Patent Number: 041892542
Section: description

In the drawings 1 designates the bedrock in which the repository is located at a certain depth below the ground level 2. This depth may be for instance 300 to 600 meters. In the bedrock 1 there is excavated an outer cavity the outline of which is designated 53 in FIG. 1, and in this cavity there is left a core 54 of rock. The space between this rock 54 and the outer rock is filled with clay 55 which forms a shell enclosing the core 54 of rock. The core 54 is positioned in relation to the outer bedrock 1 by means of supporting members 56 which may consist of reinforced concrete or of left rock. The core 54 contains an inner cavity 57 of a spherical form. Thus, the core 54 forms a shell of rock around the cavity 57. The cavity 57 communicates through a vertical shaft 58 with a horizontal tunnel 59 which is located adjacent to the ground level. The cavity 57 and the shaft 58 are lined with reinforced concrete 60. The cavity 57 constitutes the storage space for the radioactive material. A vertically standing cylinder 61 of reinforced concrete is placed within the cavity 57. This cylinder is shown in detail in FIG. 3. As seen in this figure the wall thickness of the cylinder may be larger in the central part of the cylinder and decrease towards the ends of the cylinder. At the lower end of cylinder 61 there are arranged two rows of ventilation holes 62 along the periphery of the cylinder. Adjacent to the top end of the cylinder there are also provided a row of holes 63 along the periphery of the cylinder wall. The cylinder 61 rests by its lower end on the bottom part of the cavity 57 while its upper end is at some distance from the top part of the cavity 57. Thus, the cylinder 61 divides the cavity 57 into an outer space between the outside of cylinder 61 and the wall of cavity 57 and an inner space formed by the interior of the cylinder. These spaces communicate with each other through the openings 62 in the lower end of the cylinder 61 and through the open upper end of the cylinder and the holes 63. As shown in FIG. 2 the space in cavity 57 which is not occupied by the cylinder 61 is filled with spherical bodies in the form of balls 64 of concrete which are all of the same diameter. Such a ball 64 is shown more in detail in FIG. 4. The ball is provided with a plurality of through cylindrical openings 65. In the embodiment shown in FIG. 4 there are three such openings. The openings 65 have the form of straight cylinders and seen in a cross-section at right angles to their axes they are so disposed that the center lines are at the corners of an equilateral triangle. Each ball 64 is provided with a hook or strap 66 which is anchored in the ball and by means of which the ball can be lifted and lowered. The balls 64 are so placed in the cavity 57 that the openings extend in a direction at a certain angle to horizontal plane. This angle should be such that the openings terminate in the spaces between the balls. The hook or strap 66 is so located in relation to the openings that when the ball is lowered into the cavity 57 hanging in the hook or strap 66, the openings 65 will automatically assume the desired direction. All the balls 64, both those located outside and those located inside the cylinder 61, are provided with such openings 65. The purpose of these openings is to facilitate the circulation of air within the cavity 57. The radioactive material to be stored in the repository is assumed to be solid and shaped into rods. Thus, spent fuel rods and fuel assemblies from a nuclear reactor can be stored without any further treatment in the repository according to the invention. The rods of radioactive material are entered into the openings 65 in some of the balls 64, namely those balls that are placed within the cylinder 61 and preferably only in those balls 64 which are at the lower part of the interior of cylinder 61. Preferably the cylinder 61 is filled with balls 64 containing radioactive material only to one third of its height. The rods of radioactive material are placed in the openings 65 in the balls 64 in such a way that the rods are spaced from the insides of the openings 65 so that air can freely circulate through the openings along the rods of radioactive material. FIG. 4 shows some fuel assemblies 67 placed in the openings 65 in the ball 64. The rods are positioned within the openings 65 by means of suitable support means (not shown). The cavity 57 is closed by means of a seal 68 located in the shaft 58 near its opening into the cavity 57. The cavity 57 may contain sensing means sensing temperature, pressure and radioactive radiation. These sensing means could be connected with measuring instruments located outside the repository by means of cables 69 which are drawn through the seal 68 and the shaft 58. The construction of the repository can be effected by the use of rock blasting methods well known in the art and will therefore not be described more in particular. The cavity 57 should be lined on its inside with heavily reinforced concrete. The concrete cylinder 61 is manufactured by casting on its place within the cavity 57. The space outside the cylinder 61 is filled with concrete balls 64 which are lowered through the shaft 58. Concrete balls 64 containing radioactive material are placed at the bottom of cylinder 61 and above these balls are placed concrete balls 64 not containing radioactive material. The shaft 58 opens straight above the upper opening of cylinder 61. If so desired the balls 64 can easily be removed from the interior of the cylinder, which may be desirable for instance if the stored radioactive material is to be removed for reprocessing. The tightly stacked concrete balls 64 which fill the cavity 57 contribute to preventing the cavity from collapsing. Therefore, the cavity can be given very large dimensions. The dimensions of the repository will of course be dependent on the amount of radioactive material to be stored in it. A repository for the storage of 350 metric tons of spent fuel from a reactor will for instance have the following dimensions: Radius of cavity 57=20 meters Distance from the center of cavity 57 to the inner side of the clay barrier 55=65 meters The maximum temperature in shell 54 of rock will then amount to about 200.degree. C. and the maximum temperature in the clay shell 55 to less than 50.degree. C. In the embodiment shown in FIG. 1 the clay shell 55 and the space occupied by this shell in the rock has a spherical shape. However, the clay shell 55 and the space occupied thereby could also have other shapes, e.g. cylindrical shape within the scope of the invention.