Patent Number: 046613110
Section: summary

BACKGROUND OF THE INVENTION The invention concerns a nuclear power plant arranged in an underground cavity comprising a small high-temperature pebble bed reactor; a reactor core; spherical operational elements which pass through the core more than once; a steel reactor pressure vessel in which the small high-temperature reactor and a heat exchange apparatus are installed; and a loading installation for the addition and removal of fuel elements. A nuclear powe plant of this type is described in German patent application No. P 33 35 451.0. In this nuclear power plant, all of the components of the primary loop, together with the control and shutdown installations are disposed within a steel reactor pressure vessel in such a manner that they may be installed and removed from above. This renders an economical underground construction possible. Under the small reactor, at least one discharge tube is provided for the removal of the spherical fuel elements. Another nuclear power plant with a small high-temperature reactor suitable for installation in an underground cavity is described in German patent application No. P 33 45 113.3. Here again, the spherical fuel elements introduced on top are drawn off by means of a discharge device at the bottom of the pile. Loading installations for nuclear reactors of medium capacity with spherical fuel elements which are built by the principle of modular construction are known. These installations comprise movable functional parts for the addition, removal, distribution and extraction of fuel elements by means of drives. The functional parts are provided with bores for the passage of fuel elements and are set into a block, or plate equipped with connecting bores for the fuel elements. Feed installations of this type are described in German Gebrauchsmuster No. 6 753 677, German Auslegeschrift No. 15 89 532 and German Offenlegungsschrift No. 23 57 426. The manner in which the introduction of the fuel elements into the reactor core is effected, is not disclosed in the references cited. The state of the art further includes German Pat. No. 1 281 046, which again concerns the feed installation of a so-called pebble bed reactor of intermediate capacity. The discharge and sorting installation for the spherical fuel elements is located under the reactor core. It contains a measuring device in which the fuel elements are examined with regard to their state of depletion. Depending on the results of the measurement, the fuel elements are either returned into the reactor core or eliminated from the circulation of fuel elements. The addition of fresh fuel elements is effected from above through a feed tube. The installation and removal of primary loop components is thus rendered difficult. SUMMARY OF THE INVENTION It is an object of the present invention to design the feed installation for the small high-temperature pebble bed reactor in a nuclear power plant of the aforedescribed structural type, so that access to the components of the primary loop from above is not impaired, and so that the loading installation occupies as little space as possible. According to the present invention, this and other advantageous objects are attained by providing a divided loading space under the reactor pressure vessel. A discharge installation comprising removal, depletion and sorting devices is located in the loading space, together with a conveyor installation. An addition device for fresh fuel elements and a collector vessel for used fuel elements are arranged outside the underground cavity, and are connected with the lower part of the loading space by means of vertical conduits, mounted outside the reactor pressure vessel. The conveyor installation comprises an inward transfer block for transferring fresh fuel elements from the addition device as well as partially depleted fuel elements sorted out in the depletion measuring installation. The conveyor installation is connected with a distribution installation for the reactor core by a vertical ascending line located above the reactor core, outside and to the side of the reactor pressure vessel. The depletion measuring installation is connected with an outward transfer block for depleted fuel elements by means of another line which is in turn connected with the vertical conduit of the collector vessel. In the nuclear power plant according to the invention, fresh fuel elements are therefore not added directly into the reactor core from above, but are conveyed outside the reactor pressure vessel into the lower part of the loading installation and transported to the distribution device in the same line with the partially depleted fuel elements. From there they are fed laterally into the reactor pressure vessel. Unimpaired access to the primary loop components from above is thus provided. According to a further embodiment of the invention, a column of fuel elements is always present in the conduit in order to prevent damage by impact to the fresh fuel elements in the course of their transport through the vertical conduit. The great fall height thus cannot have a harmful effect on the fuel elements, even in the absence of braking means. The aforedescribed objects of the invention may also be attained with a configuration different from the one described above. One alternative configuration according to the present invention is characterized in that a divided loading space is provided under the reactor pressure vessel; an addition device for fresh fuel elements and a collector vessel for used fuel elements are disposed in the loading space, together with a discharge installation comprising removal, depletion measuring and sorting devices and a conveyor device; the loading space is accessible through a horizontal channel and a vertical shaft; the conveyor installation comprises an inward transfer block for the introduction of fresh fuel elements from the addition device, together with partially depleted fuel elements sorted out in the depletion measuring device; the conveyor installation is connected with a distribution device for the reactor core by means of a vertical ascending line; the distribution device is provided above the reactor core, outside and to the side of the reactor pressure vessel; the depletion measuring device is connected by another line with an outward transfer block for depleted fuel elements, which, in turn, is connected with the collector vessel by a further line. The operating elements of a small high-temperature pebble bed reactor with a capacity of 100 to 200 MWe comprises one-half fuel elements and one-half pure graphite elements, or so-called blind elements. The above is true if one disregards the initial loading phase in which absorber elements are also added. As the consumption of the graphite elements cannot be measured with the depletion measuring installation, it is proposed to completely replace the graphite elements with fresh graphite elements in the course of the operational recirculation of the core load after an operating period of (n-1) years in order to avoid greater expense. The symbol n is defined as the service life of the graphite elements in years. Under the conditions prevailing in a small reactor n amounts to about 10 years. With this method, even though the quantity of fresh graphite elements is twice as high as that actually consumed, there is still a cost saving.