Patent Number: 
Section: claims

1. A grid for spacing nuclear fuel rods in a nuclear fuel assembly, the grid defining a substantially regular array of cells and comprising:a peripheral belt having an upper edge and a lower edge, the peripheral belt having guide fins extending inwardly on at least one of the upper and lower edges, the peripheral belt in plan view having a substantially square shape with four sides and four corners, the at least one upper and lower edge of the peripheral belt having setbacks formed in the at least one upper and lower edge of the peripheral belt and extending inwardly and positioned between adjacent guide fins of the same side of the at least one upper and lower edge; andcrossed inner plates surrounded by the peripheral belt,wherein the guide fins bear against regions of the inner plates that constitute stiffeners. 2. The grid according to claim 1, wherein the setbacks are formed by bosses in the peripheral belt. 3. The grid according to claim 1, wherein the peripheral belt has through orifices to allow cooling fluid to pass laterally. 4. The grid according to claim 3, wherein the regular array of cells includes peripheral cells having outside faces defined by the peripheral belt, the through orifices being positioned substantially in centers of the outside faces. 5. The grid according to claim 4, wherein the peripheral belt lacks springs for laterally holding nuclear fuel rods received in the peripheral cells. 6. The grid according to claim 1, wherein the inner plates present notches for mutually engaging the inner plates at cross-points, the notches having respective regions of reduced width. 7. The grid according to claim 1, wherein the inner plates adjacent to the peripheral belt include abutments for laterally retaining the nuclear fuel rods, the abutments projecting into the peripheral cells from inside faces facing the peripheral belt. 8. The grid according to claim 7, wherein the lateral retaining abutments are formed in the bottom edges and the top edges of the inner plates adjacent to the peripheral belt. 9. The grid according to claim 1, wherein the peripheral belt presents cups set back towards the inside of the grid, the inner plates being welded to the peripheral belt in the cups. 10. The grid according to claim 1, wherein the bottom edges of the inner plates are substantially rectilinear in register with faces of cells for receiving guide tubes of the nuclear fuel assembly. 11. A nuclear fuel assembly comprising:nuclear fuel rods;a skeleton for supporting the rods, the support skeleton comprising guide tubes;nozzles disposed on longitudinal ends of the guide tubes; andgrids for spacing the rods, the grids including at least one grid according to claim 1. 12. The grid according to claim 1 wherein one of the cells of the grid is a peripheral cell bordering the peripheral belt, the peripheral cell having a fin section of one of the guide fins extending over the peripheral cell and a further fin section of one of the adjacent guide fins extending over the peripheral cell. 13. The grid according to claim 12 wherein one of the setbacks is located between the fin section and the further fin section. 14. A grid for spacing nuclear fuel rods in a nuclear fuel assembly, the grid defining a substantially regular array of cells and comprising:a peripheral belt having an upper edge and a lower edge, the peripheral belt having guide fins extending inwardly on at least one of the upper and lower edges, the peripheral belt in plan view having a substantially square shape with four sides and four corners, the at least one upper and lower edge of the peripheral belt having setbacks formed in the at least one upper and lower edge of the peripheral belt and extending inwardly and positioned between adjacent guide fins of the same side of the at least one upper and lower edge; andcrossed inner plates surrounded by the peripheral belt,wherein the inner plates present notches for mutually engaging the inner plates at cross-points, the notches having respective regions of reduced width. 15. The grid according to claim 14, wherein the setbacks are formed by bosses in the peripheral belt. 16. The grid according to claim 14, wherein the peripheral belt has through orifices to allow cooling fluid to pass laterally. 17. A grid for spacing nuclear fuel rods in a nuclear fuel assembly, the grid defining a substantially regular array of cells and comprising:a peripheral belt having an upper edge and a lower edge, the peripheral belt having guide fins extending inwardly on at least one of the upper and lower edges, the peripheral belt in plan view having a substantially square shape with four sides and four corners, the at least one upper and lower edge of the peripheral belt having setbacks formed in the at least one upper and lower edge of the peripheral belt and extending inwardly and positioned between adjacent guide fins of the same side of the at least one upper and lower edge; andcrossed inner plates surrounded by the peripheral belt,wherein the inner plates adjacent to the peripheral belt include abutments for laterally retaining the nuclear fuel rods, the abutments projecting into the peripheral cells from inside faces facing the peripheral belt,wherein the lateral retaining abutments are formed in the bottom edges and the top edges of the inner plates adjacent to the peripheral belt. 18. The grid according to claim 17, wherein the setbacks are formed by bosses in the peripheral belt. 19. The grid according to claim 17, wherein the peripheral belt has through orifices to allow cooling fluid to pass laterally.