Method of manufacturing a grid having calibrated cells for a nuclear fuel assembly

#The present invention relates to a method of manufacturing a grid for a nuclear fuel assembly, the grid having at least two crossed sats of mutually parallel plates which define cells, of which some receive guide tubes and others receive fuel rods, the plates being provided with boss means for limiting the amount of movement available to the rod relative to the nodes of a regular array.

#One of the functions of such grids is to hold the fuel rods transversely to prevent the coolant from exciting vibrations of an amplitude which would cause wear of the sheaths of the rods by mechanical interaction with the plates.

#In most prior art fuel assemblies, the plates are provided with springs situated opposite bosses and exerting a force on the rods to hold them against the bosses. However, changes in reactor design particularly towards higher burn-up and higher thermohydraulic performance, have led to changes in the functions of the various grids. In particular, it is presently considered advantageous to use a single grid to support the fuel rods, with the other grids belonging to two possible different types. One of these types has a structural role and includes a belt, while the other type has a role that is purely thermohydraulic.

#The embodiments of the invention are described by way of example with reference to the accompanying drawings, in which:

#FIG. 1 is a plan view of a fraction of a grid having bosses to which the invention is applicable;

#FIGS. 2 and 3 are section views on line I--I of FIG. 1, showing grids in which each cell containing a fuel rod has two planes of bosses and one plane of bosses, respectively;

#The grid shown in FIGS. 1 and 2 is particularly suitable for use as a structural grid in a nuclear fuel assembly for a pressurized water reactor, however in a lighter form optionally having the belt omitted, it is also usable as a grid having a purely thermohydraulic function. Such a grid may be incorporated, in particular, in a fuel assembly of the kind described in either of U.S. patent applications Ser. Nos. 07/735,251 and 07/735,246 assigned to the same assignee and filed on the same day as the present application the contents of which are included the present specification reference.

#The grid 10 is constituted by two mutually crossed sets of parallel plates 12 and 14, made of a zirconium-based alloy such as "Zircaloy 4" that has been thermally relaxed. The plates are generally cut out from a foil, assembled to half-depth, and fixed together by welding at their cross-points. Their downstream edges are extended by mixing fins 16 that may take up any of the conventional dispositions already in use. The plates may also be provided with tongues (not shown) placed around the cells for receiving guide tubes. Such tongues are designed to be welded to the guide tubes, e.g., by resistance welding.

#Each face of each of the cells that is designed to receive a fuel rod 18 (only two of which are shown schematically in FIGS. 1 and 2) is provided with at least one inwardly-projecting boss, with the exception of the outermost cells which include bosses on only two or three of their faces.