Patent Number: 047553499
Section: summary

The present invention concerns an anti-seismic damping mechanism for the encased primary pumps of nuclear reactors, and, more particularly, such a mechanism intended to limit the pendulum effect of the pump in the case of a seismic disturbance. It is known that in nuclear reactors equipped with so-called "encased" pumps of great height, it is necessary, in the event of earthquakes, to immobilize the lower part of the pumps relative to the reactor vessel. If this precaution is not taken, the pumps could suffer significant damage from harmful effects due to flexing of the pump body, damage occurring at the area of attachment of the pump to the foundation slab, or other damage. The immobilization mechanisms provided must react instantaneously and without external intervention to interdict any significant displacement of the base of the pump. These mechanisms, moreover, must permit relative displacements of the pump and the reactor bed to accommodate differential horizontal and vertical thermal expansion between the vessel and the slab during increases in the reactor temperature. In current breeder reactors, the primary pumps are seated on the slab through the intermediary of an elastic ring or sliding mechanism, which systems allow slow movements during differential thermal expansion between the slab and the vessel. Elastic ring type arrangements allow inclination of the axis of the pump, whereas the sliding mechanism allows variable displacement of the vertical axis of the pump. The connection between this axis and the bed is fixed when the pump is seated on an elastic ring, and an articulated sleeve connecting the pump to the tank is used in the configuration comprising a sliding mechanism. Differential expansion between the vessel and the slab in the radial or vertical direction is permitted by sliding of the base of the pump within the connection between the pump and the bed, or by modification of the length of the articulated sleeve. The object of the invention is to provide an anti-seismic damping mechanism for such encased primary pumps within nuclear reactors, which enables a rigid attachment of the primary pumps directly on the slab and which furnishes a clearly more economical solution than that of familiar systems. Of course, the damper mechanism according to the invention must also permit hydraulic hermeticity, the continuity of the hydraulic seam between the base of the pump and the elastic ring regardless of the axial and/or radial differential expansion due to temperature differences between the slab and the bed, and must also hold the pump steady or fixed to avoid the pendulum effect of the pump in the event of an earthquake. Summarizing the invention, this goal is achieved by virtue of an improved mechanism for anti-seismic connection between the base of the pump and the so-called "dome" of a nuclear power station (the dome, despite its name, may include cylindrical components). The mechanism is characterized by the fact that the pump base is provided externally with a flange seated with a slight axial play and significant radial play within a socket which is closed by an immovable cover. The flange can slide axially with a small play within a cylindrical portion of the dome. A radial damping mechanism is mounted between the flange and the socket, so as to permit, without constraint, slow displacements of the flange with respect to the socket, and to block such displacements when they are rapid. At the same time, the lower end of the socket has an external collar, which, through the intermediary of a metallic journal, can slide axially upon a cylindrical bearing which is solidly bound with the internal lateral wall of the dome. Calibrated passages arranged in the periphery of the vessel enable controlled circulation of the heat-carrying fluid of the reactor between the base of the pump and the upper part of the space between the pump and the dome. It will be understood that this mechanism permits slow displacements of the base of the pump with respect to the dome in the horizontal direction by sliding of the flange in its socket and in the vertical direction, by sliding of the socket and the collar of the lower part of the pump base within the dome. In fact, the passages arranged in the journal of the collar and at the periphery of the socket have the effect that the fluid bathing the assembly offers practically zero resistance to very slow axial or vertical displacements, such as those resulting from differential thermal expansions. The same applies in the horizontal direction, since, as stated above, the damper mechanism mounted between the flange and its socket permits, without constraint, relative displacements between these two entities, so long as the displacements are very slow. In an advantageous embodiment of the present invention, the radial damping mechanism specified above is constituted by a damper ring surrounding the flange of the pump base, preserving the significant play which must exist, in the radial direction, between this flange and the socket, and, preferably, this ring displays projections spaced at regular angles, for example, four projections separated by 90.degree., which bear upon corresponding sufaces of the periphery of the flange or on the internal lateral wall of the socket, and which delimit a number of chambers between the ring and the flange and between the ring and the wall of the socket. In the case in which the ring displays four projections 90.degree. from one another, and in which the projections are arranged alternately on the interior and the exterior of the ring, there are obtained two pairs of diametrically opposite chambers, one interior and one exterior. Preferably, the projections in question terminate in a flat end which bears against a corresponding plane surface of the wall of the socket or the flange. In another embodiment of the invention, the radial damper mechanism specified above is constituted by a number of fluid-escape dampers, for example four in number, angularly spaced in a regular manner, with one end of each bearing upon the internal lateral wall of the socket, and the other end upon the periphery of the flange. It will be understood that the possibility of slow horizontal displacements, for example due to differential thermal expansions, is ensured in the first case by escape of fluid among the various chambers of the damper ring, and, in the second case, by escape passages provided in conventional fashion in the fluid-escape dampers known colloquially as "dashpots." Conversely, rapid displacements, resulting for example from earthquakes, are blocked, through the fact that the fluid cannot move very rapidly through the very narrow calibrated passages specified above. Of course, the same applies for vertical displacements, the fluid being unable to pass very rapidly through the passages provided at the periphery of the socket and within the journal fixed to the collar surrounding the pump base.