Patent Number: 052232093
Section: summary

The invention relates to a method and a system for pressure relief of the containment of a nuclear power plant, having a filter disposed inside the containment which contains a washing fluid. Pressure relief of a nuclear power plant containment may become necessary if, contrary to every expectation, the pressure threatens to exceed the design value intended for the containment. In order to reduce the pressure in the containment of a nuclear power plant, a method and an apparatus disclosed in German Published, Non-Prosecuted Application DE 38 06 872 A1 may be used, in which a wet scrubber is provided as part of a filter inside the containment. A mixture of gas and steam produced inside the containment is cleaned of toxic and/or radioactive ingredients by the scrubber before the mixture is released into the environment. A special embodiment of a wet scrubber is described in German Published, Non-Prosecuted Application DE 38 15 850 A1. In that configuration, mixing of the gas and steam mixture with the washing fluid takes place inside Venturi nozzles, thereby assuring very intimate mixing and therefore a very good washing action. Once the design pressure of the containment is reached, regardless of the cause of its creation, the gas-steam mixture inside the containment amounts to approximately 70% water steam, along with compressed air and other gases formed inside the containment, which also includes a not-insignificant quantity of pure hydrogen with a probability bordering on certainty. In previously proposed embodiments having a wet scrubber accommodated inside the containment, the wet scrubber is either insulated and unheated or not insulated and accordingly continuously heated. As a result, the temperature of the washing fluid, such as water, may be markedly lower than the temperature of the gas-steam mixture, so that at least when the system is started up, upon passage of the gas-steam mixture through the washing fluid, an undesirably pronounced condensation of the water steam can occur. Even such condensation of the water steam is technically controllable only at considerable expense. However, a further disadvantage of the condensation of the water steam in the washing fluid is that as a result the relative proportion of the pure hydrogen gas in the gas-steam mixture flowing out of the wet scrubber can become undesirably high. With a continuously heated wet scrubber, there is an additional disadvantage which is that a considerable proportion of the washing fluid evaporates continuously and must be replaced. It is accordingly an object of the invention to provide a method and a system for pressure relief of the containment of a nuclear power plant having a wet filter provided inside the containment, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type, and in which at most a non-relevant proportion of the water steam contained in the filter material condenses during the startup process. Since water is preferably used as the washing fluid, the temperature of the washing fluid must be approximately equal to the temperature of the filter material. On the other hand, as little washing fluid as possible should evaporate during continuous operation. With the foregoing and other objects in view there is provided, in accordance with the invention, a method for pressure relief of a containment of a nuclear power plant, which comprises heating a washing fluid in a filter disposed inside a containment at a relatively high rated heating power through a thermal bridge, with a gas-steam mixture filling the containment, prior to initial operation or initiation of operation of the filter; and rendering the thermal bridge substantially or virtually ineffective or even broken down completely in an operating state of the filter, leaving the washing fluid with only a remaining continuous rated heating power not being relevant for filtration. In accordance with another mode of the invention, there is provided a method which comprises setting the continuous rated heating power at less than 0.1 times the rated heating power. In accordance with a further mode of the invention, there is provided a method which comprises raising the washing fluid to an operating temperature after at most eight hours and preferably after substantially two hours of heating at the rated heating power. In accordance with an added mode of the invention, there is provided a method which comprises setting an operating temperature in a range of substantially from 100.degree. to 150.degree. C., raising the operating temperature to as high as substantially 260.degree. C. upon pressure relief directly from a primary loop of a nuclear power plant or direct pressure relief of a circuit that carries pressurized water, and supplying water to the filter as the washing fluid. In accordance with an additional mode of the invention, there is provided a method which comprises supplying a quantity of heat with the continuous rated heating power being less than a quantity of heat removed or drawn from the washing fluid by evaporation, for setting an operating temperature of the filter lower than an entry temperature of the gas-steam mixture. The method according to the invention is very advantageous, because as a result of the avoidance of concentration in the washing fluid, safe startup without significant changes of volume in the filter material is assured, and moreover a compact structure for the filter and a very small consumption of washing fluid are made possible. With the objects of the invention in view, there is also provided, in a nuclear power plant having a containment, a system for pressure relief of the containment, comprising a filter disposed inside the containment, the filter having a container, at least part of the container being double-walled or having two walls defining a chamber between the walls, and a heat-conducting fluid at least partly filling the chamber during a heating period and being at least half evaporated after attainment of an operating temperature. In accordance with another feature of the invention, the container has double-walled portions with the two walls, the double-walled portions having a heat-conducting resistance when not filled with the heat-conducting fluid being at least ten times higher than when filled with the heat-conducting fluid. In accordance with a further feature of the invention, the two mutually concentric walls define a double-walled, preferably cylindrical, middle part of the container surrounding or enclosing a vertical axis, and there is provided a single-walled curved base firmly closing the middle part toward the bottom, and a curved single-walled cap firmly closing the middle part toward the top. In accordance with an added feature of the invention, the double-walled middle part has a given height, the container includes a first inner chamber, the chamber between the walls is a second chamber having an annular cross section extending over the entire given height, and the second chamber communicates with the first inner chamber through openings formed just below the cap. This structure of the filter container makes it possible to fill the inner chamber with washing fluid to an extent that fluctuates within wide limits, and to fill the second chamber at least in its lower part with heat-conducting fluid, which represents part of a thermal bridge between the surroundings of the filter container and the filter contents, and evaporation of the heat-conducting fluid inside the second chamber is possible. Due to the disappearance of the heat-conducting fluid from the second chamber, the thermal bridge between the surroundings of the filter container and its contents is interrupted, so that the above-mentioned advantages are attained. After the disappearance of the heat-conducting fluid from the second chamber, this chamber is available to receive radioactive waste filtered out of the gas-steam mixture, so that this waste advantageously remains inside the containment. In accordance with an additional feature of the invention, there is provided washing fluid filling substantially 30 to 80% and preferably approximately 50% of the first inner chamber, the second chamber having a lower part being filled with the heat-conducting fluid and an upper part, and filter mats filling at least the upper part. In accordance with again another feature of the invention, the two walls have portions surrounding the heat-conducting fluid, and at least part of the second chamber has heat transfer fins increasing surface area in the vicinity of the wall portions. In accordance with again a further feature of the invention, both the washing fluid and the heating-conducting fluid are water. In accordance with again an added feature of the invention, there is provided a convection barrier in the form of a horizontal partition disposed in the second chamber above the heat-conducting fluid, the horizontal partition being perforated and/or slit. In accordance with again an additional feature of the invention, there are provided mist separators disposed in the upper part of the inner chamber in front of the openings leading to the second chamber. In accordance with still another feature of the invention, there is provided a vertical pipe disposed centrally in the first inner chamber for delivering a gas-steam mixture to be filtered, the vertical pipe having upper and lower ends, a horizontally extending segment through which the upper end of the vertical pipe communicates with the interior of the containment, radially disposed horizontal feed pipes into which the lower end of the vertical pipe discharges, and short Venturi nozzles communicating with the horizontal feed pipes just above the base. In accordance with still a further feature of the invention, the two walls are in the form of an inner wall and an outer wall, the outer wall has another opening formed therein leading to the outside, and there is provided a pipe penetrating the containment and communicating with the other opening for carrying a filtered gas-steam mixture out of the second chamber through the outer wall at approximately half the height of the filter mats. In accordance with a concomitant feature of the invention, there is provided an overpressure line through which the interior of the filter communicates with a primary loop of the nuclear power plant or with the interior of the containment, the overpressure line having an overpressure valve being closed during normal operation. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a method and a system for pressure relief of the containment of a nuclear power plant, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.