 with a weld nugget 2 and opening 3. Arrows illustrate the flow obtained. FIG. 2 illustrates the flow obtained when the same test is performed on a vane 4 without a weld nugget or opening, but with the weld nugget 5 "shielded" by the full vane 4 and by the thickness of the strips 6 and 7. The weld nugget 5 lies substantially within the transverse confines of the strips 6 and 7 in the case of a grid structure provided in accordance with the principles of the invention. FIG. 3 shows a nuclear reactor fuel assembly 10 comprising an array of fuel rods 12 held in spaced relationship with each other by grids 14 spaced along the fuel assembly length. Fuel assembly 10 includes, extending longitudinally therethrough, guide tubes 16. Control rods 50, in the form of neutron absorber elements, move within guide tubes 16; such control rods serving as a means for regulating the thermal output power of the reactor. The fuel assembly also includes a plurality of fuel rods 12. Each fuel rod 12 comprises a hermetically sealed elongated tube, known in the art as the cladding, which contains a fissionable fuel material, such as uranium, in the form of pellets. As may best be seen from FIG. 3, the individual fuel rods 12 are supported in the fuel assembly by means of a plurality of spacer grids 14, such that an upwardly flowing liquid coolant may pass along the fuel rods thus preventing overheating and possible melting through of the cladding. In the manner well-known in the art, the coolant, after passing through the reactor core and being heated through contact with the fuel rods, will be delivered to a heat exchanger and the heat extracted from the circulating coolant will be employed to generate steam for driving a turbine. As noted, and as may be seen from FIG. 3, the positioning and retention of the fuel rods in fuel assembly 10 is accomplished through use of a plurality of the spacer grids 14. All or several of the spacer grids 14 may be of the improved design depicted in FIGS. 4-12. It is to be noted that due to differential expansions which will not be described herein, when the reactor is running hot, the spacing between the fuel assemblies will be larger than under cold conditions. The aforementioned spacing includes the clearance left between fuel assemblies to accommodate thermal and irradiation induced growth. Under a seismic load, the fuel assemblies could, with this spacing, move about and impact against each other and against the walls of the core shroud. Such impacts could, if sufficiently strong, cause the permanent distortion of the fuel assembly spacer grids of the prior art, could also cause bending of the guide tubes and could result in damage to the cladding of individual fuel assemblies through varying the coolant flow characteristics of the fuel assembly or otherwise. Additionally, seismically induced stresses could exceed the elastic limit of the integral grid assembly springs of the prior art fuel assemblies, this being particularly true for those springs in the outer rows of the fuel assemblies. With reference now jointly to FIGS. 4-12, each of the zircaloy spacer grids 14 support and align the fuel rods 12 through the establishment of six points of contact therewith. Thus, as depicted in FIG. 4, each of the fuel rods 12 is contacted by a pair of generally transversely oriented springs 20,22 which respectively urge the fuel rod against oppositely disposed stop members 20' and 22' in each sector or cell of the grid. The stop members 20' and 22' will customarily be provided in pairs with the individual stops of each pair being respectively vertically above and below a plane through the point of contact of the springs 20,22 with the cladding of fuel rod 12. Thus, considering fuel rod 12, this element is urged by means of springs 20 and 22 against pairs of arches 20' and 22' formed respectively on upper strip members 46 and lower strip members 46'. The spacer grid 14 is assembled by interweaving of the internal strip members 46 and 46'. The ends of the strip members 46, 46' may be engaged in slots 30 provided in the spacer grid perimeter strip 32. Welds are formed at all points of the intersection within the spacer grid and the ends of the strip members 46,46' are either welded into the slots 30 in perimeter strip 32 or are butt welded to the perimeter strip. When compared to the prior art, the spacer grid 14 of the present invention has a longer internal strip to perimeter strip weld because the slots 30 run the full transverse width of the perimeter strip 32 exterior plane and thus provide greater strength. Referring to FIG. 5, perimeter strip 32 is provided with cutouts or "windows" 34 in regions corresponding to alternate sectors of the outer row of the spacer grid. These "windows" which are of smaller dimensions when compared to the windows of the prior art perimeter strips, such as shown in U.S. Pat. No. 3,607,640, enhance grid strength while maintaining coolant flow to the outer row of fuel rods. The perimeter strip 32 is also stamped so as to form, in sectors which alternate with the sectors provided with "windows" 34, inwardly extending integral springs 36. As may be seen from FIG. 4 by reference to fuel rod 12, each of the springs 36 cooperates with an internal spring 36' on a grid internal strip member to support and align a fuel rod of the outer row of the fuel assembly. The perimeter strip 32 may also be provided, above and below each of the windows 34, with inwardly extending dimples 38. Dimples 38, in the manner known in the art, enhance the rigidity of perimeter strip 32. Restated, the presence of dimples 38 increases the resistance of strip 32 to bending in response to a force component directed along the length of the perimeter strip. Additionally, as may be seen in the case of fuel rod 12, dimples 38 function as stops or arches against which the fuel rod will be urged by the internal springs 20 and 22, integral with the strip members 46 and 46'. The dimples 38 must be provided above and below each of the perimeter strip "windows" 34. Additional parts of dimples 39 may also be provided in the perimeter strip sectors which have the integral springs 36 formed therein. In the interest of facilitating understanding of the drawing, the dimples 39 have not been shown in FIG. 3. When employed, dimples 39 will not extend into the fuel assembly sector as far as the fuel rod contacting springs 36. The dimples 39 will thus function as backup arches to prevent the elastic limit of springs 36 from being exceeded should the fuel assembly be subjected to vibration in excess of that encountered during normal operation. The pairs of dimples 39, if provided, will also enhance the rigidity of perimeter strip 32. The perimeter strip 32 is also provided, above and below each of the windows 34, with an inwardly ridged horizontal rib 40, as may best be seen from joint consideration of FIGS. 5 and 6. Ribs 40, in the manner known in the art, also enhance the rigidity of the perimeter strip 32. The presence of the ribs 40 increases the section modulus of the perimeter strip and results in increased resistance to bending compared to a flat non-ribbed perimeter strip or a strip provided with a plurality of irregularities. The valleys or slots defined by tabs 44 of the serrated upper and lower edges of perimeter strip 32 function as partial lead-in tabs for the fuel rods 12 which facilitate their insertion; the bases of the slots are aligned with the center of the windows 34 and springs 36 in the perimeter strip 32. The tabs 44 function as anti-hangup devices; these tabs 44 preventing the hanging or interference between adjacent fuel assemblies during refueling. Interior strips 46,46' are provided with large unslotted sections which are kept free of large windows 34 or cutouts 33 and only contain one arch 20' or 22' in the section as can best be seen in FIGS. 7 and 8. The wide unslotted section with a minimum of windows 34 or cutouts 33 provides a larger load path than grids of prior art, which increases the resistance to bending of the strip and thus the strength of the grid. Strip slots 48 may also be tapered at the ends to facilitate the welding at the intermediate locations. The presence of intermediate welds increases the resistance to bending the strip and thus the strength of the grid. To summarize the significant features of the spacer grid of FIGS. 4-12, the perimeter strip 32 is, when compared to the prior art, wider and contains stiffening dimples 38,39. The perimeter strip 32 of the spacer grid is also characterized by inwardly ridged horizontal upper and lower ribs 40 which also add to the stiffness of the strip. The connection between the perimeter strip 32 and the internal strip members 46,46' is defined by a weld seam of increased length along slot 30, when compared to prior art spacer grids, and the serrated upper and lower edges of the perimeter strip define anti-hangup tabs 44 and fuel rod lead-in features. Interior strips 46,46' have been provided with large unslotted sections which have been kept free of windows and any unnecessary cutouts. Also, intermediate welds may be associated with the grid interior strip to increase its rigidity and strength. Tests have shown that the spacer grid of FIGS. 4-12 exhibits an improvement in impact strength and this increase in impact strength has been achieved with essentially no degradation in performance, perturbation in enrichment, or added resistance to coolant flow, i.e., no increase in pressure drop across the fuel assembly and with little change in the cost of fabrication of the grid. In assembling a fuel assembly, an array of control rod guide tubes 16, FIG. 3, having control rods 50 adapted for slidable longitudinal movement therein, are positioned to extend axially through selected sectors in the grids 14 and are thereupon welded to grid tabs or strip walls to form the fuel assembly skeleton structure. Opposite ends of the guide tubes 16 are attached to top and bottom end fittings 52 and 54 using a unique threaded fastener. Reference to the plan view of FIG. 4 illustrates the relative disposition of fuel rods 12 and guide tubes 16 and, particularly, how the fuel rods are held in a relatively immovable position in each grid. Each fuel rod 12 is biased by a spring 20 and 22 into engagement with arches 20' and 22' formed on the grid strip walls, and, as shown, project inwardly into each sector or cell 24. This construction serves to preclude axial movement of the fuel rods 12 in their grids 14 during the time the fuel assembly is being moved or transferred from one location to another. The arches are impressed in the strips 46,46' and dimples may be impressed in the peripheral strip 32 during the strip punching and stamping operation. After the appropriate grid strips 46,46' and 32 are assembled into the form of a grid 14, the arches project into each sector, except the sectors having control rod guide tubes 16, from two adjacent walls as shown in FIGS. 4 and 12. As shown in FIG. 12, the intersecting strips 46,46' are welded together at each junction with the weld nugget being designated 26. At each intersecting joint of the strips where a mixing vane is desired, there is provided a solid mixing vane 28 containing a longitudinally disposed slot 48. These mixing vanes are disposed so as to provide the desired directional flow of the fluid coolant as explained heretofore. Each vane shields a small opening or window 35 which, according to the preferred embodiment, is formed under the bottom end in each vane and directly above and adjacent to the junction of the intersecting strips. While the windows 35 are shown as oval, other shapes and configurations, such as rectangular, semi-circular, square, etc. can be employed. It is also possible to locate the window 35 and weld nugget 26 at different elevations within the grid 14. The function of the window 35 in each case is to create material and provide clearance for welding the strips 46,6' together. The placement of the weld 26, shielded by the vane 28, substantially eliminates flow separation on the downstream side of the mixing vane. This results in an improvement of the vane's fluid mixing capability between subchannels and rod heat transfer ability downstream of the vanes. The intersecting joints are formed in the usual manner by providing strips 46,46' with complementary slots 48 which are oriented as shown in FIGS. 7-11 for engagement. Slots 48 may be tapered at their ends if intermediate welds are required for additional grid strength. The vane strips 46,46' are provided with an integrally formed vane 28. The vane strips 46,46' have formed therewith in the area where the window 35 is to be provided, a consumable weld tab 60,60' complementary to the shape of the window. The complementary spacer strip 46,46' is provided at its edge with a consumable weld tab 60 similar to consumable weld tab 60', only being unslotted which comprises a continuation of the slot 48 continuation line by virtue of the orientation of the consumable weld tab directly over the slot 48. These strips, when intersected as illustrated in FIG. 10, have their tabs similarly intersected. These tabs are made of a material such as zircaloy or Inconel which is consumed during the welding of the joints. The consumable weld tabs 60,60',62,62' are dissolved to form the weld nugget 26 as best illustrated in FIG. 11. The consumable weld tabs are integral with the intersecting strips 46,46' which are made of the same material. By shielding the upper grid intersect welds 26 by the novel vane design, its reactor performance has been increased. FIGS. 1 and 2 show two different vane designs in operation. FIG. 2 illustrates the flow patterns seen when the shielded vane is employed. Flow separation on the downstream side of the mixing vane is minimized, thus improving the fluid mixing and rod heat transfer capability of the vanes. These conditions would include all anticipated flows during normal and transient core operations. By eliminating or at least reducing fluid separation, the pressure losses of the grid spacer 14 which are normally attributable to fluid friction and acceleration can be reduced, the directional movement of the fluid from one subchannel to another is improved, and the flow pattern generated by the vanes is more effective in cooling the rods. With the present invention, the fluid streamlines on the downstream surface of the mixing vane will assume a trajectory which is similar in many ways to a frictionless flow pattern. This advantage can be seen by looking at FIG. 1 which illustrates a typical prior art mixing vane which has a window at the spacer strip intersection joint. As shown, the flow streamlines have pressure differentials which result from acceleration differences between the flow on the upstream side, downstream side, and around the weld nugget thereby causing flow separation on the downstream side of the vane. This in turn reduces the extent of fluid directional change on the downstream side which is desired for purposes of obtaining uniform cooling of the fuel rods and subchannel mixing. It will be appreciated from the foregoing description that a novel and improved nuclear fuel grid spacer 14 for a nuclear fuel reactor has been disclosed and enjoys significant advantages over conventional spacers as discussed heretofore.