Patent Number: 062298682
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a fuel assembly of a boiling water type comprising an upper handle 1, a lower end portion 2 and a plurality of fuel units 3 stacked one above the 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. Further, each fuel unit 3 comprises a top tie plate 16 and a bottom tie plate 17 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 17 in one fuel unit 3 is facing the bottom tie plate 16 in the next fuel unit 3 in the stack and such that the fuel rods 4 in all the fuel units 3 are parallel to one another. A fuel rod 4 contains fuel 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 the 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 8 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 3 comprises 24 fuel rods 4 arranged in a symmetrical 5.times.5 lattice. The fuel units 3 are kept in position by being fitted onto and fixed to the water channel 14 which surrounds the vertical channel. 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 replace 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 a fuel assembly which is provided with an internally arranged vertical channel 14a through which water is conducted in a vertical direction 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 fitted onto the tube which surrounds the vertical channel. FIG. 2b shows a fuel assembly which is provided with two centrally arranged 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 17 and a bottom tie plate 16 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 fuel assembly and they are stacked in such a way that the top tie plate 17 in one fuel unit 3 is facing the bottom tie plate 16 in the next fuel unit 3 in the stack, and such that the fuel rods 4 in all the fuel units 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 4a are arranged extending through the whole fuel assembly. The control rod guide tubes 4a are intended to receive finger-shaped control rods (not shown) which are, respectively, inserted into and withdrawn form the guide tubes 4a for the purpose of controlling the power of the nuclear reactor. The guide tubes extend between a top art 4b and a bottom part 4c. The top part 4b is arranged above the uppermost fuel unit 3 in the fuel assembly and the bottom part 4c is arranged below the lowermost fuel unit 3 in the fuel assembly. The fuel units 3 are kept in position by being fitted onto and fixed to the control rod guide tubes 4a. FIG. 4a shows a fuel unit 3 for a pressurized-water reactor according to FIG. 3, which is connected at top and bottom to fuel units 3. The fuel units 3 are interconnected by way of the guide tubes 4a extending through the whole fuel assembly. The fuel rods 4 extend between a bottom tie plate 16 and a top tie plate 17. In the bottom tie plate 16 and the top tie plate 17, sleeves 18 are arranged. To the left in FIG. 4a, the sleeves 18 and the guide tubes 4a are shown in a view from the side and to the right in FIG. 4a, the sleeves 18 and the guide tubes 4a are shown in a vertical section. The bottom tie plate 16 is fixed to the guide tube 4a by means of bulging (not shown) of the guide tube 4a when this is inserted into the sleeve 18. The top tie plates 17 are freely movable along the guide tubes 4a. The top tie plates 17 may, of course, be fixed to the guide tube 4a whereas the bottom tie plates 16 are arranged freely movable in relation thereto. Admittedly, FIG. 4a refers to a fuel assembly for a pressureized-water reactor, but in those cases where a boiling water reactor is intended, the fuel assembly is designed in a corresponding manner but in that case the bottom tie plates 16 and the top tie plates 17 are instead arranged to the water channels 14, 14a, 14b with axial gaps between the fuel units. FIG. 4b shows a fuel unit 3 of the same type as in FIG. 4a but with a spacer 19 arranged between the bottom tie plate 16 and the top tie plate 17. In an advantageous embodiment, the spacer 19 is made from sleeve-formed cells with elongated contact surfaces, for example of the type indicated in SE 9303583-0. The sleeves 19a which surround the control rod guide tubes 4a have been given a larger length in the axial direction for increased mechanical guiding of the control rod guide tubes. The sleeves in the sleeve spacer 19 may possibly be provided with conventional mixing vanes for mixing the coolant flowing upwards through the fuel assembly. The top tie plate 17 and the bottom tie plate 16 are provided with a plurality of flow openings 20 intended to be traversed by the coolant flowing upwards in the fuel assembly. These flow openings 20 are thus arranged substantially between the positions of the fuel rods 4. FIG. 5a shows a flow opening 20 in a top tie plate 17. In the flow opening 20, flow tongues 21 are arranged. Between the flow tongues, spaces 22 are arranged. FIG. 5b shows a flow opening 20 in a bottom tie plate 16. In the flow opening 20, flow tongues 23 are arranged. Between the flow tongues, spaces 24 are arranged. FIG. 5c shows the bottom tie plate 16 in FIG. 5b arranged above the top tie plate 17 in FIG. 5a. The flow tongues 23 in the bottom tie plate 16 are arranged above the spaces 22 in the top tie plate 17 and the flow tongues 21 in the top tie plate 17 are arranged below the spaces 24 in the bottom tie plate 16. In a new fuel assembly, the top tie plate 17 is arranged at a definite distance A1, of the order of size of a few millimeters, from the bottom tie plate 16 (see FIG. 5d). When, during operation of the reactor, the fuel rods 4 are extended, because of the radioactive irradiation, more than the control rod guide tubes 4a and the water channels 14, 14a, 14b, respectively, and when one of the top tie plate 17 or the bottom tie plate 16 is secured to the control rod guide tubes 4a and the water channels 14, 14a, 14b, respectively, whereas the other is freely movable around the guide tubes and water channels, the distance between the top tie plate 17 and the bottom tie plate 16 is reduced gradually during the service life of the fuel assembly (see A2 in FIG. 5e). In this way, burnup-dependent and automatic flow limitation, restriction, is obtained. The arrows in FIGS. 5d and 5e indicate the path of the coolant through the top tie plate 17 and the bottom tie plate 16. FIGS. 6a-6c show an alternative form of burnup-dependent flow limitation where the flow openings 20 in the top tie plate 17 and in the bottom tie plate 16 are given eccentrically displaced center axes. FIGS. 6d-6e show how the coolant flow is gradually restricted during the service life of the fuel assembly in that the fuel rods 4 grow more in the axial direction than the control rod guide tubes 4a and the water channels 14, 14a, 14b, respectively. FIG. 7a shows an embodiment of top tie plates 17 and bottom tie plates 16 provided with flow openings 20 with center axes displaced in relation to each other. In FIG. 7a, the center axes are displaced in relation to each other in such a way that a diagonal flow is created by a mixing cross section in the fuel assembly which, for example, may consist of two adjacently located fuel assemblies or four adjacently located sub-assemblies. FIG. 7b shows an alternative embodiment of the flow control in FIG. 7a. In FIG. 7b the center axes of the flow openings have been displaced such that, within the mixing cross section, the flow is deflected through substantially 90.degree..