Patent Number: 
Section: description

FIG. 1a shows an embodiment of a fuel assembly according to the invention. During operation, the fuel assembly is arranged vertically in the reactor core. FIG. 1b shows a vertical section Ib-Ibxe2x80x2 through the fuel assembly, and FIG. 1c shows a vertical section Ic-Icxe2x80x2 through the fuel assembly. The fuel assembly comprises an upper handle 1, a lower end portion 2 and a plurality of fuel units 3a, 3b and 3c stacked on top of each other. Each fuel unit comprises a plurality of fuel rods 4 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 4 comprises fuel in the form of a stack of uranium pellets arranged in a cladding tube 7. The fuel assembly is enclosed in a fuel channel 9 of substantially square cross section. In this embodiment, the fuel assembly comprises eight fuel units. A fuel unit has 100 fuel rod positions in an orthogonal 10xc3x9710 lattice. A fuel rod position is a position in the lattice in which it is possible to arrange a fuel rod. All the positions in the lattice need not be occupied by fuel rods. The fuel unit is divided into four sub-bundles with 25 fuel rod positions in an orthogonal 5xc3x975 lattice. The lattice in one sub-bundle comprises a fuel rod position in the centre of the sub-bundle, and around this an inner square ring is arranged consisting of 8 fuel rod positions. Outside the inner ring, there is an outer square ring consisting of 16 fuel rod positions. The fuel assembly comprises three different types of fuel units 3a, 3b, 3c. In the two lowermost fuel units 3a, all the fuel rod positions are occupied by fuel rods which are arranged in parallel with the longitudinal axis of the fuel assembly. The fuel rods in the fuel units 3b and 3c are arranged so as to be inclined between the bottom tie plate and the top tie plate. In one sub-bundle, all the fuel rods in the two rings are inclined in the same direction, that is, either clockwise or counterclockwise around the centre of the sub-bundle. The purpose of inclining the fuel rods around the centre of the sub-bundle is to set water and steam, which flow upwards through the fuel assembly, into rotation, thus achieving a separation of water and steam. Such a fuel assembly is known from Swedish patent specification No. 96024476. The fuel unit 3b has 96 fuel rods distributed among four sub-bundles. Each one of the sub-bundles comprises 24 fuel rods arranged in an inner ring 11a and an outer ring 11b. The fuel rods are inclined in the direction of the arrows, that is, around the centre of the sub-bundle. The fuel rod position in the centre of the sub-bundle is unoccupied. In this way an empty volume is created in the centre of the fuel bundle. The empty volume constitutes the lower part of a vertical channel which extends through the six uppermost fuel units in the fuel assembly. There are four channels 12a, 12b, 12c, 12d extending through the fuel assembly, one channel in each sub-bundle. The inclined fuel rods in the sub-bundle bring about an eddy of water and steam around the channel. The direction of the eddies is marked by arrows in the channel. In this eddy, the water and the steam are separated from each other by the water being thrown outwards while at the same time the steam is pressed against the centre of the eddy. The four uppermost fuel units 3c each have 80 fuel rods distributed among four sub-bundles. In each sub-bundle, the fuel rod position in the centre and four positions in the inner ring are unoccupied, thus forming an empty volume which extends through the sub-bundle. The empty volumes contribute to the four channels 12a-12d which extend through the fuel assembly. In each one of the channels, four steam pipes 10a, 10b, 10c and 10d are arranged one above the other such that their longitudinal axes coincide with one another. Through the steam pipes, the steam is conducted towards the outlet of the fuel assembly. In this embodiment, all the steam pipes are designed identically. Each fuel unit 3c comprises four steam pipes arranged between the bottom tie plate and the top tie plate. FIG. 2 shows in more detail how a steam pipe may be designed. The lower end 14 of the steam pipe, hereinafter referred to as the inlet end of the steam pipe, has an opening which constitutes an inlet 14b for the steam. The upper end 13 of the steam pipe, hereinafter referred to as the outlet end of the steam pipe, has an opening which constitutes an outlet 13b for steam and water which have accumulated on the inner side of the steam pipe. The outlet end of the first steam pipe 10a is arranged at a distance from the inlet end of the next steam pipe 10b. The inlet end of the steam pipe has an outer diameter D2 which is smaller than the diameter D3 of the opening in the outlet end. In this way, the water 15 emanating from the inside of the first steam pipe is thrown out at a distance from the inlet of the next pipe, thus separating the water from the steam which continues up through the next steam pipe. The distance between two steam pipes shall be so large as to provide a sufficient inflow area for the steam while at the same time it must not be so large that the separated water has time to be deflected to such an extent that it follows the steam up into the next steam pipe. Preferably, the opening between the steam pipes shall have an area which is of an order of magnitude near the cross-section area of the steam pipe, which is determined by the diameter D1 of the steam pipe. The arrows in the figure show the direction of the steam flow. The inflow of the steam is facilitated by designing the outlet ends and inlet ends of the steam pipes such that venturi effect is obtained. For this purpose, the inlet ends and the outlet ends are arranged such that they are tapering towards the opening. The diameter D2 of the inlet and the diameter D3 of the outlet are to be smaller than the diameter D1 of the steam pipe. The inside of the outlet end 13 is provided with slightly angularly adjusted grooves 16 which open out into the outlet. The task of these grooves is to collect water from the water film 17 which covers the inside of the steam pipe and to concentrate the water to the orifices 16b of the grooves. In this way, large water drops are formed in localized paths. One advantage of this is that large water drops are not deflected as easily as smaller water drops and hence the risk of the water accompanying the steam into the next steam pipe is reduced. In addition, the localized paths with water cause formation of water-free paths between these first-mentioned paths, through which steam may flow into the next steam pipe without being obstructed. The inlet end 14 is provided with a rejection edge 18 and the outlet end 13 is provided with a rejection edge 19 for scraping off the water which is transported along the outside of the steam pipe. To reinforce the formation of the water film on the inside of the steam pipe, the inside of the steam pipe may be provided with oblique vanes 20. Alternatively, vanes may be arranged in the bottom tie plate 6. The inlet end of the steam pipe is attached to the bottom tie plate 6 with a plurality of attachment means 21 and the outlet end of the steam pipe is attached to the top tie plate 5 with a plurality of attachment means 22. FIG. 3 shows another example of how the outlet end 25 and the inlet end 26 of the steam pipe may be designed. The opening edge of the outlet end is provided with lugs 27. The lugs have the same function as the grooves in the preceding example, namely to form large water drops in localized paths. The figure shows the difference between the shape of the water drops when the opening edge is provided with lugs 27 and when the opening edge is straight 28. At the straight opening edge, a curtain with smaller water drops is formed which risk penetrating into the opening between the steam pipes. The steam pipe is provided with a rejection ring 29 on its inlet end and with an additional rejection ring 30 at its outlet end. The task of the rejection rings is to remove the water which is accumulated on the outside of the steam pipe. The outlet end of the steam pipe is attached to the top tie plate 5 with attachment means 32 and the inlet end of the steam pipe is attached to the bottom tie plate 6 with attachment means 31. Swedish patent document 9604720-4 shows a fuel assembly with fuel units stacked on top of each other, in which the fuel rods are arranged in a polar lattice comprising a number of concentric rings. In such a fuel assembly, the invention may be advantageously applied. FIG. 4 shows part of a fuel assembly according to a second embodiment of the invention. The fuel assembly comprises a number of fuel units 40a-40c which all have fuel rods arranged in a polar lattice comprising three concentric rings. In the lower part of the fuel assembly, fuel units 40a are arranged. All the fuel rod positions in the fuel unit 40a are occupied. In the fuel unit 40b, all the fuel rod positions are also occupied. The fuel rods 42 have a smaller diameter than the fuel rods 41 in the fuel unit 40a. The fuel rods in the inner rings are inclined outwards from the centre of the rings so as to form an empty volume in the centre. The fuel rods are not inclined in the direction of the ring, as is the case in the first embodiment. In this way, no eddy is formed which separates the steam from the water. One advantage of inclining the fuel rods outwards is that the water accompanies the fuel rods, and in this way a certain separation of water and steam occurs. In the fuel units 40c, the fuel rod positions in the inner ring are unoccupied and, instead, a steam pipe 10a, 10b is arranged in the centre. The steam pipes have the same design as has been described above. The advantage of this embodiment is that it is simpler to manufacture than the first-embodiment. FIG. 5 shows part of a fuel assembly according to a third embodiment of the invention. The fuel assembly comprises a top tie plate 37, a bottom tie plate 38 and a plurality of full-length fuel rods 36 which are arranged between the top tie plate and the bottom tie plate. Further, the fuel assembly comprises a number of part-length fuel rods 39 which extend from the bottom tie plate and terminate far below the top tie plate. Above these part-length fuel rods, a plurality of steam pipes 10a, 10b are arranged. The steam pipes are arranged one above the other. Between the steam pipes, spacer elements 44 are arranged to keep the steam pipes in spaced relationship to each other and to fix the steam pipes to each other. To keep the fuel rods in spaced relationship to each other, a number of spacers 45 are arranged in spaced relationship to each other along the fuel assembly in the longitudinal direction thereof. The steam pipes are attached to the spacers. The steam pipes are designed in the same way as has been described in the preceding embodiments.