Patent Number: 050892126
Section: summary

The invention relates to an apparatus for controlling the power output of a nuclear reactor, in particular for the reactor core of an advanced pressurized water reactor, having a number of control elements each being movable by a drive mechanism, and each of the control elements including a plurality of control rods and a support structure joining the control rods to one another. When taking the existing, time-tested technology of pressurized water reactors used in many nuclear power plants as a point of departure, substantially better utilization of the raw material used as a source of energy is attained in a so-called advanced pressurized water reactor by using novel fuel assemblies, along with slight changes to the pressurized water reactor core. In order to accomplish this, the average neutron energy in the reactor core must be increased. Since the water that moderates neutrons continues to be used simultaneously as a coolant, the average neutron energy in an advanced pressurized water reactor is shifted beyond the thermal range by reducing the ratio between the moderator volume and the fuel volume. In a liquid-cooled reactor core, the fuel assemblies are disposed vertically in a generally cylindrical reactor pressure vessel that has a rounded or partly spherical cover or dome and a rounded or partly spherical bottom. The coolant flows from bottom to top through the reactor pressure vessel and simultaneously acts as a moderator. Each fuel assembly includes a bundle of fuel rods, which are guided in a grid or lattice assembled from spacers and are movably supported in a shared top and bottom piece. The fuel rods can expand between the top and bottom pieces retained in grid or lattice plates and are therefore not hindered in their axial temperature expansion. In order to enable the fuel rods to receive uranium oxide as a fuel, they are constructed as tubes within which the fuel is hermetically sealed in pellet form. Controlling the power output of the nuclear reactor is performed, among other means, by control rods that are driven to a variable depth into the active part of the reactor core. To this end, certain fuel assemblies include guide rods, inside which the control rods can be driven. However, during operation of the nuclear reactor, only a selected number of fuel assemblies is equipped with control rods. In order to reduce the number of control rod drive mechanisms, the control rods that are all associated with the same fuel assembly are coupled to a single drive rod above the fuel assembly through a shared support structure, known as a "spider". The drive rods of the control elements pass through the cover of the reactor pressure vessel to individual control element drive mechanisms at the outside. The locations of the control elements are typically symmetrically distributed over the cross-sectional area of the reactor core. The number of control elements is limited by the maximum allowable number of bores that can be provided in the reactor pressure vessel cover for the passage of the drive rods therethrough. In contrast to the square cross-sectional structure of the fuel rod grid or lattice of the conventional pressurized water reactor, the fuel rod grid or lattice structure in the advanced pressurized water reactor is hexagonal in cross section. This permits a very small spacing between the fuel rods, so that on average more fuel is contained per unit of reactor core volume in an advanced pressurized water reactor than in the core of a conventional pressurized water reactor. Given unchanged dimensions of the reactor pressure vessel, the necessary compactness of the reactor core of an advanced pressurized water reactor makes for twice the number of fuel rods, as compared with a conventional pressurized water reactor, at only approximately half the active core height. In order to ensure reliable regulation and/or shutoff of the advanced pressurized water reactor, it is therefore necessary to provide a larger number of control elements per unit of cross-sectional area of the reactor core than in a conventional pressurized water reactor. On the other hand, as shown by such a configuration described in the journal "Kernreaktoren" [Nuclear Reactors] by H.-J. Zech, in Deutsches Atomforum, Bonn 1988, if the dimensions of the reactor pressure vessel are unchanged, then the number of drive rods for the control elements is limited, particularly because of the predetermined number of bores in the reactor pressure vessel cover. It is accordingly an object of the invention to provide an apparatus for controlling the power output of a nuclear reactor, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and with which more control elements can be actuated in the reactor core of an advanced pressurized water reactor per unit of cross-sectional area than in a pressurized water reactor of the previously typical type, for a given number of control element drive mechanisms. The control elements should be distributed in such a way that they cover the area uniformly and symmetrically over the entire cross section of the reactor core, in order to compel uniform power distribution in the core. With the foregoing and other objects in view there is provided, in accordance with the invention, a nuclear reactor, especially an advanced pressurized water reactor, with controlled power output, comprising a reactor core having a cross section in a given plane with a center of area and axes of symmetry passing through the center of area, a multiplicity of control elements disposed in groups, each of said groups having at least one of said control elements, said control elements of each of said groups with more than one control element being joined together, said groups being symmetrical to at least two of the axes of symmetry, drive mechanisms each moving a respective one of said groups of control elements, each of said control elements having a plurality of control rods, a support structure joining said control rods of a group to one another, fuel assemblies disposed in groups, each of said groups of control elements having a given number of control elements being associated with one of said groups of fuel assemblies having said given number of fuel assemblies, and other fuel assemblies with which said control elements are not associated, said other fuel assemblies surrounding said groups of fuel assemblies. In accordance with another feature of the invention, each of said groups of control elements has 1, 2 or 3 control elements, and each of said groups of fuel assemblies has 1, 2 or 3 fuel assemblies. In accordance with a further feature of the invention, the cross section of said reactor core is approximately circular, and each two of said axes of symmetry define an angle therebetween being an integral multiple of 30.degree.. In accordance with an added feature of the invention, the cross section of said reactor core is substantially circular, and said reactor core has a hexagonal fuel rod grid structure. In accordance with an additional feature of the invention, the groups of control elements are a total of 61 groups of control elements having a total of 151 control elements, 13 of said groups of control elements each have one control element, 6 of said groups of control elements each have two control elements, and 42 of said groups of control elements each have three control elements. In accordance with yet another feature of the invention, there are provided connecting pieces each having said control elements of a respective one of said groups detachably retained thereon and each being movable by a respective one of said drive mechanisms. In accordance with yet a further feature of the invention, there is provided a multi-armed guide plate in which said connecting piece is guided. In accordance with a concomitant feature of the invention, the multi-armed guide plate has adjacent arms defining an angle therebetween being a fraction of 360.degree., said fraction corresponding to the number of said arms. An advantage of the apparatus for controlling the power output of a nuclear reactor according to the invention, is that with little engineering effort or expense, substantially more control elements per unit of cross-sectional area can be installed in the reactor core, for an unchanged number of control element drive mechanisms, by joining the control elements into groups. A distribution of the control elements that covers the area and is symmetrical over the cross section of the reactor core is attained in accordance with the invention by providing that the drive rods of two or three control elements are joined together by being converted, through one connecting piece each, into one drive rod. Along with individually driven control elements, this enables a particularly advantageous geometrical configuration of the control elements to be provided over the reactor core cross section, with six axes of symmetry between which the angle is an integral multiple of 30.degree.. Thus, in calculating the layout and monitoring the reactor core, a 30.degree. portion can be used as the basis and then extrapolated for the entire core cross section. It is moreover possible for a number of fuel assemblies that suits given requirements to be provided with control rods, through the formation of groups having more than three control elements, locally or over the entire core cross section. The apparatus according to the invention is especially advantageous when a pressurized water reactor system is being converted into an advanced pressurized water reactor, because neither the reactor pressure vessel cover nor the control element drive mechanisms need be replaced. 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 an apparatus for controlling the power output of a nuclear reactor, 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.