Patent Number: 043057870
Section: summary

BACKGROUND OF THE INVENTION This invention pertains to racks for storing spent radioactive fuel. Such racks usually consist of a series of parallel cells whose walls are provided with a protective covering, generally on their inner side, based on a material capable of absorbing neutrons, such as for example, boron. The various cells are assembled by means of end structures or plates. Racks of this type have as their principal drawbacks their high cost, which is due primarily to the large quantity of protective material required to cover all the cells, and also the complex cell construction process and the assemblage of the entire unit. A further drawback of these racks lies in their high weight, due to the double wall of each cell. BRIEF SUMMARY AND OBJECTS OF THE INVENTION The object of this invention is to achieve a rack of much lower cost and weight than the traditional racks. This is accomplished by reducing the quantity of covering material and by simplifying the construction and assemblage of the entire unit. In accordance with the present invention, the set of cells consists of self-supporting metal tubes arranged in a staggered fashion with respect to each other and joined to each other along their longitudinal edges so as to form a rigid spatial structure. The tubular hole delimited by each four tubes has a larger cross-sectional area than the tubes themselves and is intended to receive the protection of neutron-absorbing material, while the tubes themselves are not provided with such protection. In addition, the protection provided in the tubular space delimited within each four tubes is independent of said tubes, so that it may be removed and inserted in the tubular holes as often as necessary. Each tube may consist of a single extruded piece or of two or more pieces welded together along their edges. Furthermore, in order to permit the tubes to be joined to each other, each tube may be provided along at least two of its edges with an equal number of longitudinal external projections, the longitudinal outer edge of each tube may be provided along each of its four edges with an external longitudinal projection at an angle of approximately 45.degree., the longitudinal outer edges of the projections closest to each other of adjacent tubes being joined to each other. In the first case, wherein each tube has two longitudinal projections, the longitudinal outer edge of each such projection may be joined to the nearest edge of the adjacent tube that lacks such a projection, or the projections of each two adjacent tubes may be placed against each other and the longitudinal outer edges of the abutting projections joined to each other. The tubular sections are joined to a base plate which has adjustable supports, at least some of which are provided with metal springs, intended to take up irregularities in the ground upon which the assembly rests. The construction described above greatly reduces the quantity of protective material required, since only the tubular holes defined by each four tubes are provided with the protective covering. Nevertheless, the spent radioactive fuel placed in the tubular sections will also be protected, since each tubular section is surrounded by four tubular holes provided with protective covering. In addition, the weight of the assembly may be reduced even further by providing openings in the walls of the tubes. Since all the tubes are rigidly connected among themselves, the use of the frames required to support the assembly in the traditional containers is obviated. However, this rigidity may, under some circumstances, be considered a drawback, because it reduces the safety of the rack when subjected to severe stress, such as, an earthquake, in which case the rigidity of the rack may give rise to a strong dynamic effect requiring the use of thicker plate and stronger welding beads in comparison with other racks having a longer period of vibration. To avoid this possible drawback, a second approach is provided whereby the tubes are arranged so as to be independent of each other throughout their length. Each tube has mounted at its top a frame between which and the said tube are arranged in discrete regions, adjustable devices that permit centering the element bearing the encapsulated neutron-absorbing material and eliminating the gap that may be present between the upper portion of the bearing element and the surrounding tubes. These frames may be independent of each other, in which case the rack assembly constitutes a spatial structure of multiple elastic pillars capable of withstanding earthquakes. The frames may also together comprise a unitary grid running through the entire assembly, so that the rack then forms a spatial structure of multiple elastic supports. The adjustable devices provided between each frame and tube may, for example, consist of two plates provided with surfaces facing each other in wedge-like fashion. These plates are inserted back-to-back with respect to each other along said surfaces, so as to lie between each tube and the frame of each element bearing the encapsulated neutron-absorbing material. The plates are provided with an upper elbow toward the tube side, in order to provide mutual support between the elbows and the outer edge of the tube. Adjustable support elements are provided between the elbows of both plates in order to permit adjustment of the distance between the elbows; the frame of each tube also being provided with top elements in order to limit the vertical displacement of the plates. The support elements may consist of vertical bolts threaded on the elbow of the plate located beside the tube with the elbow of the other plate resting upon said bolts. In the same fashion, the top elements may consist of vertical through bolts threaded in the frame of each tube starting on the upper surface thereof, said screws extending downward beyond the frame so as to rest on the elbow of the plate located on the side of the element bearing the encapsulated neutron-absorbing material. Regardless of which of these approaches is adopted, the reduced quantity of protective material entails a reduction in the weight of the rack and especially a reduction in the cost thereof, due to the high price of the protective materials used.