Patent Number: 053501616
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in detail wherein like numerals represent the same or like parts throughout, and referring particularly to FIGS. 1 and 2, nuclear fuel assembly zircaloy grid strip 10 has a linear series of springs 12 which are cut from sheet 14 and extend outwardly in alternating opposite directions from sheet 14. Springs 12 have an essentially uniform thickness of about 0.018 inches. Each spring 12 has first end 20 which is connected to sheet 14, and opposite second end 22, which is free. As shown in FIGS. 1 and 2, each spring 12 has first curve 24 formed near the first end 20 of spring 12, i.e., near the base of the spring, and second curve 26 formed near second end 22, the first and second curves being in opposite directions. First curve 24 has convex side 30, and opposite concave side 32 which defines a radius of curvature R, shown in FIG. 3. Convex side 30 has cold worked layer 40 having a length extending along the entire length of first end 20 of spring 12, and having a thickness of approximately 0.009 inches, i.e., about half the thickness of spring 12. The width of cold worked layer 40 preferably is sufficient to include all of convex surface 30, and more preferably includes part of the straight or unbent portions of spring 12 and sheet 14 on either side of convex surface 30. Concave surface 32 is not cold worked according to the preferred embodiment of the invention. Fuel assembly grids (not shown) are formed by interlocking a plurality of grid strips 10 in a conventional way via slots 50, and fastening grid strips 10 using weld tabs 52. Fuel rods (not shown) are mounted in the grids in a conventional manner. The preferred method of making grid strip 10 is by additionally treating a grid strip which has been conventionally fabricated, with the exception that, before treatment, springs 12 do not extend outwardly as far as the springs in a conventional strip. Untreated strip 10 is masked completely except for the convex surface 30 at the base of each spring 12, and a relatively small portion of the spring and/or sheet proximate convex surface 30 on the convex side of the spring. Strip 10 is then shot peened at appropriate conditions of shot velocity, shot diameter and time in order to cold work the material to a depth of approximately half its thickness. The shot peening causes the curvature of first curve 24 of each spring 12 to increase, so that springs 12 protrude further from the sheet. This change in curvature verifies that the intensity and coverage of the shot peening is appropriate. The mask is removed, and the grid strip is ready to be used. Suitable process conditions for shot peening a particular type of grid spring can be determined by a conventional technique, e.g., by obtaining an Almen strip which has the same thickness as the grid spring, shot peening the Almen strip at known process parameters, and then cutting the strip to determine the thickness of the cold worked portion. This procedure can be repeated on other Almen strips using different process conditions until a strip is obtained which has a cold worked layer of the appropriate thickness. The process conditions which resulted in the cold worked layer of the desired thickness are then used to shot peen the grid springs. If selectively cold worked grid strip 10 is placed in a nuclear reactor and is subjected to a neutron flux in an unloaded state, the degree of curvature of springs 12 increases (the radius of curvature of the inner side of the curve decreases), because the metal on the convex side of the curve grows faster than the metal on the concave side. The increase will begin immediately, but will be very gradual. Alternatively, if springs 12 of strip 10 are loaded with fuel rods and the reactor is operated, springs 12 retain their preload to a greater extent than untreated, but otherwise comparable springs. The improvement in preload retention is obtained because the differential growth of the spring at least partially compensates for the reduction in spring resiliency which results from radiation exposure. The difference between the reduction in spring force of a conventional spring with time, and the reduction in spring force of the selectively cold worked spring of the present invention with time, is shown schematically in FIGS. 4(a) and 4(b). As illustrated in the Figures, even when the initial load on the selectively cold worked spring is lower than the initial load on the conventional spring, the final load is higher for the selectively cold worked spring than for the conventional spring. When the load of the conventional spring has gone to zero, i.e., after about one reactor cycle, the cold worked spring continues to have a positive load. It will be appreciated from the foregoing description that a novel and improved nuclear fuel assembly grid spring has been disclosed which has significant advantages over conventional springs. It also will be appreciated that the scope of the invention is intended to include a variety of embodiments of springs and other metal components that are not specifically disclosed. The exact pattern of varied material characteristics, and the techniques used to obtain this variation will depend upon the desired type and degree of differential growth.