Patent Number: 053012119
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1 and 2 are schematic representations of the use of the so-called Linear Magnatron Sputtering technique, available from Surface Solutions, Inc., as adapted for implementing the present invention. In the preferred embodiment, a nuclear fuel assembly component such as fuel rod cladding or a control rod guide tube 10, made from zircaloy, is coated substantially uniformly along the entire inside surface, over substantially the full length of the tube. For illustrative purposes, the aspect ratio of the component tube 10 as shown in the Figures, has been substantially shortened relative to the typical tube dimensions of less than 0.5 inch ID and a length in excess of 12 ft. The component tube 10 is fixtured such that first and second end plugs 12, 14 seal the tube interior. One end plug 14 has a vacuum port 16 through which a vacuum pump 18 can evacuate the content of the tube. The working gas port 28 may be situated in either end plug. Each end plug includes means, such as first and second mounts 20,22, for supporting a source tube 100 of wear or corrosion resistant material having a smaller outer diameter than the inner diameter of the component tube, coaxially within the component tube, thereby defining a cylindrical annulus 24 between the tubes. After evacuation, an inert working gas such as argon from source 26 is backfilled in the annulus 24 to a pressure sufficient to sustain a plasma discharge. The source tube 100 in the normal implementation of the present invention, would be a homogeneous tube of the material which is desired to be coated on the inside surface, or substrate 30, of the cladding tube 10. In the preferred embodiment, a chemical reaction occurs in the space 24 between the source material from tube 100 and a reactive gas, in a manner to be described more fully below, whereby an oxide, nitride, or carbide is coated on the substrate. The source tube 100 can be at least the same length as the cladding tube 10. A power supply 32 with negative lead 34 is connected through the connector mount or otherwise, to the source tube 100, and the positive lead 36 is connected to the substrate 30, such that the source tube serves as a cathode and the substrate serves as an anode. A plasma 37 consisting of positive argon ions and electrons is established in the annular space 24, with the positive ions 38 bombarding the cathode 100 with sufficient energy to vaporize surface atoms from the source tube 100 onto the substrate 30. Because the source material is passed into the vapor phase 40 by a mechanical process (momentum exchange) rather than a chemical or thermal process, virtually any material is a candidate for coating. Thus metals, glasses and other ceramics having desirable wear and corrosion-resistant properties can be utilized. The plasma in the annular space can be established by any known means, such as direct current discharge when sputtering metals, but in order to improve the efficiency, magnatron techniques are applied to confine and shape the plasma. In the Linear Magnatron Sputtering technique available from Surface Solutions, Inc. a circumferential magnetic field 42 is established around the source tube 100 by running high currents from current source 44 axially through a copper tube 46 centered within the source tube and cooled by coolant 50, as shown in FIG. 2. This achieves a uniform rate of material evaporation along the length of the source tube 100, because of a constant plasma thickness that is uniformly excited over the whole length of the source tube. This results from the plasma drift current in the plasma surrounding the source tube, running in a direction parallel to the axis of the source tube 100. This is far superior to cylindrical post sputtering schemes, which require that the drift current run in a closed loop. In the Linear Magnatron Sputtering System of the present invention, the drift current may be boosted with an enhancer device shown generally at 48, to a very high level at one end of the source tube. It should thus be appreciate that to the extent a plasma of constant thickness can be maintained in uniform excitation over the full length of the source tube 100, the radially projecting source atoms 40 will coat the full surface 30 of the cladding tube substantially uniformly. Under many circumstances, satisfactory results are obtained if at least about ten feet along a twelve foot tube, is coated. It should also be appreciated that in the embodiment described in FIG. 1, the component tube 10 itself serves as the boundary of the evacuation chamber 24. An alternative embodiment would encapsulate the tube 10 within an outer vacuum chamber (not shown), in which case the fixturing and end plug arrangements would have analogous counterparts in the walls of the vacuum chamber. Once initiated, the flux of sputtered material leaving the source tube 100 will continue until a substantially uniform coating of, for example, a thickness of 0.0002 inch is achieved, whereupon the process is stopped, the end plugs removed, and the next tube fixtured for restarting the coating process. FIG. 3 shows an alternative embodiment of the process, and the coated tube resulting from such processes. In FIG. 3, the cladding or guide tube 200 has a non uniform but prescribed distribution of coating material, as a result of a commensurate non uniform, but prescribed distribution of material on source tube 300. For example, if component 200 is a control rod guide tube, the upper or left portion as shown in FIG. 3a can be coated at 210 over only a limited axial distance, e.g., less than one half the guide tube length, extending to just below the "withdrawn" portion of the control rod tips in the nuclear reactor. The source tube 300 has a correspondingly limited region of source material 310. Thus, the coating can be limited to partial lengths within the rod by shortening the length of source tube. As mentioned above, the process according to the present invention is amenable to the sputter coating of a wide variety of potentially useful materials. Some of these materials cannot readily be sputtered from a homogeneous source tube, but rather can be formed either chemically in the inert gas via chemical reaction, or at the internal surface of the component tube. The coatings are thus applied by analogy to reactive chemical deposition processes. A source 52 of reaction gas including nitrogen, oxygen, or carbon and associated plug port 54 are shown in FIG. 1 for this purpose. Table 1 lists a variety of metals and metallic compounds that can be sputtered in accordance with the present invention: TABLE 1 ______________________________________ Metals and Metallic Compounds ______________________________________ ZrN TiN CrN HfN TiAlVN TaN ______________________________________ Table 2 is a representative list of ceramic materials including glasses that are usable with the present invention: TABLE 2 ______________________________________ Ceramics and Glasses ______________________________________ Zr.sub.2 O.sub.3 Al.sub.2 O.sub.3 TiCN TiC CrC ZrC WC Calcium Magnesium aluminosilicate Sodium Borosilicate Calcium Zinc borate ______________________________________