Patent Number: 041892542
Section: summary

The invention relates to a system for the storage of radioactive material in rock cavities. More particularly the invention relates to a repository for the storage of spent fuel from nuclear power plants and such high level waste that is produced during the reprocessing of spent nuclear fuel. Systems for the storage of radioactive material in rock have previously been proposed. Such a system is described in the U.S. patent application Ser. No. 857,041 of Dec. 2, 1977, by Hallenius and Sagefors, and consists of an outer cavity in the rock. This cavity encloses a core of rock mass and is filled with clay so that the clay forms a shell around this core in which is provided an inner cavity which forms the storage space for the radioactive material. This inner cavity is provided with recesses for the accommodation of the radioactive material and communicates with a shaft for entering the radioactive material. This system is provided with an inner cooling system consisting of a plurality of conduits for a coolant. Each such conduit forms a closed loop which extends in a vertical plane along the inside of the inner cavity and along the outside of the core of rock. The system may also be provided with an outer cooling system consisting of a tunnel situated in the rock outside the shell of clay, said tunnel forming a helix which extends concentrically with the system in several turns along the total height of the system. The ends of the helical tunnel are joined in the rock at some distance from the repository, thereby forming a closed cooling system. Therefor, both the inner and the outer cooling system will operate according to the thermosiphon principle which means that coolant heated by the heat developed by the radioactive material rises upwards in the cooling system due to its lower density and is conveyed to a place within or outside the repository having a lower temperature where the coolant is cooled and returned to the hotter places in the repository. Thus, the circulation of the coolant is effected without the aid of any external machinery requiring the supply of energy from outside. However, this cooling system is relatively complicated. Also it is a disadvantage that it is difficult to calculate beforehand the dimensions of the cooling system and the whole repository in order that the dissipation of the generated heat shall be effective without causing dangerous temperature rises in the repository and its environment. For this reason the dimensions of the repository and the cooling system must be estimated with large safety margin which may make the construction costs unnecessarily high. The present invention relates to a repository of the kind described above for the storage of radioactive material in rock. The repository comprises a substantially spherical cavity excavated in the rock. This cavity is surrounded by a shell of rock and this shell is surrounded by a shell of clay. The clay shell is surrounded by the rock formation. It is an object of the invention to provide in a repository of this kind an effective distribution and dissipation of the heat generated by the stored radioactive material. The invention makes it possible to calculate beforehand with great accuracy the heat distribution and the temperature rise in the environment of the repository. Hereby it is also possible to calculate with great accuracy the dimensions of the repository so that the temperature in the rock and the clay shell does not reach dangerous values. The cooling system in the repository according to the invention also contributes to the mechanical stability of the repository and prevents the cavity from collapsing under the action of extremely high external forces. According to the invention the repository is characterized in that a vertically standing tube-shaped member of a heat resistant and mechanically strong material is arranged within the cavity, which tubeshaped member divides the cavity into an outer space and an inner space and is provided at its top and bottom ends with openings connecting the outer space with the inner space, that both the inner space and the outer space are filled with substantially spherical bodies of a heat resistant and mechanically stable material which bodies are provided with through openings and arranged so that these openings extend at an angle to the horizontal plane, and that the radioactive material to be stored is formed into rods which are placed within said openings in some of the spherical bodies in such a way that the rods of radioactive material are at a certain distance from the inside of the openings, and that those of the spherical bodies which contain radioactive material are situated in the bottom part of the inner space in the tubeshaped member. The tubeshaped member preferably consists of a cylindrical tube of reinforced concrete which is open at both ends and also is provided with openings around its periphery adjacent to its ends. The said spherical bodies are also preferably made of reinforced concrete. In the repository according to the invention air in the bottom part of the tubeshaped member will be heated by the radioactive material and caused to rise upwards within the tubeshaped member to its top end where the air is forced through the openings at the top end against the wall of the cavity where the air is cooled and flows downwards in the outer space between the tubeshaped member and the wall of the cavity, whereupon the air again flows into the tubeshaped member through the openings at its bottom end and again comes in contact with the radioactive material and is heated anew so that the flow cycle is repeated. The air flows through the spaces between the spherical bodies and through the openings in these bodies. Thus, the spherical bodies act as a porous mass which makes possible a relatively free and rapid air flow and simultaneously prevents the cavity from being compressed and collapsing under the action of high external forces. The heat generated by the radioactive material is thus distributed by convection nearly uniformly over the whole cavity, and large temperature peaks in limited areas of the interior of the cavity are avoided. The generated heat spreads through the rock surrounding the cavity and further on to the clay shell. Due to the spherical shape of the cavity it is relatively simple to calculate the temperature distribution in the environment of the cavity. For a given amount of stored radioactive material it is thus possible to estimate the variation with time of the temperature in the rock and the clay shell and the resulting maximum temperatures. These temperatures will of course be dependent of the dimensions of the rock mass and the clay shell, and it is therefore possible to determine beforehand these dimensions so that the temperature cannot assume critical values. By "critical values" of the temperature are meant such values which may cause undesirable changes in the rock and the clay, e.g. crumbling of the rock and drying-up of the clay so that it loses its plasticity.