NUCLEAR FUEL ASSEMBLY

Abstract: The present invention relates to a fuel assembly for a pressurized-water nuclear reactor comprising a plurality of fuel units (3) stacked on top of each other, wherein each fuel unit (3) comprises fuel rods (4) extending between a bottom nozzle (5) and a top nozzle (6). A coolant is adapted, during operation, to flow upwards through the fuel assembly. The fuel assembly has a substantially square cross section and the fuel rods (4) are arranged in an orthogonal lattice. Further, support lugs (8) are adapted to support the fuel rods (4), in four directions, in the upper or lower end of a fuel unit (3) to ensure that a certain distance is maintained between the fuel rods (4).

Description: Nuclear fuel assembly TECHNICAL FIELD The present invention relates to a nuclear fuel assembly of pressurized-water type comprising a plurality of fuel units, each comprising fuel rods extending between a top nozzle and a bottom nozzle. BACKGROUND ART In a nuclear reactor, moderated by means of light water, the fuel exists in the form of fuel rods, each of which contains a stack of pellets of a nuclear fuel arranged in a cladding tube, a column of extruded fuel cylinders or an uninter¬ rupted column of vibration-compacted powdered fuel. The cladding tube is normally made of a zirconium-base alloy. A fuel bundle comprises a plurality of fuel rods arranged m parallel with each other in a certain definite, normally symmetrical pattern, a so-called lattice. The fuel rods are retained at the top by a top nozzle and at the bottom by a bottom nozzle. To keep the fuel rods at a distance from each other and prevent them from bending or vibrating when the reactor is in operation, a plurality of spacers are distributed along the fuel bundle in the longitudinal direction. A fuel assembly comprises one or more fuel bundles, each one extending along the main part of the length of the fuel assembly. Together with a plurality of similar fuel assemblies, a fuel assembly is arranged in a core. The core is immersed in water which serves both as coolant and as neutron moderator. During operation, the water flows from below and upwards through the fuel assembly, whereby the temperature of the water increases the higher it rises in the fuel assembly but it does not boil . A consequence of this is that the corrosion of the cladding tubes increases in the upper part compared with the lower part of the fuel assembly. This difference between the upper and lower parts gives rise to special factors which must be taken into consideration when designing the fuel assembly. It is therefore desirable to achieve a flexible fuel assembly for a pressurized-water reactor which, in a simple manner, may be given a shape in which the upper part of the fuel assembly differs from the lower part thereof. UK 1 403 491 shows a fuel assembly for a pressurized-water reactor with a possibility of designing the upper part so as to differ from the lower part. This fuel assembly comprises a plurality of fuel units stacked on top of each other, each comprising a plurality of fuel rods extending between a top nozzle and a bottom nozzle. The fuel units are fixed to a centrally arranged support tube such that the bottom nozzle of one of the fuel units rests on the top nozzle of an adjacently located fuel unit and such that all the fuel rods in the fuel units are parallel to each other. The support tube extends through the whole fuel assembly and the fuel assembly has a substantially circular cross section. This fuel assembly is intended to be used in a nuclear reactor moderated by heavy water where the fuel assemblies are arranged in vertical pressure channels Another fuel assembly for a pressurized-water reactor with short fuel units with a hexagonal cross section is shown in "Improvements in Water Reactor Fuel Technology, Proceedings of a Symposium Stockholm, 15-19 September 1986, Interna¬ tional Atomic Energy Agency, Vienna, 1987". The fuel units are shown vertically arranged and stacked on top of each other. One factor which must be taken into consideration when designing a fuel assembly of pressurized-water type is that means must be arranged to adopt the forces which influence the fuel assembly. For this purpose it is known, in normal full-length fuel assemblies, to form spacers with a plurality of cells for retaining and mutually fixing the parallel elongated elements extending through the cells. The cells may be designed tubular, so-called sleeve cells. Another commonly used type of spacer comprises crossed sheet-metal strips, standing on edge, which form substan¬ tially square cells. In a reactor of pressurized-water type, in which the fuel assemblies support each other via contact between the spacers, great demands are made on the spacers which must be stiff and be able to take up great forces. In the event of an earthquake, the spacer is subjected to great lateral forces. The lateral forces cause the fuel rods to be pressed against the spacers in adjacently located fuel assemblies. The lateral force which arises when the fuel rod is pressed against the wall of the cell is transmitted to the adjacent cell which, in turn, superimposes the lateral force on an adjacent cell etc. This superimposition of lateral forces may result in one or more cells at the corners of the spacer, where the superimposed force is greatest, collapsing. The collapse means that the function of the spacer is jeopardized, that is, keeping the indivi- dual fuel rods at a distance from each other to ensure that the whole geometry is capable of cooling to avoid super¬ heating. In the event that a spacer collapses, there is a risk of one or more fuel rods being superheated and hence penetrated, causing fissionable material to enter into the core. In fuel assemblies comprising short fuel units without spacers, these forces must be taken up in some other way. It is known, in fuel assemblies with circular cross section, to weld or solder support lugs directly onto the fuel rods to keep these at a distance from one another. The object of the present invention is to provide a fuel assembly for a pressurized-water reactor comprising a plurality of short fuel units and members for absorption of forces, in particular lateral forces, which influence the fuel assembly such that a certain distance is maintained between the fuel rods. SUMMARY OF THE INVENTION, ADVANTAGES The present invention relates to a fuel assembly for a pressurized-water reactor comprising a fuel assembly with a plurality of fuel units, each comprising fuel rods extending between a top nozzle and a bottom nozzle. The features which characterize this fuel assembly are stated in claim 1. The fuel units have a substantially square cross section and comprise bottom and top nozzles which may be identically designed. According to one aspect of the invention, the bottom nozzles and the top nozzles are provided with support lugs between the fuel rod positions. The task of the support lugs is to take up lateral forces which, during an earth¬ quake, may influence the top and bottom nozzles in order thus to ensure that a certain distance is maintained between the fuel rods . The support lugs may be achieved in the top and bottom nozzles by milling out recesses, holes, therein at fuel rod positions for the insertion of fuel rods into these. Further, top and bottom nozzles are provided with flow openings to allow flow of the coolant flowing upwards through the assembly. The flow openings are arranged between the positions of the fuel rods. The support lugs are thus formed between the fuel rods and the flow openings. The top nozzles and bottom nozzles are preferably made in a zirconium alloy. In an alternative embodiment of the invention, the upper and lower ends, respectively, of the fuel rods, which are attached to a top nozzle and a bottom nozzle, respectively, may be designed with radially upwardly extending vertical flanges such that flanges in two adjacently arranged fuel rods are adapted to make tight contact with each other, thus ensuring that a certain distance is maintained between the fuel rods. The fuel rods are fixed to the top and bottom nozzles, respectively, by means of, for example, laser welding or resistance welding. The advantage of the fuel assembly according to the invention is that, because of the short fuel units, a very stable construction may be obtained which, in a simple way, may be equipped with means for taking up lateral forces which influence the fuel assembly and which ensure that a certain distance is maintained between the fuel rods . Another advantage of the invention is that strong and axially extended structures for taking up lateral forces are avoided. Further, extra structures in the fuel assembly are avoided, the task of which is to take up the lateral forces; cf. , e.g. SE 94020757, where a reinforcement part is arranged as a complement to a sleeve-type spacer. By reducing the amount of material in members for taking up lateral forces, the amount of material is reduced and so is the neutron-absorbing effect of the construction material. In addition, a small flow-preventing area is obtained, which causes the pressure drop across the fuel assembly to be decreased. Still another advantage of the invention is that the absorption of forces occurs via the solid support lugs. Consequently, considerably greater forces may be taken up than in, for example, those cases where the fuel rods constitute the force-absorbing member. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows in a vertical section a fuel assembly of a pressurized-water type with short fuel units. Figure 2a shows in a vertical section A-A in Figure 2b fuel rods arranged in a top nozzle in a fuel unit, where the top nozzle is provided with support lugs between the fuel rods. Figure 2b shows the fuel unit in Figure 2a in a view from above. Figure 3a shows in a vertical section B-B in Figure 3b a fuel unit with four adjacently located fuel rods, each with an upper end which is provided with vertical flanges extending radially outwardly. Figure 3b shows the fuel unit in Figure 3a in a view from above. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a fuel assembly of a pressurized-water type of substantially square cross section, comprising a plura- lity of fuel units 3 stacked one above the other. Each fuel unit 3 comprises a plurality of fuel rods 4 arranged in parallel and in spaced relationship to each other in a given lattice. Further, each fuel unit 3 comprises a top nozzle 6 and a bottom nozzle 5 for attachment of the fuel rods 4 in their respective positions in the lattice. The fuel units 3 are stacked on top of each other in the longitudinal direc¬ tion of the fuel assembly and they are stacked in such a way that the top nozzle 6 in one fuel unit 3 is facing the bottom nozzle 5 in the next fuel unit 3 in the stack and such that the fuel rods 4 in all the fuel units 3 are parallel to one another. A fuel rod 4 comprises fissionable material in the form of a stack of fuel pellets 7b of uranium arranged in a cladding tube 7a. A coolant is adapted to flow from below and up through the fuel assembly. A number of so-called control rod guide tubes 4a are arranged extending through the whole fuel assembly. The control rod guide tubes 4a are intended to receive finger-shaped control rods (not shown) which are, respectively, inserted into and withdrawn from the guide tubes 4a for the purpose of con¬ trolling the power of the nuclear reactor. The guide tubes extend between a top part 4b and a bottom part 4c. The top part 4b is arranged above the uppermost fuel unit 3 in the fuel assembly and the bottom part 4c is arranged below the lowermost fuel unit 3 in the fuel assembly. The fuel units 3 are kept in position by being fitted onto and fixed to the control rod guide tubes 4a. The attachment of the fuel rods 4 to a top nozzle 6 is shown more clearly in Figure 2a. The top nozzle 6 is provided with support lugs 8 arranged between the fuel rods 4 which are attached thereto. The shape of the support lugs 8 is shown further clear from Figure 2b. Figure 2b also shows that the top nozzle 6 is provided with flow openings 9 for the passage of the coolant which, during operation of the reactor, flows from below and up through the fuel assembly. These flow openings 9 are thus arranged substantially between the positions of the fuel rods 4. The flow openings 9 may be provided with mixing vanes (not shown) by the arrangement of tabs in the edge of the flow opening in such a way that the coolant flowing through these is mixed. The support lugs 8 are achieved by the arrangement of recesses, holes, 11 in a plane plate. The holes 11 are arranged between the flow openings 9 and have such a depth that the material remaining in the bottom of the hole 11 is given a thickness of the order of size of 0.7 millimetres. By this shape, the fuel rods 4, the upper ends of which are arranged in the holes 11, are given a possibility of diffe¬ rential growth by allowing the material in the bottom of the hole 11 to be deformed to a certain extent. The upper ends of the fuel rods are fixed to the top nozzle 6, for example by means of laser welding or resistance welding (se reference numeral 12 in Figure 2a) . Thus, the support lugs 8 are arranged between the fuel rods 4 and the flow openings 9. The support lugs 8 have a thickness in the axial direc¬ tion of the fuel rod of the order of size of 5 millimetres and are arranged such that each fuel rod is surrounded by four support lugs 8. In Figure 2a and in Figure 3a, the upper ends of the fuel rods 4 are shown with a somewhat rounded shape. The rounded shape further facilitates the differential growth of the fuel rods and deformation of the bottom of the hole 11. To the right in Figure 2a, a side support 13 is shown, which is arranged welded or soldered to the top nozzle 6. The top nozzle 6 is provided with a support lug 8 also between the side support 13 and the adjacently located fuel rod 4 . In this way, a strong structure is obtained, with the ability of taking up lateral forces from adjacently arranged fuel assemblies . The top nozzle 6 is arranged fixed, permanently or detach- ably, to guided to the guide tube 4a via a sleeve 10. Figure 3a shows an alternative embodiment of support lugs 8 arranged in a fuel unit 3. The support lugs 8 are arranged in the upper part of a fuel rod 4, more particularly in the top plug which is adapted to seal the fuel rod. The support lugs 8 are designed extending outwards from the outer sur¬ face of the fuel rod 4 in a direction across the direction of flow of the coolant and in a direction parallel to the centre axis of the fuel rod. The support lugs 8 are four in number and evenly distributed around the top plug of the fuel rod 4. The support lugs 8 in adjacently located fuel rod 4 are adapted to make tight contact, in four directions, with each other in the fuel unit 3 or with a side support 13 surrounding the fuel unit 3. With this design of the upper end of the fuel rod 4, very good protection is obtained against the effect of lateral forces from, for example, fuel units located adjacent to this fuel unit. The fuel rods 4 in Figure 3a are arranged attached to the top nozzle 6 by arranging pins 14 in the respective ends thereof and inserting these pins into openings in the top nozzle 6. To the right in Figure 3a, the above-mentioned side support 13 is shown, which is arranged integrated into the top nozzle and folded through an angle of the order of magnitude of 90° in relation to the rest of the structure of the top nozzle 6. The fuel rod 4 arranged adjacent to the side support 13 is provided with a support lug 8 which rests against the fuel rod. Figure 3b shows, in a view from above, part of a fuel unit comprising support lugs 8 according to Figure 3a. In connection with Figures 2a, 2b, 3a, 3b, only top nozzles 6 and upper ends of fuel rods 4 are discussed. It is, of course, possible or even advantageous to design bottom nozzles 5 and lower ends of fuel rods 4 in a manner corre¬ sponding to that described above.

Claims: CLAIMS 1. A fuel assembly for a pressurized-water nuclear reactor comprising a plurality of fuel units (3) stacked one above the other, each comprising fuel rods (4) extending between a bottom nozzle (5) and a top nozzle (6) and wherein a coolant is adapted to flow upwards through the fuel assembly, characterized in that it has a substantially square cross section and that the fuel rods (4) are arranged in an orthogonal lattice and that the top nozzle (6) or the bottom nozzle (5) is provided with flow openings (9) and recesses (11) in which the fuel rods (4) are arranged, and that support lugs (8) are formed between the fuel rods (4) and between the flow openings (9) . 2. A fuel assembly for a pressurized-water nuclear reactor comprising a plurality of fuel units (3) stacked one above the other, each comprising fuel rods (4) extending between a bottom nozzle (5) and a top nozzle (6) and wherein a coolant is adapted to flow upwards through the fuel assembly, characterized in that it has a substantially square cross section and that the fuel rods (4) are arranged in an orthogonal lattice and that it comprises fuel rods (4) , the upper or lower end of which is provided with a plug which, in turn, is provided with support lugs (8) formed as vertical flanges extending in a radial direc¬ tion outwards from the outer surface of the plug such that support lugs (8) arranged in adjacently located fuel rods (4) make tight contact with one another.

Inventors: NYLUND OLOV, SAHLIN THORBJOERN, NYLUND, OLOV, SAHLIN, THORBJOERN
Applicants: ASEA ATOM AB, NYLUND OLOV, SAHLIN THORBJOERN, ABB ATOM AB, NYLUND, OLOV, SAHLIN, THORBJOERN
Application year: 1997
Main classification: G21C3
IPC codes: G21C3