Patent Number: 061513768
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a fuel assembly according to the invention. The fuel assembly comprises an upper handle 1, a lower end portion 2 and a plurality of fuel units 3 stacked one above the other. Each fuel unit 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 tie plate 5 and a bottom tie plate 6 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 direction of the fuel assembly and they are stacked in such a way that the top tie plate 5 in one fuel unit is facing the bottom tie plate 6 in the next fuel unit in the stack and so that the fuel rods 4 in all the fuel units 3 are parallel to one another. A fuel rod 4 contains 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. FIG. 2 shows that a fuel assembly is enclosed in a fuel channel 8 with a substantially square cross section. The fuel channel 8 is provided with a hollow support member 9 of cruciform cross section, which is secured to the four walls of the fuel channel 8. In the central channel 14 formed of the support member 9, moderator water flows. The fuel channel with the support member surrounds four vertical channel-formed parts 10, so-called sub-channels, with an at least substantially square cross section. The four sub-channels each comprises a stack of fuel units 3. Each fuel unit comprises 24 fuel rods 4 arranged in a symmetrical 5.times.5 lattice. The fuel assembly in FIG. 2 comprises 10.times.10 fuel rod positions. By a fuel rod position is meant a position in the lattice. All the fuel rod positions in the lattice need not be occupied by fuel rods 4. In certain fuel assemblies, a number of fuel rods 4 are replaced by one or a plurality of water channels. The introduction of a water channel changes the number of fuel rods 4 but not the number of fuel rod positions. FIG. 2a shows an alternative embodiment of a fuel assembly of the same type as that shown in FIG. 1. The fuel assembly is provided with an internal vertical channel 14a through which water is conducted from below and upwards through the fuel assembly. The channel 14a is surrounded by a tube 9a with a substantially square cross section. The fuel units 3 are kept in position by being threaded onto the tube which surrounds the vertical channel 14a. FIG. 2b shows an additional embodiment of a fuel assembly of the same type as that shown in FIG. 1. The fuel assembly is provided with two central vertical water rods 14b through which water is conducted from below and upwards through the fuel assembly. The water rods 14b have a diameter which is somewhat larger than the diameter of the fuel rods 4 and are formed with a substantially circular cross section. The fuel units 3 are kept in position by being fitted onto the water rods 14b. FIG. 3 shows a pressurized-water fuel assembly. In the same way as the fuel assembly in FIG. 1, it comprises a plurality of fuel units 3 stacked on top of each 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. Each fuel unit 3 further comprises a top tie plate 5 and a bottom tie plate 6 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 direction of the fuel assembly and they are stacked in such a way that the top tie plate 5 in one fuel unit 3 faces the bottom tie plate 6 in the next fuel unit 3 in the stack, and so that the fuel rods 4 in all the fuel elements 3 are parallel to each other. A fuel rod 4 contains 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 upwards through the fuel assembly. A number of so-called control rod guide tubes 4b are arranged extending through the whole fuel assembly. The control rod guide tubes 4b are intended to receive finger-shaped control rods (not shown) which are inserted into and withdrawn from, respectively, the guide tubes 4b for the purpose of controlling the power of the nuclear reactor. The guide tubes 4b extend between a top part 15a and a bottom part 16a. The top part 15a is arranged above the uppermost fuel unit 3 in the fuel assembly and the bottom part 16a is arranged below the lowermost fuel unit 3 in the fuel assembly. FIG. 4 shows a fuel rod 4 for a fuel assembly according to FIG. 1, 2, 2a or 2b. The fuel rod 4 comprises, as mentioned above, a cladding tube 7a and a stack of fuel pellets 7b arranged therein. The cladding tube 7a is sealed at the top by a top plug 15 and at the bottom by a bottom plug 16. In FIG. 4 a top plug 15 is shown which is adapted to partially surround part of the column with fuel pellets 7b. The inner diameter of the top plug 15 corresponds to the inner diameter of the cladding tube 7a so as to obtain an even inner side of the fuel rod 4. The top plug 15 is formed with an internal cavity, an axial gap 15a, in which fission gases may accumulate. The axial gap 15a is also intended to allow thermal expansion of the column of fuel pellets 7a. The fuel rod 4 has a material thickness which around the axial gap 15a is larger than in the rest of the fuel rod 4 (see reference numeral 15b). In FIG. 4, the thicker material 15b around the axial gap 15a is achieved in the top plug 15 which, is thickened radially outwardly. In an alternative embodiment, the cladding tube 7a may be provide with a larger material thickness 15b in the region which is intended to surround an axial gap 15a. To reduce the release of fission gases, the fuel pellets 7b, as shown in FIG. 4, may be formed with a through-hole 17. In this way, the maximum temperature arising in the central part of the fuel pellets 7a, and hence the release of fission gases, is reduced. By providing the fuel pellets 7b with holes 17, a fission gap space, distributed in the axial direction, is also achieved, whereby the axial gap 15a in the upper end of the rod 4 may be reduced to a corresponding extent. The axial extent of the axial gap 15a is dependent on the length of the fuel rod 4. As an example may be mentioned that, for a fuel rod 4 which has a length of the order of magnitude of 300 millimeters, the axial extent of the axial gap 15a is of the order of magnitude of 2-5 millimeters. Further, the fuel pellets 7b are formed with cup-shaped upper and lower end surfaces (see reference numeral 18). Because of the thermal expansion, the fuel pellets 7b grow more in the central, warmer parts than in the outer, colder parts. The cup shape 18 thus allows thermal expansion to a certain extent before t he axial gap 15a is utilized for this purpose. It is important to form the thicker material 15b around the axial gap 15 such that it gives rise to as small a pressure drop as possible. The fuel rod 4 therefore exhibits a smooth transfer between the outer diameter of the cladding tube 7a and the largest diameter of the top plug 15. For the same reason, the upper par t of the top plug 15 is provided with rounded corners. FIG. 5 shows an alternative embodiment of the outer surface of the top plug 15. This embodiment is intended to give rise to a lower pressure drop in comparison with the embodiment shown in FIG. 4. The transition (see reference numeral 15c) between the smaller and larger diameters of the fuel rod 4 is here made longer than that shown in FIG. 4. In the same way, the upper part of the reference numeral 15d is provided with a more elongated transition. FIGS. 6a and 6b show an embodiment of the top plug 15 where the plug has been provided with mixing vanes 19. The mixing vanes 19 are formed as bars extending from the outer surface of the top plug 15 in a direction across the flow direction of the coolant and in a direction parallel to the top plug 15. The mixing vanes 19 may be one or more in number. In an advantageous embodiment of the invention, four mixing vanes 19 are arranged evenly distributed along the outer surface of the top plug 15. In FIGS. 6a and 6b, the mixing vanes 19 are shown straight and with an inclination. The inclination may be chosen to be an angle .alpha. in relation to the center axis C of the top plug 15 which is of the order of magnitude of 15.degree.-40.degree.. Alternatively, the mixing vanes 19 may be formed with a curved shape to further increase the mixing of the coolant and hence its cooling capacity. Through the otherwise coil-shaped appearance of the top plug 15, the velocity of the coolant is increased upon passage thereof, thus increasing the power of the mixing vanes 19. In FIG. 7 an embodiment is shown in which the top plug 15 is provided with flanges 20. The flanges 20 are formed so as to extend out from the outer surface of the top plug 15 in a direction across the flow direction of the coolant and in a direction parallel to the center axis C of the top plug 15. This embodiment of the top plug 15 is particularly suitable to arrange in the lower part of the fuel assembly where the need of mixing of the coolant is smaller than in the upper part of the fuel assembly. The object of the flanges 20 is to increase the hydrogen-absorbing ability of the material so that the remainder of the top plug 15 may be given a smaller material thickness. With this embodiment, the hydrogen-absorbing quantity of material may be increased, resulting in a small pressure drop. At the same time, the material thickness in the rest of the top plug may be reduced to a corresponding extent. The mixing vanes 19 and the flanges 20 also provide a larger surface transmitting heat to the coolant. FIG. 8 shows in a view from above, a top tie plate 21 for retaining the upper ends of the fuel rods 4. The top tie plate 21 comprises a plurality of flow openings 22 for passage of the flow flowing upwards through the fuel assembly. Further, top tie plate 21 comprises openings 23 for receiving and positioning pins 24 arranged in the upper part of the top plug 15. In an advantageous embodiment, the mixing vanes 19 are adapted so as to be completely or partially covered by the surfaces of the top tie plate 21 which are formed across the flow direction between the flow openings 22. In this way, the pressure loss caused by the mixing vanes 19 and the flanges 20, respectively, is limited.