Patent Number: 063320126
Section: summary

BACKGROUND OF THE INVENTION The invention relates to grids for holding fuel rods in a nuclear fuel assembly, and in particular to grids used in pressurized water reactors and serving to hold rods at the nodes of a regular array, generally a square array. The invention relates in particular to making straps or plates which are assembled together to make up such grids, generally via half-depth slots. The grids of an assembly define common cells which receive the rods and other cells having guide tubes passing therethrough, which tubes are often welded to the grids. Conventionally, in grids used for holding rods longitudinally, rigid dimples or bosses for abutment with the rods are provided in two of the four faces of a common cell for receiving a rod, and two holding springs are cut out from or fitted to the other two faces so as to press a rod onto the dimples. Between grids designed to hold fuel rods at the nodes of a regular array, it is possible to interpose grids provided with fins that serve only to improve mixing of coolant streams flowing along the assembly. It is important to reduce the headloss caused by the presence of the grids. For this purpose, proposals have already been made to round those edges of the straps which are situated upstream in the flow direction (EP-A-0 273 183). However, that convex shape gives rise to turbulence which dissipates energy. The use of straight chamfers does not solve the problem associated with such turbulence. SUMMARY OF THE INVENTION An object of the invention is to provide a grid strap that gives rise to reduced headloss, and that is easy to fabricate. To this end, the invention proposes a strap whose upstream edge presents, at least in each face that is to be adjacent to a cell, a chamfer that is of concave profile. This structure increases the "transparency" of grids and facilitates the passage of coolants along the cells around the rods. Straps or plates of a grid that is to hold rods are advantageously of the structure described in French Patent No. 2,766,003, assigned to Framatome et al. Each strap may include rod-holding springs each having two resilient strips cut out from the wall of the strap and integral therewith, each extending transversely to the strap and connected to the strap at both ends, the strips projecting towards the inside of the cell concerned, at least in the free state, and being connected to each other in the middle by a transverse bridge that projects relative to the two strips. These springs are designed to press the rods into the dimples, which are likewise in the form of bridges. Under such circumstances, it is advantageous to provide rectilinear chamfers upstream from the dimples and/or the bridges. The concave chamfers on the upstream sides of the straps can be of various shapes. They may be of constant length. They may be of a length that varies from a maximum, halfway between the slots, to a minimum close to the slots. The upstream edge of a plate may be defined by an edge which is concave between each pair of slots. In general, on a plate provided with bosses, the length of the chamfer is shorter than the distance between the bottom edge of the strap and the nearest boss. Nevertheless, the chamfer may extend over the entire strap, from the upstream edge to the downstream edge thereof. To avoid weakening the upstream edge, the strap must retain a certain degree of thickness along the upstream edge; nevertheless, the thickness is preferably no greater than about half the thickness of the strap in its ordinary portion in order to ensure that the effect of the chamfers is significant. The above characteristics which are advantageously usable in combination but which can be used separately, together with other characteristics, will become clearer on reading the following description of particular embodiments given by way of example. The description refers to the accompanying drawings.