Patent Number: 053435087
Section: description

DETAIL DESCRIPTION Referring now to the drawings there is shown in FIG. 1 a nuclear reactor fuel assembly 10 comprising an array of fuel rods 12 held in spaced relationship with each other by fuel rod spacer grids 14 and 16 spaced along the fuel assembly length. The grid assembly includes upper spacer grids 14 and a lower spacer grid 16. Each fuel rod 12 comprises a hermetically sealed elongated tube, known in the art as the cladding, which contains a fissionable fuel material, such as uranium, in the form of pellets. The individual fuel rods 12 are supported in the fuel assembly by means of the spacer grids 14 and 16, such that an upwardly flowing liquid coolant may pass along the fuel rods and thereby prevent overheating and possible melting of the cladding. In a manner well-known in the art, the coolant, after passing through the reactor core and being heated through contact with the fuel rods, will be delivered to a heat exchanger and the heat extracted from the circulating coolant will be employed to generate steam for driving a turbine. The fuel assembly 10 also includes an array of guide tubes 15, having control rods 30 adapted for slidable longitudinal movement therein, which are positioned to extend axially through selected cells in spacer grids 14 and 16. The control rods 30 serve as means for regulating the thermal output power of the reactor. Each spacer grid 14 and 16 includes straps made of a material such as Inconel or other material, interwoven to form two separate grid sections of egg-crate configurations. The openings formed are virtually aligned to form cells, and are of a sufficient size to receive fuel rods 12 or guide tubes 15. Guide tubes 15 are attached to a top nozzle 40 and a bottom nozzle 45 by means of screws or some other affixing means and, along with the spacer grids 14 and 16, form the fuel assembly skeleton structure. The spacer grids 14 are attached to the guide tubes 15 by conventional means such as spot welding. The bottom spacer grid 16 is attached to the guide tubes 15 by a retaining means which is preferably retainer 20. The retainer 20 will be described in more detail with reference to FIGS. 2a, 2b, 2c, 2d, 3, 4a and 4b. FIGS. 2a and 2b show two side views of retainer 20 rotated ninety degrees from each other. The retainer 20 is preferably made by stamping or forming an Inconel or stainless steel tube, but it is recognized that other materials and processes could be used. The retainer 20 includes an upper end 21 for receiving the lower end 17 of the bottom spacer grid 16 and a lower end 29 for receiving the lower end of the guide tubes 15. The upper end 21 of retainer 20 has four leg portions 22 which are fastened to the lower end 17 of the bottom spacer grid 16, preferably by such means as spot welding as shown at 50 in FIG. 4a. FIG. 2d shows a cross-sectional view of the retainer 20 in FIG. 2a. The lower portion 29 of the retainer 20 has an enlarged section 28, a flattened section 27 and two apertures 26. FIG. 2c illustrates the cross-sectional view of the lower portion 29 taken along line A--A. As shown in FIG. 3, the lower end of each guide tube 15 is machined and welded to an end plug 24 to mate with the lower portion 29 of the retainer 20. The end plug 24 of each guide tube 15 has a shoulder portion 13 for mating with the retainer 20. The apertures 26 receive the shoulder 13 of end plug 24. The flattened sectioned 27 mates with the end plug 24 and the enlarged portion 28 is captured between a top surface 46 of the bottom nozzle 45 and the shoulder 13 of end plug 24 so as to axially constrain movement of the retainer 20. Assembly of the skeleton structure will now be described. The legs 22 of the retainer 20 are welded to selected lower end sections 17 of the bottom spacer grid 16 such that each spacer grid cell which will later contain a guide tube 15 is attached to a retainer 20. FIG. 5 illustrates the bottom spacer grid 16 with attached retainers 20. The grid 16 is then clamped during assembly of the skeleton structure. The guide tubes 15 are attached to the top nozzle 40 in the conventional manner, or, alternatively, by means of the retainers 20 of the present invention. The guide tubes 15 pass through the selected sections of lower end 17 of spacer grid 16 and through the retainers 20. The shoulders 13 of the end plugs 24 protrude from the apertures 26 of the retainer 20. End plugs 24 are then attached to the bottom nozzle 45 by means such as attachment screws 18. The lower section 29 of the retainer 20 is thereby captured between the surface 46 of bottom nozzle 45 and shoulder 13 of end plug 24 and axial movement is thereby constrained. Since the grid 16 is clamped during skeleton fabrication, and each retainer 20 is welded to the spacer grid 16, the guide tubes 15 are restrained from rotating during torquing of the attachment screws which connect the bottom nozzle 45 to the guide tubes 15. The twisting of the skeleton is thereby minimized and the integrity of the skeleton increased while maintaining a strong and secure connection between the spacer grid 17, guide tubes 15 and bottom nozzles 45. Further, the retainers of the present invention are economical to produce and thereby provide a low cost means to increase the integrity of the skeleton structure.