Patent Number: 047584033
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS FIGS. 1 and 2 show schematically a fragment of a grid which comprises two intersecting sets of plates 10 and 12 defining pockets or openings for receiving elongate elements 14. The major part at least of these elements are fuel elements 30 as shown in FIG. 6. Others may be guide tubes 32 which slidably receive reactivity control and/or shim rods. A plurality of such grids 22 will be distributed along the elongate elements and secured to the guide tubes 32. The guide tubes 32 and end nozzles 34,36 secured thereto constitute a supporting structure of a fuel assembly 40. Each grid pocket is provided with two half-fins, placed on the same side of the grid. Because each of these half-fins only concerns a single pocket, it is possible to place all the half-fins on the same side of the grid since they do not interfere with the assembly of the half-plates by lap joint intersection. The half-fins 18 oriented in one direction may be carried by plates 12, those 20 oriented in the perpendicular direction being carried by plates 10. Thus, each pocket comprises two opposite half-fins along one of its diagonals and at each intersection of two orthogonal plates are to be found two half-fins perpendicular to each other. In FIG. 1 it can be seen that the amount of material represented by the half-fins is the same for all the pockets. This amount of material is also the same for all the hydraulic cells, such as cell 16 shown by broken lines in FIG. 2. The hydraulic balance is achieved in all the cells, except possibly for the edge cells, and comprises two incoming flows and two outgoing flows as can be seen in FIG. 2. Finally, there is direct circulation in the constrictions in two directions. The half-fins may be formed by simply bending flaps stamped in the plates before assembling these latter. In the variant shown in FIG. 3, each cell such as 16a only comprises a single half-fin 18a or 20a. The half-fins of two adjacent cells are orthogonal and the diagonal symmetry of the hydraulic flows is maintained. The hydraulic balance is maintained, with one incoming flow and one outgoing flow. The amount of material represented by the fins remains the same for all the hydraulic cells. FIGS. 4 and 5 show a possible grid construction corresponding to the diagram of FIG. 3. Grid 22 shown in these Figures has a general conventional construction. It comprises plates 10 and 12, generally made from a zirconium based alloy, assembled by lap jointing and welded at their intersection points. Each plate comprises fuel element bearing bosses 24 and different openings cooperating with springs for applying the elements against the bosses. In the case illustrated, these springs are inserted. Some of the springs, for example spring 26, are double and are inserted in straddling relation in indentations opening in one of the edges of the plates. Other springs, such as spring 28, are single and so of dissymmetrical construction. Grids of this kind are described in prior documents, particularly French Patent No. 82 17717. The grid of FIGS. 4 and 5 comprises a half-fin per hydraulic cell. Each plate 10 carries half-fins 18a with a pitch double that of the pockets. All the half-fins of the same plate 10 are slanting in the same direction, opposite that of the half-fins of the two adjacent plates 10. The half-fins 18a are thus disposed in quincunx arrangement. One pocket out of two is thus provided with two half-fins 18a placed at the ends of its diagonal, at least the current part of the grid, and the pockets respectively provided with and devoid of fins are distributed in a checkerboard arrangement. This arrangement must sometimes be modified at the edge of the grids where certain half-fins 18a may have a reversed arrangement, symmetrical with respect to the external plate. The same arrangement is to be found for the half-fins 20a carried by plates 12. The choice between the arrangement shown in FIG. 1 and that of FIGS. 3 to 5 will depend on the relative importance which is attached to homogenization of the mixture and reduction of pressure losses. In the case of a single half-fin per hydraulic cell, mixing of the fluid streams by deflection is not as complete, but on the other hand the pressure drop is appreciably reduced since the number of half-fins is reduced practically by half. The invention is susceptible of numerous variants and it applies to grids of very varied construction. In particular, the invention may be used when the springs for holding the fuel elements in the grids are formed by parts stamped out from the plates, as is described for example in French Patent No. 1,536,258.