Patent Number: 047568780
Section: summary

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.