Patent Number: 048266523
Section: summary

The subject matter contained herein is related to U.S. application Ser. No. 866,921, now U.S. Pat. No. 4,701,298, the disclosure of which is expressly incorporated herein. BACKGROUND OF THE INVENTION 1. Field of The Invention The invention relates to a low capacity nuclear reactor housed underground in the cavity of a cylindrical pressure vessel. More particularly, the invention relates to a reactor with a core or pile of spherical fuel elements, a blower forced downward, cooling gas flow, a removable metal core vessel, and a cover closing off the opening of the pressure vessel, wherein the cover is removable for installation and dismantling of the core vessel. The installation also includes a graphite reflector surrounding the pile on all sides and comprising a bottom, side and roof reflector arranged in part within the core vessel, and with a plurality of absorber rods displaceably located in the side reflector. 2. Description of the Related Technology DE-OS No. 30 16 402 shows a high temperature modular reactor with a metal vessel containing the core of fuel elements located in the cavity of a concrete vessel closed off by a cover. The metal vessel, together with a metal base plate and the bottom, side and roof reflectors may be lifted from the concrete vessel following the shutdown of the reactor, discharge of the spherical fuel elements, dismantling of the control rods and opening of the cover. At least one pipe leading downward is connected to the base plate to remove heated cooling gas, which flows through the pile from bottom to top. The hot gas line leads to a second cavity, located parallel to the first cavity in the concrete vessel and in which a heat consumer, such as a steam generator, is installed. As an alternative to a second cavity, DE-OS No. 30 16 402 further proposes to extend the cavity containing the nuclear reactor downward and to place the heat consumer under the metal vessel. German Application P No. 35 18 968.1, corresponding to co-pending U.S. Application Serial No. 866,921, discloses a further nuclear reactor. This nuclear reactor has a compact design and a far-reaching elimination of active operation installations, such as a charging apparatus, gas purification means, control systems, and safety systems. It is particularly suitable for the generation of thermal energy for heating purposes. The core vessel containing the pile of spherical fuel elements houses only part of the graphite bottom and side reflectors. Channels are provided in the part of the side reflector located within the core vessel for absorber rods, which serve trimming and shutdown purposes only. The roof reflector rests directly on the stationary pile of fuel elements. The entire core vessel and the components located therein are removed following burnup of the fuel elements. The heat generated in the core is transferred to a cooling system mounted on the inside of a pressure vessel enclosing the nuclear reactor by cooling gas flowing through the pile from top to bottom. The state of the art further includes the nuclear reactor plant described in DE-OS No. 33 35 451, which relates to a high temperature reactor with spherical fuel elements. In this plant, all of the components of the primary loop, together with the control and shutdown devices are located inside a steel reactor pressure vessel in a manner such that they may be installed and dismantled from above. This enables use of an economical underground location. At least one removal tube is provided under the high temperature reactor for discharge of the fuel elements, said tube leading laterally out of the reactor pressure vessel. DE No. 1 034 784 shows a gas cooled nuclear reactor with a pile of fuel elements of uniformly shaped bodies and similarly shaped pourable bodies for the reflector jacket. SUMMARY OF THE INVENTION It is an object of the invention to provide a pebble-bed nuclear reactor in which active operating devices, such as a charging apparatus, gas purification plant, control systems and safety systems may be disposed of to a great extent, so that replacement of the core vessel and its discharge may be carried out in simpler manner, and the negative effect of the metal core vessel on the neutron economy of the fuel element pebble-bed may be reduced. The afore-defined object is attained according to the invention by means of the following characteristic elements: (a) the side reflector comprising a solid outer jacket located outside the core vessel and an inner part located inside the core vessel and containing a pile of spherical graphite elements having the same diameter as the fuel elements; (b) the roof reflector and the bottom reflector also comprise a pile of spherical graphite elements of the same diameter as the fuel elements, with the graphite elements of the roof reflector resting directly on the fuel elements, which form a stationary pile; (c) a cage comprising lattice work or perforated sheet metal, open at the top, is provided as the core vessel, with the mesh or holes of the cage smaller than the diameter of the graphite and fuel elements and with the cage capable of supporting the entire weight of the graphite and fuel elements piles; (d) a plurality of cylindrical sleeves made of lattice work or perforated sheet metal mounted in a uniform distribution on the inside of the cage serving as the core vessel, said sleeves extending over approximately the entire height of the cage and containing the absorber rods, which are provided only for trimming and shutdown purposes; (e) a closeable opening is provided in the bottom of the core vessel for the removal of burnt fuel elements and graphite elements; (f) a blower is mounted vertically in a center position on the cover of the pressure vessel. (g) a gas conduction jacket is set on the top of the core vessel, which separates the suction and compression sides of the blower in the free space above the roof reflector. The use of spherical, i.e., pourable, elements not only for the active core zone, but also for the inner part of the side reflector and the bottom and roof part of the graphite reflector surrounding the active core zone, renders the removal of the graphite reflector from the core vessel following the extraction of the core vessel from the nuclear reactor much easier. In contrast to the configurations used heretofore, the core vessel is not made of a solid material, but of perforated sheet metal or a metal lattice, whereby two essential advantages are attained. The weight of the core vessel is significantly reduced, which is important in view of its removal, and the effect of the metal core vessel as a neutron absorber is diminished. The nuclear reactor according to the invention is further characterized by its compact structural configuration and it is protected against external effects (aircraft crashes, pressure waves, sabotage, etc.) and accidents in the conventional part (piping fractures, etc.) by its underground placement. In addition, the surrounding soil provides excellent shielding against radioactive radiation. This simple and economical concept yields a power capacity of approximately 10 to 20 MW. Higher power capacities may be obtained by a multiplication of the unit reactor. All necessary accessory devices are provided only once in order to further improve the economy of the reactor. This simple configuration keeps the cost of generating energy low enough so as to be able to compete with the present fossil fuels. The stationary fuel element pile makes power operation for approximately 10 to 40 years possible. After this, the fuel elements, together with the core vessel, the graphite pebbles, and the trim and shutdown rods are replaced. Apparatus for continuous and discontinuous charging may be eliminated. The subcriticality of the fuel elements in the core vessel during the in and out movement is assured by the retention of the trim and shutdown rods in the core vessel. The possibility of removing the core vessel with its associated components extends the life of the entire plant, as the highly stressed structural parts, such as the inner side and roof reflectors, may be simply replaced. The cooling gas, preferably helium, is transported by the blower, the rotor of which projects into a free space between the cover of the pressure vessel and the roof reflector, from top to bottom through the pile of fuel elements. After passing through the pile, the cooling gas is distributed over the bottom of the pressure vessel and then flows in an annular space between the core vessel and the inside of the pressure vessel in an upward direction. It then flows along the outside on a gas conduction jacket and re-enters the blower. Advantageous further developments of the invention will become apparent from the claims and the description below of an embodiment with reference to the schematic drawings.