Patent Number: 048270636
Section: summary

The invention relates to a nuclear reactor fuel assembly having mutually parallel fuel rods and guide tubes for control rods, two lattice-like spacers with mesh openings through each of which one fuel rod is guided and retained in a force-locking manner or one guide tube is guided and secured, an additional lattice secured at least to one of the guide tubes and disposed between the two spacers as seen in the longitudinal direction of the fuel rods and the guide tubes, the additional lattice having mesh openings, and turbulence-promoting vanes protruding beyond the sides of the mesh openings, one fuel rod being guided with play or one guide tube being guided through each of the mesh openings. A nuclear reactor fuel assembly of this type is known from European Patent Application No. 0 148 452, corresponding to U.S. Application Ser. No. 567,448, filed Dec. 30, 1983. The additional lattice of this prior art fuel assembly has square mesh openings formed therein and is made of inner sheet-metal ribs that pass through one another at right angles. On the outside of the fuel assembly, the additional lattice has four outer ribs that define a square periphery of the additional lattice and are secured on the inner sheet-metal ribs that are at right angles thereto. The surface of the additional lattice formed by the inner sheet-metal ribs has rigid bearing nubs thereon for the fuel rods in the mesh openings, and each trailing edge of the holes for the fuel rods formed by an inner sheet-metal rib is provided with a single turbulence-promoting vane thereon. The turbulence-promoting vanes serve to mix the coolant, such as water, that flows longitudinally through the nuclear reactor fuel assembly in a nuclear reactor, for example a pressurized water reactor. Mixing of the coolant is intended to prevent the coolant from being unevenly heated over the cross section of the fuel assembly and to prevent the particular fuel rods of the fuel assembly that are heated to the greatest extent and thus are undergoing the severest stress from being inadequately cooled. However, the mixing lattice leads to considerable pressure losses in the coolant. 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 which reduces the pressure losses. With the foregoing and other objects in view there is provided, in accordance with the invention, a nuclear reactor fuel assembly, comprising mutually parallel fuel rods and guide tubes for control rods, two lattice-like spacers having mesh openings formed therein, one of the fuel rods or one of the guide tubes being guided and secured in each of the mesh openings, an additional lattice secured at least to one of the guide tubes between the two spacers as seen in the longitudinal direction of the fuel rods and the guide tubes, the additional lattice having mesh openings formed therein defining sides of the mesh openings and a smooth and flat surface of the additional lattice in the mesh openings, turbulence-promoting vanes protruding beyond the sides of the mesh openings formed in the additional lattice, one of the fuel rods being guided with play or one of the guide tubes being guided through each of the mesh openings formed in the additional lattice, and brackets gripping the additional lattice between at least one of the fuel rods or the guide tubes at the outside of the fuel assembly. It has been demonstrated that in this way, uniform heating of the coolant in the nuclear reactor is attained even if the size and/or the number of turbulence-promoting vanes on the spacers is reduced. The spacers may even not have any turbulence-promoting vanes at all, which leads to particularly low pressure losses in the coolant. Spacers that do not have turbulence-promoting vanes can also be manufactured more economically. In accordance with another feature of the invention, the additional lattice has mutually parallel leading and/or trailing edges with zig-zag portions, each of the zig-zag portions being compactly disposed in the plane of one of the sides of one of the mesh openings formed in the additional lattice. In accordance with a further feature of the invention, the additional lattice has mutually parallel leading and/or trailing edges on which the turbulence-promoting vanes are disposed, each of the edges having ends and being associated with one of the sides of the additional lattice, each two adjacent turbulence-promoting vanes on one of the mutually parallel edges of the additional lattice being twisted in mutually opposite directions about the longitudinal direction of the fuel rods and the guide tubes, located on one of the edges of one of the mesh openings of the additional lattice for a fuel rod, and tapered to a point and protruding beyond the side of the mesh opening associated with the edge at one of the ends of the edge. This structure produces a more extensive evening out of the temperature of the coolant flowing through the fuel assembly in a nuclear reactor, while at the same time having low pressure losses for the coolant. In accordance with an added feature of the invention, the additional lattice is spaced apart from the two first-mentioned spacers by unequal distances. As a result, turbulence caused by spacers in the liquid coolant flowing through the fuel assembly in a nuclear reactor calms down, so that the coolant once again flows to the additional lattice in a uniform flow, and the turbulence-promoting vanes of the additional lattice can become optimally effective. A better transfer of heat to the coolant then takes place on the way to the next spacer. In accordance with a concomitant feature of the invention, the zig-zag portions of the leading and/or trailing edges are mutually staggered. 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. The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.