Patent Number: 042749212
Section: description

Referring now to the drawing and first, particularly, to FIG. 1 thereof, there is shown part of a reactor core having fuel assemblies of heretofore conventional construction. Each fuel assembly is surrounded by a hexagonal box or chest 3 and is formed of a large number of individual fuel rods 2 disposed in a triangular grid. The hexagonal fuel assemblies are disposed close to one another in the manner shown in FIG. 1 and thus form the reactor core per se. The individual fuel assemblies are obviously not stacked together without gaps, since they could otherwise note be assembled and disassembled without difficulty. The spacing between adjacent fuel assembly boxes or chests is about 3 to 5 mm. It consequently also affords the hereinaforedescribed plastic deformation, which can be kept within limits only by suitably increasing the thickness of the walls of the boxes 3. FIG. 2 shows the construction of a reactor core by means of fuel assembly boxes or chests 4 which are of circular cylindrical shape. From the viewpoint of strength, this is the optimal shape, so that, for the same strength, considerably smaller wall thicknesses are required. The wedge-shaped spaces 5 between the fuel assemblies are practically without significance from a neutron-physics point of view. FIGS. 3 and 4 show two possible distributions or arrays of the fuel rods 2 within the boxes 4, all of the fuel rods 2 being located on concentric circles. A decided disadvantage of this fuel assembly construction appears initially to lie in the fact that, for the same flow cross section, in comparison to the state of the art, the area per fuel assembly, inclusive of the gaps therebetween, is about 5% greater. This increase in the total core volume by about 5% resulting from the use of round assemblies, however, is more than made up for the decrease in the steel content of the box or chest from 6% to 1.5% for round assemblies. Depending upon the diameter of the round fuel assemblies, higher temperatures can develop in the central zone if fuel rods of the same enrichment are used. However, this can be prevented by appropriately large diameters of the fuel assemblies or by equipping the central zone with rods of lesser enrichment. It is also possible to provide a separate, correspondingly matching flow configuration for the central zone. This can be effected, for example, by appropriate formation of the coolant supply at the inlet thereof to the fuel assembly. A further possibility, according to FIG. 5, is to omit the fuel rods of the central zone, for example, at the central rod and of the two inner rings of rods. A second, likewise circular-cylindrical inner guide tube 41 is then provided for the flow configuration. The interior thereof can then contain, for example, a control element 6 formed of absorber material or an additional breeder element 7. The entire core of the reactor can consequently be constructed of identical fuel assemblies. In this connection, it should be noted that if the boxes have the shape of circular cylinders, the head piece and carrier or base member of the fuel assemblies can be formed especially simply as turned parts whereas, in the case of hexagonal fuel assemblies, complex transitions between the hexagonal cluster and the round connecting parts had to be effected. In summary, it can be concluded that the circular fuel assembly according to the invention of the instant application is less expensive and simpler to fabricate than the conventional hexagonal assembly heretofore; that the thickness of the box or chest wall and, thus, the steel content of the assembly are considerably smaller than in the hexagonal fuel assembly; and that the matching of the cooling of the inner rows of rods is less complicated and can be accomplished by the hereinaforedescribed measures. The simplest measure is that which provides a suitably large diameter of the fuel assemblies. In conclusion, it should be mentioned that it is known from other types of nuclear reactors to use fuel assemblies with circular cross section, although, respectively, within special cooling channels. For fast nuclear reactors, however, the efforts have been to fill the entire core cross section with fuel rods leaving as few gaps as possible, for which purpose the hexagonal geometry of the individual assemblies was available. It was not realized that the constructional and manufacturing engineering as well as neutron-physical advantages of circular fuel assemblies for fast nuclear reactors more than make up for the apparent disadvantages due to the empty wedge-shaped spaces between the individual fuel assemblies. Likewise, it was not heretofore recognized that the use of such fuel assemblies prevents the occurrence of long-term damage to the boxes inhibiting the operation of the reactor.