Patent Number: 052271293
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to the drawings wherein like numerals represent like parts throughout the figures, a fuel assembly for which the present invention has application is generally designated by the numeral 10. Fuel assembly 10 is employed in a nuclear reactor and includes a plurality of fuel rods 20 which are mounted to a lower support grid 30. The fuel rods have a cladding tube 40 which contains fissionable fuel pellets 50. The cladding tube 40 is manufactured from a zirconium-alloy or other suitable alloy. The invention will be primarily described with reference to a zirconium-alloy cladding tube. In accordance with the invention, the cladding tubes have a coating 60 on at least a portion of the outside surface. The coating 60 is a thin film comprised substantially of zirconium nitride. The film may have a thickness on the order of approximately 5 microns. The relative dimension of the coating 60 is exaggerated in FIG. 2. The thin film of zirconium nitride is wear resistant, and constitutes a barrier which resists corrosion of the tube substrate. Because of the relatively thin film thickness, e.g., approximately 5 microns, the outer diameters of the coated cladding tubes or fuel rods do not significantly reduce the coolant pressure drop through the fuel assembly 10. While the thin film of zirconium nitride may be applied to the zirconium-alloy cladding tube along substantially the entire length of the tube, the thin film is especially advantageous in the region 42 below the support grid where the tube is particularly susceptible to debris fretting due to the metallic particles and the high pressures and high temperatures of the surrounding water. In addition, the zirconium nitride coating may be applied in the region 44 of the cladding tube which is engaged by the lower support grid 30 to enhance the corrosion and wear resistance of the tube. The zirconium nitride coating 60 is reactively deposited on the zirconium-alloy cladding tube 40 by means of a cathodic ion plating process in apparatus 70 such as, for example, a Vac-Tec ATC 400 ARC Coating System. The zirconium source material is evaporated in a vaccum chamber 72 by arc source 74. A zirconium bar functions as the cathode 76 in the arc circuit. The zirconium arc spots typically are 1 to 3 microns in diameter and have current densities as high as 10 amp per square micron. The high current density causes flash evaporation of the zirconium material. The cladding tube 40 is the anode of the arc circuit. A fixture assembly 78 which is capable of positioning and rotating the tube 40 is employed to ensure a uniform coating on the tube substrate. Depositions of the coating were performed with tube substrate temperatures in the range of 300.degree. to 400.degree. C. in the presence of nitrogen at a total chamber pressure of 0.26 to 1.3 Pa. Typical deposition rates achieved were 200 to 300 nm/min. The thickness of the zirconium nitride film was approximately 3.0 to 7.0 microns. An established negative characteristic of the cathodic arc process is that macro-particles emitted from the source material may be directly incorporated into the growing film. The macro-particles are composed primarily of unreacted material. An analysis of samples of zirconium nitride coating 60 deposited on a sample substrate revealed that the concentrations and the sizes of the zirconium macro-particles were not significant. For example, significantly lower concentration and smaller sizes of macro-particles were present for a thin coating of zirconium nitride in comparison to an analogous coating of titanium nitride. The relatively favorable insubstantial concentration and size of the zirconium macro-particles may result from the relatively high melting point of zirconium, the low vapor pressure of zirconium and the higher arc velocity on the target surface. If the mean resident times of the cathodic arc spots are relatively small, individual vaporization events on the surface circuit may not provide sufficient time to achieve sufficient localized heating to melt the zirconium source material and thus result in emission of zirconium in the form of macro-droplets. In one example, for a zirconium nitride coating 60 of 5 microns, the microhardness of the zirconium nitride film was measured. The hardness measurements are summarized in Table 1. TABLE 1 ______________________________________ Load Microhardness (g) (kg/mm.sup.2) ______________________________________ 10 3,300 20 3,296 50 1,850 100 1,505 ______________________________________ The measured microhardness values for zirconium nitride (ZrN) deposited by cathodic arc plasma deposition are apparently larger than those observed for similarly formed titanium nitride (TiN) films and also appear to be siginificantly larger than those reported for bulk zirconium nitride. Wear resistant measurements of zirconium nitride coatings wherein test samples were subjected to typical fretting conditions present in a fuel rod/grid interaction indicate that a wear factor increase of in excess of 60 may be achieved by a thin coat of zirconium nitride in comparison to an uncoated zirconium-alloy fuel tube. The zirconium nitride coating 60 also functions as a corrosion resistant barrier for the zirconium-alloy cladding. The zirconium nitride coating apparently functions as a barrier to oxygen diffusion, thus enhancing the corrosion resistance of the zirconium-alloy tube. A zirconium nitride coating may also be deposited on Inconel-alloy tubes by a cathodic arc deposition process. Test results for a zirconium nitride coating deposited on an Inconel 178-alloy test substrate revealed a wear resistance of approximately 6 when compared to the wear resistance of the test substrate. The test samples were subjected to wear conditions characteristic of fretting conditions at the fuel rod/grid interface of a reactor. While a preferred embodiment of the invention has been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the scope of the invention. Accordingly, various modifications, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.