Patent Number: 048448613
Section: claims

1. A fuel assembly for a nuclear reactor comprising a top end piece, a bottom end piece, a plurality of elongate elements extending between and interconnecting said top end piece and bottom end piece, a bundle of parallel fuel elements located between said top and bottom end pieces, and a plurality of grids spaced apart along said elongate elements and forming cells for retaining said fuel elements at the nodes of a regular lattice, said grids including a plurality of top grids, a plurality of bottom grids and a plurality of median grids, wherein: said median grids are constructed and arranged to withstand lateral shocks and provided with turbulence creating fins,  said bottom grids are all located between said median grids and bottom end piece, are devoid of fins and are arranged fro cross-bracing said fuel elements, and  said top grids are all located between said median grids and top end piece, are provided with turbulence creating fins and are arranged for cross-bracing said fuel elements,  each of said bottom and top grids being constructed to impress to a coolant flow along and within said fuel assembly a head loss smaller than any one of said median grids.  said median grids are constructed and arranged to withstand lateral shocks and provided with turbulence creating fins,  said bottom grids are devoid of fins and are arranged for cross-bracing said fuel elements, and  said top grids are provided with turbulence creating fins and are arranged for cross-bracing said fuel elements,  each of said bottom and top grids being constructed to impress to a coolant flow along and within said fuel assembly a head loss smaller than any one of said median grids,  wherein said top end piece comprises a first plate having wide openings for passage of the coolant, fixed to said elongate elements and a second plate fastened to said first plate and formed with a checker board pattern having nodes coinciding with the axis of the fuel elements and forming means for limiting longitudinal movement of the latter. 2. The fuel assembly as claimed in claim 1, wherein each said median grid has a height greater than that of the top and bottom grids and comprises cell defining walls and wherein each of said walls is formed with bosses contacting said fuel elements, each of said walls being provided with two bosses offset in the longitudinal direction of the fuel element received in the cell, said bosses being the only fuel element contacting means in said median grids. 3. The assembly as claimed in claim 1, wherein each of said median grids is provided with tongues projecting upstream with respect to the direction of coolant flow, each formed with a stiffening rib transverse to the direction of coolant flow. 4. The assembly as claimed in claim 1, wherein the grids are spaced apart at progressively decreasing intervals in the flow direction. 5. The assembly as claimed in claim 1, further comprising an additional grid devoid of fins, located between said top grids and said top end piece and of a material having a greater neutron absorption than said top, bottom and median grids. 6. The assembly as claimed in claim 5, wherein said additional grid is secured to some at least of said elongate elements and is provided with bosses for supporting shoulders formed on plugs of the fuel elements and the upper end piece is provided with means limiting the upward movement of the elements so that these latter can only extend downwardly by sliding in the other grids. 7. The assembly as claimed in claim 6, wherein said top end piece comprises a first plate having wide openings for passage of the coolant, fixed to said elongate elements and a second plate fastened to said first plate and formed by a checker board pattern having nodes coinciding with the axis of the fuel elements, and forming means for limiting the movement of these latter. 8. The assembly as claimed in claim 7, wherein said elongate elements consist of only some of a plurality of guide tubes and constitute a framework for said fuel assembly with said bottom end piece and a top end piece. 9. The assembly as claimed in claim 1, further comprising a grating located immediately above the lower of said bottom end pieces, said grating being arranged for guiding extensions of plugs of the elements. 10. The assembly as claimed in claim 9, wherein the grating is fixed at its periphery to a skirt belonging to the bottom end piece. 11. The assembly as claimed in claim 9, wherein said extensions of plugs of the fuel elements have a sliding fit in said grating, some at least of said plugs being formed, at their end, with a bulge for retaining the grating. 12. A fuel assembly for a nuclear reactor comprising a top end piece, a bottom end piece, a plurality of elongate elements extending between and interconnecting said top end piece and bottom end piece, a bundle of parallel fuel elements located between said top and bottom end pieces, and a plurality of grids spaced apart along said elongate elements and forming cells for retaining said fuel elements at the nodes of a regular lattice, said grids including top grids, bottom grids and median grids, wherein: