Patent Number: 050911422
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 shows a fuel assembly 1 consisting of a bundle of parallel fuel rods 2 held by struts 3 arranged with certain spacing along the length of the rods 2. The struts 3 consist of grids, the cells of which each receive a fuel rod. Certain positions in the latticework of the grids are occupied by guide tubes 4, which are longer than the fuel rods 2. The guide tubes 4 are connected at one of their ends to an end block 5 forming the top end block of the fuel assembly and at their other end to a second end block 6 forming the bottom end block. When the assembly is in the underwater storage position inside a well, the top end block 5 is accessible from the top of the well. This top end block 5 carries leaf springs 7 ensuring holding of the assembly inside the reactor core, the upper core plate of which rests on the springs 7. The end block also comprises studs 8 projecting relative to its upper surface. FIG. 2 shows the framework 9 of the fuel assembly comprising guide tubes 4, struts 3 and the end blocks 5 and 6. This framework 9 serves as a housing for the fuel rods 2 of the bundle which may be introduced or extracted from the framework when the top end block 5 is removed. In order to effect replacement or removal of rods, demountable connections between the end of the guide tubes 4 and the top end block 5 are provided. FIGS. 3 and 4 show a top end block of a fuel assembly comprising an adapter plate 10 into which the guide tubes engage, inside openings 11 passing through this adapter plate and accessible from the top part of the fuel assembly. The instrumentation (side tube of the fuel assembly situated in the central part is received inside an opening 12 having a special shape. The top end block of the assembly consists of the adapter plate 10 and a frame 14 connected together by means of a skirt 13 welded onto the plate 10 and onto the frame 14. The frame 14 has bosses 8 comprising centering openings and flanges 8' for holding the springs 7. As can be seen in FIG. 4, the through-holes 11 allowing fixing of the guide tubes 4 are arranged in defined positions corresponding t the positions of the twenty-four guide tubes of the assembly. Water flow holes 15 pass through the adapter plate 10 of the end block between the through-openings 11 of the guide tubes. FIG. 5 shows a locking sleeve 20, facilitating implementation of the method according to the invention. This locking sleeve comprises a lower part 24 having an external surface of frustoconical shape which forms the part of the locking sleeve ensuring expansion of the guide tube and an upper part consisting of a cylindrical ferrule 25 forming the ferrule for fixing the locking sleeve in the end block of the fuel assembly. The ferrule 25 consists of six cylindrical segments 25a, 25b, 25c, 25d, 25e and 25f separated from one another by slits 26 arranged in the direction of the generatrices of the ferrule 25. The cylindrical segments 25a to 25f are identical, the slits 26 being arranged at 60.degree. from one another about the axis of the locking sleeve 20. The expansion bush 24 of the locking sleeve, which is frustoconical in shape, comprises an internal bore of substantially cylindrical shape, such that the thickness of the wall of this bush 24 decreases from its large base at the level of which the ferrule 25 is connected, along the line 30, to its small base, at the level of which an inclined frustoconical surface 24a is provided, facilitating engagement of the locking sleeve inside the guide tube. The expansion bush 24 of the locking sleeve, which is relatively thick, has sufficient rigidity to ensure effective holding of the guide tube, as will be explained herein below. The ferrule 25 is substantially less thick than the expansion bush 24, in particular in the vicinity of the junction line 30, i.e., the large base of the part 24. The locking sleeve 20 has an internal radially projecting shoulder 28 ending in an inclined frustoconical surface 28a directed towards the inside of the sleeve. The sleeve 20 may also be made as a single machined piece or, alternatively, the ferrule 25 may be mounted on the top part of the frustoconical holding part 24. The slits 26 may be cut from the ferrule 25, after machining or assembly of the sleeve. In all cases, the cylindrical segments 25a to 25f forming the ferrule 25 may be deformed by pushing inside locking cavities in the end block of the fuel assembly, and by folding about a line situated in the vicinity of the line 30. FIG. 6 shows the adapter plate 10 of a top end block of a fuel assembly, in the vicinity of a through-opening 11 in which the end 4a of a guide tube 4 is engaged. The upper end 4a of the guide tube 4 has been formed prior to introduction into the opening 11, so as to fit perfectly in the inlet part of this opening which has a slightly frustoconical shape and which comprises an annular enlargement 16. The upper part 4a of the tube 4 has a frustoconical shape corresponding to the shape of the inlet part of the opening 11, and an annular securing part 17, radially projecting outwards, intended to fit into the annular enlargement 16 of the plate 10, when the tube 4 is positioned inside the opening 11. The frustoconical inlet part of the opening 11 ends in a shoulder 21 projecting radially towards the inside of the opening 11. It should be noted that the end of the tube 4, when the latter is completely engaged inside the opening 11, does not rest on the shoulder 21. As a result of preforming, the tube car: be arranged in place without difficulty and very precisely inside the end block without the formation of any tearing support. The opening 11 has, at its inlet end on the lower surface of the adapter plate 10, a frustoconical enlargement 19 for facilitating introduction of the tube 4. The upper part or outlet part of the opening 11, situated above the shoulder 21, comprises a ring-shaped radial cavity 22. Above the cavity 22, the opening 11 forms an outlet part 23 widening out and emerging on the upper surface of the adapter plate 10. The reassembly of a fuel assembly comprising guide tubes and connection means as shown in FIGS. 6 and 7 may be performed underwater in the storage well of the nuclear reactor fuel assemblies, as follows. The fuel assembly rests via its bottom end block on the bottom of the well, the guide tubes of the assembly being in the vertical position. The top part of these guide tubes onto which the adapter plate 10 of the end block 5 is engaged is located at a water depth sufficient to protect the operators carrying out the reassembly. Engagement of the adapter plate onto the ends 4a of the guide tubes does not pose any difficulty, the guide tubes being held in a precise transverse arrangement by the struts of the assembly. Moreover, as a result of the preformed ends of the guide tubes, the guide tubes can be engaged and arranged in position precisely inside the through openings of the adapter plate. The top part 4a of each of the guide tubes is made so as to be deformable radially in order to allow it to be engaged inside the opening 11 and then expanded so as to secure the projecting parts 17 inside the annular enlargements 16. To this end, at least two slits such as 18 arranged at 180.degree. relative to one another separate the top part 4a of the guide tube into at least two frustoconical segments deformable in the radial direction. After arranging in position the top end block on the end of the guide tubes, these guide tubes are fixed by means of locking sleeves 20, such as those shown in FIG. 5 and using a tool 31 such as that shown in FIGS. 6 and 7. The tool 31 consists of a pole of considerable length, the bottom part 32 of which can be seen in FIGS. 6 and 7. The pole of the tool 31 is fixed at the top to a holding and raising means such as a rolling bridge serving the fuel well, and may be displaced inside well, with its axis ZZ' in the vertical position. The bottom part 32 of the pole comprises, in sequence from top to bottom, a cylindrical part, a frustoconical bearing part 33 widening outwards, a frustoconical thrusting surface 34 directed inward, and a cylindrical end part 35. The end part 35 of the bottom part 32 of the pole comprises means for retaining a locking sleeve (not shown). These retaining means may consist, for example, of curved leaf springs, the convexity of which is directed outwards and which are fixed in the axial direction, on the external surface of the end part 35. Such retaining devices are described in a copending patent application filed by applicant on the same day as the present application. The tool 31 comprises two tubular-shaped components (37) and 38, the first of which 37 is slidably mounted on the cylindrical part of the pole situated above the frustoconical surface 33, and the second of which (38) is slidably mounted around the first. The first sliding component 37 comprises slits in its bottom part defining radially deformable segments 39, the bottom part 40 of which forms a gripping jaw. The component 37 on which the component 38 is slidably mounted forms a clamping chuck, the bottom parts 39 of which may be displaced radially between a first position shown in FIG. 6, and a second position shown in FIG. 7. The sliding jacket 38 is connected, at the top (not shown), to an axial displacement device allowing it to be displaced between its respective positions shown in FIGS. 6 and 7, respectively. Each of the gripping jaws 40 of the component 37 comprises a frustoconical internal bearing surface 40a and a frustoconical external bearing surface 40b. The sliding jacket 38 comprises a frustoconical bearing surface 38a at the bottom. In order to carry out reassembly of the end block of a fuel assembly, use is made of a sleeve supplying device arranged inside the well, in the vicinity of the end block to be reassembled. Such a supply device has been described in a copending patent application filed by applicant on the same day as the present application. The tool 31 is positioned so as to be able to introduce the end part 35 of the pole into the cylindrical internal bore of the holding bus 24 of a sleeve located in the supply device. The end part 35 engaged inside the sleeve, as a result of its retaining devices, enables the sleeve to be removed and introduced, via the widened out end 23, into an opening 11 of the adapter plate 10 inside which a guide tube 4 is engaged. A thrusting force is exerted on the pole of the tool 31, the bottom part 32 of which is in contact with the shoulder 28 and the surface 28a of the sleeve 20, by means of a shoulder and the frustoconical bearing surface 34, respectively. The thrusting force transmitted to the sleeve 20 enables the latter to be introduced inside the tube 4 in the locked position, as shown in FIG. 6. An adjusting wedge enables the position of the pole of the tool 31 to be precisely determined in order to introduce the sleeve 20. The pole of the tool 31 is then raised so as to disengage its end part 35 from the sleeve 20, which remains in position inside the guide tube which it fixes. Using a known type of tool, a part of each of the cylindrical segments 25a to 25f is pushed into the cavity 22 of the top part of the opening 11 of the plate 10 of the end block. The deformed parts 29 thus obtained, engaged inside the cavity 22, enable the sleeve 20 to be axially locked relative to the end block. The guide tube 4 is thus perfectly fixed inside the demountable end block. In order to carry out demounting of the end block, the locking sleeves 20 engaged inside the guide tubes and fixed inside the adapter plate of the end block must be extracted. For this purpose, a tool 31 is used, which initially is brought underneath the assembly located underwater inside the well and engaged inside the sleeve 20, as shown in FIG. 6. The pole of the tool 31 abuts against the shoulder 28 and the bearing surface 28a via the shoulder of the part 33 and the bearing surface 34. The gripping jaws 40 of the chuck member 37 are then engaged inside the frustoconical opening 23, around the top end of the ferrule 5. The actuating jacket 38 is lowered, thereby radially folding the segments 39 of the component 37 inwards. FIG. 7 shows the final position of the segments 39, at the end of the downward movement of the jacket 38. During their pivoting movement inwards, the segments 39 come into contact with the top part of the cylindrical segments 25a to 25f of the ferrule 25, which they fold inwards until the segments come into contact with the frustoconical surface 33. Towards the end of its downward movement, the actuating jacket 38 comes into contact, via its surface 38a, with the bearing surface 40b of the component 37, which has a certain freedom of sliding movement around the end 32 of the pole. As a result of the downward sliding movement of the component 37, folding of the segments of the ferrule 25 is facilitated and the ferrule 25 is properly gripped by the gripping jaws 40, as shown in FIG. 7. Folding of the segments of the ferrule 25 has allowed the deformed parts 29 of these segments to be disengaged from the cavity 22. The locking sleeve 25 is therefore no longer held inside the opening of the adapter plate by the fixing ferrule. The sleeve 20 may be easily extracted by raising the pole of the tool 31, using the raising means fixed to the top part thereof. It is sufficient, in fact, to overcome the clamping forces of the frustoconical holding bush 24 inside the tube 4, these forces being very much less than those required for disengaging the deformed parts such as 29 from the radial cavity. Moreover, the ferrule 25 is firmly held by gripping between the bearing surfaces 40a of the gripping jaws 40 and the frustoconical surface 33. Extraction of the locking sleeves is therefore obtained without difficulty as a result of the method of the invention. It is possible to use the method according to the invention to perform the extraction of locking sleeves of a type different from that which has been described in which the fixing ferrule consists of successive cylindrical segments separated by slits. The method according to the invention can also be used in the case of sleeves comprising deformable tongues, cut in the fixing ferrule, which may be pushed outwards into cavities of the end block when the locking sleeve is fixed, and folded inwards in order to disengage them from the cavities, when the sleeve is extracted. It is also possible to use a fixing ferrule having folding lines allowing the parts of the ferrule between two folding lines to be folded down and the deformed parts to be disengaged from the corresponding cavities. In the case where the fixing ferrule consists of cylindrical segments, separated by slits, there may be any number of segments, and only part of these segments may comprise parts deformed inside cavities provided in the end block. The tool for performing extraction of the locking sleeves may be different from that which has been described. This tool may carry out only extraction of the sleeve, or both positioning and extraction. The tool may also consist of a plate carrying a set of extraction spindles arranged in a latticework reproducing the arrangement of the guide tubes of one or more fuel assemblies and each formed in the same manner as the extraction tool described above. The plate is associated with means for displacement in the vertical direction and comprises means for simultaneous operation of the folding and gripping components of the set of spindles. The tool is arranged in position on the top end block of at least one fuel assembly, such that the spindles are each engaged onto a sleeve for locking a guide tube of the assembly. The disengagement and extraction of the locking sleeves from the guide tubes of one or more fuel assemblies may therefore be performed simultaneously, thereby ensuring a considerable saving in time. Finally, the method according to the invention applies to any demountable fuel assembly for a lightwater nuclear reactor, in which the guide tubes are fixed inside an end block by means of locking sleeves comprising a ferrule for fixing the sleeve inside the end block.