Patent Number: 059057700
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a storage framework for nuclear reactor fuel elements, having a plurality of structural wells of essentially rectangular cross section fastened on a baseplate. In order to provide for the intermediate storage of spent fuel elements from a nuclear power plant, in particular a pressurized water or boiling water reactor, the fuel elements are stored within the nuclear power plant, in particular within fuel element storage basins. It is desirable for technical and economic reasons to utilize the available storage space as efficiently as possible. In order to increase the storage capacity, storage frameworks with incorporated nuclear poison, for example with boron-containing material, are employed. The storage framework, while having sufficient mechanical stability and neutron-absorbing properties, should require as little space as possible for itself, in order to allow a high loading density for spent fuel elements. Published European Patent Application 0 537 615 A1, corresponding to U.S. Pat. No. 5,311,563, specifies a storage framework for nuclear reactor fuel elements, in which a plurality of wells of essentially rectangular cross section are fastened on a baseplate, the wells standing vertically on the baseplate and in each case being disposed diagonally opposite one another in a checkered manner. Some of the wells located diagonally opposite one another are in each case connected to one another, along their mutually adjoining longitudinal edges with an offset, through the use of at least two connecting elements that bridge a gap formed by the offset. The pair of longitudinal edges is assigned at least one first connecting element of high rigidity, in each case in a first direction parallel to the baseplate, and at least one second connecting element of high rigidity, in each case in a second direction parallel to the baseplate. Due to the high rigidity of the connecting elements, internal transverse forces acting on the storage framework can be absorbed, without the need for an additional supporting grid in the upper region of the storage framework. It is thus also possible to load the interspaces of the storage framework which are not provided with wells, so that intermediate positions or intermediate locations for fuel elements are formed. In order to provide neutron absorption, the walls of the wells of the storage framework are composed of austenitic boron steel with a boron content of up to 2%, and the connecting elements are composed of a soft austenitic steel, the carbon content of which is lower than 0.1%. As a result, when extreme external forces act on the storage framework, the boron steel wells experience virtually no deformation, since the external forces are absorbed, where appropriate, by virtue of plastic deformations of the connecting elements. The use of an austenitic boron steel as a load-bearing structure for the wells makes those wells particularly complicated to manufacture, in order to ensure the required mechanical stability of the boron-treated steel. Further embodiments of a storage framework for spent fuel elements, in which the respective storage framework has a neutron-absorbing material, are disclosed, for example, in U.S. Pat. No. 4,088,897, U.S. Pat. No. 4,630,738, U.S. Pat. No. 4,695,424 and U.S. Pat. No. 4,119,859. A fact common to the storage frameworks known from those publications is that the neutron-absorbing material is integrated firmly into the load-bearing structure of the storage framework. In the storage framework disclosed in U.S. Pat. No. 4,088,897, boron-containing material is disposed between an inner and an outer wall of a well for receiving a spent fuel element. In the storage framework known from U.S. Pat. No. 4,630,738, a plurality of square neutron-absorbing wells parallel to one another on a baseplate are connected firmly to the baseplate, with the neutron-absorbing material, in the form of plates made from sintered boron-treated aluminum, being fastened firmly to the respective sides of the rectangular well. The boron-treated plates are disposed between an inner and an outer well. U.S. Pat. No. 4,695,424 discloses a storage framework, in which the neutron-absorbing material, for example a boron carbide, in the form of plates is non-displaceably fastened, particularly welded, to the outside of a well for receiving spent fuel elements. U.S. Pat. No. 4,119,859 describes a storage framework for fuel elements of a nuclear power plant, with wells for receiving a fuel element in each case. There, the wells have a sandwich structure with an inner wall and an outer wall, between which a neutron-absorbing material, for example boron carbide, is intercalated. The known measures for introducing neutron-absorbing material into a fuel element storage framework involve integrating the neutron-absorbing material firmly into the load-bearing structure of the fuel element storage basin, thus necessitating a considerable construction outlay in production terms and, where appropriate, requiring a monitoring of the neutron-absorbing material. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a storage framework for nuclear reactor fuel elements, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which ensures easier assembly, a high stability of an entire interconnected well system, a high loading density as well as uncoupling of neutron-absorbing material from load-bearing structures of the interconnected well system. With the foregoing and other objects in view there is provided, in accordance with the invention, a storage framework for nuclear reactor fuel elements, comprising a baseplate; a plurality of structural wells of substantially rectangular cross section fastened on and standing upright on the baseplate, the structural wells disposed approximately diagonally opposite one another in a checkered manner, and each two of the structural wells disposed diagonally opposite one another spanning an interspace; a neutron-absorbing structure with boron-treated steel disposed in at least one of an interspace and a structural well for displacement relative to the structural wells, and the neutron-absorbing structure having at least a partial region with spacers, such as strips, plates or edge beads, toward a respective one of the structural wells. Due to the above-described separation of the load-bearing function in the structural wells from the function of neutron absorption, the structural wells intended for receiving the fuel elements can be manufactured simply from known non-boron-treated austenitic steel or other steels permitted for this purpose, in particular in mass production. The neutron-absorbing structure has no load-bearing function of any kind, so that the use of boron-treated steel presents no problem at all both from a production standpoint and for reasons of mechanical stability. By virtue of an appropriate shaping of the neutron-absorbing structure, the interspace may likewise be used for receiving spent fuel elements. The use of boron-treated steel additionally has the advantage that there is no need to monitor the neutron absorption capacity. The boron-treated steel has, for example, a boron fraction of up to 2%. It is likewise possible to place the structure in the structural wells, in which case a configuration both in the structural wells and in the interspaces is particularly effective, since two layers of, for example, plates made from boron-treated steel are thereby laid one behind the other, with the result that it is also possible to store fuel elements having relatively high radioactive radiation capacity. Since the neutron-absorbing structure is constructed so as to be displaceable relative to the structural wells, an exchange of the structure can be carried out, for example if it becomes necessary to adapt the shape of the structure to the fuel elements to be received. Furthermore, this ensures that mechanical forces are transmitted to the structure at most to only a slight degree. Moreover, a displaceable structure is suitable for the retrofitting of existing fuel element storage frameworks. In accordance with another feature of the invention, the neutron-absorbing structure has plates made of boron-treated steel which, particularly for especially effective neutron absorption, are disposed essentially parallel to the side walls of the structural wells. In accordance with a further feature of the invention, the plates are connected to an essentially rectangular absorber well which, as a non-load-bearing element, merely has to have sufficient self-supporting stability. Such an absorber well may also be inserted into a formed interspace after an interconnected storage framework system has been produced from the structural wells. In accordance with an added feature of the invention, the absorber well is formed in each case from four plates which are connected releasably to one another, in particular intermeshed. As a result, the absorber well formed from the plates connected releasably to one another is held in its position by the structural wells surrounding it, thereby forming a preferably rectangular cell which is loosely installed and is not subjected to load. This cell formed by the absorber well is, in turn, suitable for receiving a spent fuel element. In accordance with an additional feature of the invention, in order to support and position the absorber well, at least some of the plates adjoin a respective structural well in at least a part region. For this purpose, the plates have suitable spacers, such as strips or distance plates. The plates may also have edge beads that are bent outward, that is to say bent toward the side walls of the structural wells, particularly in order to improve the utilization of the corners formed by the structural wells. In accordance with yet another feature of the invention, a structural well has, at an end located opposite the baseplate, a guide strip which, in particular, runs parallel to the baseplate and points into the interspace and through the use of which an unintentional removal of the neutron-absorbing structure from the interspace, for example when a fuel element disposed therein is extracted, is avoided. In accordance with yet a further feature of the invention, in order to increase the mechanical stability of the fuel element storage framework which is constructed from the structural wells, structural wells located diagonally opposite one another are in each case connected at the corresponding longitudinal edges through at least one connecting element, the connecting element having high rigidity and running in a first direction parallel to the baseplate. The connection, in particular a welded connection, of the structural wells to the connecting element, forms a mechanically stable interconnected system of load-bearing structural wells for receiving spent fuel elements. In accordance with a concomitant feature of the invention, in order to provide a further increase in mechanical stability, a further connecting element of high rigidity, which runs along a second direction parallel to the baseplate, is provided at the longitudinal edges. There is an angle which is preferably between 70.degree. and 90.degree. between the first direction and the second direction. The connecting elements accordingly run largely perpendicularly to one another and intersect within the gap present between the longitudinal edges of the structural wells located diagonally opposite one another. The structural wells are thereby fixed within the plane spanned by the baseplate and forces occurring between them are absorbed by the connecting elements. 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 a storage framework for nuclear reactor fuel elements, 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.