Patent Number: 047818859
Section: summary

The invention relates to a nuclear reactor fuel assembly including an elongated fuel channel with a square cross section and channel walls, the fuel channel having an imaginary lattice disposed therein with a box-shaped cross section having mesh openings and sides parallel to the channel walls, fuel rods containing nuclear fuel being mutually spaced apart in the mesh openings, and a water pipe spaced apart from the fuel channel by a given spacing and having a cross section spanning more than one of the mesh openings of the imaginary lattice, the given spacing being completely filled with the mesh openings of the imaginary lattice and the fuel rods disposed therein. A nuclear reactor fuel assembly of this type is known from U.S. Pat. No. 3,808,098. The known nuclear reactor fuel assembly is intended for a boiling water reactor. The water pipe thereof is a hollow cylinder and is not centrally disposed in the imaginary lattice. The water pipe forms an internal channel in the fuel assembly, through which water flows that does not boil over the entire length of the fuel assembly in the boiling water reactor. As a result, the water in the water pipe causes a better distribution of the flux of thermal neutrons and thus brings about improved reactivity in the reactor core of the boiling water reactor. In the prior art nuclear reactor fuel assembly, some subsidiary flow channels in the longitudinal direction of the fuel assembly have a different cross section from other subsidiary flow channels, which are formed only by the interspace between adjoining fuel rods. Such flow channels are formed by the intermediate space between the water pipe and adjoining fuel rods and in a boiling water reactor, a water and steam mixture flows through them in the longitudinal direction of the fuel assembly. The difference in the cross sections becomes greater as the size of the cross section of the water pipe and the the number of mesh openings or apertures of the imaginary lattice or grid encompassed by this cross section increases. Because of this difference in cross sections, the water-steam mixture flowing through the nuclear reactor fuel assembly is not uniformly distributed, which results in uneven cooling of the fuel rods of the fuel assembly. It is accordingly an object of the invention to provide a nuclear reactor fuel assembly which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and to make the cooling of the fuel rods uniform. With the foregoing and other objects in view there is provided, in accordance with the invention, a nuclear reactor fuel assembly, comprising an elongated fuel channel with a square cross section and channel walls, the fuel channel having an imaginary lattice disposed therein with a box-shaped cross section having mesh openings and sides parallel to the channel walls, fuel rods containing nuclear fuel being mutually spaced apart in the mesh openings, and a prismatic water pipe spaced apart from the fuel channel by a given spacing and having a cross section spanning more than one of the mesh openings of the imaginary lattice, the given spacing being completely filled with the mesh openings of the imaginary lattice and the fuel rods disposed therein. The cross-sectional outline of the prismatically constructed water pipe which is defined by straight lines, makes it possible to dimension the cross section of subsidiary flow channels which are partly defined by the water pipe in such a way that a largely uniform cooling of all of the fuel rods in the nuclear reactor fuel assembly is attained. Moreover, the prismatically constructed water pipe is capable of effecting a more uniform distribution of water, which acts as the moderator substance, and nuclear fuel, and thus of increasing the reactivity of the boiling water reactor. Pages 4-7 of the ASEA Journal, 3/84, do disclose a water pipe disposed in the center of a nuclear reactor fuel assembly for a boiling water reactor, which is a hollow body having a cross-shaped cross section. However, this hollow body is not spaced apart from the elongated fuel channel; instead, its longitudinal edges are welded directly to the inside of the fuel channel. As a result, although relatively uniform cooling of the fuel rods in the fuel assembly is attained, nuclear fuel and water are nevertheless unevenly distributed in the reactor core of the boiling water reactor, since a majority of the water in the water pipe having the cross-shaped cross section is located on the inner surfaces of the fuel channel, while in the reactor core of the boiling water reactor, water is already present as the moderator substance, on the outer surfaces of the fuel channel. The nuclear reactor fuel assembly according to the invention not only provides uniform cooling of the fuel rods and increased reactivity in the boiling water reactor, but it also has the further advantage of permitting the fuel assembly components, such as spacers, fuel assembly top fitting and fuel assembly base to be adapted to the water pipe without special expense. No adaptation of the fuel channel, of any kind whatsoever, is needed. In accordance with another feature of the invention, the water pipe is centrally disposed in the imaginary lattice. In accordance with a further feature of the invention, the cross section of the water pipe is square. In accordance with an added feature of the invention, the cross section of the fuel channel has cross-sectional sides, the cross section of the water pipe is square and has cross-sectional sides, the cross-sectional sides of the water pipe are each parallel to a respective cross-sectional side of the fuel channel, and the cross-sectional sides of the water pipe are each spaced apart from a respective one of the cross-sectional sides of the fuel channel by the same distance. In accordance with a concomitant feature of the invention, the imaginary lattice (10, 11) has n.times.n mesh openings, where n 8, the cross section of the central water pipe spanning (n-6).times.(n-6) mesh openings leaving remaining mesh openings between the water pipe and the fuel channel being occupied solely by fuel rods. These advantageous embodiments of the nuclear reactor fuel assembly provide better symmetry in the distribution of the moderator substance in the form of water and of the fuel in the reactor core of the boiling water reactor and therefore permit more uniform distribution of the enrichment with fissionable isotopes in the fuel rods of the nuclear reactor fuel assembly. 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 nuclear reactor fuel assembly, 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.