Patent Number: 050154354
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 shows a fuel assembly 1 consisting of a bundle of parallel fuel rods 2 held by spacers 3 arranged at certain intervals along the length of the rods 2. The spacers 3 consist of grids, each of whose cells receives a fuel rod. Certain positions in the grid lattice are occupied by guide tubes 4, which are longer than the fuel rods 2. At one of their ends, the guide tubes 4 are connected to an end fitting 5 forming the upper end fitting of the fuel assembly and, at their other end, to a second end fitting 6 forming the lower end fitting. When the assembly is in storage position under water in a pool, the upper end fitting 5 is accessible from above the pool. This upper end fitting 5 carries blade springs 7 responsible for holding the assembly in the reactor core, whose upper core plate comes to rest on the springs 7. The end fitting also comprises studs 8 projecting from to its upper face. FIG. 2 shows the fuel assembly framework 9 comprising the guide tubes 4, spacers 3 and the end fittings 5 and 6. This framework 9 serves as a housing for the fuel rods 2 of the bundle, which can be introduced into or extracted from the framework when the upper end fitting 5 is removed. Demountable couplings are provided between the ends of the guide tubes 4 and the upper end fitting 5 for performing the replacement or the removal of the rods. FIGS. 3 and 4 show an upper end fitting of a fuel assembly comprising an adapter plate 10 in which the guide tubes can engage inside openings 11 passing through this adapter plate and accessible from the upper part of the fuel assembly. The guide tube for the instrumentation of the fuel assembly, located in the central part, is received in a specially shaped opening 12. The upper end fitting of the assembly consists of the adapter plate 10 and a frame 14, connected together by means of a skirt 13 welded to the plate 10 and to the frame 14. The frame 14 carries the protuberances 8 comprising openings for centering and flanges 8' for holding the springs 7. As can be seen in FIG. 4, the passage holes 11 permitting the guide tubes 4 to be fastened are arranged in defined positions corresponding to the positions of the twenty-four guide tubes of the assembly. Water passage holes 15 pass through the adapter plate 10 of the end fitting between the passage openings 11 for the guide tubes. As can be seen in FIG. 5, the upper part 4a of the tube 4 has a frusto-conical shape widening upwards, corresponding to the shape of the passage hole 11 in its lower part which receives the end of the tube 4. The upper part 4a of the tube 4 additionally comprises two slots such as the slot 18, arranged in the direction of the generatrices of the conical frustum and in diametrically opposite positions. These two slots, such as the slot 18, define two frusto-conical sectors in the upper part 4a of the guide tube 4, which thus has a certain scope for radial distortion. Furthermore, in the upper part 4a of the guide tube 4 there is provided, on each of the radially distortable sectors, an outwardly projecting part 17 in the shape of a part of a ring, which can be housed inside an annular widening 16 of the hole 11 when the guide tube 4 is fitted into the opening 11 of the adapter plate 10. Radial expansion of the upper part 4a of the tube 4 is provided by a locking sleeve 20. The interlocking and the fastening of the tube 4 in the adapter plate 10 and the demountable end fitting 5 are thus effected. The part of the passage opening 11 of frusto-conical shape receiving the guide tube 4 and comprising the annular widening 16 has a widened entry part 19 facilitating the introduction of the tube 4 and ends in a shoulder 21. Part 17 of the tube 4 of annular shape and projecting outwards is preformed before the introduction of the tube into the opening 11 and permits the axial locking of the tube 4 in the annular cavity 16 owing to the elasticity of the frusto-conical sectors. It should be noted that the upper part of the tube 4 does not abut on the shoulder 21 in its engagement position inside the adaptor plate. This arrangement makes it possible to facilitate fitting and improve the positioning of the guide tube 4 in the adapter plate 10. The part of the opening 11 which is located above the shoulder 21 comprises cavities 22 in the shape of ring portions, machined radially inside the adapter plate 10. Above the cavities 22, the opening 11 comprises a frusto-conical part 23 widening upwards and opening onto the upper face of the adapter plate 10. FIGS. 6 and 6A show a locking sleeve 20 of a device for fastening a guide tube such as shown in FIG. 5, which comprises a lower part 24 consisting of a shell of frusto-conical shape and an upper part 25 consisting of a shell of cylindrical shape. The frusto-conical shell 24 which in its lower part comprises an engagement surface 24a is intended to come into a locking position inside the end 4a of the guide tube 4, as shown in FIG. 5. The shell 24 is responsible for the radial expansion of the frusto-conical sectors forming the upper part 4a of the tube 4 and the holding of the projections 17 in the annular widening 16 of the opening 11 of the adapter plate 10 of the end fitting. The upper shell 25 forms the shell for fastening the locking ring which can be housed in the upper part of the hole 11 of the adapter plate 10 when the sleeve 20 is in locking position, as shown in FIG. 5. According to the invention, the external surface of the locking sleeve is of cylindrical and frusto-conical shape and has no part protruding outwardly in the radial direction. The locking sleeve 20 comprises a one-piece tubular wall 26, the outer surface of which has a cylindrical upper part and a frusto-conical lower part which are connected by means of the large base of the frusto-conical part. The thickness of the wall 26 is variable over the height of this wall. The upper part 25 of the wall forms a collar of small thickness. The inner bore of the lower part 24 of the locking sleeve is of cylindrical form, with the result that the wall has a thickness decreasing from the top downwards in this lower part 24. The part 24 of the wall has an annular groove 29 in the vicinity of its upper end, in its part of large thickness. The thickness of the wall of the part 24 is substantially larger than the thickness of the shell 25, at least in its upper part of large thickness. The collar 25 is separated from the frusto-conical part 24 by a rim 30 projecting radially towards the inside of the sleeve. The groove 29 is delimited on the opposite side to the rim 30, i.e., downwards, by a shoulder 31. The groove 29 can be used to put the locking sleeve 20 in place in a guide tube by introducing a gripping tool into the groove 29. An axial push can be exerted on the sleeve 20 by means of the shoulder 31, in order to obtain the locking of the guide tube which is kept apart by means of the frusto-conical part 24 engaged in the guide tube, this part 24 constituting the ring for expanding the tube. The collar 25 is then located in a part of the bore of the connector of the assembly which is not occupied by the guide tube. One or more radial indentations are machined in this part of the bore of the connector. After locking of the guide tube, the collar 25 is pushed into the radial indentations of the bore of the connector, and it ensures that the locking sleeve is fastened in these. FIGS. 7 and 7A shown an alternative embodiment of the locking sleeve of FIGS. 6 and 6A. The sleeve 20' is identical to the sleeve 20, except that it has a frusto-conical shoulder 31' instead of the plane shoulder 31 used during the installation of the blocking sleeve in the guide tube. As an alternative embodiment, during the installation of the sleeve, the thrust on the sleeve can be exerted through the radially protruding rim 30. The fastening of the locking sleeve can be ensured by distortion of the regions 32 of the shell 25 inside the cavities 22, using tools of known type, introduced into the upper part of the sleeve. The axial and rotational locking of the sleeve 20 is thus produced. To perform the dismantling of the end fitting of the assembly it is necessary, in a first step, to perform the extraction of the locking sleeves 20 introduced into the end of the guide tube. In the case of a fastening device such as shown in FIGS. 6 and 7, the extraction of a sleeve is effected thanks to interlocking members such as the claws of a traction tool that may be introduced into the groove 29 or 29'. The interlocking members of the tool for pulling the sleeve can be easily fitted through the inside of the sleeve, with a control from inside the pool. The traction on the sleeve effected through a groove in its inner surface allows the dismantling to be effected more easily by extracting the fastening distorted zones 32 from the cavities 22. The sleeve is no more retained in the end fitting and the ring 24 for expanding the tube can be extracted from the guide tube 4. It is possible to conceive interlocking grooves accessible through the inside of the sleeve of a different shape. Finally, the invention applies to the case of any fuel assembly for a light-water reactor comprising a demountable end fitting in which the guide tubes of the assembly are fastened by locking sleeves.