Patent Number: 
Section: description

FIGS. 1, 2a and 2b show a fuel assembly 1 according to a first embodiment of the invention. FIG. 2a is a cross section Axe2x80x94A through the lower part of the fuel assembly in FIG. 1. FIG. 2b is a cross section Bxe2x80x94B through the upper part of the fuel assembly in FIG. 1. The fuel assembly is of boiling-water type and comprises a long tubular container, of rectangular cross section, referred to as a fuel channel 2. The fuel channel 2 is open at both ends, thus forming a through-going flow passage through which coolant flows. The fuel channel 2 is provided with a hollow support means 11 of cruciform cross section, which is secured to the four walls of the fuel channel. The support means comprises four hollow wings and a hollow enlarged cruciform centre. The support means 11 forms a vertical cruciform channel 8 through which non-boiling water flows upwards through the fuel assembly. The fuel channel 2 with support means 11 surround four vertical channel-formed parts 12a-12d, so-called sub-channels, with a substantially square cross section. Each sub-channel contains a fuel bundle comprising a plurality of parallel and spaced apart fuel rods 3a, 3b, 3c. A fuel rod comprises a number of cylindrical pellets 4 of uranium dioxide, stacked on top of each other and enclosed in a cladding tube. The fuel rods in the lower part of the fuel assembly, FIG. 2a, are arranged in a symmetrical 5xc3x975 grid in which all the fuel rod positions except one are occupied by fuel rods. The non-occupied fuel rod position is located inside the cruciform centre of the support means. The spaces between the fuel rods in positions adjacent each other are traversed by coolant and are referred to hereinafter as coolant channels 14. The coolant channels formed between four fuel rods in positions adjacent to each other have a cross section area A1. The fuel rods are of three different types, full-length straight fuel rods 3a, part-length fuel rods 3b, and full-length bent fuel rods 3c. The part-length fuel rods 3b have a height which at least corresponds to half the height of the fuel assembly, but may constitute as much as 80% of the height of the fuel assembly. That part of the fuel assembly in which the part-length fuel rods are arranged is referred to in this patent application as the lower part 10 of the fuel assembly. Each sub-bundle comprises two part-length fuel rods 3b and three bent fuel rods 3c. Part-length fuel rods are marked with a P in this figure and in the following figures. Bent fuel rods are marked with a B in this figure and in the following figures. All the fuel rods 3a, 3b, 3c in a fuel bundle are retained at the bottom by a bottom tie plate 6. The part-length fuel rods 3a, 3c in the fuel bundle are retained at the top by a top tie plate 5. The fuel rods are kept spaced apart from each other by means of spacers 7a, 7b. In the lower part 10 of the fuel assembly, the fuel rods 3a, 3b, 3c are kept in position by the spacers 7a and in the upper part of the fuel assembly the fuel rods 3a, 3c are kept in position by spacers 7b. The part-length fuel rods 3b terminate below the top tie plate 5, usually in or in the vicinity of a spacer. According to the invention, the bent fuel rods 3c are arranged in positions adjacent the part-length fuel rods 3b and are bent in a direction towards these. The bending begins at the uppermost one of the spacers 7a in the lower part of the fuel assembly and terminates at the lowermost of the spacers 7b in the upper part of the fuel assembly. Before and after the bending, the fuel rod is straight. In this embodiment, the bending takes place between two consecutive spacers, which is an advantage since it is then sufficient with two different spacer types. If the bending is to be large, however, it may be necessary to distribute the bending between several spacers. According to the invention, the bent fuel rods 3c are arranged in positions adjacent the part-length fuel rods 3b and are bent in direction towards these. The bending begins at the uppermost one of the spacers 7a in the lower part of the fuel assembly and terminates at the lowermost of the spacers 7b in the upper part of the fuel assembly. Before and after the bending, the fuel rod is straight. In this embodiment, the bending takes place between two consecutive spacers, which is an advantage since it is then sufficient with two different spacer types. If the bending is to be large, however, it may be necessary to distribute the bending between several spacers. Bending may be in the interval of 1-10 mm, preferably in the range of 2-4 mm. Also, the spacers are arranged to take up the bending forces during the bending of the fuel rods. From FIG. 2b it is clear that the grid in the upper part of the fuel assembly is no longer regular. Above the part-length fuel rods an open region 15 is formed. The bent rods 3c are bent inwards towards the open region. The coolant channels which adjoin the open region will thus have a cross-section area A2 in the upper part-of the fuel assembly which is larger than their cross-section area A1 in the lower part of the fuel assembly. The other coolant channels have a cross-section area A1 which is substantially constant in its longitudinal direction. From FIG. 2b it is clear that the grid in the upper part of the fuel assembly is no longer regular. Above the part-length fuel rods an open region 15 is formed. The ben rods 3c are bent inwards towards the open region. The coolant channels which adjoin the open region will thus have a cross-section area A2 in the upper part of the fuel assembly which is larger than their cross-section area A1 in the lower part of the fuel assembly. The other coolant channels have a cross-section area A1 which is substantially constant in its longitudinal direction. The second cross-sectional area A2 is more than 10% larger than the first cross-sectional area A1. Moreover, the second cross-sectional area A2 may be between 10% and 40% larger than the first cross-sectional area A1. FIGS. 3a and 3b show the invention applied to a different type of fuel assembly. This fuel assembly comprises two vertical water channels 16a, 16b with a substantially circular cross section. FIG. 3a shows a cross section through the lower part of the fuel assembly. The fuel rods are arranged in a symmetrical 9xc3x979 grid. The fuel assembly has eight part-length fuel rods 3b and the other fuel rods 3a, 3c are full-length rods. FIG. 3b shows a cross section through the upper part of the fuel assembly. The four fuel rods which are arranged immediately adjacent to a part-length fuel rod are bent inwards towards the open region which is formed above the part-length fuel rod. The coolant channels which adjoin the open region above the part-length fuel rod have a cross-section area A4, A6 in the upper part of the fuel assembly which is larger than the cross-section area A3, A5 in the lower part of the fuel assembly. FIGS. 4a and 4b show a further embodiment of the invention. This embodiment differs from the preceding one in that the bent fuel rods 3c have a diameter d1 which is larger than the diameter d2 of the straight, full-length fuel rods 3a. One advantage of arranging the bent rods with a larger diameter is that the distance between the fuel rods is reduced, which results in a reduced transverse flow from the adjoining coolant channels to the open region above the part-length fuel rods. In this embodiment the part-length fuel rods 3b have the same diameter d2 as the straight full-length fuel rods 3a. In another embodiment, the part-length fuel rods 3b may have the same diameter d1 as the bent fuel rods 3b. A disadvantage of also the part-length fuel rods also having a larger diameter is that a larger quantity of uranium is obtained in the lower part of the fuel assembly where the moderation is good. FIG. 5 shows in more detail a bent fuel rod 3c and the design of the spaces which surround the bent fuel rod. The bent rod is fixed to the bottom tie plate and the top tie plate and the rod is bent between two spacers 7a and 7b. The bending forces from the bent rod must be taken up by the spacers, and primarily by the two spacers 7a, 7b which are positioned nearest the bent part 20 of the rod. Most spacer types keep the rods in position by one or more fixed supports as well as by one or more resilient supports. According to the invention, the fixed support are arranged so as to absorb the greatest bending forces, that is, on the concave sides of the rod. Fixed supports 21a, 21b are mounted on one side each of the bent rod, one support 21a being arranged in the spacer 7b above the bent part 20 and the other support 21b being arranged in the spacer 7b below the bent part 20. On the opposite side of the fixed supports, resilient supports 22a, 22b are arranged.