Patent Number: 042749212
Section: summary

The invention of the instant application relates to a fuel assembly for fast nuclear reactors, especially of the gas-cooled type, which is formed of a cluster of fuel rods disposed parallel to one another and enclosed by a casing or box serving to guide coolant flow. At the present state of the art of fast nuclear reactors, it is generally known that the fuel assemblies are enclosed by boxes or chests for guiding coolant flow. Each fuel assembly therefore has a separate, adjustable coolant flow configuration, which is necessary for ensuring reliable heat removal from the fuel assemblies that are extremely highly stressed thermally. In general, the fuel element boxes or chests are of hexagonal cross section, since such a cross section is readily derived from the triangular division of the fuel rod grid. This triangular division is selected because the rod spacing, for the same flow area, is considerably greater than, for example, with the square pitch customary in pressurized-water reactors. Since, as aforementioned, the boxes serve to conduct the flows in the fuel assembly so that all of the rods therein are subject to the same cooling conditions, it is necessary to manufacture the hexagonal boxes with great precision. With a rod spacing of theoretically 1.12 mm between the casing tube and the inner surface of the box, an accuracy of +1 mm, which is currently attainable for hexagonal tubes of about 170 mm width across the flats, already means a considerable increase in the outer-lying subchannels. This results in a flow distribution within the fuel assembly, by means of which the inner rods are cooled relatively less than the outer rods. Currently, this effect is taken into consideration in the layout or construction of the fuel assembly. During the life or duration of such an hexagonal fuel assembly, there is the danger that the fuel element will tend to deform to a circular cross-sectional shape as much as possible due to internal pressure exerted therein. Even though the elastic expansion is insignificant, a plastic deformation which increases during the life time can result, however, from creep due to irradiation and temperature. Increases of the width across the flats by more than 1 mm, however, which must be expected, render the homogeneous cooling of the fuel rod cluster questionable. It follows therefrom that, in the heretofore conventional hexagonal boxes, dimensional accuracy can be ensured only by increasing the wall thickness accordingly. This, however, not only makes the manufacture of the fuel assemblies more expensive but also involves neutron-physical disadvantages. It is accordingly and object of the invention to provide a fuel assembly for fast nuclear reactors that is of such shape that the hereinaforementioned difficulties do not occur or are very considerably reduced and which, in addition, affords simpler and more accurate production possibilities than heretofore, together with an effective conservation of material. With the foregoing and other objects in view, there is provided, in accordance with the invention, a fuel assembly for fast nuclear reactors comprising a cluster of parallel-disposed fuel rods and a casing surrounding the fuel rods and defining a coolant flow configuration, the fuel rods being disposed in a manner that the axes thereof intersect imaginary coaxial circles, and the casing being of circular cylindrical shape. In accordance with another feature of the invention, the fuel rod axes are disposed on V-shaped lines extending symmetrically from the center of the fuel assembly and constituting radii defining an angle of 60.degree.. In accordance with a further feature of the invention coolant subchannels extend in the middle of the cluster and are bordered by fuel rods of reduced output. In accordance with an additional feature of the invention, the fuel rods of reduced output have a lower enrichment than the remaining fuel rods of the fuel assembly. In accordance with an added feature of the invention, a central guide tube is disposed coaxially to the casing, and a breedermaterial element is received in the interior of the central guide tube. In accordance with a concomitant feature of the invention, a central guide tube is disposed coaxially to the casing, and a control element formed of absorber material is axially adjustably disposed in the interior of the central guide tube. 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 fuel assembly for fast nuclear reactors, 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.