Patent Number: 047284889
Section: description

DETAILED DESCRIPTION OF THE INVENTION Shown in FIG. 1 is a water displacement rodlet 1 which is composed of an elongate tubular zirconium base alloy (preferably Zircaloy-2 or 4) or a Zirconium-niobium alloy member 2 hermetically sealed at both ends and containing ZrO.sub.2 pellets and helium. The rodlet is one of a number of rodlets which is connected to a spider assembly 3. The spider assembly 3 has a drive line 5 for moving the rodlets into and out of the thimble tubes of a nuclear fuel assembly. Rodlet 1 is connected to the spider assembly by a rodlet flexure member 7 located at the end of one of the spider vanes 9. Distributed along the length of the rodlet 1 are plates 11 which exend transversely with respect to the longitudinal axis of the rodlet 1. Preferably there are six such support plates 11. Each support plate 11 is perforated through its thickness by holes 13 which are connected to each other by slots 15. Slots 15 and holes 13 are configured and sized to provide support to the rodlets while allowing the rodlets 1 and the spider assembly 3 to readily travel up and down through the support plates 11 without binding. The support plates are composed of a non-zirconium base alloy, preferably a stainless steel such as AISI 304 stainless. The rodlet flexure member 7 provides flexibility to prevent jamming of adjacent rodlets in the support plates. These flexures then permit small gaps to further reduce wear. As shown in FIG. 1 the rodlet in accordance with the present invention has been coated with a wear resistant ESD coating on the outside diameter surface of the zirconium base alloy tubular member 2 in those areas of the tubular member in the vicinity of the holes in support plates 11. This is more clearly shown in FIG. 2 which is an enlarged view of the zirconium base alloy tubular member 2 in the vicinity of a support plate 11. It is preferred, as shown in FIG. 2, that the length of tube, L, having tube ESD coating 16 thereon at any one location should exceed the thickness, T, of the support plate 11, and more preferably, the coated portion of the tube should extend below plane of the bottom face 17 and above the plane of the top face 19 of support plate 11. Most preferably the coated length, L, is at least about three times the thickness, T, of plate 11. As also shown in FIG. 2, it is preferred that surface wall 21 of hole 13 which communicates with top surface 19 and bottom surface 17 of the support plate 11, also have an ESD coating 23 on it. ESD coating 23 may be selected from those described in the aforementioned Johnson patent application relating to ESD coatings. Alternatively, an improved ferrous alloy for wear may be substituted for the 304SS support 11 eliminating the need for ESD coating 23. A transverse cross section through the tubular member 2 and the support plate 11 is shown in FIG. 3. The tubular member is preferably Zircaloy-2 or 4 in a fully recrystallized, partially recrystallized or cold worked and stress relief annealed condition. Metallurgically bonded to the outer surface of tube 2 is a relatively thin ESD coating 16. The Zircaloy tube may have an outside diameter of about 0.91 inches and a wall thickness of about 0.055 inches while the ESD coating has an average thickness between about 0.001 and 0.002 inches. At and near the metallurgical bond 25 between the coating 16 and the tube 2 is a diffusion zone and heat affected zone that may extend a few thousands of an inch into the wall of the tube 2, but leaves the bulk of the wall thickness in its original metallurgical condition. The ESD coating 16 is preferably obtained from a cemented Cr.sub.2 C.sub.3 electrode as described in the aforementioned Johnson patent application. The ESD coating 23 on the wall 21 of the aperture 13 in support plate 11 also has a thickness of about 0.001 to 0.002 inches, but does not necessarily have the same composition as that found in coating 16 on tube 2. In addition to differences in coating composition caused by the diffusion of base metal into the coating (e.g. Zr in the case of tube 2 and Fe in the case of plate 11), the coating composition itself may differ in order to optimize the resistance of the wear couple to fretting wear under water reactor operating conditions. For example the coating 23 may be selected from those ESD coatings in accordancw with the invention described in the aforementioned Johnson patent application. While FIGS. 2 and 3 indicate that the surface of wall 21 of hole 13 has been entirely coated, it is also contemplated that wall 21 may remain uncoated or coated in only certain areas, for example in the areas of wall 21 wich is in close proximity to the juncture 27 of wall 21 and top face 19 and the juncture 29 of wall 21 and bottom face 17. The preceding examples have clearly demonstrated the benefits obtainable through the practice of the present invention. Other embodiments of the invention will become more apparent to those skilled in the art from a consideration of the specification or actual practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.