Patent Number: 051065720
Section: summary

FIELD OF THE INVENTION The present invention relates to a connection device for the centering and fixation of the upper core plate which closes the core cavity of a nuclear reactor, particularly of a PWR reactor, and a flange extending above said plate and on which are secured against movement the ends of guide tubes in which move cluster assemblies of neutron absorbing rods, adapted to be inserted in the core between the nuclear fuel elements, for ensuring the regulation of the fission reaction. More particularly, the invention relates to a device allowing relative positioning of two horizontal parallel plates by providing for perfect alignment of passages extending through them for the engagement or retraction in the core of the absorbing rod clusters, by eliminating any risk of jamming or blockage of said clusters during their movements, while allowing a slight relative axial displacement. BACKGROUND OF THE INVENTION In the prior art, the connection and accurate positioning of these two members is provided by means of spindles, one of the ends of which extends into an appropriate housing of the core plate by being axially slit in order to impart to it a relative resiliency in the transverse direction, the other end extending with a clearance through a bore of the flange of a cluster guide and being secured against movement with respect to the latter by screwing a nut coming in abutment against an inner shoulder of the bore so as to exert a reaction during the screwing on a threaded portion of the spindle. The nut is thereafter blocked on the flange, e.g., by soldering. By distributing over the flange periphery an appropriate number of such spindles and by providing for their coordinated tightening via the nuts associated thereto, accurate positioning of the flange with respect to the plate is achieved, notably so as to bring into alignment the vertical axes of these two members and to adjust to an accurately determined value the horizontal clearance between them. The axially slit end is formed of two flexible branches the outer diameter of which is slightly greater than the receiving bore, formed in the upper plate of the core, assuring sure that after mounting there is no remaining clearance subject to vibrations and wear, and providing an adaptation to the position tolerances of the two spindle axes respectively on the core upper plate and on the cluster guide lower flange. The two spindles are generally disposed in such manner that the slits are positioned perpendicularly with respect to one another. This state affords the best compromise between strength and flexibility. However, the centering of the two parallel members takes place against the resisting section of the spindles, which must provide not only for the orientation and coincidence of the cluster guides with respect to the network of fuel elements underneath the core plate, but also for the holding of the lower ends of these guides, while opposing the vibrations, which are sometimes significant, created by the hydromechanic stresses resulting from the axial flow of the cooling water flowing through the plate and flange and which is collected above said plate in the upper portion of the reactor vessel, and then is discharged through appropriate outlet nozzles. The prior art solutions only partly meet these requirements, and cannot provide assurance of reliability over time. SUMMARY OF THE INVENTION The object of the present invention is a device mitigating the disadvantages of the standard solutions by allowing dissociation of the two functions which consist respectively in centering the flange with respect to a common axial reference, and relative fixing of the two members by providing between them a spacing which is always unifrom whatever the operational speeds of the reactor and resulting stresses on the cluster guides and their connections with the plate. To this effect, the device includes at least two axial guiding spindles of the flange with respect to the core plate, diametrically opposite and rigidly fixed to the plate or to the flange in order to extend in a housing in alignment formed in the flange or in the plate respectively, and an assembly of self-locking shoes, adapted for sliding with a clearance in bores formed in the flange in order to apply against the plate surface, said shoes being each associated with a position control mechanism, carried by the flange and exerting on the shoes a force having a transverse component so as to provide by reaction the immobilization of the flange with respect to the plate. Thanks to these dispositions, the spindles ensure on the one hand the guiding in axial translation of the flange with respect to the upper core plate, save the mounting clearances of these spindles in their housings. Simultaneously, lateral self-locking shoes provide for the and axial retention of the flange with respect to the plate, the locking mechanism associated with each of said shoes allowing adjustment of their relative positioning in the bores of the flange, and therefore positioning of the latter, independently of the dimensional variations to which they may be subjected during the reactor operation, their structure being such that they readily absorb the effects of the vibrations created by the cooling water flow from the core through the plate and flange. According to a particular feature of the invention, the self-locking shoes have a general cylindrical shape and include a plane bearing face, applied against the core plate, and an opposite face slanting with respect to the horizontal. The control mechanism cooperating with each shoe is preferably made of a push-piece extending into a bore of the flange and including a convex face, bearing on the slanting face of the shoe, said push-piece being prolonged outside the flange bore at the end opposite to the plate by an elongated rod, located on the axis of a sleeve with an inner screw thread at its end for screwing a hollow calibration bushing, through which the rod extends freely and exerts a vertical force on the latter via a spring cartridge, bearing on the one hand against the bushing and on the other hand against a washer through which extends the rod, and in abutment on a shoulder of the latter. Advantageously, the spring cartridge includes a pack of conical washers mounted between the washer bearing against the rod shoulder and a complementary washer, sliding freely inside the sleeve, parallel to the rod axis, under the effect of the bushing being screwed on the sleeve inner thread. The spring cartridge thus permanently exerts on the shoe a force which is adjusted and develops a friction force between the plate and the shoe which opposes lateral displacement of the flange with respect to the plate in a selected direction. The arrangement of several shoes on the same flange cooperates for opposing any lateral displacement in any direction, while allowing the effects of any accidental vibrations to be absorbed, the flange resuming its its predetermined position with respect to the plate once these vibrations have disappeared or have been eliminated. According to another feature of the invention, the threaded sleeve includes, in its lower end, a hollow end-piece through which extends the stem and the end of which is in part inserted in the flange bore and then is secured against movement in position with respect to the flange, e.g. by soldering. According to still another feature, the elongated rod has, at its upper end extending beyond the calibration bushing, a transverse slit identifying its axial orientation and therefore that of the convex face of the push-piece with respect to the flange axis and allowing, by reaction on the shoe slanting face, relative adjustment of the push-piece and therefore of the flange connected thereto with respect to the core plate. Advantageously, each shoe has a configuration in the shape of a clevis, the two parallel sides of which are disposed on either side a plane central rib extending the push-piece downwardly, the clevis and the rib being connected by a transverse peg carried by the clevis sides and engaged with clearance in a hole of the rib so as to allow axial and radial movement of the push-piece with respect to the shoe when the flange bears on the plate. The mounting of the shoe and the push-piece thus formed allows making these two elements captive by connecting them to one another via a connection with a clearance which does not impede the relative displacement of these members for the flange centering and immobilization when the convex face of the push-piece bears against the shoe slanting face. The shoe can usefully carry a surface coating, such that the face of this shoe which is in contact with the core plate has a high coefficient of friction. On the contrary, the push-piece, the inner surface of the flange bore receiving the shoe, as well as the respective slanting and convex faces of the shoe and the push-piece, respectively carry a surface coating with a reduced coefficient of friction, facilitating the relative displacement of these parts. Advantageously, this coating can be made of chromium and "Stellite" or any similar material, conventional in the art and adapted to the operating conditions in the reactor vessel.