Patent Number: 
Section: description

The rod 10 shown in FIG. 1 has cladding 12 closed by a connection plug 14 and by a top plug 16, and it contains a column of absorber material held pressed against the bottom connection plug 14 by a spring 20 which is compressed between the column and the top plug 16. The top plug 16 enables the rod to be secured to a finger of a spider 22. The top plug 16 and the means enabling it to be fixed to the spider 22 can be of the structure described in particular in French patent application No. 95/15488 (which issued as French Patent No. 2,742,912 and which issued as U.S. Pat. No. 5,889,832), to which reference can be made. The rod is of conventional shape, having a diameter that is constant except that its bottom end is bullet-shaped to make it easier to insert rods into the guide tubes of an assembly when an absorber cluster is lowered. The cladding 12, the connection plug 14, and the top plug 16 are advantageously made of austenitic stainless steel of a grade that enables them to be welded electrically with a tungsten electrode (TIG welding). The outside surface of the cladding, of the top plug, and of the connection plug is advantageously subjected to nitriding treatment prior to assembly so as to increase resistance to wear. An austenitic steel makes it possible to perform ionic nitriding treatment of good quality with low sensitivity to corrosion. The nitriding can be performed by the method described in document FR-A-2 604 188, to which reference can be made. In general, the pellets 24 occupying the cladding are made of boron carbide. Their diameter is slightly smaller than the inside diameter of the cladding 12, so as to allow them to be insetted and so as to accommodate swelling. The cladding can have an outside diameter of 9.68 millimeters (mm) and it can be about 1 mm thick, as is common in pressurized water reactors. In conventional manner, the spring 20 can be made of an xe2x80x9cInconelxe2x80x9d type alloy. The connection plug 14 is fixed to a bar 26 of hafnium, which is solid in the example shown in FIG. 1. The total length L0 of the bar 26 generally lies in the range 25% to 35% of the length L1 of the column of pellets 24. The hafnium bar 26 is fixed to the connection plug 14 by a connection that is purely mechanical. In the example shown in FIGS. 1 and 2, this connection is performed by crimping in the zone outlined by box 28. For this purpose, the connection plug 14 has a top portion which is engaged inside the cladding and a flange 30 which bears against the bottom edge of the cladding and which is welded to the cladding. The plug has an extension with circumferential grooves or channels machined therein, there being two such grooves in FIG. 2. The bar 26 is terminated by a thin tubular zone 31 constituting a skirt, which zone is deformed into the grooves of the connection plug 14 once they have been assembled together. It can be seen that the absorber column thus presents only very little discontinuity, because of the presence of the skirt. An axial bore is formed at the end of the skirt and opens out into a hole which is used for suspending the bars during oxidation treatment; this hole allows water to flow inside the connection and regenerate the oxide layer constantly. The crimping is advantageously performed by cold isostatic compression, for example using apparatus of the kind shown in FIGS. 3 and 4. This method and apparatus can be used to assemble together parts made of two different materials that are unsuitable for conventional thermal welding, which materials can be other than hafnium and stainless steel. The components to be crimped together (the hafnium bar and the cladding provided with plugs as shown in FIG. 2) are placed in such a manner that the zone to be crimped is in register with a ring 32 of a material which is deformable but incompressible or only very slightly compressible, such as certain elastomers. At rest, the inside diameter of the ring 32 is slightly greater than the outside diameter of the cladding to be deformed. The length of the ring 32 matches the length of the crimping that is to be performed. When crimping cladding to a hafnium bar as shown in FIG. 1, the length of the ring lies in the range a few millimeters to about 15 mm. Its outside diameter is about 10 mm greater than its inside diameter. It is radial deformation of the ring under the effect of axial compression that performs the crimping. In FIG. 4, it can be seen that the ring 32 is enclosed in a chamber defined by a high strength steel sleeve 34 and an annular piston 36 which slides in a bore of the sleeve. The sleeve is pierced by a hole for inserting the bar 26. The inside diameter of the piston is designed to allow the cladding 12 to pass through it. The apparatus includes a mechanism for urging the piston 36 into the sleeve. This mechanism is carried by a frame 40 to which there is secured a housing 42 for receiving the sleeve. The frame carriers a hydraulic actuator 44 whose plunger 46 bears against a rocker arm 47. The rocker arm bears against the piston 36 via an adjustment module constituted by two washers 48 that are screwed one in the other. These washers are pierced by a central hole and the arm 47 has a slot so as to allow the components for crimping together to pass through them freely. When the actuator is powered via pipe 50, it compresses the ring 32 which swells inwards so as to deform the tubular zone 31 of the bar and convert it from the shape shown in FIG. 4 to the shape shown in FIG. 2. The apparatus described above can be varied in numerous ways. Crimping can be performed in a single groove, thus enabling the length of the ring 32 to be shortened. Two rings separated by a spacer (or more than that) can be provided so that each ring acts over a groove. Crimping can be performed from the inside, in which case the ring 32 is placed inside the two tubular portions that are to be assembled together so as to give rise to expansion. The mechanical connection can also be provided by a screw connection, as shown in FIGS. 5 to 7 where members corresponding to those of FIGS. 1 and 2 are referenced by the same reference numerals. In this case, the connection plug 14 comprises in succession a portion which engages in the end portion of the cladding and which is terminated by a bearing shoulder, a tapped portion, and a thin deformable skirt 52. This connection plug 14 is welded to the cladding. The bar 26 is terminated by three zones of decreasing diameters. The first zone 54a has longitudinal notches (two such notches in the example shown) for receiving deformed zones of the skirt in register therewith so as to prevent the bar from turning. It also has centering zones so as to facilitate assembly. The second zone 54b is threaded and enables the bar to be assembled to the connection plug. It is designed to be screwed into the tapped portion of the plug and to be tightened with determined torque. The thread is dimensioned in such a manner as to provide sufficient mechanical resistance to the fatigue stresses to which it will be subjected in a reactor. Finally, the third zone 54c is constituted by an extension which engages inside the plug so as to ensure axial continuity of neutron absorption. After the bar has been fixed, it is locked against rotation by deforming the skirt 52 using a punch of suitable shape to press the skirt into the notches. Water can penetrate into this connection and can serve continuously to regenerate the protective oxide layer on the hafnium. Finally, the mechanical connection shown in FIG. 8, where members corresponding to those of FIG. 6 are referenced by the same references, has a connection plug 14 with a chamber for receiving a reduced-diameter terminal portion of the bar 26. A pin 60 is engaged in two aligned transverse bores in the connection plug 14 and the terminal portion of the bar.