Patent Number: 058964304
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows part of a nuclear power plant comprising a reactor vessel 1 with a removed reactor vessel cover, not shown in the figure, arranged in a water-filled reactor pool 2. Further, a so-called fuel pool 3 is shown arranged adjacent to the reactor pool 2. The fuel pool 3 is substantially designed for temporary storage of new and completely or partially burnt-up fuel assemblies 4, respectively. The reactor vessel 1 comprises a core 5 with a plurality of fuel assemblies 4 and control rods 11 and a core grid 6 arranged above the core 5. The reactor pool 2 is connected to the fuel pool 3 via a closable opening 7. The other internal parts 8 of the reactor can be temporarily arranged in the reactor pool 2. During shutdown of the nuclear reactor, the reactor vessel 1 is filled with water and the reactor vessel cover removed. Then, the reactor pool 2 above the reactor vessel 1 is filled with water, and the port 7 between the reactor pool 2 and the fuel pool 3 is opened. Internal reactor parts 8 arranged above the core 5 are lifted out and arranged in the reactor pool 2. The core grid 6 and the fuel assemblies 4 arranged below the grid are now available for a gripper 9 arranged in the reactor hall. A fuel cassette 12 is arranged in the reactor vessel 1 above the core grid 6. The gripper comprises, for example, a telescopic arm 10 for lowering into the reactor vessel 1 and raising one or more fuel assemblies 4 therefrom and loading these in the fuel cassette 12. When the fuel cassette 12 is filled with the desired number of fuel assemblies 4, it is transported to the fuel pool 3 by the gripper 9. FIG. 1 shows the gripper 9 in dashed lines during transportation of a fuel cassette 12 comprising a group 4a of fuel assemblies 4 and possibly control rods 11. The fuel assemblies 4 are lifted out through the openings in the core grid 6. Also FIG. 2 shows, in principle, the appearance of the core grid 6. The core grid 6 comprises a grid having an opening in the grid which corresponds to the size of a core module, i.e., four adjacently located fuel assemblies 4 and one cruciform control rod 11 arranged therebetween. The control rods 11 may either be lifted out together with the fuel assemblies 4, or be left in the reactor core 5. The removed control rods are placed temporarily in the fuel pool 3 together with the fuel assemblies 4. FIG. 3 shows a two-row fuel cassette 12 intended for eight fuel assemblies 4 or 8 core modules and loading via an opening 13 open at the top. Fuel cassette 12 comprises eight substantially vertically arranged sleeve-formed spaces having a substantially square cross section. Each sleeve-formed space is limited by walls 12a made of a neutron-absorbing material and a bottom part 12b. The sleeve-formed spaces are joined to each other form one unit for transport and storage of fuel assemblies 4 and possibly control rods 11. When a fuel assembly 4 or a group of fuel assemblies 4, which is not a core module, is lifted out of the reactor core 5, these are each arranged in a sleeve-formed space in the cassette 12. Then, an additional fuel assembly 4 or another group 4a is lifted out of the reactor core 5 and is arranged in the fuel cassette 12. When lifting of a core module is made, this is preferably arranged in a sleeve-formed space surrounding the whole core module. The lifting operation continues until the fuel cassette 12 is filled with the desired number of fuel assemblies 4 or core modules, whereupon the fuel cassette 12 is transported to the fuel pool 3 for temporary storage therein. Possibly, one or more of the fuel assemblies 4 are replaced in the fuel cassette 12 while this is stored in the fuel pool 3. Fuel assemblies 4 may also be transferred in or between the fuel cassettes 12 when these are placed in the fuel pool 3. When it is time again for inserting the fuel assemblies 4 into the reactor core 5, the fuel cassettes 12 are transported from the fuel pool 3 to a location in the reactor vessel 1 above the core 5. Thereafter, the fuel assemblies 4 are lifted one by one, by groups, or by core modules out of the fuel cassette 12 and are arranged in the reactor core 5. FIG. 4 shows a single-row fuel cassette 12 intended for eight fuel assemblies 4 or for eight core modules and loading via a vertical opening 14, each arranged in the wall of a sleeve-formed space. Each vertical opening 14 is provided with a stop means 15 for fixing the fuel assemblies 4 in the fuel cassette. FIG. 5 shows a two-row fuel cassette 12 with twelve sleeve-formed spaces intended for twelve fuel assemblies 4. Each sleeve-formed space is provided with a vertical opening 16 extending along the substantial length of the sleeve-formed space. The opening 16 is provided with a port 17 intended for sealing the sleeve-formed space during transport and storage of the fuel assembly 4 arranged therein. During a shutdown of the reactor, the fuel assemblies 4 are normally lifted out whereas the control rods 11 are left in the reactor vessel 1. If it is desired also to lift out the control rods 11, this can be done either by lifting them out together with the core modules as mentioned above, or in a work operation separate from the lifting of the fuel assemblies 4. According to one aspect of the invention, the control rods 11 are arranged in control rod cassettes 18 in a way corresponding to the arrangement of the fuel assemblies 4 in fuel cassettes 12. FIG. 6 shows an embodiment of a control rod cassette 18. The control rod cassette 18 comprises a frame structure 18 formed with eight control rod positions 19. Alternatively, the control rod cassette 18 may be designed so that the control rods 11 can be arranged with their control rod blades lib overlapping each other. The control rod cassette 18 is preferably designed so that it can be loaded laterally, that is, with a vertical opening corresponding to the openings 14, 16 in the fuel cassette 12 in FIGS. 4 and 5, respectively. Preferably, the same gripper 9 is used for lifting the control rods 11 as for lifting the fuel assemblies 4. By arranging a plurality of control rods 11 in one control rod cassette, the emptying of the reactor vessel 1 is further accelerated. The control rod cassette 18 is arranged in the fuel pool for temporary storage in the same way as the fuel assembly cassettes 12. FIG. 7 shows how the fuel pool 3 is divided into a number of positions. Each of these positions consists of a square in the grid shown. When a fuel cassette 12 is arranged in the fuel pool 3, this can be arranged at an arbitrary location therein; for example, in the dashed position relating to a two-row fuel cassette with eight fuel assemblies 4 according to FIG. 3. In one embodiment of the invention, the fuel cassettes 4 are filled and arranged in the fuel pool 3 in such a way that each fuel assembly 4, removed from the reactor core 5, in the fuel pool 3 is given a position which, in relation to the other removed fuel assemblies 4, is the same as in the reactor core 5. In this way, the same geometry is obtained in the fuel pool 3 as in the fuel core 5. In those cases where refuelling or fuel transfer is to take place, this is suitably performed while the fuel cassettes 12 are placed in the fuel pool 3. At the same time as the refuelling and/or the fuel transfer takes place in the fuel pool, the reactor vessel 1, or parts connected thereto, is/are freely available for servicing. When the servicing is completed and the fuel assemblies 4 are possibly replaced or transferred, the fuel assemblies 4 are transferred in their respective fuel cassettes 12 again to a location in the reactor vessel 1 whereupon the fuel assemblies 4 are moved from the fuel cassette 12 and filed down onto their position in the reactor core 5. It is self-evident that the different types of fuel cassettes 12 and control rod cassettes 18 may be arbitrarily provided with any of the openings 12, 14, 16 which are shown and be provided in a suitable way with ports 17 or stop means 15.