Patent Number: 046577335
Section: description

DETAILED DESCRIPTION FIG. 1 shows a fuel assembly employed in a nuclear reactor of improved yield as described in French No. 2,535,508. This assembly comprises a set of fuel rods 1 which consist of a sheath tube enclosing pellets of fuel material. The tube is closed at its lower end by a plug 2 and at its upper end by a plug 3. The rod zone 1a located just above the lower plug 2 is filled with fertile material which replaces the fissile fuel material filling the median zone of the rod. The same applies to the zone 1b located below the upper plug 3 of the rod. Thus in each of the assemblies and in the entire core, an upper blanket and a lower blanket of fertile material is formed, allowing the neutron yield of the reactor to be improved, as described in French No. 2,535,508. The rods 1 are held in a structure consisting of the spacer grids 5 for transversal positioning of rods 1, the lower end plate 6, the upper end plate 8 and the guide tubes 7 which are connected to the grids 5 and which are fixed at one of their ends on the lower end plate 6 and at their other ends on the upper end plate 8. The guide tubes 7 thus ensure the positioning and the rigidity of the support structure of the rods 1. The guide tubes 7 are fixed in openings which penetrate the end plates 6 and 8, by means of a tapped end portion on which may be engaged the threaded portion of a bush 9 (for the lower end plate) or 10 (for the upper end plate). After screwing the bushes 9 and 10 into the tapped ends of the guide tubes 7, locking of these bushes in rotation is carried out. The fuel assembly is fixed by its lower part on the lower core plate 12 by a device with a central control 13 shown in outline in FIG. 1. This device for fixing the lower part of the assembly was the subject of French No. 2,517,867 in the name of the assignee of the present application. With reference to FIGS. 1 to 5, a description will now be given of the device for fixing the lower part of the fuel rods on the lower end plate 6. The lower plug 2 of the fuel rods is machined over the majority of its length in a section which is seen in FIG. 2 and which has three radial branches 2a, 2b and 2c at 120.degree.. Each of the radial expansions corresponding to the branches 2a, 2b and 2c comprises in addition in its lower portion a shoulder 15 projecting relative to the circular section of the rod in its cylindrical part in which is inscribed the section of the plug, with the exception of the parts projecting in the radial direction 15. An attachment plate 14 rests with its lower face on the upper face of the end plate 6 on which this plate 14 is fixed by threaded hollow bushes 16 engaged in openings 17 which penetrate the lower end plate 6. Each of the openings 17 comprises a shoulder 17a on which bears the head 16a of the bush 16. The threaded part 16b of the bush 16 is screwed into a tapped hole provided in the plate 14 in the extension of the opening 17 of the end plate 6. After the bush 16 is screwed in, its locking in rotation is carried out by deformation of an upper skirt 16c extending the head 16a of the bush, into grooves provided at the entry of the opening 17. It is possible in this way to fix the plate 14 in a perfectly stable manner on the end plate 6. The plate 14 comprises openings 18 with a diameter which is substantially equal to the diameter of the cylindrical part of the fuel rods 1, and in locations which correspond to the locations of the rods of the bundle. In the extension of each of the rods and of the openings 18, the end plate 6 is also pierced by holes 20 of a diameter near to that of the rods. The end plate 6 comprises therefore three types of perforations which penetrate right through it: the fixing openings of the guide tubes, the passage openings for the hollow attachment bushes 16 of the plate 14 and the holes 20 allowing the passage of the cooling water of the reactor around the lower part of the rods 1. The hollow bushes 16 also allow the passage of the cooling water. Similarly the attachment plate 14 comprises three types of perforations penetrating right through it: the holes for passing the guide tubes 7, the openings 18 for fixing the fuel rods 1 and the tapped holes for the screwing in of bushes 16. The lower surface of the attachment plate 14 has furthermore a set of grooves 21 which are parallel to each other and each of which corresponds to a row of fuel rods. The openings 18 for the passage of the lower part of the rods lead out at their lower end into the interior of the grooves 21. As may be seen in FIG. 1, the depth of the grooves 21 corresponds to the height of the radial shoulder 15 provided on the plugs 2 of the fuel rods 1. In the case of an assembly for a nuclear reactor with improved yield as described, this depth is 5 mm. The dimension of the shoulders 15 in the radial direction is 2 mm. The width of the grooves 21 machined on the lower surface of the plate 14 is chosen so that one of the radial branches of the lower part of the rod is perpendicular to the axis of the groove, as shown in FIG. 2. The three branches being at 120.degree., the axis of the rod is offset to a slight extent relative to the median plane of the groove, so that it is necessary to provide cylindrical recesses 24 in a wall of the groove, to make it possible to place the rods on the plate 14. In the case of an assembly with rods of a diameter of 9.5 mm, the width of the groove is 8 mm. We shall now describe the operations required for the fitting and the fixing of the rods in the assembly. This fitting is carried out on an assembly the structure of which comprises the upper end plate, the guide tubes and the spacer plates, with the lower end plate not yet being placed in position. To begin with, the attachment plate 14 is placed in position on the lower end of the guide tubes, the plate 14 comprising a system of holes corresponding to the position of these guide tubes. The rods are then inserted through the positioning plate 14 and through the spacer grids 5, these rods being stopped by the shoulders 15 of the lower plugs 2 which abut on the bottom of the grooves 21 machined in the attachment plate 14. The rods can be placed in position in only one way in the grooves which ensure their locking in rotation. The lower plate 6 is then placed in position on the ends of the guide tubes, the plate 14 then resting on the upper surface of the end plate 6. The lower end plate is then attached to the guide tubes by virtue of the threaded bushes 9 which are locked in rotation when they have been screwed into the ends of the guide tubes. The plate 14 is then fixed on the end plate 6 by virtue of the threaded bushes 16 which are also locked in rotation when they have been screwed in, by virtue of the skirt 16c which is deformed inside the grooves provided in the end plate 6. The rods 1 are then perfectly immobilized in the assembly in axial translation as well as in rotation. These rods are held laterally by the grids 5 which comprise only small bosses for holding the rods, and no longer the Inconel springs, as did the spacer grids of the assemblies of the prior art. Applicant's tests have shown that the rods fixed in this way are not subjected to abnormal modes of vibration in the reactor during operation. Furthermore, these rods are perfectly held in the axial direction by the attachment plate 14. Nevertheless, the cross-section of flow of the cooling water in the assembly remains considerable by virtue of the shape of the lower plugs which permits the flow of the water through the end plate and the attachment plate, in a cross-section which is comparable to the cross-section of the rod and by virtue of the hollow bushes 16. Finally, the dismantling of the lower end plate and of the rods is extremely easy because it is sufficient to unscrew the assembly bushes 16 from the plate 14 and the assembly bushes 9 from the guide tubes 7. During the unscrewing, the bush skirt is deformed to undo the locking in rotation of these bushes. It is possible to devise other means of fixing the plate 14 on the end plate 6, so long as these means of fixing permit easy dismantling as do the bushes which have just been described. It is equally possible to devise a shape of the profiled part of the lower plug of the rods which is slightly different from that which has been described, provided that this shape allows an orientation and a locking in rotation of the rods in the grooves. Finally, the assembly according to the invention applies not only in the case of the nuclear reactors with improved yield where the core comprises a layer of fertile material in its upper part and in its lower part, but also in the case of all the nuclear reactors in which the neutron balance is improved by reducing the quantity of material absorbing the neutrons in the fuel assemblies.