Patent Number: 052326586
Section: description

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a cross section through a fuel assembly according to the invention, which is covered laterally by walls 2 and 3 of a fuel assembly box, at the bottom by a bottom or base part and at the top by a top or cap part. A coolant, serving as a moderator, is introduced through flow openings in the bottom part, flows upward in interstices between fuel rods 4 and 5, and emerges again through flow openings in the top part. It may be advantageous to place an elongated tube 6 in the interior of the fuel assembly box. This tube 6 carries a flow of liquid coolant through it over its entire length, while the coolant flowing along the fuel rods is heated by the rods and is present in the upper part of the fuel assembly as a mixture of liquid and steam. In the box, the tube or water tube 6 assures that sufficient liquid coolant, as the moderator, will be present in the upper part of the fuel assembly as well. In the most common box, the fuel assembly has a square cross section, in which reference symbol d indicates the maximum spacing between the outer surfaces of opposite box walls, which is predetermined by the geometry in the core zone of the reactor. This geometry also determines a radius of curvature r at rounded corners of the box. The invention provides flat surfaces for the outsides of the box walls, the spacing of which have the value d. The corners are rounded in accordance with the predetermined radius r. As is typical in the prior art, the fuel rods are each disposed in rows in such a way that the minimum spacing between two adjacent fuel rods of a bundle is equal to the spacing of two fuel rods that are adjacent one another in a row parallel to one box wall. These lengthwise and crosswise rows, which are defined by the minimum fuel rod spacing, accordingly extend parallel to the box walls 2 and 3. In the exemplary embodiment of FIG. 1, the lengthwise rows and crosswise rows are each formed by 11 fuel rods. In order to keep a minimum spacing m as great as possible despite this high number of fuel rods, the spacing between the inner surfaces of the opposite box walls is increased by reducing the thickness of the box walls 2 and 3. While wall thicknesses of 3 mm have previously been typical, and even uniform wall thicknesses of 2.7 mm and 2.54 mm have been considered adequate, wall thicknesses of less than 2.4 mm, and in particular between 1.5 and 1.7 mm, are used in this case. With previously known materials, this would not have been adequate for the stability of the box. Nevertheless, this stability is attained, because the box walls are thickened in the region of the rounded corners by a reinforcement that protrudes into the interior of the box. Thicknesses between 2.5 mm and 3.0 mm and preferably 2.7 to 2.9 mm, are adequate for this thickening. These thickenings in the wall regions reduce the space available for a corner rod and the intersections of the rows of fuel rods bordering the walls 2 and 3. However, in many boxes, it is readily possible for such corner rods to be thinner than the others, which is already known. In the present box, no fuel rod whatsoever is provided at the aforementioned points of intersection of the two peripheral rows, such as at a position A. It is therefore unnecessary with the selected configuration to manufacture and use fuel rods having a different diameter in order to nevertheless assure that a predetermined minimum value for the flow cross section, or in other words a minimum spacing between the inner surfaces of the box and the adjacent fuel rods, is also adhered to in the region of the rounded corners for the coolant flowing along the vicinity of the box wall. As compared with a configuration of 9.times.9=81 fuel rods, despite the tube 6 which occupies the cross section of 3.times.3 fuel rods and despite the four missing corner rods, the present geometry still has a total number of 11.times.11-(3.times.3)-4=108 fuel rods. An in-between number of 100 fuel rods can advantageously be attained in accordance with FIG. 2. In the FIG. 2 embodiment, the distribution of the fuel rods is especially uniform, even in the corner regions. In this box, the fuel rods 4 and 5 next to the box walls are again disposed in a row parallel to the box walls, and the other fuel rods are disposed in lengthwise and crosswise rows parallel to them. However, a spacing n of the fuel rods in these rows is greater than the minimum spacing m between adjacent fuel rods. In other words, fuel rods having the minimum spacing m are located in rows that are inclined relative to the box walls. This geometry permits a configuration in which there is already no fuel rod provided in the position A. Accordingly, no fuel rod needs to be left out in order to adhere to a minimum spacing between the box wall and adjacent fuel rods. Accordingly, in its outer dimensions, the novel fuel assembly meets all of the demands specified by the geometry of the reactor core. The fuel rods themselves are distributed in a simple, clear and advantageous way over the cross section, and make optimal use of the available cross-sectional area with a view to wide spacings between one fuel rod and adjacent fuel rods or walls. The total fuel provided for one fuel assembly is distributed over a desired higher number of fuel rods, each with a correspondingly smaller diameter.