Patent Number: 051805480
Section: summary

FIELD OF THE INVENTION The present invention relates to a grid having mixing fins for a nuclear fuel assembly, the grid having the particular function of improving the mixing of streams of coolant flowing upwards through the assembly and of making the temperature within the assembly more uniform. BACKGROUND OF THE INVENTION The invention relates more particularly to mixing grids of the type comprising at least two sets of crossed plates fixed together at their crossover points, delimiting cells some of which are for receiving fuel rods and the other are for receiving guide tubes, the grids being provided with coolant-stirring fins extending the plates downstream and disposed to deflect the coolant transversely to its general flow direction, each plate being provided with abutment means projecting inwards from each of the faces of the cells for receiving fuel rods. The invention is particularly applicable to fuel assemblies in which the support structure includes guide tubes interconnected two end fittings and, in addition to mixing grids of the above kind that do not participate in supporting the rods (i.e. to supporting them vertically), at least one additional grid for carrying the rods. For this purpose, the grid is provided with springs cut out from the plates or fixed to the plates and designed to urge the rods against abutment means constituted by bosses situated opposite. Development in reactor characteristics, in particular towards higher thermohydraulic performance and combustion rates gives rise to the use of grids other than supporting grids having abutment means delimiting passages that are larger in size than the rods, thereby facilitating coolant flow and avoiding damage to the rod sheaths during insertion. This clearance is small enough to limit rod vibration to an amplitude smaller than that which could give rise to the sheath being damaged and to the stirring fins being hammered by the rods. The present invention seeks in particular to provide a grid of the above type in which the abutment means participate in making the temperature within an assembly more uniform, without giving rise to excessive headloss. SUMMARY OF THE INVENTION To this end, the present invention provides a mixing grid in which the abutment means comprise, in each wall separating two internal cells occupied by fuel rods, two portions of plate that are cut out and deformed into a scoop-shape, and that are offset relative to each other in the coolant flow direction, with the scoops projecting in opposite directions and tending to cause coolant to pass from one cell to another. The term "internal" designates a cell surrounded on all of its sides by plates, consequently it applies to all of the cells of a belted grid, but it excludes the outermost cells of a grid that does not have a belt. Such a structure can be used both for structural grids that provide mechanical strength to the assembly as a whole and are provided with an outer belt that may possibly extend further in the flow direction than the other plates, and also for grids hat have a thermodynamic function only and do not have a belt. The scoop-shaped abutment means are constituted by deforming approximately semicircular zones of the plates, opening either to an edge of the plate or else to a slot formed through the plate transversely to the flow direction. The two scoops may be in the general form of half a truncated cone. The scoops offset in the coolant flow direction may occupy a fraction only of the width of the plate (i.e., they may optionally extend up the entire height of the grid). In an advantageous embodiment, each of the fins engages only one of the cells in the grid (i.e., extends to one side only of the line of intersection with another plate), and each plate includes no more than one half-fin at each cell corner. It may then be advantageous either to place the scoops on a line parallel to the general flow direction through the assembly but offset from the middle of the cell wall, or else to dispose them on a line that slopes relative to the general flow direction. For example, the fins may be disposed as described in French Pat. No. 84 16803 (published under No. 2 572 837).