Patent Number: 045227829
Section: description

DETAILED DESCRIPTION FIG. 1 shows a fuel assembly constituted by a bundle of parallel fuel rods 1 whose spacing is maintained by means of spacer grids 2 regularly spaced along the fuel assembly and constituting, as FIG. 2 shows, a square-mesh lattice in which some fuel rod 1 locations are occupied by guide tubes 5. The guide tubes 5 are longer than the rods 1, so that these guide tubes can be fixed on the end plates 3 to assure the rigidity of the assembly. The tubes 5 serve both the keep the assembly rigid and to guide the absorbant rods constituting the control rod associated with the assembly. FIG. 3 shows the end of two guide tubes 6 and 7 of a fuel assembly whose upper end plate, or upper tip, is made of stainless steel and whose spacer grids 9 and 10 are made of a nickel alloy with high elasticity. The guide tube 6 is a stainless steel tube and it is connected to the spacer grids 9 and 10 and to the upper tip 8 by direct welding to these parts. The tube 7 is made of zirconium alloy called "Zircaloy" which has a low neutron capture cross section. This tube 7 is simply engaged in the spacers 9 and 10 and in the upper tip 8, inside openings allowing longitudinal displacement of the tube 7 with respect to these spacer grids and to this upper tip, when the tube 7 lengthens under the action of irradiation and the temperature in the reactor. The whole assembly comprises four guide tubes made of stainless steel such as the tube 6 and twenty tubes made of zirconium alloy such as the tube 7. At their lower part, the tubes 7 are fixed to the lower tip of the assembly, by mechanical connection. It is therefore clear that, in an assembly such as that described with reference to FIG. 3, the mass of material with a high neutron capture cross section is therefore reduced, as twenty out of twenty-four guide tubes are made of zirconium alloy with a low neutron capture cross section. In addition, the assembly is very easily constructed, since the four stainless steel tubes can be directly welded to the end tips and the spacer plates, without the need for intermediate parts. In addition, free sliding of the zirconium tubes in the openings provided in the upper tip and in the spacer grids allows expansion of the zirconium tubes under irradiation to be accommodated, without lengthening of the assembly occurring. FIG. 4 shows two guide tubes 11 and 12 engaged at their upper part in the sleeves 13 and 14 fixed by welding to the upper tip 15 and to the upper spacer grid 16 of the assembly. These stainless steel tips allow connection of the upper spacer grid and the upper tip and thus increase the rigidity and strength of the assembly with respect to axial forces. The stainless steel tube 11 is welded at its upper part to the tip 13 and to each of the spacer grids such as 18 disposed at regular intervals over the height of the assembly. The guide tube 12 made if zirconium alloy with a low neutron capture cross section is simply engaged in openings in the spacer grids such as 16 and 18 and in the upper tip 15. At its lower end, the tube 11 bears a plug 19 made of stainless steel which allows it to be fixed by screw means to the lower end plate 20. The lower end grid 60 is connected to the lower tip 20 by a sleeve 61 to which it is welded. The guide tube 11 is inserted in this sleeve. The sleeve is secured to the end plate when the tube is fixed. The lower part of the zirconium alloy tube 12 is fixed to the lower plate 20 by means of a zirconium alloy plug 23 closing the lower end of the tube 12 and allowing the tube 12 to be fixed by screw means to the lower tip 20. In the complete assembly, four stainless steel tubes similar to the tube 11 are used, fixed in the same way to the end tips and the spacer grids. The other twenty tubes used are zirconium alloy tubes such as the tube 12, only fixed to the lower tip 20. FIG. 5 shows that the stainless steel tip 13 is square-sectioned which allows perfect engagement of this tip in the grid 16 and very strong fixing of the tip to the grid 16 without interfering with the fuel rods thereby. Similarly, the tips 14 are square-sectioned, allowing perfect engagement in the cells of the grid 16. FIG. 6 shows an embodiment of the connections between the stainless steel guide tubes and the tip by direct welding of these tubes to the upper tip 15 and to the upper spacer grid 16, without using a sleeve 13. On the other hand, the Zircaloy tubes 12 are engaged in sleeves 14 identical to those described with reference to FIG. 4, assuring connection between the plate and the upper end and the upper spacer grid. This embodiment of the assembly allows the mass os stainless steel and therefore the absorption of neutrons by the structures of this assembly to be reduced. FIG. 7 shows the upper part of a stainless steel guide tube 24, assuring the rigidity of the assembly, in the case of an easily removable fuel assembly, like that described in French Pat. No. 2,368,785. In such an assembly, the upper end of the guide tubes has widened-out, prismatically shaped part 25 which can be introduced into a recess 26 provided in the lower part of the upper tip 27, to fix the tube to this tip, by means of a hollow bushing 28 including a threaded part 29 which screws into a corresponding screw-threaded part provided on the inner surface of the part 25 of the guide tube. The bushing 28 is engaged in an opening passing through the plate 27 and including a shoulder 31 and an upper part 32 of larger diameter than the lower part in which recesses 33 are provided, for locking the bushing rotationally by expansion of the collar constituting the upper part. In this way, the tube is perfectly locked rotationally and fixed rigidly on the upper tip 27. In the case of such a removable assembly, the provision of four stainless steel tubes 24 like those represented in FIG. 7, welded to the spacer grids such as 35, is all that is necessary to obtain rigid assembling of the guide tubes, spacer grids and tips, while retaining the possibility of removing the upper tip by unscrewing a bushing 28. In an assembly with twenty-four guide tubes, the other twenty tubes are made of zirconium alloy and are only fixed to the lower tip of the assembly by plug and screw as described with reference to FIG. 4. The zirconium alloy tubes are also introduced into openings in the spacer grids and the upper tip, allowing them to move in the event of these tubes expanding. When the upper end plate 27 is removed, it is possible to have access to the fuel rods and to extract these selectively from the assembly, to replace or examine them. In all cases, it is preferable to provide Zircaloy tubes which are sufficiently long for these tubes to open as near as possible to the upper face of the upper tip of the assembly. More efficient retention and guiding of these tubes is thus provided. FIG. 8 shows a variant of the fixing of a stainless steel guide tube, in the case of an easily removable assembly. A cylindrical tip 36 having a prismatically shaped upper end 39 engaged in a correspondingly shaped opening provided in the upper tip 38 is fixed. A bushing 37 can be fixed, as before, inside the tip 36 to fix the upper plate 38 with respect to this tip 36. The tip 36 is also fixed by welding to the spacer grids such as 40 and the stainless steel guide tube 41 is fixed by welding to the end of the tip 36, immediately below the first spacer grid 40. In this embodiment, the connection between the upper plate and the first spacer grid 40 is thus strengthened since it is achieved by means of tips 36, of larger diameter than the guide tubes 41, which can be square-sectioned, as in FIG. 4. As in the case of the apparatus described with reference to FIG. 7, the collar constituting the upper part of the bushing 37 can be deformed so as to enter the recesses 43 by expansion and lock the bushing rotationally. FIG. 9 shows a variant in mounting the zirconium alloy tubes 44 in which these are guided and held at their upper part by a sleeve 45 fixed to the upper plate of the assembly 47 through a bushing 48 which also serves to retain and guide the upper part of the tube 44. The sleeve 45 has an upper part 46 whose outer surface is prismatically shaped to engage in a correspondingly shaped opening in the plate 47. The sleeve 45 is also connected by welding to the upper spacer grid 49 and thus forms the connection between the upper end plate 47 and this spacer grid 49. The tubes 44 are connected at their lower part to the lower end plate of the assembly and engage in openings in the various grids of the assembly so that guiding is assured for them, while retaining the possibility of displacement of the zirconium tubes with respect to these spacer grids, in the longitudinal direction. FIG. 10 shows a variant in mounting the zirconium alloy guide tubes 50 in which these have a widening 51 in their upper part in which the lower part of the sleeve 45, fixed on the upper end plate 47 by means of a bushing 48, as described with reference to FIG. 9, then engages. This type of mounting of the zirconium alloy tubes allows sleeves with a greater thickness to be used, with a given inner diameter of guide tube and a given outer diameter of sleeve 45, since the inner diameter of the sleeves can be reduced, as it is no longer required to pass the guide tubes. FIG. 11 shows a fuel assembly according to a second embodiment of the invention. The spacer grids 65, 66, 67 are made of zirconium alloy (Zircaloy). The guide tubes are also made of the same alloy. The guide tube 68 is connected to the upper tip 15 in a similar way to that represented in FIG. 7, i.e., by using a hollow bushing 74 screwing into a corresponding screw-threaded part provided inside the guide tube. Although it has not been specifically represented, connection of the guide tubes 68 to the tip can be obtained as in all the embodiments represented in FIGS. 7, 8, 9, 10, provided that care is taken to use the materials which allow the necessary welded connections to be made. The freely mounted guide tubes 69 are also made of Zircaloy and are introduced into a blind bore 70 in the lower end plate 20 and into a bore 71 in the upper plate. At its upper end, this bore 71 includes a circular projection 73 for fixing the guide tube. FIG. 12 shows an embodiment of the connection between the grid 65 and the tip 15 in which the guide tube 68 is strengthened in this part and has an outer section which is greater than its normal section over a height between its upper end and the lower level of the end grid 65. This section can be circular or square, identical to that represented in FIG. 5. The guide tubes have the same reinforcement in their lower part, between the end grid and the lower tip. It is clear that the apparatus according to the invention allows simplification of the structure and manufacture of the assembly, less longitudinal deformation of this under irradiation, particularly in the embodiment in which the spacer grids are made of stainless steel and a very strong structure to be obtained, despite use of a small mass of material with a high neutron capture cross section. In addition, having only a small number of connections between the guide tubes and the grids allows an improvement in the thermohydraulic conditions, since the cells adjacent to the freely mounted rods in the grids are very well irrigated. The invention is not limited to the embodiments described; it includes all the variants thereof. Other means of connection, removable or not, between the guide tubes and the end plates and other means of connection between the lower ends of these guide tubes and the lower end plate and thus conceivable. The use of materials other than stainless steel for constituting the guide tubes assuring the rigidity of the assembly and other materials than zirconium alloys for the other guide tubes fixed only to the lower plate of the assembly is also conceivable. Lastly, the fuel assembly according to the invention can be used in all nuclear reactors in which the fuel is in the form of very long rods constituting bundles of parallel rods connected together by spacer grids and end plates.