Patent Number: 063320126
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The strap 12 shown in part in FIG. 1 has a structure very similar to that described in the abovementioned French patent. It has notches 16 designed to co-operate with straps constituting a second set. Each zone situated between two notches and designed to separate two cells for rods carries springs 22 and bearing bosses 28. Each spring is made up of two resilient strips 24 cut out from the strap and interconnected in the middle by a transverse bridge 26. In the flow direction, each spring 22 is positioned between two bosses 28, likewise in the form of bridges. The bridges 26 and the bosses 28 may be of the same shape. The edge of the plate that is to be situated downstream in the flow direction, once the plate is in provided in a grid, may carry mixing fins 29. To increase the transparency of a grid built up from crossed straps, concave-shaped chamfers are provided at least in the vicinity of the upstream edge of the strap. In general, the flow in a pressurized water reactor is upwards and the upstream edge is constituted by the bottom edge. FIGS. 1 and 2 show chamfers 31 that are relatively easy to make and that do not weaken the edge excessively. The chamfering of each face extends over a distance b that is slightly less than the thickness a of the strap. For a grid made up of straps of the zirconium alloy known as Zircaloy 4, and having a thickness a=0.425 mm, the length b may be 0.35 mm. The upstream edge must not be too thin, as otherwise the edges will be weakened and may become indented in the event of an impact. However, the chamfers must be large enough to increase the transparency of the grid to a significant extent. In practice, the thickness along the edge will be selected to be close to half the common thickness a. Although various concave profiles can be used to make the chamfer, it is advantageous to use a shape that is in the form of a circular sector having its center situated beyond the upstream face of the grid constituted by the straps of the invention. In particular, it is possible to use a cylindrical shape of radius R and center C situated at a distance c beneath the upstream edge of the strap, where c is of the same order as b, and at a distance d from the axis of the strap that is equal to several times a. In practice, good results have been obtained with a strap of thickness a=0.425 mm, when c=0.3 mm, and d=2.1 mm. The chamfers may be provided either on either side of the crossings with orthogonal straps, or else along the full length of a strap. The straps may be attached together by welding either on the downstream face, or using beads along the dihedral angles, or indeed in windows such as 33. In the variant shown in FIG. 3, which is not to scale for reasons of clarity, the chamfer is of varying height. It is defined by a line 30 starting from the upstream edge close to each slot 16 and reaching a maximum distance from said edge in the middle of the face of a cell. Under such circumstances, the thickness of the edge is at a minimum in the middle of the wall of a cell and reaches the common thickness at the points where the strap crosses other straps. In the embodiment shown in FIG. 4, again not to scale, each chamfer for a strap 12 or 14 is again defined by a line 30 extending from the edge of the strap at its cross-points with two other straps. In addition, the upstream edge of the strap is neither of constant thickness nor rectilinear. This upstream edge has a curved shape serving to recenter the streams of coolant liquid towards the rods, such as the rod 32 shown diagrammatically in FIG. 2. To further improve the transparency of a grid as shown in FIG. 1, the bridges 26 and the bosses 28 may also be chamfered. However, under such circumstances, it is possible to use chamfers that are straight and plane, given the small sizes thereof. By way of example, FIG. 5 shows chamfers that can be formed on the upstream and downstream edges of a bridge, prior to the sheet being deformed, so as to constitute the bosses and the springs. An angle .alpha. of the chamfers equal to about 20.degree. has given good results. The thickness e of the edges can be that of the upstream edge of the strap, i.e., no more than 0.5 mm for a=0.425 mm, for example.