Patent Number: 047770139
Section: summary

The invention concerns a nuclear reactor, in particular a high-temperature reactor, with a reactor protecting building and therein a reactor-pressure container, in particular, one made of concrete and equipped with at least one safety valve in the form of a spring-valve to limit--in the event of reactor overheating--the pressure in the reactor-pressure container which comprises an inside liner associated with cooling ducts connected to at least one liner cooling equipment. In high-temperature reactors of the above kind, the entire primary closed circuit is integrated into the pressure container. The pressure container consists of pre-stressed concrete. To achieve adequate gas hermeticity against the pressurized helium acting as the primary coolant, the reactor pressure container is equipped on the inside with a steel lining, the so-called liner. On the gas side, the liner is provided with thermal insulation and on the concrete side with cooling ducts connected to one or more water cooling systems. In this manner the liner and the concrete are protected against the high gas temperatures. Nuclear reactor overheating takes place when the gas cooling fails. Those malfunctions are prevalent which occur for pressurized primary circulation. A special role falls also to those malfunctions related to additional failure of the liner cooling, and which may have the worst consequences imaginable. This is due to the fact that in the later stages of the malfunction, when already substantial quantities of radiological fission products have been released from the fuel elements, there will be liner destruction and thermal dissociation of the concrete on the inside of the reactor pressure container. Ordinarily, the reactor pressure container is protected against excessive pressure by one or more safety valves staggered with respect to triggering pressure. Preceding or following check valves offer an additional way of blocking in the case a safety valve in the open position should fail. As regards nuclear reactor overheating taking place under pressure, the thermal expansion of the primary gas causes the safety valves to respond when the pressure increase is not compensated for by helium withdrawal through the gas purification plant. It is desired that the safety valves act in different ways. If the liner cooling is operative, their operation should be regular, that is, the safety valve(s) should open when the response pressure has been exceeded and close again when there is a subsequent decrease below the operational pressure. In the event of a failure of the liner cooling, the safety valve(s) should remain open as, in such an event, total pressure relief of the primary circulation is desired. These goals can be achieved by safety valves which permit opening against spring force by means of their own drives. This design however incurs the drawback that control errors may result in accidental opening of the primary closed circuit. Furthermore, auxiliary power often is required to open them. Moreover complete pressure relief of the primary circulation in the event of failed liner cooling is not necessarily desirable. Thus, if the reactor were totally pressure-relieved, the fuel elements would be raised to higher temperatures whereby more fission products would be released by the fuel elements. Further, significant amounts of fission products are released by the fuel elements already during the pressure relief phase and are immediately carried by the outflowing helium into the reactor protecting building. Again, it might happen that pressure should build up behind the liner in the concrete because of the evaporation of the water in the concrete, whereby the liner might fail prematurely if a matching opposing pressure were absent on the inside. Lastly, the evacuation of the gas from the primary circuit is provided in only the open relief line. This line is incapable of retaining significant quantities of fission products. Accordingly, it is the object of the invention to so design the pressure relief in a nuclear reactor of the initially stated type that it is better matched to the particular situation in a nuclear reactor overheating case and that it operates more reliably. This problem is solved by the invention in that the valve-spring(s) of the safety valve(s) consist(s) of a material of which the spring-constant decreases with temperature and that they are exposed to the gas flowing out of the particular safety valve, and that provision is made for at least one cooling apparatus to cool the valve spring(s) and/or the gas before the outflow, where this cooling apparatus is connected to at least one liner cooling equipment. In the invention, depending on the kind and time sequence of the malfunction, the valve spring of a safety valve is variably thermally stressed by the gas flowing out of the now open safety valve, the particular valve spring having a decreasing spring constant as the temperature rises. The thermal loading of the particular valve spring is controlled by a cooling system connected to the liner cooling equipment. If the liner cooling equipment operates normally, then the cooling apparatus it feeds ensures that the particular valve spring remains relatively cold, and its closing force therefore corresponds to the operational pressure or is somewhat higher. This takes place alternatively or in combination in that the particular valve spring and/or the gas of the primary system are cooled before passing through the particular safety valve. Accordingly, if there is nuclear reactor overheating and thereby an opening of the particular safety valve while the liner cooling equipment keeps operating normally, then there follows merely a pressure drop down to the operational pressure because the particular valve spring is designed for this pressure and its spring constant is kept fixed by the cooling equipment. If the liner cooling equipment were to fail, then the particular valve spring and the housing surrounding it is exposed to the hot, discharging gas and is raised to a temperature at which its spring constant drops considerably. In this manner, the closing force of the particular safety valve is automatically lowered, and as a result a corresponding pressure relief of the reactor pressure container is achieved. However, the pressure relief is not total. Therefore the primary circuit remains at a lower pressure, whereby the fission products released by the fuel elements remain enclosed for many days in the primary circuit. They can escape from the primary circuit only after the thermal destruction of the liner. By that time many have disappeared entirely by radioactive decay or have deposited themselves on the colder sites of the primary circuit, whereby they are no longer available for release from the primary circuit or only conditionally. Furthermore, the time when the liner is destroyed thermally can be extended considerably by the pressurization in the primary circuit because the inside pressure counteracts any liner failure by collapse. The pressure in the primary circuit also prevents massive water and steam penetration from the liner cooling equipment and from the heated concrete after liner destruction. Moreover, the gas escaping through the concrete after the liner has been destroyed no longer arrives unfiltered into the reactor protecting building. A large part of the entrained fission products is retained in the narrow and wide flowpaths in the concrete. A significant contribution to this result comes from the condensation of steam caused by the strong cooling of the gas inside the concrete. The body of concrete therefore serves as a sink for both fission products and heat. In an embodiment of the invention, the cooling apparatus are connected to all the liner cooling equipment so that heating of the valve spring due to the discharging gas takes place only when all cooling apparatus have failed. Preferably the cooling apparatus are connected to the intake of the particular cooling equipment to achieve adequate cooling. The safety valve(s) appropriately are designed in such a manner that any excess pressure in the reactor pressure container would lead to closing the safety valve(s) no earlier than after one hour. Also, the material of the valve spring(s) shall evince a spring constant which drops sharply above 150.degree. C., for instance the spring material 50CrV4. A further embodiment of the invention provides that the valve spring(s) of the safety valve(s) is designed in such a manner that their closing pressure in the event of malfunction of the liner cooling equipment and for open safety valve(s) is in the range of the pressure level of the liner cooling equipment. In a further development of the invention, the safety valve(s) are such that their valve spring(s) are exposed to the heat from the gas discharging from the pressure container only after the safety valve(s) have opened. As a result, the particular safety valve will not yet respond when there is a single failure of the liner cooling equipment, rather it will only respond when there shall be a pressure in the reactor pressure vessel which exceeds the triggering pressure. Accordingly, there cannot be an accidental opening of the particular safety valve in the event of liner cooling equipment failure. In a further feature of the invention, the safety valve(s) are enclosed by a thermally insulating housing. Such a heat insulating housing reduces heat losses when the valve spring is heated, and thereby assures a rapid drop of the closing force. The invention further provides that each safety valve housing is enclosed in the region of the valve spring(s) by welded-on cooling coils of the cooling apparatus. To solve the problem stated above, the invention further provides that each liner cooling equipment includes in its discharge at least one safety valve so that primary system gas can be evacuated through the liner cooling equipment when the liner is destroyed. It is possible then to reduce any excess pressure setting in on account of the evaporation of concrete water through the safety valve(s). In the process, the gas flows through a plurality of thin tubes which may extend several meters through the concrete. The gas then cools within these tubes and loses part of the entrained fission products capable of deposition. Thereby thermal relief is provided also to the subsequent system of pipes. By using the liner cooling equipment to ensure the already damaged reactor pressure container against excess pressure, it is possible to eliminate those safety valves of which the response pressure, in the event of a nuclear reactor overheating malfunction together with failure of the liner cooling equipment, is reduced. In the liner cooling equipment, the safety valves anyway already are set for the required low pressure. Because it does not communicate in normal operation with the primary circuit, the accidental opening of a safety valve cannot lead to unchecked activity into the reactor protecting building. The above concept of the solution can be implemented in especially advantageous manner because the two malfunctions, namely the pressure rise in the reactor pressure container above the operational pressure and the failure of the liner cooling equipment within the scope of a nuclear reacting overheating case, can take place sequentially, so that the problem under consideration herein is solved especially advantageously by combining both concepts of the invention. Appropriately the safety valve(s) have a closing force somewhat above the operational pressure of the particular liner cooling equipment. It is especially advantageous to provide each safety valve with a venting conduit terminating in a water seal. As the liner cooling equipment almost always is a water system, the connection to such a water seal presents no problems. The volatile and solid fission products contained in the discharging gas are retained in the water seal. Lastly, the invention provides that each safety valve is bridged by a remote-controlled bypass valve allowing complete pressure relief of the primary circuit through the liner cooling equipment.