Patent Number: 052805107
Section: summary

BACKGROUND OF THE INVENTION The present invention pertains to nuclear fuel assemblies, and more particularly, to the coating of fuel assembly components. Although coating the inside of fuel assembly components has been recognized as providing significant advantages, the small inside diameters, high aspect ratios, and high costs of conventional coating processes, have stymied commercialization of such improved fuel assembly components. Two pending U.S. applications represent a significant advance in the state of the art in this respect. U.S. patent application Ser. No. 07/924,731, "Sputtering Process Burnable Poison Coating" (Bryan et al), is directed to a method for sputter coating the inside surface of a fuel assembly tubular component with absorber material such as a burnable poison or hydrogen getter. U.S. patent application Ser. No. 07/924,732, "Fuel Assembly Sputtering Process" (Bryan et al), is directed to a method for sputter coating the inside surface of fuel assembly tubular components such as nuclear fuel rods or control rod guide tubes, with wear or corrosion resistant material. The foregoing U.S. applications, the disclosures of which are hereby incorporated by reference, describe the so called Linear Magnetron Sputtering process as the preferred sputtering technique. The sputtering process disclosed in these applications requires active confinement of a plasma within the tubular substrate to be coated, as well as an active cooling system. SUMMARY OF THE INVENTION It is thus an object of the present invention to provide an apparatus and method for coating burnable poison material, wear resistant and corrosion resistant materials, or hydrogen getter materials, on the inner surface of nuclear fuel assembly tubular components such as fuel rods, control rods, poison rods instrument thimbles, and the like. It is a related object of the invention, to deposit microparticle coatings on the inside of nuclear fuel assembly components, by an apparatus and a process that is simpler and more cost effective than prior art sputtering. These objects are achieved by a method and apparatus for coating the inside surface of a tubular component of a nuclear fuel assembly, in which the component is supported within a vacuum chamber, and a source rod having an evaporatable field emitter structure is positioned within the component. The field emitter is formed of a material to be coated on the inside surface of the component. An electrical current is passed through the rod at an amperage sufficient to evaporate at least a portion of the emitter structure. The evaporated material of the emitter then deposits on and adheres to the component surface, as a microparticle coating. In some embodiments, the desired coating is a compound, such as BN, which cannot itself constitute the source rod. In such a case, the vacuum chamber is backfilled with a reactive gas, such as nitrogen, and the source rod material, such as boron, when evaporated from the emitter structure chemically reacts with the gas to form the compound, which adheres to the component inner surface. The invention facilitates coating the inside of fuel rod cladding and other components, with a high degree of uniformity. Moreover, the invention allows the simultaneous deposition of multiple atom species to form alloys. The crystal instructure of the coating can be adjusted by adjusting the incident ion energy with respect to the work pieces (i.e., component inner surfaces) to be coated. The small size and variable density of emitter arrays afford great flexibility in the nature of the coatings that can be deposited.