Patent Number: 051065720
Section: description

DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 shows schematically the vessel 1 of a pressurized water nuclear reactor with including above the core (not shown) a closing plate, called core upper plate 2, placed horizontally. Vessel 1 extends above plate 2 via an enclosure 3 terminating in a flange 4 on which a cover 5 is placed and latched. Nozzles 6 of large diameter are provided in the side wall of enclosure 3 for the outflow of the core cooling water. In enclosure 3, a guide assembly 7 is mounted above the upper core plate, each guide being provided at its lower portion with a circular guide flange 8 adapted for bearing on plate 2 while forming with it a narrow transverse clearance 9. Guides 7 extend with their vertical axes 10 perpendicular to plate 2 and are provided for supporting clusters of absorbant rods (not shown in detail), the extent of whose insertion inside the reactor core through plate 2 via an operating member 11 permits the nuclear reaction to be controlled. In FIG. 1, reference 12 denotes schematically the circulation direction of the cooling water which, flowing from bottom to top, flows through the upper plate 2 prior to being discharged outside the guides 7 and then from enclosure 9 through nozzles 6. In a conventional construction of the above type, the accurate positioning of the flanges 8 of the guides 7 for the absorbing clusters with respect to the upper core plate 2 is, whereby each guide has to notably positioned perfectly with respect to a corresponding opening of plate 2 for the passage of the absorbing rods. Simultaneously, the transverse dimension of clearance 9 between plate and flange must be rigorously determined, irrespective of the vibrations created by the flow of cooling water and by the hydrodynamic stresses exerted on the connection means which are used. In particular, the assembly formed by the core plate 2 and the flanges 8 which are above it must be such that possible excessive forces can be absorbed by subsequent return of the two parts to their optimal and predetermined relative positions, and this without prejudice to the mechanical structures used. To this effect and according to the invention, each flange 8 of any guide 7 is positioned with respect to the core plate 2 by means of two diametrically opposite massive spindles 13 and 14 (FIG. 2), these spindles being implanted in the thickness of the flange while protruding outside said flange in the direction of plate 2 and extending inside housings 15 and 16 provided in register in plate 2 with a convenient mounting clearance 17. In a particular embodiment of the invention, spindles 13 and 14 emerge from the lower face of each flange 8 along a height which is sufficient for to permit precise positioning of guide 7 connected to this flange with respect to core plate 2, of the order of about 0.5 mm. These spindles 13 and 14 extend into their housings 15 and 16 with a diametrical clearance 17 which in practice is about 0.3 to 0.4 mm. The two positioning spindles 13 and 14 are associated with an assembly of four self-locking shoes 18, placed as shown in FIG. 2. These shoes are so designed that, as will be seen hereafter, they can exert on flange 8 suitably distributed forces, while allowing in combination with the centering obtained by spindles 13 and 14 the indispensable precise positioning and the permanent alignment of the vertical axes of the passages reserved for the absorbing clusters. Preferably, the four self-locking shoes 18 are distributed by pairs, two on either side of the diametrical plane connecting spindles 13 and 14, the shoes of the same pair exerting on flange 8 forces which are respectively in directions 19a and 19b, converging at a point 19c situated on the transverse axis perpendicular to the direction of the plane connecting spindles 13 and 14. The conjunction of the forces of the four shoes 18 and of spindles 13 and 14 allows them to resist displacement of the cluster guide with respect to upper core plate 2 without permanent and fluctuating dragging effects, thereby ensuring the desired centering of axis 10 of guide 7 connected to flange 8, in alignment with that of the passage (not shown) formed in plate 2 underneath flange 8, through which the cluster of absorbing rods will be more or less permanently inserted in the core underneath the plate, for control of the nuclear reactivity. FIGS. 3 and 4 show a particular embodiment of the self-locking shoe 18 and of the control mechanism associated therewith. This assembly includes a block or shoe 22 as such, mounted with a clearance in a transverse bore 21 formed in flange 8, said shoe 22 having a plane lower face 23 in contact with the upper surface 24 of the core plate 2. Shoe 22 protrudes slightly underneath flange 8 over a height corresponding to the clearance 9 provided between plate 2 and flange 8. At its end which is opposite face 23, shoe 22 has a slanting face 25 for cooperating with a push-piece 26, also mounted with a clearance in bore 21. Shoe 22 and push-piece 26 are connected to one another through a non-rigid connection, permitting their relative axial displacement while also permitting the shoe to be suspended underneath the push-piece, rendering it captive. To this effect, the push-piece is extended downwardly by a flat central rib 27, engaging between the two parallel sides 28 and 29 of a clevis (FIG. 4) formed in the corresponding upper portion of the shoe, the connection between rib 22 and sides 28 and 29 of the clevis being provided by a transverse peg 30, extending through a cylindrical hole 31 of larger diameter formed in rib 27. Push-piece 26 is extended upwardly, particularly toward the upper portion of transverse bore 21, by a narrower stem 32, extending over a distance outside this bore, the stem 32 including two successive axial portions 33 and 34 of different diameters, portion 33 having a diameter larger than that of portion 34 so as to form a shoulder 35 at the junction of the two portions. At its upper end opposite shoulder 35, stem 32 is formed with a slit 36 allowing the relative orientation of the stem with respect to the vertical to be indentified and, the direction of the force exerted by push-piece 26 on stem 22 by the cooperation of the respective surfaces of the two elements mutually bearing on one another to be varied. To this effect and in order to allow an orientation of the resultant force created on shoe 22, push-piece 26 is formed at its lower end with a convex face 37, substantially as a portion of cylinder, and adapted to come to bear against the slanting surface 25 of the shoe, the application of these surfaces on one another producing a resultant of the forces created, particularly for the four associated shoes of the same spindle, according to directions 19a and 19b of FIG. 2. Portion 33 of stem 32 extends with a clearance through an axial passage 38, formed in an end-piece 39 the lower end 40 of which engages over a short distance inside bore 21 of flange 8, the end-piece being then secured against movement with respect to the flange, e.g., by a weld 41. End-piece 39 is extended upwardly by a sleeve 42, defining a chamber 43 in which is also mounted, with a clearance, a spring cartridge 44, including a lower support washer 45 through which extends the smalles diameter portion 34 of stem 32 and which bears at the base of this lower portion on shoulder 35. A pack of conical washers 46 is disposed about portion 34, these washers being thus contained between the lower ring 45 and a complementary washer 47. A variable force can be exerted on the spring cartridge 44, and due to the abutment of washer 45 on shoulder 35, on stem 32 and thense on push-piece 36. To this effect, sleeve 42 is formed with an interior screw thread 48, cooperating with a screw thread 49 having the same pitch 49 formed on the outer surface of a calibration bushing 50. The latter is formed with an axial bore 51 through which extends with a clearance the portion 34 of stem 32, and which includes at its upper end a head nut 52 allowing control of the rotation of the bushing in the sleeve screw thread, by exerting on spring cartridge 44 the required force, of the order of 1000N per shoe. The structure of the spring cartridge may be varied, for example by using, as shown schematically in FIG. 4, double washers 46a or other means allowing an elastic transmission of the efforts of the bushing toward the push-piece while ensuring, in the case of a force in the opposite direction, the absorption of the latter by simple compression of the washer pack between washer 47 which remains immobile and washer 45 which is subjected to a telescoping movement within sleeve 42 and which follows the corresponding limited displacement of stem 32. Shoes 22 carry an appropriate surface coating, which is preferably different for the plane lower face 23 and for the slanting upper face 25 of these shoes. Notably, the surface coating of face 23 is chosen in such manner that it imparts to the surface 24 of upper plate 2 a coefficient of friction which is the highest possible. On the contrary, the coating of convex face 37 of face 25, as well as that of push-piece 26, is chosen with a coefficient which is as small as possible in order to facilitate sliding motion of the respective faces on one another. Preferably also, the outer surfaces of shoe 22 and of push-piece 26, as well as the inner surface of bore 21, carry a surface coating similar to that of faces 25 and 37 for facilitating in the same manner their relative sliding motion when the device is used. The operation of the centering and fixation device according to the invention is the following: Flange 8 with its two fixed spindles 13 and 14 (FIG. 2) is positioned on the upper plate 2 with a clearance in their positioning which corresponds to the clearance 17 of these spindles in their respective housings 15 and 16. After mouthing of the spindles and self-locking shoes on the flange of the cluster guides, the spring cartridges are calibrated, and the cluster guides are then mounted on the support structure above the upper core plate. After mounting of the two fixed spindles 13 and 14 on flange 8, one disposes on end-piece 39 the push-piece 32, the washer 45, the spring cartridge 44, the washer 47, the calibration sleeve 50 (without tightening it), the shoe 22 on push-piece 32, and its axis 30. The four end-pieces thus fitted out are positioned on flange 8 and attached, e.g., by soldering. Underneath flange 8 is fixed a wedge of a thickness corresponding to the predetermined space 9, then the tightening is carried out with the assistance of a calibration sleeve 50 of spring cartridge 44 to a predetermined value, taking into account the relaxation of the service elements, the calibration tolerances, the variations of clearance 9, so that the force exterted by the push-piece on the shoes always remains greater than to a calculated value, i.e., about 600N. The tightening of the calibration sleeve is carried out by one of the conventional methods for obtaining a preload: either a torque or an angular stroke after being in contact, or a force cell placed underneath the shoe, or a combination of these methods. The shoes 23 and the push-pieces 32 are oriented with the assistance of slit 36 according to directions 19a and 19b, and then the rotation of calibration nut 32 is braked. The setting plate is then removed, to release the preload of the spring cartridges. The cluster guides are then mounted on the support structure; This positions flanges 8 above the core plate 2. About 20 mm prior to reaching clearance 9, the fixed spindles 13, 14 engage into the respective housing 15 and 16 with a small clearance 17, thereby ensuring a correct orientation of the cluster guide on the upper core plate 2. About 5 mm prior to reaching clearance 9, face 23 of the shoes comes in contact with the core plate 2, the shoes push back push-pieces 32 upwardly, and the latter compress the spring cartridges to the preestablished mounting value. The four shoes 18 are controlled so that the resultant of the forces exerted on these shoes taken two by two, as is also shown in FIG. 2, converges at points symmetrical to one another in a median diametrical plane, perpendicular to that which joins spindles 13 and 14. To this effect, shoes 22, the lower plane faces 23 of which are in contact with surface 24 of plate 2, are subjected by the calibration bushings 50 to an appropriate force in the direction of the plate, stems 32 being previously oriented by their slit 36 in the desired direction. The screwing of bushings 50 is pursued until, due to the lateral displacement of shoes 22 created by the cooperation of the convex face 37 of push-piece 26 with the slanting face 25 of the corresponding shoe, the initially defined diametrical clearance between the shoe and its bore is adjusted, the resilient washers 46 of cartridge 44 being compressed within the corresponding limit. In this position, flange 8 is perfectly positioned with respect to the upper core plate 2, while leaving between these members only the clearance 9 which is necessary for the differential expansion of flange 8 and of the structures connected thereto. The calibration of spring cartridge 44 takes into account the relaxation of the elements in service and the calibration tolerances, so that the force exerted by the push-piece on the self-locking shoes remains substantially uniform, even when clearance 9 varies during the thermal cycles resulting from the operation of the reactor and of the differential expansions resulting therefrom. This axial force, multiplied by the coefficient of friction between the shoes 22 and the upper core plate 2, thus creates a lateral component opposing the transverse displacement of flang 8 under the effect of the hydrodynamic stresses of which these members are the seat, particularly due to the flow of cooling water through them. The respective positions of the shoe and of the push-piece change from those of FIG. 6 to those of FIG. 5, due to the inclination of the bearing faces 25 and 37, the effect of which is to cancel the clearance of shoe 22 and of its housing within flange 8. The invention is not limited to the embodiment more especially described and shown hereabove. In particular and as is shown in FIG. 7, the mounting in two portions of shoe 22 and of push-piece 26 connected via their transverse peg 30 can be replaced by another embodiment shown schematically in FIG. 7, where the lower end of portion 33' of the stem again includes a push-piece 26' acting on the shoe 22' bearing on surface 24 of plate 2, but in which the shoe is made captive due to a retracted bearing surface 53 formed in the bottom of the flange and against which bears, notably in the flange transportation position, a collar having the same profile 54 as said shoe.