Patent Number: 043127053
Section: summary

The invention relates to a spacer for fuel rods assembled into a fuel assembly for nuclear reactors formed of a grid structure of sheetmetal webs passing edgewise perpendicularly through one another and having resilient and rigid contact elements fastened thereto. A great number of constructions have become known heretofore for such spacers; in particular, reference may be had to German Pat. No. 1,589,051 wherein a so-called three-point support is proposed in which a fuel rod bears on one side against two rigid contact elements disposed one above the other and is pressed against them by a resilient contact element which engages the fuel rod on the opposite side thereof at half the height between the two rigid contact elements. Rigid and resilient contact elements are always thus located in the middle of the mesh walls of the spacer grid i.e. at the narrowest location between the fuel rod and the wall of the mesh, so that especially unfavorable cooling conditions prevail at this location. It has already been proposed heretofore also to place these contact elements in part at the intersection points of the spacer webs (note German Published Non-Prosecuted Application DE-OS Nos. 1 764 396 and 1 930 981). In these cases, the resilient contact elements, of themselves, form complete parts which are suspended into the web grid. The rigid contact elements, however, continue to remain at the narrowest location between the fuel rod and the wall of the mesh. This construction makes it possible to reduce the neutron absorption of such spacer grids with respect to previous constructions by making the web grid per se of a material having a low neutron absorption cross section, such as Zircaloy, for example, and only the spring contact elements per se of appropriately spring-hardened material, such as, Inconel, for example. The invention of the instant application is also based upon this principle. The two last-mentioned proposals for spacer construction provide one spring contact element and four rigid contact elements for each spacer mesh. The rigid contact elements are located in the zone of the narrowest coolant gap and reduce heat removal by the coolant at these locations. It is further possible that, with these constructions, the fuel rod can be pushed out of the centered position thereof if strong lateral forces should occur, such as in the event of an earthquake, for example, and the fuel rod will then directly contact the wall of the mesh which, due to the degradation of the heat removal connected therewith, can lead to clodding tube defects. It is accordingly an object of the invention to provide a spacer of the foregoing general type wherein cooling conditions are made more uniform over the entire circumference of the fuel rods at the height of the spacer grids and, at the same time, the pressure drop due to the latter is reduced. It is a further object of the invention to provide such a spacer wherein excursions of the fuel rods from the centered position thereof due to strong lateral forces are prevented as much as possible, while providing a construction of adequate strength with reduced material expenditure. With the foregoing and other objects in view, there is provided, in accordance with the invention, a spacer for fuel rods assembled into a fuel assembly for nuclear reactors formed of a grid structure of sheetmetal webs passing edgewise perpendicularly through one another comprising resilient and rigid contact elements fastened to the sheetmetal webs, each of the fuel rods being supportable by two diagonally opposing three-point contact systems, each of the systems being formed of one of the resilient contact elements and two of the rigid contact elements, the one resilient contact element being centrally disposed, as viewed in axial direction, and the two rigid contact elements being oppositely disposed and being ring-shaped. In accordance with another feature of the invention, the sheetmetal webs pass through one another at intersecting points and are formed with respective slots at the upper and lower edges thereof adjacent the intersecting points, the ring-shaped rigid contact elements being located at the intersecting points at the upper and lower edges of the sheetmetal webs and being engaged in the respective slots and welded to the sheetmetal webs, the ring-shaped rigid contact elements being formed of short tube sections having a diameter smaller than the diameter of the fuel rods to be supported in the spacer, at least the sheetmetal webs passing in the one direction through the other sheetmetal webs being formed with cutouts, members having a substantially triangular cross section respectively suspended in the cutouts and being held in position by the other sheetmetal webs, the members having two of the resilient contact elements extending in axial direction and projecting diagonally into two adjacent meshes of the grid structure. In accordance with a further feature of the invention, the two resilient contact elements of the members, respectively, project laterally into the mesh beyond the respective ring-shaped rigid contact elements at the upper and lower edges of the sheetmetal webs so as to contact a respective fuel rod, whereas the ring-shaped rigid contact elements are spaced from the respective fuel rod. In accordance with an additional feature of the invention, the grid structure has a multiplicity of meshes respectively defined by wall portions of the sheetmetal webs, the wall portions, respectively, being formed with large-area openings. In accordance with a concomitant feature of the invention, the members suspended in the cutouts are formed of one-piece stamped-out blanks of resilient material having respective head and base parts formed by bends into a substantially triangular cross section, the head and base parts being formed with slots for guidance into cutouts and engaging a mesh wall forming part of the respective sheetmetal webs. 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 spacer for nuclear reactor fuel assemblies, 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.