Patent Number: 047449411
Section: summary

BACKGROUND OF THE INVENTION The present invention applies to the antiseismic protection of solid blocks, located in a building well and able to move with respect to the building if seismic waves arrive. It more particularly, but not exclusively, relates to an antiseismic supporting device for a pile block of a fast neutron nuclear reactor. Nuclear reactors generally have to be protected against the prejudicial effects of earthquakes, which can produce more or less violent waves in the ground and whose horizontal and vertical components can, if no precautions are taken, have highly prejudicial, or even catastrophic effects on the devices used for supporting the pile block and on the different components thereof. The present invention more particularly relates to fast neutron nuclear reactors, such as the French Superphenix reactor in Creys-Malville, in which the pile block is closed in its upper part by a concrete slab, which bears directly on the building of the reactor. In constructions of this type, the reactor vessel and the components of the pile block are suspended on said slab and extend into a well of the building structure. In the case of the Creys-Malville Superphenix power station, the reactor building rests by its floor directly on the ground and is consequently subject to all the horizontal and vertical components thereof in the case of an earthquake. In order to improve this, it has already been proposed (EDF - SPIE Batignolles French Patent No. EN 75/20654 of 1.7.1975) that the reactor buildings be placed on fretted elastomer supports in order to reduce the effects of the horizontal components of seismic shocks. This method has already been used by Electricite de France for the pressurized water nuclear reactor in Cruas and, combined with sliding plates, in the pressurized water nuclear power station in Koeberg, South Africa. These supports, which have a very considerable transverse flexibility, act in the horizontal plane in a manner of a filter, which only permits the passage of the frequency to which it is tuned. As the other frequencies have been eliminated, there is no longer any amplification of the movements of the oscillating path of the construction, which are then solely subject to the acceleration given by the spectrum of the ground for the tuning frequency. The interest of such supports having a limited horizontal rigidity, but a high vertical rigidity, is to largely uncouple the horizontal vibration modes from the vertical vibration modes, so that in the case of horizontal excitation, the complete building virtually behaves as an oscillator with a single degree of freedom, which gives a horizontal translation movement which, in itself, absorbs 98% of the energy and almost completely eliminates the rocking movements. However, although the reduction in the stresses due to the horizontal components of the earthquake is spectacular, this device does not make it possible to reduce the influence of the vertical component. In general, vertical stresses are less prejudicial (they only constitute a supplementary percentage to be added to the gravity) and it is easier to adapt thereto. In the special case of fast neutron reactors, for which the pile block is suspended on the slab, the aforementioned procedures may in certain cases prove inadequate, because such reactors are very sensitive to vertical stresses. SUMMARY OF THE INVENTION The present invention relates to an antiseismic support structure making it possible to more completely overcome that in the prior art described hereinbefore the harmful effect of horizontal and vertical components of seismic waves. In the special case of a pile block of a fast neutron nuclear reactor, said antiseismic support structure comprises in per se known manner, a concrete covering slab resting on the structure of the reactor building and in which the vessel and the components of the pile block are suspended in a well of said structure, whereby the building rests on the ground by means of two superimposed floors, namely an upper floor and a lower floor, in which the two floors are separated from one another by elastic supports, particularly fretted elastomer supports. The antiseismic support structure is characterized in that the assembly of the pile blocks rests on the structure by a series of elastic supports, which absorb the vertical components of possible seismic waves, and in that it is vertically guided in the building well by vertical guidance means preventing any horizontal displacement or rocking of said block relative to the building under the action of the horizontal components of these same seismic waves. Thus, as it is difficult to envisage supports which can both filter the horizontal components and the vertical components of the earthquake without any risk of having a significant rocking of the building, the structure according to the invention, makes it possible to minimize the horizontal stresses for the complete building and the equipment contained therein, while reducing the vertical stresses solely for the pile block and its components (pump, exchangers), which are the most sensitive points. Thus, the structure according to the invention combines three means for combatting the harmful effects of seismic shocks. The first of these means consists of placing the reactor building on supports, which are either elastic, or electroplastic, or elastic associated with sliding means in order to obtain, when appropriate, an oscillatory translation movement of the complete building in the horizontal plane, without any corresponding rocking, which could impart very dangerous pendulum-like movements to the pile block. The effect of these elastic supports is to filter, with respect to the vessel, the horizontal components of the earthquake. They are optionally supplemented by viscous absorbers. According to a special construction according to the invention, the reactor building rests on the ground by means of two superimposed floors, which are separated from one another by elastic supports, such as e.g. fretted elastomer supports. The second means used in the structure according to the invention consists of a series of elastic supports, which absorb the vertical components of the seismic waves and which are placed between the pile block assembly and the building structure. This series of elastic supports consequently supports the components of the pile block and is optionally supplemented by viscous absorbers, which filter the effects of an earthquake in the vertical direction. Finally, the third protection means consists of vertical guidance means, such as e.g. fretted elastomer plates, which are positioned between the pile block and the wall of the building, and which act as vertical guides for preventing any horizontal dynamic displacement, as well as any rocking of the pile block with respect to the remainder of the building, so that within the horizontal plane, the pile block has to follow the movement of the building. According to a first embodiment of the support structure according to the invention, the series of elastic supports is positioned between the upper floor and the bottom of the concrete cylindrical structure of the pile block, while the guidance means are distributed along the building well between the top and bottom thereof. In a second embodiment of the support structure according to the invention, the elastic supports and the guidance means are located at the actual covering slab, namely between the latter and the upper part of the building well. Moreover, this guidance system permits the free thermal expansion of the slab, which, in a subsidiary but very important manner, makes it possible to particularly simply solve the problems of the design of said slab, no matter whether it is fixed or suspended.