Patent Number: 047012986
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a low capacity nuclear reactor. More particularly, the invention relates to an underground reactor in the cavity of a cylindrical pressure vessel with a pile of spherical fuel elements through which cooling gas flows top to bottom aided by a blower. The reactor has a removable steel core vessel with a graphite reflector comprising bottom, side, and roof reflectors surrounding the pile. The side reflectors exhibit channels to contain a plurality of absorber rods. 2. Description of the Related Art A nuclear reactor of the type shown in West German DE-OS No. 30 16 402 is a high temperature reactor of modular configuration. A metal vessel, containing a fuel element pile located in a cavity of a concrete vessel closed off by a cover may be lifted out from the concrete vessel together with a metal base plate and the bottom, side and roof reflectors. The removal may be effected only following shutdown of the reactor and discharge of the spherical fuel elements and removal of the control rods and opening of the cover. At least one line connected to the base plate leads downward to remove the heated cooling gas which flows through the pile from bottom to top. The hot gas line leads to a second cavity containing a heat consumer such as a steam generator arranged parallel to a first cavity in the concrete vessel. DE-OS No. 30 16 402 alternatively proposes to extend the cavity containing the nuclear reactor and to arrange the heat consumer under the metal vessel rather than utilize a second cavity. The state of the art also includes the nuclear reactor installation described in DE-OS No. 33 35 451, a high temperature reactor with spherical fuel elements. In this installation all of the components of the primary loop, such as the control and shutdown devices, are arranged within a steel reactor pressure vessel so that they may be installed or removed from above. Economical subterranean construction is thus possible. At least one discharge tube under the high temperature reactor is provided for removal of fuel elements. The tube leads laterally out of the reactor pressure vessel. SUMMARY OF THE INVENTION It is an object of the invention to provide a nuclear reactor which for the most part eliminates active operating installations such as charging devices, gas purification means, control systems, and safety systems. A reactor of this type is suitable for use in less industrialized areas and is operable with a reduced number of personnel and low maintenance requirements. Reactors of this type are particularly suited for applications such as generation of heat. According to the invention as installation with the following elements achieves the desired object: (a) side and bottom reflectors divided into an inner and outer reflector, whereby the inner reflector is arranged within a removable core vessel; (b) a roof reflector is located entirely within the core vessel, resting directly on a stationary pile of fuel elements and which may be replaced by removing the core vessel; (c) absorber rods provided in a displaceable manner for trimming and shutdown purposes only within a plurality of channels in the inner side reflector; (d) a central opening closed by a cover located in the roof area of the pressure vessel for installation and removal of the core vessel and components and fuel elements installed therein; (e) a cooling gas blower vertically mounted in a central position in the cover; (f) a cooling system mounted on the entire inner side of the pressure vessel, for removal of heat generated in the fuel element pile from the pressure vessel; (g) a gas tight jacket arranged in front of the cooling system on the side of the cavity, and a free annular space for the transport of cooling gas between the gas tight jacket and outer side reflector; (h) the primary gas and the cooling system are designed in a manner such that safe removal of the decay heat is assured even in case of accidents; (i) a primary loop structurally sealed, preferably by weld lip seals, eliminating requirement of forced ventilation and filter devices. The nuclear reactor according to the invention has a compact configuration and is protected against external effects (crashing aircraft, pressure waves, sabotage, etc.) and accidents in the conventional part (pipeline fractures, etc.) by virtue of its subterranean location. Furthermore, the surrounding soil provides excellent radioactive radiation shielding. This simple and economical concept yields an approximate capacity of 10 to 20 MW. Higher capacities may be obtained by a multiplication of the unit reactor. All necessary auxiliary devices are provided once only in order to further improve the economy of the installation. This simple configuration results in low energy generation costs, capable of competing with the present fossil energy carriers. Power operation of approximately 10 to 40 years is possible by the stationary fuel element pile. Subsequent to the reactors' operational lifetime, the fuel elements are replaced by removal together with the core vessel, the inner reflectors, and the trim and shutdown rods. An installation for continuous and discontinuous charging may thus be eliminated. Retention of the trim and shutdown rods in the core vessel assures maintenance of a subcritical state of the fuel elements in the core vessel during their installation and removal. The service life of the entire installation is extended according to the invention by enabling removal of a core vessel and the components in that highly stressed structural parts such as, for example, the side and the roof reflector, may be replaced. These structural parts additionally may be repaired. The pressure vessel may be a prestressed cast vessel, a prestressed concrete vessel, a reinforced concrete vessel or a steel pressure vessel. The cooling gas, preferably helium, is transported by a blower with a rotor protruding into a free space located between the cover and the roof reflector. The gas traverses the fuel element pile from top to bottom. The cooling gas is distributed over the bottom of the pressure vessel following its passage through the pile, and subsequently flows in an annular space between the outer side reflector and the gas tight jacket upwards into a free space above the roof reflector. It then reenters the blower. A cooling system mounted on the inner side of the pressure vessel is capable of removing all of the heat generated. Heat is transferred by conduction and radiation to the cooling system. The medium circulating in the cooling system (for example, water if the nuclear reactor is used as a heating reactor) is conducted in its own piping. The piping is completely separated from the primary loop by a gas tight jacket so that leakage from the cooling system cannot penetrate into the primary loop. Requirements necessitating a rapid reactor protection system to control reactivity accidents are therefore not present. Gas purification installations may be eliminated due to the absence of water carrying components in the primary loop and the fact that during operation no additions of fuel elements are taking place and no contaminations are able to penetrate into the primary loop. Advantageously, the gas pressure in the primary loop may be chosen so that it is higher than the pressure of the medium in the cooling system. This provides an additional degree of safety in view of a potential entry of the medium into the primary loop. The fuel elements have a high heavy metal content, enabling an extended retention time of the fuel elements in the core. Drives for absorber rods are conveniently provided in passages in the outer periphery of the cover. It is advantageous to locate a gas conduction jacket in a free space over the roof reflector separating the low pressure and the high pressure parts and the suction and compression sides of the blower, respectively. The gas conduction jacket is connected to the core vessel. The gas tight jacket may be equipped with ribs on its side facing the annular space in order to improve heat transfer from the heated cooling gas to the cooling system. The outer side reflector may rest directly on longitudinally arranged ribs. In this manner, cooling channels are provided for upward flow of cooling gas. A metal support installation may be provided to support the core vessel. The support installation in turn rests directly on the bottom of the pressure vessel. The blower motor may advantageously be located in a passage through the cover which may be equipped with a removable closure for the center installation and removal. This facilitates the maintenance of the motor and the blower. The pressure vessel may be supported on a foundation. A concrete cover may be conveniently located above the pressure vessel with good accessability. The cover protects the nuclear reactor in combination with the subterranean construction from external effects. A light construction hall may be provided over the cover to house auxiliary and supply systems. In a nuclear reactor according to the invention the absorber rods located in the inner side reflector are intended only for trimming and shutdown. The reactor power is regulated solely by the rpm or speed of the blower and the secondary flow of the cooling system, by utilizing the stabilizing property of the negative temperature coefficient. Active controls by absorber rods may be eliminated. The trim and absorber rods serve in combination with a burnable neutron poison (for example gadolinium) to absorb the initial excess activity. The variation of the excess activity taking place during the operation of the reactor is compensated by the displacement of the trim and shutdown rods. In the process, the trim and shutdown rods are extracted manually from time to time gradually. No regulation or automatic controls are required for these slow variations in reactivity. The trim and shutdown rods are moved only if, due to a decline in the reactor output, not enough heat is being generated, for example if the quality of the hot water is inadequate. Brief fluctuations of fuel element temperatures are tolerated over a relatively broad range without difficulty due to the high temperature resistance of the ceramic fuel elements. The removal of the nuclear reactor after burnout of the fuel elements may be advantageously effected by initially placing a shielding bell directly onto the pressure vessel and then, subsequent to raising the cover, drawing the core vessel into the shielding bell. The primary loop and the cooling system are designed so that the decay heat is removed safely even in case of accidents. In case of a blower failure decay heat is transferred by natural convection to the cooling system, whereby the direction of flow of the cooling gas in the fuel element pile is reversed. This does not endanger the blower and its drive motor by their exposure to heat. A potential pressure rise in the primary loop may be taken into consideration in the layout of the primary loop, or it may be compensated by an overflow of the cooling gas into gas reservoirs. The cooling system design provides for an adequate volume of the cooling gas circulating in the cooling system piping for removal of the decay heat by natural convection. Decay heat is transferred to the cooling system by heat conduction to the graphite reflector and by thermal radiation from the grahite reflector to the cooling system in case of a pressure relief accident. Normally prevailing temperatures in the reactor core are not appreciably exceeded. Decay heat is safely removed without damage to the fuel elements or the release of activity from the fuel elements even in case of the failure of the cooling system. The decay heat is removed in this case by conduction through the pressure vessel into the surrounding soil and into the atmosphere. If a steel reinforced or prestressed concrete vessel is used as the pressure vessel, thermal conduction may be affected favorably by a special layout of the steel reinforcements. This simple construction requires only a very slight monitoring effort.