Patent Number: 044903286
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention concerns a gas cooled high temperature nuclear reactor and a bottom shield therefor having a base plate, arranged under the reactor core and over the bottom of the prestressed concrete pressure vessel in a horizontal manner. 2. Background of the Prior Art The dead weight of the reactor core and of the bottom and side reflectors and the thermal side shield are supported by a bottom shield in one type of nuclear reactor in a concrete pressure vessel. The loads resulting from rod forces in the reactor and possible external forces such as earthquakes must be absorbed by the bottom shield and transmitted to the prestressed concrete pressure vessel. The bottom shield also serves to protect the bottom liner of the prestressed concrete pressure vessel against excessive nuclear radiation and temperatures. SUMMARY OF THE INVENTION It is an object of the invention to provide a gas cooled high temperature reactor with a bottom shield capable of functioning in a manner so that the above-cited thermal, nuclear and weight considerations are provided for as ideally as possible. Thus, any detrimental action by thermal,nuclear and weight forces are presented by an economical and simple construction of a bottom shield. The objects are attained according to the invention by a bottom shield comprising at least two plates arranged over and coaxially with respect to each other. These two plates will be referred to hereinafter as the first and second plates or as the upper and lower plates respectively. Each of the plates comprise several parts partially connected with each other with the lower plate being arranged at an axial and vertical distance, respectively, both from the bottom liner of the prestressed concrete pressure vessel and the upper plate. In advantageous embodimentsof the invention, the bottom shield may contain a first plate above the second plate with a second plate having a greater thickness than the thickness of the first plate. In addition, a means for spacing the first plate from the second plate may comprise a plurality of bearing supports. Similarly, a means for spacing the second plate from the bottom liner of the cavity in the concrete pressure vessel of a nuclear reactor installation advantageously comprises a plurality of bearing supports. The bearing supports between the bottom liner and the second plate are spaced at a greater distance from one another than the bearing supports between the first and second plates. The individual plate members are comprised of circular segments having overlapping areas which form the plate member seams. In the overlapping area, pins are positioned forming the seams of the interconnected plate members. An annular supporting ring formed of a plurality of interconnected supporting ring members surrounds the second or lower plate in an advantageous embodiment of the instant invention. The first or upper plate member may advantageously display a ribbed lower surface to aid in the circulation of cooling gas between the first and second plates. In addition, the upper plate may advantageously carry a plurality of sheet metal plates secured in spaced relationship to one another on the upper surface. As an advantageous alternative, the upper surface of the first plate may display a waffle like pattern. The bottom shield arranged in this manner including the support arrangement provides the essential advantage that the loads to be transmitted are transferred to the prestressed concrete pressure vessel in a uniform distribution. Furthermore, the distances of the upper and the lower bearing supports are chosen so that the different stiffnesses of the two bottom shield plates are balanced with respect to each other. Further advantages of the invention are described in the following. During the operation of the nuclear reactor according to the present invention, the temperature profile over the plate thicknesses is substantially reduced as the result of the division in a thin upper and a thick lower plate with a cooling gap provided between them. Because of the segmented construction with concentric partition gaps, whereby the segments are mechanically joined only within the inner half of the radius, the individual segments are able to expand freely in relation to each other, since the radial temperature profile may be maintained very small. The division of the lower plate into a circular plate and an annular ring further results in the fact that parts stressed differently by the temperature and the imposition of loads are allowed to move freely. By virtue of the special configuration of the top side of the upper plate, which may be provided both with a flat or a waffle structure, an additional bypass flow may be effected for cooling purposes. When the top side of the upper plate has a flat configuration and consists of individual cover plates with expansion gaps arranged between them, this may be used as thermal insulation and to facilitate the sliding of the graphite structures.