Patent Number: 042784980
Section: summary

The invention relates to an earthquake-proof mounting support control rod drives of nuclear reactors, preferably pressurized-water reactors, including a reactor pressure vessel with a convex vessel cover and also control rods and control rod drive shafts which are coupled thereto and are mounted so as to be movable in axial direction thereof within tubular drive housings extending pressure-tightly through the pressure vessel cover and sealed against the outside, the drive housings including control rod drive stub tubes forming respective feed-through passageways and pressure tube sealingly connected thereto, such as by a flange connection, especially, the control rod drive stub tubes extending with different stub tube lengths above the spherical vessel cover. From the journal "Kerntechnik, Isotopentechnik und Chemie," 1968, No. 4, pages 217 to 225, especially FIG. 9 (see also the associated reprint pages 1 as well as 7 to 9), control rod drives of the hereinaforementioned type have become known heretofore. The problem underlying the invention is to construct these heretofore known control rod drives with respect to earthquake safety in such a manner that undesirable earthquake vibrations of the control rod drives, including the stub tubes thereof are reliably avoided. A basic beginning is from the realization that certain groups of control rod drives with the same stub tube length, if struck in the resonance-frequency range, can be excited to especially heavy vibrations by the foundation shocks due to an earthquake, whereas others, which are outside the resonance frequency range, are not so excitable. This involves especially transversal vibrations, longitudinal vibrations being only of secondary importance. It is accordingly an object of the invention to provide an earthquake-proof mounting support which will take such transversal vibrations into account. With the foregoing and other objects in view, there is provided in accordance with the invention, an earthquake-proof mounting support of the type hereinbefore defined which offers a solution for the problem that is presented providing that the upper ends of the pressure tubes are flexibly connected to each other by grid bars of a support grid. Thus, use is made of the fact that the control rod drives individually or in groups vibrate differently with respect to frequency, amplitude and direction in the event of an earthquake or of another phenomenon initiating vibrations. Mutual cancellation of the undesired vibration is thus achieved by means of the invention, or at least a reduction to smaller amplitudes is achieved by mutual attenuation. Through the use of grid link bars, unimpeded thermal movement of the drive housings in the axial direction thereof without mutual interference is achieved whereas, in the event of excitation in transversal direction due to an earthquake, the vibrations are cancelled due to coupling. In accordance with another feature of the invention, the support grid comprises two subgrids respectively disposed in a lower and an upper grid plane. The vibration suppression is thereby even more effective, and the grid stiffness in the transversal direction is improved. In accordance with a further feature of the invention, the grid bars of the one grid plane are disposed in rotated relationship to the grid bar direction of the other grid plane. By the term grid, there is understood herein that, in one plane, there are at least two different grid bar directions. The grid bars can therefore be disposed in rectangular, square, rhombic or zig-zag configuration, wherein the hereinaforementioned rotation of the grid direction from the one to the next plane (statically defined truss structure) is especially advantageous. This structure is then stiff or rigid not only in two principal directions of the one grid, but also in other vibration directions which form an angle therewith. In accordance with an added feature of the invention, one subgrid is disposed in the form of a rectangular raster or screen, especially a square screen, and the other subgrid is formed of diagonal bars, the joints of which lie approximately in a projection of the corners of the one subgrid onto the other subgrid. The square raster or screen for the one subgrid represents the preferred embodiment since, in most cases, also, the control rod drives and the pressure tubes thereof are disposed in the form of such a raster or screen. In accordance with an additional feature of the invention, the grid bars are divided into halves, and the two grid bar halves are connected together by turnbuckle means. On the one hand, this affords equalization of tolerances in the assembly and, on the other hand, vibration-damping coupling between respective adjacent tubes can be made strong enough without adverse effect on the flexible connection for the relative thermal expansions. In accordance with another embodiment of the invention the earthquake-proof mounting support when applied to reactor pressure vessels wherein the control rod drives, including drive housings and associated drive shafts, extend pressure-tightly through a spherical bottom wall of the pressure vessel and are disposed with the pressure tubes thereof protruding downwardly and/or in which the free axial length of the drive housings of the control rod drives are approximately equal to one another, preferably for boiling-water reactors, the pressure tubes are articulatingly connected to each other in the vicinity of the lower ends thereof by the grid bars of the support grid in such a manner that the free axial lengths of the drive housings, measured from the outside of the spherical bottom wall or the outside of the spherical cover, respectively, to the grid joint, are different as viewed over the cross section of the support grid. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in an earthquake-proof mounting support for control-rod drives of nuclear reactors, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.