Patent Number: 
Section: description

To be able to design the fuel assembly such that water and steam are separated in an efficient way, it is desirable for the fuel assembly to be so flexible that it may be given different shapes in the axial direction in a simple manner. Such a fuel assembly is shown in international patent document PCT/SE95/01478 publication number WO 96/20483. This fuel assembly comprises a plurality of fuel units stacked one above the other, each one comprising a plurality of fuel rods extending between a top tie plate and a bottom tie plate. The fuel units are surrounded by a common fuel channel with a substantially square cross section. A fuel assembly of this type may given different shapes in the axial direction in a simple manner. FIGS. 2 and 3a-3f show a fuel assembly according to the invention. During operation, the fuel assembly is arranged vertically in the reactor core. FIG. 2 is a vertical section Fxe2x80x94F through the fuel assembly. FIGS. 3a-3f show a number of horizontal sections Axe2x80x94A, Bxe2x80x94B, Cxe2x80x94C, D1xe2x80x94D1, D2xe2x80x94D2, D3xe2x80x94D3 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, 3c and 3d stacked one above the 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 column 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 contains eight fuel units which are each about 0.5 meters high. A fuel unit has 100 fuel rod positions in an orthogonal 10xc3x9710 lattice. A fuel rod position is a position in the lattice and in these it is possible to arrange a fuel assembly, but 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 center 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 rods in the fuel unit have an upper end arranged at the top tie plate and a lower end arranged at the bottom tie plate. A fuel rod belonging to the inner or the outer ring has its lower end arranged in a first fuel rod position and its upper end arranged in a second fuel rod position. The upper and lower ends of the fuel rod are thus arranged in separate fuel rod positions. The first and second fuel rod positions are positioned side-by-side and, in addition, belong to the same ring. There are two positions in the lattice which fulfil both of these conditions. The fuel rods are thus inclined between the bottom tie plate and the top tie plate, and a fuel rod may be inclined in two different directions within the same ring. In a sub-bundle all the fuel rods in the two rings are inclined in the same direction, that is, either clockwise or counterclockwise around the center of the sub-bundle. The purpose of inclining the fuel rods around the center of the sub-bundle is to set the water and the steam, flowing upwards through the fuel assembly, in rotation, thus achieving an eddy with a center in the center of the sub-bundle. The eddy may be directed in the clockwise or counterclockwise direction depending on in which direction the fuel rods in the two rings are inclined. The angle between the longitudinal axis of the fuel assembly and the longitudinal axis of the inclined fuel rods is determined by the distance between the bottom tie plate and the top tie plate and the distance between two fuel rod positions close to each other in the lattice. The fuel assembly comprises four different types of fuel units 3a, 3b, 3c, 3d. The two lowermost fuel units 3a are identical and a horizontal section Axe2x80x94A through these is shown in FIG. 3a. The fuel unit 3a has 100 fuel rods arranged in a 10xc3x9710 lattice, and is divided into four sub-bundles 15a, 15b, 15c, 15d with 25 fuel rods in each sub-bundle. All the fuel rod positions in the lattice are occupied by fuel rods. In the fuel rod position in the center of each sub-bundle, a straight center rod 4a is arranged. The center rod is parallel to the longitudinal axis of the fuel assembly and has the same fuel rod position in both its upper and lower ends. The figure shows by means of arrows in which direction the fuel rods in the inner ring 20a and the outer ring 20b are inclined. In two of the sub-bundles 15a, 15c the fuel rods in the rings are inclined clockwise around the center rod and in the other two sub-bundles 15, 15d the fuel rods in the rings are inclined counterclockwise around the center rod. FIG. 4 shows the fuel unit 3a in a view from the side in a section Exe2x80x94E through the fuel assembly. The figure shows that the fuel rods in the sub-bundle 15a are inclined to the right and that the fuel rods in the sub-bundle 15b are inclined to the left. By inclining the fuel rods in different directions in the different sub-bundles, four eddies are achieved in the fuel assembly during operation of the reactor, two being directed counterclockwise and two being directed clockwise. The sub-bundles which are arranged along the same diagonal have fuel rods which are inclined in the same direction. It is an advantage if some of the eddies are directed counterclockwise and some are directed clockwise, because in that case the rotational effects which arisexe2x80x94both mechanical and thermohydraulicxe2x80x94may counterbalance each other. The following two fuel units 3b in the stack are of the same type and a horizontal section Bxe2x80x94B through these is shown in FIG. 3b. The fuel unit 3b has 96 fuel rods divided into four sub-bundles. Each one of the sub-bundles contains 24 fuel rods arranged in an inner ring 20a and an outer ring 20b. The fuel rod position in the center of the sub-bundle is unoccupied. In this way, an empty volume is formed in the center of the fuel bundle. Otherwise, the fuel unit 3b is arranged in the same way as the fuel unit 3a. The empty volume constitutes the lower part of a vertical steam channel which extends through the six uppermost fuel units in the fuel assembly. In the two lowermost fuel units 3a, no steam channels are needed since there is no steam there, but on the other hand it is an advantage to initiate the eddy formation at this early stage. There are four steam channels 16a, 16b, 16c, 16d in the fuel assembly, one in each sub-bundle. The inclined fuel rods in the sub-bundle achieve an eddy of water and steam around the steam channel. The directions of the eddies are marked with arrows in the steam channel. In these eddies, the water and the steam are separated from each other by throwing the water outwards and, hence, away from the steam channel whereas the steam is pressed against the center of the eddy. Because of the low density of the steam and the low flow resistance in the steam channel, the steam will flow upwards at great speed through the steam channel and disappear out through the top of the fuel assembly. In this way, the percentage by volume of steam in the coolant is reduced. On top of the fuel units 3b in the stack, two fuel units 3c are stacked. A horizontal section Cxe2x80x94C through these is shown in FIG. 3c. The fuel unit 3c has 88 fuel rods and each sub-bundle contains 22 fuel rods. In one sub-bundle, the fuel rod position in the center is unoccupied and, in addition, two positions in the inner ring are unoccupied. Otherwise, the fuel unit 3c is arranged in the same way as the fuel unit 3a. By increasing the number of unoccupied fuel rod positions, the steam channels 16a, 16b, 16c, 16d will have a larger cross-section area in these fuel units compared with the fuel units 3b further down in the fuel assembly. In this way, the steam channel will have an increasing cross-section area towards the top of the fuel assembly and hence an increasing volume, which is necessary since the percentage of steam which is to be transported away increases towards the top of the fuel assembly. At the top of the fuel assembly, two fuel units 3d are stacked on top of each other. A horizontal section D1xe2x80x94D1 through the fuel unit 3d immediately above the bottom tie plate is shown in FIG. 3d. The fuel unit 3d has 80 fuel rods and each sub-bundle contains 20 fuel rods. In one sub-bundle the fuel rod position in the center and four positions in the inner ring are unoccupied. The unoccupied fuel rod positions are those which are closest to the center of the fuel unit. Otherwise, the fuel unit 3d is arranged in the same way as the fuel unit 3a. The steam channels 16a, 16b, 16c, 16d have their largest cross-section area in these two uppermost fuel units. The steam channels have their outlets 21 through holes in the top tie plate in the uppermost fuel unit in the stack. To illustrate how the lattice positions of the fuel rods are displaced between the top tie plate and the bottom tie plate, FIG. 3e shows a horizontal section D2xe2x80x94D2 through the fuel unit 3d on half its height. In FIG. 3f a horizontal section D3xe2x80x94D3 through the fuel unit immediately below the top tie plate is shown. A fuel rod displaces its lattice position one step in the clockwise or the counterclockwise direction within the ring to which it is associated. It is especially shown how the fuel rods 4b, 4c, 4d, 4e in the inner ring are displaced to the next lattice position one step in the counterclockwise direction in the inner ring. The embodiment described so far is based on an orthogonal lattice with top tie plates and bottom tie plates identical as regards lattice positions. However, the invention may very well be applied also if the lattice is irregular and nor do the lattice positions need to be identical in the top tie plates and the bottom tie plates. The fuel rods may also be inclined to differing degrees in the same fuel unit. Such embodiments may be preferable, for example for increasing the distance between fuel rods which change their direction of inclination in the corners. The bottom tie plate and the top tie plate may be provided with enlarged holes 16 to allow the passage of the steam in the steam channel. To intensify the eddies around the steam channel, both the bottom tie plate and the top tie plate may be provided with fins around these enlarged holes which are oriented such that the eddy is intensified. FIG. 5 shows part of the bottom tie plate for the fuel unit 3b in a section Gxe2x80x94G through FIG. 2. Around the hole 16, fins 17 are arranged to control the water and the steam in the direction of the macroscopic eddy. It is important to note the difference between these fins 17 and know fins which are often arranged on spacers in both boiling water and pressurized-water reactors to mix the coolant in a sub-channel between four adjoining rods and hence improve the dryout margin. In these cases the known fins are to be arranged so as to intensify the microscopic eddy. There are further possibilities of achieving the above-mentioned intensification of the eddy, for example by turning around ligaments in the top tie and bottom tie plates into an inclination of 45xc2x0 with the horizontal plane. The top tie plate and/or the bottom tie plate may be provided with a frame which, in turn, may carry obliquely positioned fins or folds. In another embodiment of the invention, all the fuel rods may be straight and the eddies may be achieved by other means, for example fins on the bottom tie plate and the top tie plate. To seal the fuel rods, they are provided at their upper end with a top plug and at their lower end with a bottom plug. These bottom plugs and top plugs may also be provided with fins or other devices to bring about an eddy in the sub-bundle.