Patent Number: 047524391
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention concerns gas cooled high temperature reactors having a cylindrical steel pressure vessel, heat exchanger devices, cooling gas circulation devices and circulating blowers. 2. Background of the Prior Art The state-of-the-art includes installations wherein a high temperature reactor for the nuclear generation of heat and the devices serving the utilization of the heat obtained are installed together in a pressure vessel. The heat is removed by means of a cooling gas which is being circulated with the aid of blowers in a closed loop or primary loop through the reactor core and the heat exchanger devices. For the removal of the residual heat, special devices, such as auxiliary heat exchangers and auxiliary blowers are often provided It is also possible to eliminate these special devices by means of a particular arrangement and layout of the primary loop components. Thus for example in the thorium high temperature reactor (THTR-300), the heat exchangers and the blowers together with the pipe circuits on the secondary side and their components are laid out so that the entire secondary heat is removed by means of the operating systems of the heat exchangers on the primary side The flow of the cooling gas from top to bottom through the reactor core and from bottom to top through the heat exchangers in this case is similar to that in normal operation. To assure the removal of secondary heat, however, the blowers must at all times be ready to function so that the area of the cold gas will not be endangered by the hot gas rising in free convection. In a further nuclear reactor installation with a gas cooled high temperature reactor, the so-called AVR plant, the heat exchanger is arranged above the nuclear reactor and the cooling gas flows from bottom to top both through the reactor core and through the heat exchanger. In the event of a failure of the blower located underneath the core, the residual heat is removed by natural convection to the structures surrounding the reactor core. The latter include in addition to a reflector jacket of graphite, a carbon brick enclosure surrounding the graphite jacket and providing shielding and thermal insulation. To safely contain the fission products released, the aforementioned structures are surrounded by a double, gas-tight steel pressure vessel. A layer of magnetite and limonite between the two steel pressure vessels serves as a biological shield. In the above mentioned THTR-300 the function of the biological shield is effected by the prestressed concrete pressure vessel, which houses in a centercavity, the reactor core and the heat exchangers. The prestressed concrete pressure vessel not only serves as the radiation shield, but also provides a complete, pressure resistant containment of the nuclear reactor installation. SUMMARY OF THE INVENTION The present invention is based on a nuclear reactor installation of the type described hereinabove and arranged in a steel pressure vessel. It is an object of the invention to provide such an installation that safely protects the outside and the environment against radiation and the consequences of accidents which may occur within the plant. Another object is to assure the removal of secondary heat in the event of an accident. According to the invention, the nuclear reactor installation is characterized in that the steel pressure vessel is tightly enclosed in a safety enclosure comprising two essentially cylindrical concrete shells in a spaced apart arrangement, a concrete cover monolithically joined with the outer concrete shell and a cantilever ring monolithically joined with both concrete shells and supporting the steel pressure vessel and a concrete cooling system in the innerconcrete shell. The concrete cooling system operates by natural means and comprises cooling water circulating in a closed loop and a second cooling water system to provide for the recooling of the cooling water circulating in the concrete cooling system. An adequate radiation shielding of the nuclear reactor and the components of the primary loop is obtained by means of the safety enclosure according to the invention. The enclosure also functions as a biological shield. Secondly, in the case of a possible release of radiation from the steel pressure vessel, the safety enclosure assures the safe containment of the installation against leakage from the primary loop. The safety enclosure thereby forms a containment barrier for the cooling gas in the steel pressure vessel and an additional barrier to retain fission products. (A first barrier to retain fission products is the fuel elements themselves. A nuclear reactor with spherical fuel elements contains the fissionable substance in the form of coated particles.) By means of the safety enclosure according to the invention, leakages of the primary loop may be retained until a controlled removal of the cooling gas to the environment through filters or a gas purification installation is assured. The safety enclosure further protects the nuclear reactor installation against external effects. These effects may consist, for example, of earthquakes, aircraft crashes or pressure waves in the case of explosions. At the same time, the safety enclosure serves as a supporting structure of the steel pressure vessel. The outer concrete shell has the further function of a protective reactor building, while the inner concrete shell provides protection against debris and fragments. In order to enable the safety enclosure to perform these different functions, the concrete material of the enclosure must be protected against excessive heating. For this reason, a concrete cooling system is provided within the inner concrete shell. In normal operation, the concrete cooling system removes the heat generated in the concrete by radiation. The heat loss of the steel pressure vessel is also removed by the concrete cooling system. Heat is transferred from the steel pressure vessel primarily by thermal radiation while it is removed from the concrete by direct contact. This concrete cooling system is further used according to the invention for the removal of the secondary heat. In the event of an accident, for example, the devices normally eliminating the secondary heat are rendered ineffective and the concrete cooling system provides a backup. Initially, the devices for the removal of secondary heat consist of the heat exchanger blower units with the operational secondary loop and possibly an auxiliary cooling system. Even in the case of a failure of the blowers, the removal of secondary heat on the primary side may be assured if the cooling gas pressure in the primary circuit is high enough so that natural convection is adequate and may be maintained as such. If, however, the heat sink on the primary side is eliminated, the secondary heat is conducted according to the invention by means of natural convection, conduction and radiation of the steel pressure vessel. The heat is then transferred from the steel pressure vessel essentially by radiation to the concrete cooling system located in the inner concrete shell. Even in the case of the loss of cooling gas (pressure relief incident) and the failure of all cooling in the primary loop, the secondary heat is transferred from the surface of the steel pressure vessel to the concrete cooling system. In this event, no increased release of fission products by the fuel elements is experienced. The concrete cooling system comprises preferably an elevated annular reservoir placed onto the inner concrete shell and maintained under atmospheric pressure, together with ascending pipes and downpipes. Ascending pipes are arranged on the side facing the steel pressure vessel and the downpipes are arranged on the side facing the outer concrete shell of the inner concrete shell. The recooling of the water circulating in the concrete cooling system is effected by a second cooling water system, which removes the heat generated to the elevated reservoir and then to the outside. In the event of partial or complete failure of the auxiliary cooling of the concrete cooling system, the water content of the elevated reservoir and the pipe system evaporates at a rate of approximately 2 to 3 t/h. Depending on the volume and water supply of the elevated reservoir, the removal of secondary heat may thereby be assured for several days without any active measures. The temperatures of the steel pressure vessel remain in such a hypothetical incident clearly under 400.degree. C. Preferably, a device for the continuous supply of water is provided on the elevated reservoir. By the actuation of this device either the removal of the secondary heat may be continued following the evaporation of the concrete cooling system or the concrete cooling system may be operated with a higher heat removal capacity. It is appropriate to connect the elevated reservoir with a blow-off line and to arrange a pressure relief valve in the blow-off line. As the transfer of heat from the steel pressure vessel to the ascending pipes of the concrete cooling system takes place essentially by radiation, it is advantageous to equip the ascending pipes with azimuthal fins or a finned wall. Cooling plates of a cast material maybe applied further to the ascending pipes. In order to be able to perform maintenance and repair work on the primary loop components installed within the steel pressure vessel, such as heat exchangers and circulating blowers, several large passages are provided conveniently in the safety enclosure. These passages permit the dismantling of the components. The passages are closed off by removable pressure resistant and gastight covers, placed onto the outer concrete shell or set into it. In its center area, the inner concrete shell may be equipped with a thickened part directed inwardly. This thickened part is preferably in the form of a flange upon which a supporting ring is resting. The supporting ring is mounted on the jacket of the steel pressure vessel. The supporting ring has the function of securing the steel pressure vessel in case of exposure to an earthquake. The pressure vessel is thereby supported only by the inner concrete shell. Additional support on the outer concrete shell would result in a direct impact of a crashing aircraft on the steel pressure vessel. The safety enclosure in turn may rest on concrete support rings joined monolithically with both concrete shells as well as on the cantilever ring upon which the steel pressure vessel rests. The annular space between the two concrete shells, which is accessible to a limited extent during operation, may beused advantageously as a working space.