Abstract:
A marked reduction in the pressure drop of cooling liquid through a grid spacer of a nuclear fuel assembly is attained by convexly contouring the upstream (usually the lower) edges of the grid members. Preferably, they are made streamlined or semicylindrical. This can be done by first beveling and then etching them, by directing a stream of mixture of abrasive and an organic polymer against them, or by traversing an electron or laser beam along them at a power and velocity such as to cause local melting. A lesser improvement is secured by beveling alone. A still further improvement can be obtained by also tapering the downstream (usually upper) edges.

Description:
INTRODUCTION AND BACKGROUND 
     This invention is directed to an improvement in spacer grids for nuclear reactor fuel assemblies. 
     Nuclear reactors for the production of power are, today, almost all provided with rod-type fuel elements containing uranium oxide pellets including fissionable isotopes, such as uranium-235 or uranium-233, and sometimes plutonium-239 oxide. These rods are in fuel assemblies, which comprise a large number of rods spaced from each other and held in position by suitable spacing means. 
     These spacing means ordinarily take the form of grids, such as are shown in U.S. Pat. No. 3,379,617, granted Apr. 23, 1968 to Harry N. Andrews and Herbert W. Keller. An improved form is shown in U.S. Pat. No. 4,077,843, granted Mar. 7, 1978 to John F. Patterson and Barney S. Flora. These grids may be formed of a single metal, such as stainless steel or one of the zircaloys, or, as shown in the Patterson and Flora patent, they may be made of two different metals. The common characteristic is that they are made up of a large number of thin metallic straps, or plates, which cross each other, usually at right angles, and contain springs which press against the fuel rods. The plates are positioned so that they present edges to the flow of coolant, which may be water or a liquid metal, such as molten sodium. 
     In the United States, the flow of liquid in power reactors is invariably vertical and in an upward direction. In other types of reactors used in other countries, the flow may be horizontal. The flow of coolant is at a high velocity, and the power required to pass it through the assemblies is considerable. An appreciable proportion of the resistance to flow in an assembly is caused by the spacer grids. 
     SUMMARY OF THE INVENTION 
     This invention relates to an improvement in the grids which materially reduces the resistance to flow and, therefore, the power required to drive the cooling fluid through the assembly. Despite the fact that the members making up the grids are only a few hundredths of an inch thick and have depths of about an inch, and that the meshes of the grids are occupied by rods, springs, etc., we have found that a marked reduction in the pressure drop through the grid is secured by convexly contouring the edges which are positioned upstream relative to the flow of the cooling fluid. In American power reactors, this will be at the bottom edges of those plates. 
     Most desirably, the edges are rounded to a streamlined, approximately semiellipsoidal form. This may be obtained in several ways. The edges may be coined or clipped to a rough approximation of that shape, and they then may be etched, which gives a rounded form. Another method is to direct a stream of a stiff mixture of abrasive and an organic polymer through the grid. This has been found to give the greatest reduction in resistance of any method. Another method is to direct an electron beam, focused to a narrow width, along the edges of the plates at sufficient power to produce local melting of the edges. This has been found to produce a nearly-perfect semicylindrical form which is also very desirable. It has the further advantage, when the plates are zircaloy, of improving their resistance to corrosion in a boiling water reactor environment. A laser beam can be substituted for the electron beam if desired. 
     An additional improvement can be obtained by tapering the downstream edges of the plates. A considerable improvement, though less than that attained by rounding, can be secured by beveling the upstream edges of the plates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
     FIG. 1 is an elevation of a typical nuclear reactor fuel assembly for pressurized water reactors. 
     FIG. 2 is a partial plan view of a grid spacer for the assembly shown in FIG. 1. 
     FIG. 3 is a partial perspective view of a grid spacer of the type shown in FIG. 2, shown with the bottom uppermost in the drawing. 
     FIGS. 4a and 4b are partial sections of a strip from a grid spacer such as is shown in FIG. 3, showing steps in one method of forming the grid member. 
     FIGS. 5a, 5b and 5c are partial sections, greatly enlarged, showing the results of other methods of forming grid members. 
     FIG. 6 is a partial perspective view of another form of grid spacer. 
     FIG. 7 is a broken view showing the structure and method of assembly of the grid spacer shown in FIG. 6. 
     FIGS. 8a and 8b are partial sections showing the formation of grid members for the grid spacer shown in FIGS. 6 and 7. 
    
    
     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&#39;. 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 &#34;phantom&#34; 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 &#34;flow-through&#34; 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&#39; and 34&#39;. It will be noted that the leading edge 32&#39; has assumed, essentially, a semi-cylindrical cross-section, while the end of tapered portion 34&#39; 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 &#34;X-Y table.&#34; 
     In FIGS. 6 and 7, we show a deflecting-type grid. This grid is disclosed and claimed in application Ser. No. 838,768, entitled &#34;Mixing Grid,&#34; 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&#39;,40&#39;, which may be welded together at their intersections. These plates are formed with matching channels 42,44, and 42&#39;,44&#39;, 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 &#34;Test Piece&#34; were as follows: 
     A. A 6×6 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 6×6 test spacer made from a 17×17 spacer for a pressurized water reactor fuel assembly. Rounding was by electron-beam melting. 
     C. A 5×5 test spacer made from a 9×9 spacer for a boiling water reactor fuel assembly. Rounding was by electron-beam melting. 
     
                       TABLE I______________________________________                    STRIP  ROD      ROD      THICK-  % REDUCTIONTEST   PITCH    DIAM.    NESS    IN SPACERPIECE  (IN.)    (IN.)    (IN.)   PRESSURE DROP______________________________________A      0.496    0.376    2 × 0.012                            10B      0.496    0.376    0.020   7C      0.572    0.424    0.020   8______________________________________ 
    
     EXAMPLE II 
     A full-sized 14×14 fuel assembly of the type used in pressurized water reactors (designated &#34;D&#34;) 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-  % REDUCTIONTEST   PITCH    DIAM.    NESS    IN SPACERPIECE  (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 6×6 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-  % REDUCTIONTEST   PITCH    DIAM.    NESS    IN SPACERPIECE  (IN.)    (IN.)    (IN.)   PRESSURE DROP______________________________________E      0.496    0.376    0.020   20______________________________________ 
    
     EXAMPLE IV 
     A full-size 15×15 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-  % REDUCTIONTEST   PITCH    DIAM.    NESS    IN SPACERPIECE  (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.