Patent Number: 041359720
Section: summary

BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to the supporting of fuel elements within a nuclear reactor and particularly to the attachment of web members comprised of a first metal to guide tubes comprised of a second metal to define a fuel element supporting spacer grid. More specifically, the present invention relates to high strength nuclear reactor fuel assemblies characterized by the use of different metals to define the fuel element receiving grids and the guide tubes on which the grids are supported and in which neutron absorber elements move for control purposes. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character. (2) Description of the Prior Art The functions performed by and the considerations which enter into the design of spacer grids for nuclear reactor fuel assemblies are discussed in detail in U.S. Pat. Nos. 3,607,640 and 3,664,924 issued to Donald M. Krawiec and assigned to the assignee of the present invention. Prior art spacer grids have, in many cases, been fabricated substantially entirely of a zirconium alloy; i.e., zircaloy. The use of annealed zircaloy has been dictated by its desirable combination of mechanical strength, workability and low neutron capture cross-section. Some designers, however, have favored fuel assemblies wherein the guide tubes are comprised of a first metal, for example zircaloy, while the grid defining members are fabricated from a second metal; the second metal typically having a higher neutron capture cross-section when compared to zircaloy but also having a greater stiffness than annealed zircaloy. Typical of the materials employed in spacer grids and having a greater stiffness than zircaloy is the steel alloy known as Inconel. A fuel assembly for a nuclear reactor will, in most instances, comprise a plurality of guide tubes which extend between upper and lower support plates; the support plates maintaining the requisite parallelism of the guide tubes. Intermediate the support plates, and mounted on the guide tubes, will be a plurality of grids each of which define an "egg crate" type structure. The fuel elements, which typically will comprise zircaloy tubes containing pellets of enriched uranium, are frictionally engaged in the spacer grids and are held in position thereby in parallelism to the guide tubes. As noted above, it is often considered desirable to employ a material such as Inconel to define the spacer grids while utilizing zircaloy guide tubes. Inconel, however, cannot reliably be welded to zircaloy and thus the use of dissimilar materials for the spacer grid and guide tubes precipitates a problem in the mounting of the spacer grids on the guide tubes. There have, in the prior art, been a number of techniques proposed for the joining of spacer grids comprised of a first metal to guide tubes comprised of a second dissimilar metal. These prior art techniques generally provide for a friction fit and/or mechanical stops between the grids and tubes and have proved to be a less than satisfactory solution since the inherently present clearances between parts have made possible vibration induced movement of the spacer grids relative to the guide tubes with the inherent possibility of fretting the fuel element cladding incident to any such vibration. Restated, the prior art joining techniques have provided only for mechanical capture of the grids on the tubes and have not guaranteed tightness between the parts as is required to insure against relative axial, radial and azimuthal motion. U.S. Pat. No. 3,920,516 discloses a prior art mechanical capture technique for use in mounting Inconel grids on zircaloy guide tubes. SUMMARY OF THE INVENTION The present invention overcomes the above briefly discussed deficiencies and disadvantages of the prior art by providing a novel and improved technique for the interconnection of components formed of dissimilar metals and particularly for the attachment of web members comprised of a first metal to tubes comprised of a second metal. The present invention also encompasses an improved nuclear reactor fuel assembly utilizing non-zircaloy spacer grids and zircaloy control element receiving guide tubes. In accordance with the invention web-like spacer grid defining members of a first metal are mechanically coupled to a guide tube comprised of a second metal by means of a cylindrical sleeve comprised of the same metal as the spacer grid defining web members. The cylindrical sleeve is continuous at its upper and lower ends and has an axial length greater than the width of the spacer grid. The cylindrical sleeve initially has, along at least a portion of its length extending from a first end along an axial distance greater than the width of the spacer grid, an outer diameter commensurate with the outer diameter of the guide tubes. The sleeve is provided with a plurality of cutouts or windows, typically four, having a length substantially equal to the width of the spacer grid web members; these windows being centered about the midpoint of the length of the sleeve. The width of these windows in the sleeve is selected so as to insure, when the sleeve is installed, contact between the side edges of each window and a grid web member whereby relative radial and azimuthal motion between the grid and tube is precluded. In accordance with a first embodiment of the invention, the sleeve is also provided with a plurality of apertures, spaced about the circumference of the sleeve, adjacent the oppositely disposed ends thereof. In use of the first embodiment of the invention, the sleeve is inserted in the spacer grid with the windows in registration with those portions of the spacer grid web members which would normally contact the guide tube. The sleeve is then expanded radially outwardly, for example by swaging, so as to have along its entire length an inner diameter slightly larger than the outer diameter of the guide tube. The outward expansion of the sleeve results in a guaranteed tight fit between the spacer grid web members and all four edges of each window intercepted thereby. This tight fit, in turn, produces a definite mechanically locking of the sleeves to the grid and, when the sleeves are attached to the guide tubes, prevents any axial, radial or azimuthal motion between the grids and guide tubes. Since the spacer grid web members and the sleeve are of the same material in the first embodiment, the interconnection of the sleeves and web members can be enhanced by welding these components to one another. The retention of the spacer grid at the proper point along the guide tube is accomplished by subsequently inserting the guide tube through the sleeve and thereafter expanding the guide tube radially outwardly in the areas where the apertures have been formed in the sleeve to lock the guide tube to the sleeve both above and below the spacer grid. In accordance with a second embodiment of the invention, the sleeve is comprised of the same material as the guide tubes; i.e., zircaloy; and thus the sleeve may be welded to the guide tube subsequent to the expansion of the sleeve which permits passage of the guide tube therethrough. In view of the attachment of the sleeve to the guide tube by welding, the sleeve need not be provided with the apertures of the first embodiment and no expansion of the guide tube is required. In the second embodiment, as in the first embodiment, the sleeve is provided with windows located intermediate the length of the sleeve; these windows having a length commensurate with the grid width and a width appropriate to achieve anti-rotation. As a result of the expansion of the sleeve, the top and bottom edges of the windows extend outwardly over the grid web members thus locking the spacer grid in place axially and the loading of the side edges of the windows in the expanded sleeve against the grid web members prevents radial and azimuthal relative motion between the guide tubes and grids.