Patent Number: 048448613
Section: summary

BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The invention relates to fuel assemblies for nuclear reactors of the type comprising a skeleton having two end pieces connected together by elongate elements such as guide tubes and grids spaced apart along the guide tubes and forming cells for holding a bundle of fuel elements in position at the nodes of a regular lattice. It is particularly suitable for use in spectral shift light water reactors in which the initial reactivity excess is compensated by undermoderating the reactor and hardening the neutron energy spectrum. An increase fraction of the neutrons is then absorbed by fertile material. In such a reactor, it is possible to increase the moderation rate for a predetermined fuel burn-up by removing rods which contain neutron transparent or fertile material (depleted uranium, for example) from guide tubes which are then invaded by water. 2. PRIOR ART In fuel assemblies of the above-defined type, the grids fulfill a number of separate functions. They guide and support the fuel elements containing fissile material. They provide resistance to transverse shocks which the assemblies may undergo during handling, accidents and, possibly, seisms. In most cases, the grids cause turbulence in the coolant flow and deflect coolant streams within the assembly to homogenize the temperature and avoid the formation of hot spots which could lead to local boiling and to sheath failure. A description of such fuel assemblies may be found in numerous reference, for instance European No. 54,236 (Westinghouse) and U.S. Pat. No. 4,059,483 (Anthony). The grids required for fulfilling all these functions impress on the flow of the coolant in the assembly a pressure drop which should be reduced as much as possible. But, concurrently, research work carried out for further increasing the thermal performances of nuclear reactors have led to increasing the coolant flow rate and, therefore, the forces exerted by the coolant on the components of the assemblies. The problem becomes more acute in spectral shift reactors since part of the cross-section should be reserved for guide tubes associated with water displacer rods and the lattice pitch should be decreased for locating the same amount of fissile material in the fuel assembly. As a result, the cross-sectional area available for coolant flow is reduced, which causes an increased pressure drop and a reduction of the protection margin before nucleated boiling in the top part (i.e., the downstream part) of the assemblies. SUMMARY OF THE INVENTION It is an object of the invention to provide a fuel assembly of the above-defined type having, compared with a conventional assembly, a reduced pressure drop while having a comparable mechanical strength. To this end, there is provided a fuel assembly in which the grids are of several different types and may be considered as comprising median grids, designed for withstanding the lateral shocks, and having fins for creating turbulences in the coolant flow along the assembly, bottom grids without fins and providing cross bracing of the elements, and top grids with fins and ensuring cross bracing of the elements, the top and bottom grids imposing on the coolant a lower pressure drop than the median grids. This arrangement considerably reduces the total pressure drop for a given flow rate since the median grids alone, which must have a high rigidity, impose a pressure drop comparable to that of the grids conventionally used in prior fuel assemblies (slightly smaller, however, if the usual fuel element supporting springs are omitted), while the top and bottom grids may impress a pressure loss much smaller than the median grids. In practice, the median grids will generally have a height greater than that of the top and bottom grids, and each will of a median grid cell will have two bosses for bearing on the corresponding fuel element, said bosses being offset in the flow direction. The top and bottom grids may have a single boss per wall. For increasing the rigidity of the median grids, they may be extended upstream (i.e., towards the bottom) by tongues having a bend in the longitudinal direction and playing the role of stiffener. Fuel assemblies have already been proposed in which the fuel elements are supported axially by the two end grids only, situated close to top and bottom end pieces (French No. 2,496,316). The two end grids may be made from a material different from the others, having a higher neutron absorption (for example, the "INCONEL" nickel-chrome base alloy) instead of a zirconium based alloy, because they are in end zones of the core. The purpose, however, is then only to improve the neutron balance by removing as much as possible materials having a considerable neutron absorption cross-section from the core. A comparable result may be attained, in a fuel assembly according to the invention, by providing an additional grid for supporting the elements above the top holding grids mentioned above and made from low neutron absorption material. Due to the reduction in the pressure drop imposed to the flow across each grid, the total number of grids may be increased in the fuel assembly; in addition, it will often be of advantage to space these grids not at equal intervals, but at progressively decreasing intervals in the direction of flow, so as to increase the turbulences in the downstream part and consequently to counter nucleate boiling.