Patent Number: 063273241
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a fuel assembly according to the invention. During operation, the fuel assembly is arranged vertically in the reactor core. The fuel assembly comprises an upper handle 1, a lower end portion 2 and a plurality of fuel units 3a, 3b, 3c and 3d stacked one above the other. The fuel unit comprise a plurality of fuel rods 4a, 4band 4c arranged between a top tie plate 5 and a bottom tie plate 6. The fuel units 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. A fuel rod contains fuel in the form of a stack of uranium pellets 8 arranged in a cladding tube 7. The fuel assembly is enclosed in a fuel channel 9 with a substantially square cross section. In this embodiment, the fuel assembly comprises four parallel stacks with ten fuel units in each stack. FIG. 2a shows a section B--B through the fuel assembly in FIG. 1. The fuel channel 9 is provided with a hollow support member 10 of cruciform cross section, which is secured to the four walls of the fuel channel. In the central channel 11 formed of the support member 10, moderator water flows. The fuel channel with the support member surrounds four vertical channel-formed parts 12a, 12b, 12c, 12d, so-called sub-channels, with an at least substantially square cross section. The four sub-channels each comprises a stack of fuel units. Each fuel unit comprises 24 fuel rods arranged in a symmetrical 5.times.5 lattice. 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. The fuel assembly has three different types of fuel rods 4a, 4b and 4c. In FIGS. 2a-2d the fuel rods 4a are designated M and the fuel rods 4a are designated P. The fuel rods 4b are not marked in the figures. A fuel rod 4a has a diameter d.sub.1. A fuel rod 4b has a diameter d.sub.2 which is about 8% larger than d.sub.1 and contains about 15% more fuel than the fuel rod 4a. A fuel rod 4c has a diameter d.sub.3 which is about 8% larger than d.sub.2 and contains about 15% more fuel than the fuel rod 4b. By varying between the three fuel rod types in the different lattice positions, a great variation of fuel units may be created. The fuel rods with the largest diameter, 4c, have a relatively larger fission gas space than the fuel rods with the smallest diameter, 4a, in order thus to take into account different linear loads because of rod diameters and typical neutron-flux ratios. It is not sufficient that the diameter is larger but also the height of the fission gas space should be larger. In this embodiment, the fuel assembly is composed of four different types of fuel units 3a, 3b, 3c, 3d at ten different levels. FIG. 2d shows a section E--E through the fuel unit 3a. The fuel unit 3d is formed to fit into the lower part of the fuel assembly where the neutron flux tends to be high for a large part of the operating cycle. This fuel unit almost exclusively comprises fuel rods of the 4c type, which is that of that fuel rods which has the largest cross-section area and contains most fuel. In the lowermost part of the fuel assembly, the significance of a reduced flow area because of the large cross-section area of the fuel rods is not so great since both the moderation and the cooling are good and the pressure drop is still low because of a low steam content. The higher up in the fuel assembly, the fewer are the fuel rods with the largest diameter 4c and instead the number of fuel rods with a smaller diameter 4a and 4b increases. FIG. 2b shows a section C--C through the fuel unit 3c and FIG. 2c shows a section D--D through the fuel unit 3bFIG. 2a shows a section B--B through the uppermost fuel unit 3d, which comprises only fuel rods of types 4a and 4b, which both have a diameter and a fuel content which are smaller than those of the fuel rod 4c. In addition, the lattice positions nearest the water channel 11 are unoccupied. One advantage of the unoccupied positions is that the shutdown margin increases. In the upper part of the fuel assembly, the optimization of the fuel units takes place in order to minimize the risk of dryout and to obtain a low pressure drop. To absorb part of the surplus reactivity in the fuel when it is fresh, certain of the fuel rods may contain a burnable absorber, for example gadolinium oxide. Such a fuel rod will be referred to below as an absorber rod. The diameter of the absorber rod determines its burnup rate. The absorber rods 13a, 13b, 13c are available in three different sizes with three different diameters d.sub.1, d.sub.2, d.sub.3 which are the same as for the fuel rods. By arranging absorber rods with different diameters in the lattice, the content of burnable absorber may be finely-divided both axially and laterally with respect to reactivity, burnup behavior and power distribution. FIG. 3 shows an absorber rod 13c in cross section. The absorber rod comprises a plurality of fuel pellets 15 and 8a stacked on top of each other in a cladding tube 7 and a top plug 16 and a bottom plug 17 seal the absorber rod. The fuel. pellets 15 contain a certain part of a burnable absorber. The two end pellets 8a in the absorber rod only contain fuel and lack burnable absorber. The end pellets in both the fuel rods and the absorber rods adjoin axial gaps which arise between the fuel units in the stack. Because of the axial gap, the moderation and hence the reactivity become higher in the end pellets compared with the other pellets in the stack. By not adding any burnable absorber to the fuel in the end pellets, the end pellets in the fuel unit are burnt up faster than other pellets. The burnup takes place at the beginning of the operating cycle while the total power of the fuel assembly is still limited by the burnable absorber. Since it is necessary in some way, for example by a lower enrichment or by providing them with holes, to limit the power in the end pellets of the fuel rods, it is an advantage that all the end pellets are identical so that the manufacture is simplified. In this embodiment, all the fuel units have the same kind of lattice. It is an advantage that all the fuel units have the same lattice because then the same bottom tie plates and top tie plates may be used for the different fuel units, which minimizes the number of components which need to be manufactured and kept in stock. It is also possible, while maintaining the same lattice, to carry out optimizations by limited displacements of the positions of the rods. In another embodiment, fuel units at the same level in the fuel assembly may have different distribution of fuel rods. This may, for example, be advantageous in a reactor where the fuel assembly is surrounded by water gaps with different widths. The moderation becomes different depending on which gaps a fuel unit is facing, which may be compensated for by arranging fuel rods with larger or smaller diameters in lattice positions adjacent the gaps.