Patent Number: 047284889
Section: summary

The present nvention pertains to ESD (electrospark deposited) coated zirconium base alloy structural elements for use in water reactors. It especially relates to water displacer rods having an ESD coating to minimize wear. In pressurized water reactor internals design it is required that Zircaloy components come into contact with AISI 304 stainless steel and other non-zirconium base alloy components in an environment of flowing high temperature, high pressure borated and non-borated water. Under these conditions, vibrations may be produced in these contacting components causing repeated impacts and fretting wear to occur in the Zircaloy component, thereby reducing the in reactor lifetime of the Zircaloy component or causing complex and costly structural design changes to reduce the wear rate (if feasible). Water displacer rodlets are low neutron cross section rods having a tubular Zircaloy cladding containing ZrO.sub.2 pellets and helium. These rods may be moveably held above a fuel assembly in a water reactor and are lowered into guide thimbles within the fuel assemblies as needed during reactor operation to displace water (coolant). Typically 16 to 24 water displacer rods are held suspended above a fuel assembly by a spider vane. There may be 88 such vanes in a reactor. The tubular Zircaloy cladding is hermetically sealed at both ends and may typically have an outer diameter of about 0.914 inch and a length in excess of about 140 inches. It is now readily apparent that because of the slender structure of the water displacement rods, support is required along their length to minimize vibration due to the flowing coolant while also keeping them aligned with their respective thimble tubes in the fuel assembly. Examples of water displacer rods, spider vanes and their use are provided in Trevor A. Francis' U.S. patent application Ser. No. 595,154 filed Jan. 13, 1984, and Robert K. Gjertsen et al. U.S. patent application Ser. No. 570,551 filed Jan. 13, 1984 (both assigned to the assignee herein,the Westinghouse Electric Corporation. These patent applications are hereby incorporated by reference. One area where we have found wear rates to be particularly significant is where water displacer rodlets (i.e. rods) come into contact with AISI 304 stainless (304SS) steel guide supports. We have observed, in fretting wear tests, that while the primary purpose of the 304SS guide supports is to support and protect the rods from excessive wear, the Zircaloy tubular member forming the rod is susceptible to wear due to the relatively poor wear characteristics of the Zircaloy on the 304SS in the pressurized water reactor environment. ESD coatings are being evaluated as a means to reduce wear between rubbing ferrous base articles in elevated temperature liquid sodium environments. In this regard the reader is referred to: Roger N. Johnson, U.S. patent application Ser. No. 703,856 filed on Feb. 21, 1985; and Gary L. Sheldon, U.S. Pat. No. 4,405,851. The foregoing documents are hereby incorporated by reference. We propose that the foregoing water reactor wear problems can be minimized by metallurgically bonding as ESD coating to the zirconium base alloy member in the area of contact with the non-zirconium base alloy member. Preferably the zirconium base alloy member is Zircaloy-2 or 4 and the ESD coating is a Cr.sub.2 C.sub.3 coating of about 1-2 mils in thickness. Preferably that non-zirconium base alloy component is also coated in the area of contact with an ESD coating.