Patent Number: 047755091
Section: summary

BACKGROUND OF THE INVENTION Technical Field The invention relates generally to fuel assemblies for nuclear reactors of the type incorporating a bundle of fuel rods disposed at the nodal points of a regular array and spacer grids distributed along the bundle. The invention relates more particularly to grids for use in fuel assemblies, which include at least two series of mutually parallel plates or straps defining cells which receive fuel rods and possibly elements substituted for them at certain locations in the array. Although the invention is general in scope, it is particularly suitable for use in fuel assemblies whose rods are arranged in a triangular array whose pitch is "tight", i.e., only a little greater than the diameter of the rods. The use of such a triangular lattice is desirable in an undermoderated reactor core, which means that the coolant streams between the rods must be narrow. However, the construction should allow the required coolant flow and provide adequate mixing of the coolant streams, without causing an excessive head loss. The grids of nuclear fuel assemblies for water moderated and cooled reactors, and particularly PWRs, generally retain the rods at the nodal points of a square array. The grids typically include a girdle and two sets of orthogonal plates. A fuel assembly has also been proposed U.S. Pat. No. 3,068,163 having grids for maintaining the rods at the nodal points of a square array, which grids are formed by a flat or undulating endless strip passing between the rods, with intermeshing at the crossing points. This solution, if applied to a "tight" pitch fuel assembly, does not allow satisfactory coolant flow, conditions, mixing of the different streams and an acceptable pressure loss. Support grids for retaining a triangular array of fuel rods have also been proposed, for instance in European No. 0 065 613 (Downs), however including a single layer of intersecting corrugated strips arranged along two different directions. SUMMARY OF THE INVENTION It is an object of the invention to provide a grid for a nuclear fuel assembly which retains the rods efficiently and accurately and achieves mixing of the fluid streams, while including a small amount of neutron-absorbing material. To this end, there is provided a spacing grid for a nuclear fuel assembly including a peripheral girdle and at least two sets of mutually parallel plates, the plates of one set being oblique with respect to the plates of the other set so that the sets define cells for the rods passing therethrough, wherein the parallel plates are distributed in at least two beds spaced apart in the longitudinal direction of the assembly. In a fuel assembly having a triangular array of fuel rods, the plates of each set will be generally parallel to a respective one of the imaginary lines joining the centers of aligned rods, so that three sets of plates will be required. In a first embodiment, the grid supports the rods: the plates are then provided with rod support means, such as bosses, dimples or tongues, for supporting each rod in each bed at at least two diametrically opposed points, the support points of a rod in a grid being distributed between several beds in which the plates belong to different sets. In other words, the support points of the same rod on the plates of one bed are offset angularly with respect to the supports by the plates of another bed. In another embodiment, the grid is mainly intended to fulfill essentially a function of mixing the fluid streams. Then the two beds of the grids, (or two beds at least if the grid includes more of them) are provided with half-fins having different angular positions in the two beds. In such a grid, whose function is essentially thermo-hydraulic, the plates may be thin and have a small height, as compared with the height of the usual grids which support the rods, center them and have a mixing function. With the arrangement of the plates in several successive beds, the pressure loss is reduced as compared to that caused by a grid where all plates are in the same plane, for the same cumulative cross-section of all grids. Because the plates of several beds include mixing fins, the latter acquire a three-dimensional effect. Depending on the type of array and the arrangement of the fins, different types of mixing and flow distribution may be provided by selecting the most appropriate arrangement in each case. In a grid having a mixing function, the girdle may be formed of several successive sections each associated with a bed, which sections are joined together solely by longitudinal junction elements, such as rods placed in the angles of the girdle. In yet another embodiment, the grid simultaneously fulfills all different functions mentioned above. The plates may be formed of metal strips which are flat or, for increased strength, corrugated to the spacing pitch of the rods along the plates. The metal strip, whether flat or undulated, may include bosses directed from one side and/or from the other side of the plate, towards the centers of the cells defined by the plate. The invention also provides a fuel assembly of the above defined type in which some of the grids at least, intended to support and center the rods, are secured to tubes connected to end nozzles for forming an assembly skeleton. Some at least of the grids then have one of the above defined constructions providing support and are connected to the tubes of the skeleton by sleeves each of which is common to all beds in the grid.