Patent Number: 041359720
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference now to FIG. 1, a spacer grid for a nuclear reactor fuel assembly is indicated generally at 10. Spacer grid 10 is defined by a plurality of intersecting web-like members such as indicated at 12. The configuration of web members 12, which are provided with integral springs such as indicated at 14, may be clearly seen from a joint consideration of FIGS. 1 and 2. The web members 12 define an "egg crate" structure through which the fuel rods or elements, such as those fuel elements indicated at 16, pass. It will be understood that, in a single fuel assembly, there will be a plurality of spatially separated spacer grids which locate and support the fuel elements. The spacer grids of a fuel assembly are mounted on the exterior of guide tubes. Absorber or moderator elements, also known as control rods, which are movable into and out of the fuel assembly to control fission rate, are positioned within the guide tubes. In a typical fuel assembly there will be five guide tubes extending the length of the fuel assembly between upper and lower support plates. In FIG. 1 only a single guide tube 18 has been shown in cross-section. Guide tube 18 will customarily be comprised of zircaloy while the spacer grid defining members 12 may be comprised of Inconel. Inconel can not reliably be joined to zircaloy by conventional fusion bonding techniques, such as welding, or by brazing. In accordance with the present invention the spacer grids are mechanically connected to the guide tubes through a cylindrical intermediary sleeve. The configuration of such a sleeve as employed in a first embodiment of the present invention may be clearly seen from FIG. 4; the sleeve 20 of FIG. 4 also being visible in FIGS. 1, 2 and 3. In accordance with the first embodiment of the invention, the sleeve 20 is comprised of the same material as the web members 12; i.e., sleeve 20 will be Inconel in the example to be first described. Continuing to refer to FIG. 4, it may be seen that sleeve 20 is continuous at its oppositely disposed ends. Sleeve 20 is provided, intermediate its length, with cutouts or windows 22 of generally rectangular shape. Additionally, in the regions between the upper and lower terminations of the windows 22 and its opposite ends, sleeve 20 is provided with circumferentially offset apertures of holes 24. For reasons which will become apparent from the discussion below, the outer diameter of sleeve 20 is initially commensurate with the outer diameter of the guide tube 18. It should be observed that design factors, which will not be discussed herein, dictate the size of the guide tube 18 and the dimensions of the opening in the spacer grid through which the guide tube passes. Accordingly, it is not possible to simply insert a sleeve having an inner diameter which exceeds the outer diameter of the guide tube into the opening in the spacer grid since insufficient clearance exists for such a sleeve. Similarly, design factors preclude the reduction, either wholly or at portions along its length, of the inner diameter of the guide tube. Restated, using a smaller guide tube would affect the control rod worth while employing a larger opening in the spacer grid would require complete grid and possibly complete fuel assembly redesign. In accordance with the present invention the sleeve 20 is inserted in the opening in the spacer grid with the windows 22 located in those regions where the guide tube will contact the web members 12. Thereafter, the sleeve 20 is radially expanded by swaging, typically in a two step process, using mandrels of different diameter. During the radial expansion step the portions of the sleeve between the windows 22 will be expanded outwardly into the corners of the spacer grid guide tube opening where clearance exists. The continuous upper and lower portions of the sleeve; i.e., those portions which have the holes 24 formed therein; are freely expandable since these portions are located above and below the spacer grid. As these continuous upper and lower portions of the sleeve are expanded outwardly the upper and lower edges of the windows 22 will pass outwardly over the web members 12 thus mechanically capturing the web members in the axial direction. At the same time, the loading of the side edges of the windows against the grid web members will prevent relative radial and azimuthal motion and will aid in preventing relative axial motion. Since the web members 12 and sleeve 20 of the embodiment of FIGS. 1-4 are comprised of the same material, immobilization of the sleeve in the grid may be enhanced by spot welds 26 as indicated in FIG. 2. The results of the expansion of sleeve 20 may be best seen from joint consideration of FIGS. 1 and 3 which respectively are cross-sectional views taken through an expanded sleeve above the spacer grid and intermediate the width of the spacer grid. After the sleeves have been installed in the spacer grids, the fabrication of the fuel assembly is continued by passing the guide tubes 18 through the expanded sleeves 20 as shown in FIGS. 1-3. When the spacer grids are properly positioned along the length of the guide tubes the guide tubes are expanded, using a mandrel, only in those regions which are in alignment with the holes 24 in the sleeve 20. Expansion of the wall of guide tube 18 into the holes 24 in sleeve 20 results, as may be clearly seen from FIG. 1, in the mechanical locking of the sleeve to the guide tube both above and below the spacer grid. This manner of locking, in combination with the capture and welding of the spacer grid web members to the sleeve provides a fuel assembly wherein vibratory movement of the spacer grid relative to the guide tube can not occur. In the embodiment of FIG. 5 the sleeve 20' is comprised of the same material as the guide tube 18. Accordingly, in the embodiment of FIG. 5 a fusion bond can not be established between the spacer grid web members 12 and sleeve 20'. Sleeve 20' of the FIG. 5 embodiment is provided with windows, identical in size, shape and position to windows 22 of the sleeve of FIG. 4. Sleeve 20' is not provided with the apertures 24 in the continuous portions thereof located above and below the windows. In the assembly of a spacer grid in accordance with the FIG. 5 embodiment the sleeve 20' will be inserted in the guide tube opening in the spacer grid in the same manner as discussed above. Thereafter, the sleeve is expanded along its entire length thereby resulting in the mechanical capture of the web members 12 in the windows 22. As noted above, the swaging operation which results in the expansion of sleeve 20' effectively preloads the sleeve onto the spacer grid in the radial direction. It is known that Inconel will experience greater thermally induced expansion when compared to zircaloy. The preloading in the radial direction insures that any differential expansion will not result in the loosening of the mechanical joint between the sleeve and grid. In both embodiments of the invention the dimensions of window 22 are selected so as to establish a very close fit between the upper and lower edges thereof and the top and bottom edges of the web members 12 subsequent to expansion of the sleeves. As a consequence of this close fit, any differential expansion in the lateral direction will result in the tight fit of the spacer grid web members in the sleeves being enhanced. In the FIG. 5 embodiment, since sleeve 20' and guide tube 18 are comprised of the same material, the fabrication of a fuel assembly may additionally comprise the step of spot welding the expanded sleeves to the guide tubes as indicated at 28 in FIG. 5. While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.