Patent Number: 042657075
Section: summary

This invention concerns a method for the separation of nuclear fission and activation products from a gas containing the same and apparatus for carrying out the method in and about a reactor building of a gas-cooled nuclear reactor. The process of this invention is particularly intended to make possible the separation of fission and activation products that occur within the containing walls of gas-cooled high temperature reactors in the case of serious so-called "hypothetical malfunctions," as malfunctions with small likelihood of occurrence are called. Nuclear fission or activation products can be set free in the rooms of the reactor building, with a large part of the space in the reactor building becoming radioactively contaminated, as a consequence of a serious hypothetical malfunction in which, after failure of the emergency cooling system, gas conduit pipes, or other components of a gas-cooled high-temperature reactor, burst. Nuclear fission and activation products contained in the gas atmosphere do lose their activity by physical and chemical degeneration processes, but the radionuclides remaining in the gas atmosphere, nevertheless, are a great danger to the enviroment. This is particularly the case if, as the result of leakages in the outer wall structure of the reactor building resulting from the malfunction, nuclear fission and activation products can escape into the outside environment. The requirements for separation of the nuclear fission and activation products, in addition to physical and chemical boundary conditions for the degeneration processes, depend above all on the geometrical dimensions and shapes of available space enclosed by the reactor building, particularly the ratio of surface of the enclosed space to the volume of the building. The liberated fission and activation products are present in the form aerosols, partly even in elemental form, which is to say atomic or molecular, so that the settling velocity for the fission and activation products resulting from diffusion and sedimentation are greatly delayed compared to the desired settling rates. In order to increase the settling rate, it is known from German published patent application No. OS 20 50 152 to innoculate the cooling stream with inactive isotopes of the fission and activation products that are produced in order to bind these fission and activation products, that might penetrate into the cooling medium circulation path of a nuclear reactor facility. This process, however, requires, independently from the occurrence of a malfunction, a continuous addition of dilute solutions containing the inactive isotopes, even during normal operation of the reactor. In the case of water-cooled nuclear reactors, it is known, after the fracture of a part of an apparatus or a pipeline of the reactor, to condense the vapor coming out of the fracture quickly by squirting in condensation nuclei, such as carbon dioxide snow or silver iodide, and in this way to bind the escaping radioactive substances (see German published patent application OS No. 20 57 593). For gas-cooled high-temperature reactors, the addition of such water condensing nuclei is either useless or uneconomic, however, because the gases escaping from the cooling medium circulation path in most malfunctions are dry gases. In the case of gas-cooled high-temperature reactors, filter systems are provided in the air circulation system of the reactor building, by which the fission and activation products are intended to be separated after occurrence of the malfunction by drawing off the atmosphere by suction from the radioactively contaminated chambers. There is the disadvantage, however, that such installations, as the result of their technically limited start-up and heat capacity filter out the liberated fission and activation products only relatively slowly and, on account of their dependence upon the supply of external energy--according to the gravity of the malfunction--are under certain conditions not at all capable of going into operation. THE PRESENT INVENTION It is an object of the invention to provide a process for separation of nuclear fission and activation products from a gas atmosphere by which high settling rates for the products contained in the gas atmosphere are obtainable, without the necessity of withdrawing the gas atmosphere out of the radioactively contaminated chambers or spaces. Briefly, at least 0.5 kilogram, preferably more than one kilogram per 50 m.sup.3 of atmosphere volume, of dust particles having a grain size distribution with an average particle size between 0.3 and 5 .mu.m is introduced in fine dispersion into the gas atmosphere. By "average particle size" is meant the particle size average value that is obtained as the characteristic size d' of the dust particle accumulation at the intersection point of the RRS straight line for a residue value R=36.8% in the grain size matrix according to Rosin-Rammler-Sperling. See "Verfahrenstechnik" [process technology] by Kiesskalt, published by Carl Hanser Verlag, Munich, 1958, pp. 61ff. By means of the dust particles (i.e., fine dry particles), the surface available for the settling of the fission and activation products in radioactively contaminated spaces is substantially magnified. At the same time, in the case of homogeneous particle distribution in the spaces in question, the critical free path length for the adsorption or settling of the fission and activation products is drastically reduced, and these fission and activation products are bound to the surface of the dust particles. This advantageously leads to a substantial increase of the settling or adsorption velocity which now is no longer dependent on the magnitude of the fission and activation product itself, but rather upon the size of the dust particles. The limits of the average particle diameter of the aggregate cloud of particles, therefore, are determined at the low extremity by the desired settling or adsorption velocity and at the upper extremity by the distribution of the particles and the aerosol formation in the gas atmosphere in the radioactively contaminated space. The binding of the fission and activation products to the dust particles diminishes at the same time the probability of escape of fission and activation products from leaks in the outer wall structure of the reactor building. As a further development of the method of the invention, the dust particles that are introduced comprise ceramic materials that are inert with respect to oxygen. Suitable dust size particles of this type consist of bentonite or clay. Cement or silica gel powders are also useable. The so-called extinguisher powders are preferred, these being powders commonly used in fire fighting. Graphite dust, because of its adsorptive capability, is also useable for inert gas atmospheres in the reactor building. In this case it is necessary to blow the material into the gas atmosphere by means of an inert gas in order to prevent explosions of the graphite dust. For carrying out the process according to the invention in a reactor building of a gas-cooled nuclear reactor having an outer pre-stressed concrete wall structure equipped with a liner, apparatus with the following features is provided according to the invention: The outlet of a dust hopper container feeds into a pneumatic feed line for drawing off dust particles stored in the container. The pneumatic feed line passes through the pre-stressed concrete wall structure and at its extremity outside of the wall structure is connectable to gas compression equipment that is situated in the neighborhood of the reactor building. The end portion of this pneumatic feed line that leads into the internal space of the reactor building has at least one dusting nozzle, and, preferably, several of them. The apparatus according to the invention, is advantageously capable of being put into operation within the reactor building independently of the apparatus destroyed as the result of the malfunction that has produced the radioactive contamination. To this extent, therefore, a passive safety system is provided. It is useful to locate the dust hopper also outside of the reactor building, so that the quantity of dust to be introduced into the reactor building can, from time to time, be increased beyond the amount contained in the hopper by way of supply. As mentioned before, in order to prevent dust explosions in the use of graphite dust, inert gas should be introduced as the compressed gas for the pneumatic feed. A homogeneous distribution of dusting material and aerosol formation in radioactively contaminated spaces is enhanced by providing a number of feed ducts or feed duct branches leading to dusting nozzles in the internal space and which are uniformly distributed over the ceiling surface of the reactor building.