Patent Number: 041347919
Section: summary

This invention relates to a nuclear reactor fuel assembly comprising a stack of parallel fuel plates disposed vertically in uniformly spaced relation and surrounded by cladding. Said fuel plates are joined together and maintained in position with respect to each other by connecting means which extend at right angles to the plane of the fuel plates and are disposed at intervals in the vertical direction on the lateral edges of said plates. As disclosed in particular in American Patent Application Ser. No. 484,743 of July 1, 1974 fuel assemblies of the type mentioned above are already known. Each clad fuel plate of a fuel assembly is made up of small parallelepipedal plates of fuel material which are formed especially of uranium oxide and each covered with thin metal foil. Said small plates are suitably disposed in spaced relation by means of metallic strips and distributed over the entire surface of the fuel plate to be formed. The complete set of small plates which are each covered with foil is enclosed along the lateral faces of each fuel plate between two metallic cladding sheets which are separated by the thickness of said small plates. By way of alternative, consideration has also been given to the possibility of covering each small plate with a thin metallic strip, in which case the cladding sheets are directly in contact with the uncovered faces of the small plates. In another patent Application which was filed in France on Oct. 2nd, 1975 under No. 75 30247 in the name of Commissariat a 1'Energie Atomique, a different type of fuel assembly made up of a stack of clad plates was also disclosed. The means employed for interconnecting these plates consisted of lateral combs, cross-members or wires which served to connect the parallel fuel plates together so as to constitute a single-unit structure. In this design, provision is made for at least two of these cross-members or the like at the end of the stack in order to be welded or made integral with an end component of parallelepipedal shape which is provided with means for supporting the fuel assembly thus formed. This latter may or may not be associated with a laterally closed outer fuel wrapper, the clad fuel plates being cooled by circulation of a fluid which usually consists of water and flows under pressure in contact with said fuel plates within the wrapper. In other design solutions which are also known in the technique and generally applicable to clad fuel plates of all types, the plates can be maintained within the fuel wrapper by forming parallel longitudinal grooves in the lateral internal faces of the fuel wrapper so as to permit engagement of said plates in said grooves. In contradistinction to clad fuel plates, other forms of nuclear reactor fuel assemblies are also known in which the fuel assembly consists of a cluster of fuel pencils of appreciable length and of generally cylindrical shape. These fuel pencils are maintained in parallel relation and disposed on a uniform lattice by means of spacer grids traversed by the fuel pencils and disposed at intervals along the height of these latter. The fuel-pencil cluster rests on a bottom support component and this latter is in turn connected to a top component in parallel relation thereto by means of tubular connecting-members or tie-rods which are located at intervals in the stack at certain nodes of the lattice. Said tubular tie-rods are advantageously designed to serve as guides for the rods of neutron-absorbing material which are displaced in sliding motion for controlling neutron flux and making reactivity changes during reactor operation. As has already been disclosed in American Pat. No. 3,954,560 of Dec. 11, 1972 consideration has already been given to a particular solution of this type in which the spacer grids are freely mounted in a floating assembly both with respect to the coupling tie-rods and with respect to the clad fuel pencils. Displacement of said spacer grids is limited by spacing sleeves which are so designed as to provide a suitable clearance space between said grids and also between these latter and the end components of the fuel assembly. The aim of the present invention is to make an improvement in plate-type fuel assemblies for nuclear reactors. The primary objective of this improvement is to give these fuel assemblies a general shape which is similar to that of a fuel-pencil assembly of the type recalled above, especially in regard to the external contour of said assemblies. Within a reactor core formed by the side-by-side arrangement of fuel assemblies of a first type such as fuel-pencil assemblies, plate-type assemblies can accordingly be substituted for one or a number or even all these latter if necessary without having to modify the environment and structures of the reactor core. In particular, the invention is intended to permit the substitution mentioned above while permitting adaptation of the plate-type assemblies employed to the mechanical means adopted for handling fuel-pencil assemblies and while also permitting accommodation of the reactivity control systems which are designed and employed for these latter. A further aim of the invention is to ensure substantially uniform cooling of the reactor core assemblies while making it possible, especially in the event that a plate-type assembly is placed next to a pencil-type assembly, to prevent any unbalance in the flow of coolant as a result of the different structures of these two types of fuel assembly. The plate-type fuel assembly under consideration is accordingly distinguished by the fact that at least a number of plates of the fuel stack are provided with hollow sleeves which are rigidly fixed to the fuel plates and extend vertically in the plane of said plates, said sleeves being uniformly spaced in the transverse direction of said plates. In a first embodiment of the invention, the sleeves which are rigidly fixed to the fuel plates have a longitudinal dimension which exceeds that of the fuel plates and are secured at the ends thereof to two support components of the fuel assembly. In another alternative embodiment, tubular tie-rods are capable of passing freely through the sleeves which are rigidly fixed to the fuel plates and are greater in length than said plates, said tie-rods being secured to the support components. In either of the two particular embodiments of the invention for producing in one case a single-unit structure in which the fuel plates are rigidly fixed to the sleeves and for producing in the other case a floating assembly in which the fuel plates are permitted to slide with respect to the tubular tie-rods, at least a certain number of the sleeves or tubular tie-rods serve to guide the reactor control rods of neutron-absorbing material as said control rods are displaced in sliding motion within the fuel assembly. In accordance with a particular feature, the means for interconnecting the fuel plates of the stack are constituted by combs having teeth which are flush-mounted in the cladding of said fuel plates or by transverse rods which are welded to the edges of said fuel plates. In accordance with another distinctive feature, the parallel fuel plates of the stack are braced with respect to each other by means of transverse spacers located at right angles to the plane of the fuel plates and distributed over the surface of said plates. As an advantageous feature, the transverse spacers are constituted by flat lugs having a width equal to the spacing between two fuel plates, said flat lugs being joined together in pairs by means of cylindrical connecting portions having a length equal to the thickness of the fuel plates. Preferably, the transverse spacers tranverse the fuel plates through elongated slots formed in said plates in a zone which does not contain fuel and are positioned by means of a movement of rotation through an angle of 90.degree. in order to bring the plane of the spacing lugs into position at right angles to the direction of the elongated slots. In accordance with another alternative embodiment, the hollow sleeves extend in a single piece to the full height of the fuel plates or else are constituted by spaced tubular elements located in the line of extension of each other. As an advantageous feature, the hollow sleeves project from the contour of the fuel plates at that end of these latter which is located opposite to the bottom support component so as to form a given spacing between said component and said fuel plates. In the particular case in which the plate-type fuel assembly is intended to be placed within the reactor in the vicinity of at least one assembly of parallel fuel pencils, the means for interconnecting the fuel plates are provided with extensions in the form of lateral sheet-metal strips of small thickness applied against the edges of the fuel plates in order to limit the flow of coolant which penetrates between said fuel plates through the sides of the fuel assembly. Moreover, the stack of fuel plates is provided with means for inducing turbulence in the flow between the fuel plates, said means being such as to comprise thin metallic cross-strips which are parallel to the fuel plates or which have a wavy shape. In accordance with a distinctive feature, the thin metallic cross-strips have at least one cut-out edge, the portions of strips thus formed being folded-back in one direction and in the other or in one direction alone.