Patent Number: 041727614
Section: summary

BACKGROUND OF THE INVENTION This invention relates to cellular grids for positioning nuclear fuel rods, or the like in close proximity to one another. As will be known a nuclear fuel element comprises an assembly of nuclear fuel rods held together in a cluster so that it may be handled as a unit and, in order to ensure correct spacing between adjacent rods over their length, cellular grids are employed. Generally these grids are usually one of two kinds. Those of one kind are constructed from metal strips which run parallel to one another in two directions at right angles so as to form cells between intersecting strips of adjacent rows. Those of a second kind are constituted by a group of co-joined cylindrical or tubular ferrules each ferrule being adapted to receive a nuclear fuel rod and having received it, to hold it in its correct position. The present invention is concerned with grids of the second kind and aims to provide an improved construction of the ferrule type grid. SUMMARY OF THE INVENTION According to the invention a cellular grid structure of the kind employing ferrules to define respective openings for nuclear fuel rods comprises a plurality of tubular ferrules grouped within an encircling band, with at least some of the adjacent openings framed by twin ferrules formed from one piece of metal strip and a bridge piece dividing means dividing the interior of the twin ferrule into two similar openings. The twin ferrules are preferably brazed together at their points of contact and also to the encircling band. Any suitable means may be provided for positively locating fuel rods in the grid openings but it is preferred to use a modification of a method proven in conventional ferrule type grids and to this end a bowspring or leaf spring would be associated with the bridge piece for urging a fuel rod in contact with a pair of co-planar dimples formed in an opposite face of the ferrule. As will be known ferrule type grids for locating nuclear fuel are especially suitable for fuel assemblies in which it is desired to incorporate auxiliary coolant tubes or sparge pipes which, when connected to a source of auxiliary coolant, can play a part in supplementing the flow of main coolant which passes through the fuel element cluster in a direction parallel to the axis of the tubular ferrules. The auxiliary coolant may also replace the main coolant on occasion. The grid according to the present invention is especially adaptable to receive and locate sparge pipes of substantially the same outer diameter as the fuel rods. It is preferred to fit the part of the ferrule wall which is to locate a sparge pipe with an inner lining.