Patent Number: 052746860
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to fuel rods and other components employed in nuclear reactors. More particularly, the present invention relates to fuel rod cladding tubes and other zirconium-alloy components. Fuel rods having outer cladding tubes are mounted in support grids of nuclear reactor fuel assemblies. Because of the harsh environment of the fuel assembly where the surrounding water temperature is typically 400.degree. C. and the water has a relatively high pressure, the cladding tube is susceptible to wear and corrosion. At the lower portions of the reactor assembly, the cladding tubes can also be exposed to debris fretting. In addition, severe wear forces can arise at the location of the grid support. Practitioners in this field are aware that a thin coating of zirconium nitride on a zirconium alloy tube, can dramatically improve wear and corrosion resistance. Such coatings can be applied by any one of a variety of techniques such as ion implantation or plasma spray as disclosed, for example, in U.S. Pat. No. 4,724,016 (Anthony) and U.S. Pat. No. 5,026,517 (Menken et al). More recent attempts include cathodic arc reactive deposition as disclosed in copending U.S. Ser. No. 514,870 (Bryan et al). These techniques, however, exhibit relative strengths and weaknesses, so that none has emerged as clearly superior. SUMMARY OF THE INVENTION An object of the present invention is to provide a new and improved wear and corrosion resistant coating for a cladding tube employed in a nuclear reactor. Another object of the invention is to provide a new and improved coating which may be applied to a zirconium-alloy nuclear cladding tube in an efficient and cost effective manner. A further objective of the invention is to provide a new and improved coating for a cladding tube which has enhanced resistance to debris fretting and corrosion and has an outer diameter which does not significantly increase the coolant flow resistance in the fuel assembly. It is a more specific object of the present invention to provide a cladding tube for a nuclear fuel assembly component, particularly a fuel rod, which has a thin film of zirconium nitride coating the outside surface. The zirconium nitride coating may be applied on a portion of the tube which will be located below or in the vicinity of a particular fuel assembly support grid, or on substantially the entire outside surface of the tube. A film having a thickness of approximately 5 microns is effective in resisting corrosion and wear of the cladding tube, which usually has a zirconium-alloy composition. The thin film of zirconium nitride is applied to the cladding tube by reactively depositing zirconium nitride on the surface of the cladding tube by an anodic arc plasma deposition process. The cladding tube is heated to a temperature in a range of approximately 300.degree. to 400.degree. C. in the presence of nitrogen. The anodic vacuum arc differs from the cathodic vacuum arc in that the arc is sustained by material evaporated from the anode, as opposed to the cathode. In the anodic vacuum arc, the cathode is either totally inactive without eroding cathode spots or has many rapidly moving spots on the cathode surface. All of the material that sustains the arc is emitted by the anode. Until recently, steady state arcs sustained by anodic material were only known for currents exceeding 400 A. In these high current arcs, anodic evaporation occurs in large luminous spots at the surface of the anode. High current anodic arcs were known predominantly in vacuum breakers where the aim of investigations was to minimize the erosion of the anode through elimination of the anodic arc. Several investigators have reported on the characteristics of low current microsecond-duration anode spots. In these experiments the arc was pulsed for approximately 10 .mu.sec at currents as low at 20A. Due to the short pulse length, very little material evaporation from the anode occurred and the usefulness of anodic evaporation for the deposition of coatings was not investigated. Recently, however, several investigators have found a new steady state mode of operation of the anodic vacuum arc at much lower currents, typically less than 100 A. In these low current anodic arcs the anode is tailored in order to enhance evaporation. The cathode is designed either for minimal erosion, by using a refractory material such as carbon or tungsten, or is manufactured of the same material as the anode. In either case the cathode is designed so as not to heat up appreciably. By tailoring the anode it has been possible to achieve rapid evaporation of the anode material without macroparticle inclusion that occurs in the cathodic arc. The anodic vacuum arc produces a metal vapor plasma that, unlike the fully ionized plasma of the cathodic arc, is only partially ionized (.about.20%). In the anodic arc, the ions are singly ionized and have a directed energy of approximately 5 eV while the electrons have a temperature of less than 1 eV. Near the anode, the density of the expanding plasma is approximately 1.times.10.sup.18 /cm.sup.3 while the neutral density is an order of magnitude higher. Coatings deposited with the low current anodic vacuum exhibit all of the desirable qualities of coatings deposited with plasma assisted deposition techniques, and cathodic arcs in specific, but do not suffer from many of the problems that these methods entail. In particular, the anodic arc rapidly produces coatings that are of a very high quality and do not suffer from macroparticle inclusion. Other advantages of the invention will become apparent from the drawings and the specification.