Patent Number: 053012119
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to nuclear fuel assemblies, and more particularly, to applying wear and/or corrosion-resistant coatings on fuel assembly components. It is well known among nuclear fuel engineers, that corrosion and wear can occur in a variety of locations on a nuclear fuel assembly. These locations include the inside surface of nuclear fuel rods (i.e., the fuel cladding ID surface), and the inside surface of control rod guide tubes. Attempts to reduce corrosion and wear within fuel rod tube cladding, are represented by U.S. Pat. No. 4,541,984 (Palmer), which discloses a getter-lubricant coating, preferably with a burnable poison material. The lubricant reduces stresses on the cladding resulting from the expansion or misalignment of fuel pellets acting against the inside clad surface. Furthermore, the getter material absorbs hydrogen atoms which would otherwise contribute to the initiation and propagation of corrosion, particularly at the areas of high stress within the cladding. U.S. Pat. Nos. 4,376,092; 4,135,972; 4,284,475; 4,333,798 all disclose a sleeve of one form or another secured within a control rod guide tube, to protect the guide tube from excessive fretting wear. This wear results from vibratory movement of the control rod leading end, which remains in the upper portion of the guide tube even when the control rod is fully withdrawn from the reactor core. Typically, these wear sleeves are coaxially placed within the guide tubes in the upper end (top) of the fuel assembly. To accommodate these sleeves the guide tube is bulged to a larger diameter and the top grid likewise requires additional operations to open its guide tube cells to a larger size. A number of techniques have been disclosed in the prior art, for coating the outside of fuel assembly components, for corrosion and/or wear resistance. U.S. Pat. Nos. 4,724,016 (Anthony) and 5,026,517 (Menken et al), are representative of these techniques. Such techniques are not readily adaptable for coating the inside surfaces of fuel assembly tubular components, which are generally at least 10 feet long, and have IDs which are often less than 0.5 inch. SUMMARY OF THE INVENTION The present invention is directed to an improved nuclear fuel assembly tubular component with a wear or corrosion-resistant internal coating that can maintain its integrity during operation. The coating can be metal or ceramic that is sputtered onto to the inside surface of the tube. The coating is applied so that it bonds to the zircaloy surface of the tube and is chosen to have a thermal expansion co-efficient similar to the zircaloy tubing to enhance adhesion to the surface. Although sputtering processes for applying coatings have been in general use for many years, sputtering has not previously been considered suitable for applying coatings to the internal surfaces of tubular components of nuclear fuel assemblies. This may be due in part to the very small inside diameter of the tubes, and the difficult task of adapting sputtering processes to depositing coatings on the inside of tubular members. These problems have been overcome in accordance with the present invention, by the use of a magnatron enhanced plasma sputtering technique, in which a uniform coating can be applied on the inside tube surface, as a result of establishing a circumferential magnetic field to confine and shape the plasma. The present invention is also directed to a method for sputter coating the inside surface of a fuel assembly tubular component with wear or corrosion resistant material. The steps include supporting the component tube in a fixture and supporting a source tube of e.g., wear resistant material, coaxially within the component tube, thereby defining a cylindrical annulus between the tubes. The annular space is evacuated and backfilled with an inert working gas such as argon, to a pressure sufficient to sustain a plasma discharge. The component tube is positively biased as an anode, and the source tube is negatively biased as a cathode, such that a plasma of the working gas is established in the annular space. A circumferential magnetic field is generated around the source tube to confine and shape the plasma whereby the source tube is bombarded with ions from the plasma substantially uniformly over the length of the source tube. Wear resistant material is thereby sputtered substantially uniformly from the source tube onto the inside surface of the component tube, to form a coating thereon. In the preferred embodiment, the invention is directed to a method for coating the inside surface of a control rod guide tube with a wear resistant material, by the so-called Linear Magnatron Sputtering process. In this process, available from Surface Solutions, Inc., Boulder, Colo., the plasma between the source tube and the substrate (e.g., guide tube) is confined and shaped by a circumferential magnetic field around the source tube. A major advantage of this technique, is that it allows uniform high-rate sputtering radially outward from the surface of drawn, extruded, or cast tubing stock used as the source material. The circumferential magnetic field is generated by running high currents axially through a cooled copper tube centered within the source tube. This plasma shaping effects a constant plasma thickness that is uniformly excited over the whole length of the source tube. This is achieved because there is a plasma drift current, running in the plasma surrounding the source tube, parallel to the axis of the source tube. Other magnatron sputtering systems require that the drift current run in a closed loop. External magnets are not required for establishing the magnetic shaping field. In the Linear Magnatron Sputtering System, the drift current may be boosted to a very high level at an initiating end, allowing the use of relatively simple tubular source cathodes. In addition to achieving higher sputtering rates, the sputtered material can also arrive at the substrate surface having higher impingement energy. Both the sputtering rate and the energy of the impingement can be independently controlled. Since the inside surfaces of the tubes can be coated in a static mode, the requirement for either moving or rotating the substrate during processing is largely reduced. This enables the use of a much simpler vacuum system, with longer processing periods between maintenance, less time required for maintenance activities, and fewer non-scheduled maintenance interrupts. The process is also similar for sputtering a wear resistant material on the zircaloy fuel rod cladding. The interaction of fuel pellet holddown springs against the fuel cladding causes the wearing away of the cladding on the surface of the fuel rod. This condition can be eliminated by the coating one such material, ZrN on the inside of fuel rod in the area where the pellet holddown spring can interact with the cladding.