Patent Number: 048448605
Section: summary

FIELD OF THE INVENTION The present invention relates to nuclear reactors and more particularly to supporting spaced fuel elements in bundles or assemblies in the reactor by means of a welded fuel element support grid with integral flow directing vanes which direct fluid flow for increased heat transfer. BACKGROUND OF THE INVENTION Fuel assemblies for nuclear reactors are generally provided in the form of fuel element or rod arrays maintained by a structure which includes a plurality of welded spacer grids, a lower end fitting and an upper end fitting. Guide thimbles provide the structural integrity between the lower end fitting, the upper end fitting and the spacer grids intermediate the ends of the fuel assembly. The spacer grids define an array of fuel rods which, typically, may be rows and columns of up to 20 rods each. One such spacer and support grid is disclosed in U.S. Pat. No. 3,481,832. The typical fuel element support grid for supporting a spaced array of nuclear fuel elements or rods intermediate their ends includes a generally quadrangular or other polygonal perimeter. A plurality of fuel element compartments or cells within the perimeter are defined by first and second grid-forming members or strips welded to the perimeter and joined to each other at their lines of intersection. The grid-forming members of the fuel element support grid are slotted for part of their width along lines of intersection with the other grid-forming members of the array such that they may be assembled and interlocked at their lines of intersection in what is termed "egg-crate" fashion. The grid-forming members of one embodiment of the present invention are also bent at points corresponding to intermediate points of the compartments for reasons discussed in U.S. Pat. No. 3,423,287. The wavy-strip structure of this embodiment has been utilized because it provides a good strength-to-weight ratio without severely affecting the flow of cooling or moderating fluid through the grid of the nuclear reactor. The grid strip bends typically act as internal arches and act with integral projecting springs for engaging and supporting the fuel elements within the compartments. Thus, at each fuel rod grid position in the fuel assembly, axial, lateral and rotational restraint is provided against fuel rod motion due to coolant flow, seismic disturbance or external impact. The spacer grids also act as lateral guides during insertion and withdrawal of the fuel assembly from the reactor. All of the elements of the fuel lattice, including the springs and the arch forming bends within the compartments, are arranged with respect to the fuel coolant flow in order to minimize pressure drop across the grid. Since separate arches out of the plane of the grid-forming members are not necessary, a minimum pressure drop is accomplished. In U.S. Pat. No. 3,764,470, a flow twister, mixing vane, or fluid flow directing vane was disclosed for redirecting the cooling fluid in the channels between the spaced parallel nuclear fuel elements. Those twisters where U-shaped metal sheets which straddled one grid member at an intersection with the free ends of the "U" folded on themselves to form two pairs of oppositely directed spirals and a pair of slots receiving the other grid member. The purpose of the twisters was to direct cooling fluid inwardly toward and spirally around the adjacent fuel rods. The desirability and theory of their use is described in the "Background of the Invention" of U.S. Pat. No. 3,764,470. The same background is applicable to the invention described herein. This patent also shows bent or "wavy" grid-forming members which define integral arches. SUMMARY OF THE INVENTION Fluid flow directing vanes or "mixing vanes" provided according to the principals of the invention are integral to the strips and provide improved strength for the grid and improved hydraulic performance of the type previously provided by the separate "twisters" of U.S. Pat. No. 3,764.470. A major advantage of the fluid flow directing vanes being integral is that there is little chance of them becoming loose parts or debris within the flow stream circulating in the reactor in a manner which would damage the internals of the reactor. Moreover, the particular design of the integral fluid flow directing vanes of the instant invention where present provide the grid with increased strength over conventional grids with or without integral fluid flow directing vanes because the vanes themselves are "contained" and provide a strong means of attaching the strips of an intersecting pair to each other. The advantages provided by the invention are accomplished in a spacer grid assembly of typical egg-crate assembly but with strips intersecting at additional points for some or all intersections formed by two strips. Individual strips of only four different types are required to produce the interior area of the grid using wavy strips but additional types to produce special fuel rod support features or special cells to accommodate guide thimbles or guide tubes can be made compatible with the four basic strips. The attachment welds at any such reinforced pair of intersecting strips in the region of the flow directing vanes can be, optionally, made at either one, two, or three locations in a manner that will be described hereinafter. A grid constructed according to the principles of the invention, with its novel integral flow directing vanes and bends provides superior performance over other designs of grids during seismic events and other off-normal conditions and during normal reactor operation. During fabrication, the strip shape is stamped and bent into wavy shape. No manual or other post assembly bending requirements to form and position the vanes is required. Because of the particular shape of the integral flow directing vanes, they pass the fuel rod support springs and arches or bends more readily during assembly than do bent mixing vanes of a conventional design as for example seen in U.S. Pat. No. 4,576,786. Moreover, because of the particular design of the grid and "contained" integral fluid flow directing vanes, there is easier access to the welds and less criticality in the least accessible or intermediate weld, if it is chosen to use one, in the area adjacent the integral attachment of the vanes to the strips. Since the vanes are integral and "contained" within the normal width of the strips without projecting beyond the strip edges, the flow directing vanes are less likely to be damaged during use and during fuel assembly fabrication than are the projecting types of integral vanes previously utilized. If desired, fluid flow directing vanes according to the invention can be provided on both the upstream and downstream side of the grid structure. The novel flow directing vanes' performance during seismic events or other off normal conditions provides resistance to lateral loading because they are "contained" and not projecting. The grid reinforcement is in part due to the fact that span lengths of the thin cross-sections are reduced at some corners by means of the shape of the fluid flow directing vanes. Accordingly, a reduced strip cross-section will provide a resistance to lateral loading that is equivalent to that achieved with a conventional design. The benefits of the reduced cross-section can be utilized elsewhere. For example, the thinner cross-section will effect a lower pressure drop for a given strength or given resistance to damage from seismic events or it will permit the use of larger diameter fuel elements with no net effect on pressure drop. Alternately, a change could be made from a conventional grid design of a given material to the current invention with an inherently weaker, but more economical material, while maintaining the cross-section of the strips. The structural improvement afforded by the design would offset the inferior material properties. During normal operation, the fluid flow directing vanes provide a good mix from the open or corner areas of the fuel element cells to the tight areas. This mix affords better heat transfer and a better "thermal margin" for reasons discussed in U.S. Pat. No. 3,764,470. This is accomplished with an acceptable pressure drop because of the reduced cross-section of the strips for a given required grid strength. The "contained" integral flow directing vanes also reduce pressure drop from conventional grids by permitting smaller than conventional weld nugget sizes. Moreover, the weld geometry at the intersection of two strips in a conventional grid structure provides greater flow restriction and undesirable turbulence than welds required at the vane's intersections of the present invention. This is true whether the strips are straight or bent. Also, without the abrupt flow control surface bends which integral flow control vanes have exhibited in the past, a lower pressure drop across the grid than would otherwise be the case is produced by the "contained" vanes of the invention. During fuel assembly reconstitution, individual fuel elements may be removed from and reinserted into the assembly. Individual mixing vanes which project beyond the strip edges, as in a conventional design, can become bent during the reinsertion process as the tip of the fuel element first approaches the grid. This bending can lead to blockage of further insertion, or to contact with the reinserted element or adjacent elements during subsequent operation. Such contact can initiate local wear and possibly breaching of the fuel element cladding tube. Also, if the bending of a conventional vane is severe enough, the vane could fracture and become debris within the circulating fluid of the nuclear reactor. Such debris is a common source of fuel element breaching in operating reactors. The "contained" vanes of the instant invention provide a geometry which is impervious to damage by fuel elements during reconstitution, thereby eliminating concerns of contact or debris during subsequent operation. The spacer grid embodiment utilizing wavy strips represents an advantage over designs which use straight strips. The hydraulic performance of the wavy grid is such that a grid with straight strips which has both a compatible pressure drop and an equivalent strength has not been found. The embodiment of wavy strips described in this continuation-in-part application provides a potential means to further improve this hydraulic advantage, and at the same time to improve the corrosion behavior and DNB performance of the fuel.