Patent Number: 048287920
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to fuel assemblies for nuclear reactors and it is particularly suitable for use in light water cooled and moderated reactors. 2. Description of the Prior Art Most presently used nuclear fuel assemblies include a bundle of fuel rods (this term designating not only rods loaded with fissile material but also rods loaded with a fertile material) and a structure for holding the bundle, comprising upper and lower end pieces or nozzle connected together by tie rods (frequently serving as guide tubes for control rods) which carry grids for holding the fuel rods at the nodes of a regular lattice, and resilient means for transmitting forces to the upper end piece, forcibly applying the fuel assembly on a core support plate and having a hold down function. A nuclear reactor core typically consists of such assemblies, of hexagonal or square cross-section, carried by a core support plate. The elements of the structure are fixed rigidly to each other. The end pieces have passages for the coolant and the core support plate has openings for coolant flow into the assemblies. The support plate has centering studs for engagement in the lower end piece for indexing it. An upper core plate over the assemblies defines the core and has openings for discharging the coolant after it has been heated in the core. This upper plate constitutes, with elements for guiding control rod clusters, upper internal equipments of the reactor. Due to the hydraulic thrust of the pressurized coolant on the fuel assemblies under normal operation and/or under transitory operating conditions, the assemblies must be provided with hold down devices for retaining them in contact with the core support plate. Such devices generally use resilient means. The most widely used approach consists in using springs fixed to the upper end piece of the assembly, on which the upper core plate bears so as to exert a force applying the assembly on the core support plate against the hydraulic thrust of the coolant. These springs may take on very different shapes, for example those described in French Nos. 1 536 257, 2 326 010, 2 412 142 and 2 409 576 to which reference may be made. These spring devices operate substantially satisfactorily. They nevertheless raise problems. They are complex to manufacture. Their efficiency decreases progressively during irradiation. The space required for them decreases the coolant flow area in the upper end piece and increases the head loss. The springs hinder the introduction, into the upper end piece, of the tool for handling the assembly. The problems, or at least some of them, increase in seriousness on assemblies of great length. A major problem is created by the growth differential under irradiation and at high temperature, between the different constituents of the core and the assemblies, due to different materials being used (as zirconium base alloy and stainless steel). In present assemblies, having a length of several meters, the difference in length between adjacent assemblies, one of which is new and another has undergone irradiation, may be as high as several centimeters. The differences in length make it difficult to provide springs capable of permanently applying under all circumstances the assemblies against the core support plate, while this is necessary to avoid shocks and excessive vibrations of the rods. In addition, the steady increase in the power of nuclear reactors results in an increase of the hydraulic forces on the springs, which forces may become greater than the weight of the assembly, and require the use of larger and larger springs more and more difficult to integrate into the upper end piece. SUMMARY OF THE INVENTION It is an object of the invention to provide a nuclear fuel assembly of the above defined type, in which the hydraulic thrust on each assembly is taken under better conditions than in the past, particularly in that the forces to be transmitted by the resilient means are reduced. For that, the invention uses the results of an analysis of the distribution of the hydraulic thrust applied to the assembly. This analysis shows that the thrust is mostly exerted on the rod bundle, the guide tubes and the holding grids. The thrust in these elements represents 80 to 85% of the hydraulic forces applied to the springs of a conventional assembly, the rest of the forces being exerted substantially equally on the end pieces. With the above object in mind there is provided a fuel assembly of the above defined type in which the structure for holding the fuel elements comprises two separate substructures, which may slide vertically with respect to each other, one of which is arranged so as to be applied against the upper core plate by the hydraulic thrust in operation and the other of which is then held applied against the lower core plate by the resilient means. The resilient means may consequently be springs of smaller size as compared to resilient means of a conventional PWR fuel assembly. In practice, the structure in abutment against the lower core plate may be provided so as to receive only about 10% of the total hydraulic thrust, which allows to reduce the number and/or size of the hold down springs forming the resilient means substantially. According to another aspect of the invention, there is provided an assembly of the above defined type, in which the structure includes two substructures movable vertically with respect to each other, one of the substructures including the upper end piece arranged to come into direct abutment against a fixed member, such as the upper core plate, and some of said tie rods, whereas the other substructure includes the lower end piece and a structure member axially movable with respect to the upper end piece, said resilient means being placed so as to exert on the movable member and the upper end piece a force tending to spread apart the end pieces of the assembly. The first substructure may comprise, in addition to the upper end piece and some of the tie rods, a unit consisting of the fuel rod supporting grids and the rods. The second substructure will then be formed of the other tie rods and the lower end piece, secured together permanently. With this arrangement, the resilient means will only have to exert the force required for holding down the second substructure in contact with the core support plate. Direct abutment of the upper end piece with the upper core plate has an additional advantage: it is much more favorable than a resilient abutment in that it does not amplify vibrations in the top part of the assembly. With the other substructure held in direct abutment against the core support plate by the resilient means, the risk of vibration in the lower part of the assembly will also be attenuated.