Patent Number: 051805480
Section: claims

1. A mixing grid for a nuclear fuel assembly, the grid comprising at least two sets of crossed plates fixed together at their cross-points and defining cells distributed in a regular array, some for receiving fuel rods and the others for receiving guide tubes, the plates 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 inwardly from each of the sides of those cells which are to receive fuel rods, thereby defining a passage that is larger than the size of the rods but small enough to prevent a rod contained in the cell coming into contact with the fins, wherein said abutment means comprise, in each plate separating two cells other than peripheral which receives a rod, two portions of said plate that are cut out and deformed into two scoops, and that are mutually offset in the coolant flow direction, said scoops projecting in mutually opposite directions and tending to cause coolant to pass from one cell to another across said plate. 2. A grid according to claim 1, wherein the scoops are formed by pressing out zones of the plates which are approximately semicircular and open either to an edge of the plate or to a slot formed in the plate transversely to the flow direction. 3. A grid according to claim 2, wherein the scoops are in the shape of truncated cone halves. 4. A grid according to claim 1, wherein two said mutually offset scoops occupy a fraction only on the width of the respective plate. 5. A grid according to claim 1, wherein the two mutually offset scoops have respective semicircular bases on their downstream sides and wherein the dowstream scoop is defined by a slot cut out in the plate while the upstream scoop is terminated by a deformed portion of the upstream edge of the plate, the downstream scoop projecting in a direction with respect to the plate which is opposite to the direction into which a fin carried by the same wall of the cell is sloped. 6. A grid according to claim 1, wherein the downstream scoop of said mutually offset scoops opens in the direction opposite to that of the upstream scoop and extends up to the downstream edge of the plate, the downstream scoop projecting from the plate in the direction opposite to the direction into which a fin carried by the same wall of the cell is sloped. 7. A grid according to claim 1, devoid of peripheral belt, wherein the plates are of such a length that they have an envelope smaller than the envelope of the fuel cells in the assembly, and wherein the walls separating the outermost cells include abutment means projecting from that side of the plate towards which a fin of the respective cell is sloped. 8. A nuclear fuel assembly having a bundle of mutually parallel fuel elements and a support structure including guide tubes and a plurality of grids, wherein each one of only some of said grids comprise at least two sets of crossed plates fixed together at their cross-points and defining cells distributed in a regular array, some for receiving fuel rods and the others for receiving guide tubes, the plates 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 inwardly from each of the sides of those cells which are to receive fuel rods, thereby defining a passage that is larger than the size of the rods but small enough to prevent a rod contained in the cell coming into contact with the fins, wherein said abutment means comprise, in each plate separating two cells other than peripheral which receives a rod, two portions of said plate that are cut out and deformed into two scoops, and that are mutually offset in the coolant flow direction, said scoops projecting in mutually opposite directions and tending to cause coolant to pass from one cell to another across said plate. 9. A mixing grid for a nuclear fuel assembly, said mixing grid comprising two sets of crossed plates fixed together at their cross-points and defining cells distributed in a regular array, some of said cells for receiving fuel rods and the others for receiving guide tubes, wherein said plates are formed with coolant mixing fins extending the plates on their downstream side and angularly disposed with respect to said plates to deflect coolant transversely to its general flow direction along the fuel rods; wherein each of said plates further comprises abutment means projecting inwardly from each of the sides of those cells which are to receive fuel rods by such amounts that they define in each said cell a passage that is larger than the size of the rod but small enough to prevent the rod contained in the cell from coming into contact with the respective fins; and wherein said abutment means comprise, in each portion of one of said plates which separates two cells which each receives one fuel rod, except peripheral cells, two cut out portions of said plate mutually offset in the coolant flow direction, projecting in mutually opposite directions and forming scoops having mutually confronting openings separated by a slot formed in the plate transversely to the general coolant flow direction. 10. A mixing grid according to claim 9, wherein each of said scoops merges with a current portion of the respective plate along a semi-circular line.