Patent Number: 047175338
Section: summary

FIELD OF THE INVENTION The invention relates to nuclear reactor fuel assemblies of the type incorporating a cluster of fuel rods (also known as fuel elements) arranged in a regular network and held in position by grids distributed and spaced apart along the assembly. The invention relates more particularly to grids for such assemblies comprising a peripheral girdle frame and at least two series of parallel elements defining openings for fuel rods therethrough and elements substituted therefor at certain points of the network. Although the invention is general in scope, it is particularly suitable for use in fuel assemblies in which the fuel rods are disposed in a triangular network with a pitch which is only slightly greater than the diameter of the fuel rods. The use of such a triangular network is desirable for an undermoderated reactor core since the amount of moderating coolant between adjacent rods must be small, while the required coolant flow should be passed and the coolant streams should mix without an excessive pressure loss. PRIOR ART The grids for nuclear fuel assemblies used in water cooled and moderated reactors are generally provided for retaining the rods at the nodes of a square lattice. They comprise a girdle and two sets of mutually orthogonal plates. A fuel assembly described in U.S. Pat. No. 3,068,163 comprises grids for holding the rods at the nodes of a square lattice. Each grid comprises a straight or undulating strip passing between the rods. The strips interengage at the crossing points. If used in a fuel assembly with a "close" pitch, that design would not ensure passage of the coolant flow under satisfactory conditions, mixture of the different streams and an acceptable pressure loss. French patent No. 2,509,078 discloses a spacing grid with a square network which differs from that described in the above-mentioned U.S. patent in that the orthogonal strips for holding the rods are superimposed and connected together by split tongues. A close pitch cannot be adopted with this solution either. There is disclosed in U.S. Pat. No. 3,158,549 (Fowler) a fuel assembly having fuel rods retained by grids each having cross wires so located that each gridwork provides three of more wires in contact with each fuel elements. However, that arrangement is not sufficient for safely and accurately maintaining the fuel elements in position without impressing an excessive head loss to the coolant flow. SUMMARY OF THE INVENTION It is an object of the invention to provide a nuclear fuel assembly grid for resiliently and efficiently holding the rods in position which however causes a relatively low pressure loss, which ensures good mixing of the fluid streams and which represents a small amount of neutron absorbing material. For that purpose, there is provided a spacing grid comprising a peripheral girdle or frame and at least two series of parallel wires defining passage pockets for the rods. The parallel wires are distributed into at least two beds spaced apart in the longitudinal direction of the assembly and each comprising at least two series of intersecting wires whose ends are fixed to the girdle. When a grid of the invention is used in a fuel assembly with a triangular lattice, each side of the triangular lattice will be parallel to the wires of at least one series. The wires of each bed may provide either two supporting points per rod, the wires of two successive beds providing to a same rod support points at diametrically opposite positions, or four support points per rod. For mixing the coolant streams, each bed may typically comprise two series of wires, the wires of one of the series of each bed forming an angle of 60.degree. with the wires of the two series of the preceding or following bed. Thus swirls are created. If the beds have a distribution which is reproduced cyclically, an overall flow having an helical shape is induced. The term "wire" should be interpreted as meaning an elongated element, whose cross section has in all its directions a dimension of the same order of magnitude less by several orders of magnitude than its length so that this element is deformable in all directions. A wire will generally have a circular section. However, ovalized or flattened sections may also be used. The invention will be better understood from the following description of particular embodiments given by way of examples.