Patent Number: 047568780
Section: description

DETAILED DESCRIPTION Referring to FIG. 1, the fuel assembly includes an upper tie plate 2, a lower tie plate 4, and numerous fuel rods 6 extending between the tie plates. There are also a number of guide tubes 8 for control rods, which bind the upper and lower tie plate together. There is also a top-end assembly 12 which may include hold-down springs 14, which maintain the assembly in its proper position in the reactor. Cooling water flows upwardly through the assembly, as indicated by arrow 16. Spaced at intervals along the heighth of the assembly are a number grid spacers 18. A partial plan view of such a grid spacer is shown in FIG. 2. This spacer is of the type described and claimed in U.S. Pat. No. 4,077,843, granted Mar. 7, 1978 to John F. Patterson and Barney S. Flora, and assigned to the assignee of the present application. This patent may be consulted for a detailed description and is expressly incorporated by reference in this disclosure. FIG. 3 shows a partial perspective of this grid spacer with the bottom shown uppermost, the flow of water being indicated by the arrows 20,20'. Referring to FIGS. 2 and 3, the grid spacer includes a perimeter strip 22, which is in the form of a square encircling the entire member. There are also a plurality of grid members 24 and a second set of grid members 26 arranged at right angles to members 24. These two sets of grid members define grid apertures through which the fuel rods, some of which are shown in "phantom" lines, extend. Mounted on certain grid members are spring strips 28 (FIG. 3) which carry springs 30, one of which extends into each of the apertures formed by the grid members. The grid members 24,26 are deformed to produce dimples 31, which are of the "flow-through" type, i.e., open at their tops and bottoms. The bottom or leading edges of grid members 24,26 are convexly contoured, while the top or trailing edges (shown at the bottom of FIG. 3) are preferably tapered. FIGS. 4a and 4b are illustrations of this. During manufacture of the strips, the edges which are to be the leading edges may be coined as shown at 32 in FIG. 4a, while the edges which are to be trailing edges may be coined to a more acute angle, as shown at 34 in the same Figure. If used in this form, a reduction in flow resistance will be attained. However, improved results are secured by etching each of the edges in acid. This produces, very nearly, the shapes shown in FIG. 4b at 32' and 34'. It will be noted that the leading edge 32' has assumed, essentially, a semi-cylindrical cross-section, while the end of tapered portion 34' has been rounded. Another method of rounding the leading edges is illustrated by FIGS. 3, 5a, and 5b. In this method, after the grid spacer has been assembled, an electron beam 36 (FIG. 3) is focused on the edges and traversed therealong. The power of the beam and the rate of travel are correlated to produce a local melting. The surface tension of the molten metal draws it into, very nearly, a perfect semicylindrical shape. FIGS. 5a and 5b reproduce greatly enlarged photographs of cross-sections of such a strip after treatment by the electron beam. The shape shown in FIG. 5a was produced by a beam of 0.5 milliampere moving at a speed of 20 inches per minute along the strip of zircaloy-4 which was about 0.02 inch thick. The shape shown in FIG. 5b was the result of using a beam of 0.6 milliampere on the same type of strip at the same velocity. A suitably-powered laser beam could be substituted for the electron beam with the same results. The beam can be traversed relative to the grid members by the use of a conventional "X-Y table." In FIGS. 6 and 7, we show a deflecting-type grid. This grid is disclosed and claimed in application Ser. No. 838,768, entitled "Mixing Grid," filed on Mar. 12, 1986 by John F. Patterson, et al., and assigned to the assignee of this application. That application is expressly incorporated herein by reference. In this type of grid, the grid members are made up of pairs of strips 38,40 and 38',40', which may be welded together at their intersections. These plates are formed with matching channels 42,44, and 42',44', which are curved to deflect the cooling fluid as more fully-described in the above cited application. During their manufacture, these strips may be sheared at an angle, as shown in FIG. 8a. When placed together, the composite strip will assume the form shown in FIG. 8b, approximating that of FIG. 4a. If desired, the assembled strip may be etched to approximate the form shown in FIG. 4b. However, the form of grid spacer shown in FIG. 6 has, inherently, a very low resistance, and the etching step may not be worthwhile. Still another method of producing the rounded edge or edges is to direct a stream of a thick mixture of abrasive and an organic polymer through a grid, the strips preferably having the form shown in FIG. 4a or FIG. 8b. If the stream is surged back and forth, both edges of the strips will be rounded to the form shown in FIG. 5c. This is a convenient way of producing a form of strip, approximately that shown in FIG. 4b. FIG. 5c, which is based on a photograph, show on a greatly enlarged scale, the cross-sectional shape of the edges produced by this method. EXPERIMENTAL EXAMPLES EXAMPLE I In the following tests, grid spacers were incorporated in test fuel assemblies, including fuel rods of standard size and spacing (pitch) and the pressure drop across a spacer was measured at a water velocity typical of that present in operating nuclear reactors. The spacer was then removed from the assembly, the lower edges of all grid members rounded, and then replaced in the assembly. The pressure drop was again measured, and the reduction in pressure drop produced by rounding was determined. Results are shown in Table I, where in the types of spacer designated under "Test Piece" were as follows: A. A 6.times.6 intermediate grid of the type shown in FIGS. 1, 1a, and 1b of application Ser. No. 838,768, having double strips. The edges were rounded by hand grinding. B. A 6.times.6 test spacer made from a 17.times.17 spacer for a pressurized water reactor fuel assembly. Rounding was by electron-beam melting. C. A 5.times.5 test spacer made from a 9.times.9 spacer for a boiling water reactor fuel assembly. Rounding was by electron-beam melting. TABLE I ______________________________________ STRIP ROD ROD THICK- % REDUCTION TEST PITCH DIAM. NESS IN SPACER PIECE (IN.) (IN.) (IN.) PRESSURE DROP ______________________________________ A 0.496 0.376 2 .times. 0.012 10 B 0.496 0.376 0.020 7 C 0.572 0.424 0.020 8 ______________________________________ EXAMPLE II A full-sized 14.times.14 fuel assembly of the type used in pressurized water reactors (designated "D") was utilized for test purposes. Some of the grid strips were 0.026 inches and some 0.020 inches thick. Some of the spacers were of standard form and some had the lower edges of the grid strips rounded by electronbeam melting. The pressure drop, under rates of water flow typical of reactor operating conditions, was measured across various spacers, and the average reduction obtained by rounding was determined. Results are shown in Table II. TABLE II ______________________________________ STRIP ROD ROD THICK- % REDUCTION TEST PITCH DIAM. NESS IN SPACER PIECE (IN.) (IN.) (IN.) PRESSURE DROP ______________________________________ D 0.550 0.417 0.026/0.020 6.5 ______________________________________ The following experiments involved the use of a stiff but flowable abrasive composition comprising a viscous carrier laden with abrasive granules, such as is described in U.S. Pat. No. 3,909,217, granted Sept. 30, 1975, to Kenneth E. Perry. The mixture was of doughlike consistency and contained 16 mesh and 36 mesh grains of silicon carbide. It approximated the composition of Example 3 of the above patent. EXAMPLE III A 6.times.6 test spacer of the type shown in FIGS. 2 and 3 having an overall height of three inches was made up, and its pressure drop determined in the same manner as described in Example I above. An identical test spacer (designated Test Piece E) was abraded by pushing the composition described in the preceeding paragraph back and forth through it at 100 pounds per square inch for 150 cycles over a period of 80 minutes, in directions perpendicular to the plane defined by the lower edges of the grid members. After cleaning, it was found, on inspection, that the top and bottom edges of grid members 24,26, dimples 31, and to a lesser degree, springs 30, received the form shown in 5c. Results, tabulated in the same manner as in the previous examples, are shown in Table III. TABLE III ______________________________________ STRIP ROD ROD THICK- % REDUCTION TEST PITCH DIAM. NESS IN SPACER PIECE (IN.) (IN.) (IN.) PRESSURE DROP ______________________________________ E 0.496 0.376 0.020 20 ______________________________________ EXAMPLE IV A full-size 15.times.15 spacer of the type shown in FIGS. 2 and 3 was abraded in the manner described in Example III. Since the spacers are subjected to a force on the order of 5,000 pounds in each direction during processing, it is important to incorporate support across the spacer span. This spacer was processed for 145 cycles at 100 psi over a period of 196 minutes. This grid (designated Test Piece F) was mounted with other spacers in a test assembly described in Example II and the pressure drop across it determined. Results are shown in Table IV. TABLE IV ______________________________________ STRIP ROD ROD THICK- % REDUCTION TEST PITCH DIAM. NESS IN SPACER PIECE (IN.) (IN.) (IN.) PRESSURE DROP ______________________________________ F 0.550 0.417 0.018 22.5* ______________________________________ *Average of two runs. While we have described several embodiments of our invention in considerable detail, it will be apparent to those skilled in the art that various changes can be made. We therefore wish our invention to be limited solely by the scope of the appended claims.