Patent Number: 042886996
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a storage rack for the storage of fuel elements of nuclear reactors consisting of a sheet metal lattice arrangement constituting a plurality of abutting similar vertical storage cases or tubes having in general a rectangular cross section. In anyone of the cases of this storage rack a rod-shaped fuel element may be stored. 2. Description of the Prior Art Within or in the neighbourhood of the reactor building there are available storage pools for storing spent or new thermal reactor fuel elements during a long or only a short period. Therein the fuel elements are placed in racks below water in such an arrangement that the necessary heat dissipation is warranted and no nuclear chain reaction may occur. Generally for such a design there are included very broad margins with respect to the mutual distances between the elements. When the storage capacity of such pools has been exhausted and the transfer of the spent fuel elements to a regeneration plant is not yet possible a temporary solution is found by placing the rods closer to each other in the pool in which case however, much more attention should be given to warrant sufficient criticality margins for the ensemble. A possibility for reducing the mutual distance or the pitch with which the fuel elements are being placed is the inclusion in the construction of a so-called neutron poison, i.e. a material having a very high effective cross section value for the absorption of neutrons. Such a material is the boron isotope B.sup.10 that may be included as an alloying component in the stainless steel, mostly used for the manufacture of these racks. When utilising such a material for the manufacture of the storage racks as a whole or part thereof the possibility arises of a much smaller pitch of the fuel elements and consequently of a much more compact storage than in the conventional racks. From nuclear physic calculations it is apparent that dependent on the chosen construction, the geometry of the ensemble and the specific fuel element, there may be found an optimum ratio of the construction material and the surrounding water with respect to the fuel elements. In case of the storage racks of the above-mentioned type there exists therefore a certain interstice between the cases determining the final storage capacity. At the found optimum any further increase of the amount of boron, i.e. the increase of the thickness of the borated sheet steel of the cases does not lead to an increase of the absorption capacity and consequently not to a decrease of the water gap or the mutual distance between the elements; that is to say that when the "saturation value" of the boron content is reached a decrease of the mutual distance between the elements will bring the ensemble closer to the criticality. Hence the effective multiplication factor will more closely approach the limit value usually assumed at 0.95. In case of storage rack arrangements including a neutron poison the water gap between the fuel rods play an important role as a moderator for the neutrons leading to a relatively high peak of the neutron flux between the said cases. This "surge" of the neutron flux at the location of the gap and consequently also in the case walls of borated steel leads to an increased capture of neutrons and consequently to a decrease of the multiplication factor of the ensemble. Also in case of a rack not provided with a neutron poison a decrease of the mutual distance between the elements leads to an increase of the multiplication factor after all. In case of a given rack construction and given fuel elements a theoretical minimum distance or gap width constituted by metal and water should be maintained between the fuel elements in order to satisfy the criterion for the multiplication factor. However, theoretical gap widths may only be applied when the storage rack may be manufactured within very close tolerances. Up till now these storage racks have been made as a welded construction. Notwithstanding the use of welding jigs the accurate gage maintenance and the attainable straightness of the cases and other construction parts constituting the storage rack are limited by the deformations inherent to each welding process. Therefore in case of a welded construction the theoretically permissible minimum gap width has to be increased with additional safety margins in view of the necessary manufacture tolerances. SUMMARY OF THE INVENTION The invention now provides a storage rack of the above-mentioned type, the manufacture of which may be carried out within very close tolerances, whereby the minimum distance between the fuel elements will nearly not be affected by the manufacturing process. If for a certain fuel element having a length of for instance 4 m the width of the water gap may be for instance 25 mm, the tolerance in case of a welded construction with respect to the case walls will be .+-.3 mm. Due to both the adjoining cases the tolerance with respect to the water gap will consequently be .+-.6 mm or about 25%. In accordance with the present invention for fuel elements having the same length, these tolerances will be .+-.5 mm with respect to the case walls and 1 mm with respect to the water gap, this corresponding with 4%. In the storage rack according to the present invention the lattice arrangement has been constructed of a plurality of mutually substantially perpendicular sheet elements having sharply shaped contours, all sheet elements in one of the two perpendicular directions completely or partially traversing the lattice arrangement at a mutual distance substantially corresponding to the width of a storage case and all sheet elements in the other of the two perpendicular directions each extending only substantially over the width of one storage case, the latter narrow sheet elements at the opposite edges thereof having been provided with lugs fitting in accurately formed openings in the traversing sheet elements, said lugs having been immobilized by means of a keying, the arrangement being such that the long edges of the narrow sheet elements may be pressed tightly against the traversing sheet elements. In accordance with one preferred embodiment of the invention all storage cases within the lattice arrangement have been separated from each other by a wall consisting of two parallel sheet elements at a short distance from each other inclusive the interstice between these sheet elements, the said interstice housing the lugs and the keyings. The keyings may then be embodied in such a manner that of the narrow sheet elements constituting together a partition wall, the lugs have been provided at the same level, said lugs having been connected with each other by means of a connecting strip running parallel to the traversing sheet elements and that between this connecting strip and the opposite wall of the cooperating traversing sheet element a key has been pinched. The connecting strips between the lugs may be secured by welding. These welds are no such critical welds that these will cause a deformation of the rack construction. Moreover each lug may be provided with an opening having an edge running parallel to the wall of a cooperating traversing sheet element while a key may be pinched between this edge and the wall of the traversing sheet element. In general the lugs will consist of an extended part of the sheet elements and slidably fit into slots in the cooperating sheet elements. For securing after assembling, all keys may be welded to the traversing sheet elements, the connecting strips and the lugs, respectively. These welds are not critical either. Such securing may also be attained by deformation of the keys. In a rectangular lattice arrangement the storage cases at each of the four outer walls of the lattice may be enclosed by a traversing sheet element extending over the entire side wall, while the sheet elements of the side walls extending parallel to the narrow sheet elements have been provided with slots for slidably receiving lugs formed on the edges of the traversing sheet elements perpendicular thereto, which lugs may then be fastened by means of a keying as described above. For the present invention it is in particular of importance that there are manufactured sheet elements having very accurate contours and openings for the lugs, for instance with an accuracy up to 0.1 mm. Hereby and by the use of keyings instead of welding combined with bending there may be obtained particularly dimensionally stable constructions throughout the entire height and width of the storage rack. The said accuracy applies in particular to the longitudinal edges of the sheet elements. By the use of the lattice arrangement according to the present invention in combination with a particular locking construction for the top plate and the bottom plate of the lattice, wherein no welding joints are employed either, there is provided a storage rack having a very stable construction. The storage racks according to the present invention entail the further advantages of enabling a very accurate prefabrication of all sheet elements, an easy cleaning of all parts of the storage rack separately, this being a necessity for the storage of nuclear fuel elements. Further the absence of welds, always resulting in uncertain material properties, excludes all worries about structural changes when exposed to the ionising radiation of the fuel elements. By keyings a very accurate assembly is possible. For the maintenance of the structural shape of the arrangement there is no necessity to use expensive assembly jigs as are required when welding. The keyings and the special construction of the storage rack by means of traversing and narrow sheet elements furthermore open the possibility of a rapid and simple exchange of parts of the rack. Moreover separate parts of a rack may easily be controlled with respect to the cleanness thereof. There are no critical welding constructions whereby expensive weld controls may be omitted and possible faults are eliminated.