Protective coatings against liquid metal embrittlement-Master Thesis OLGA CHERENKOVA

Author: OLGA CHERENKOVA ; Type of thesis: Master Thesis
Abstract: A nuclear reactor is a system that contains and controls sustained nuclear chain reactions. Reactors are used for generating electricity, moving aircraft carriers and submarines, producing medical isotopes for imaging and cancer treatment, and for conducting research. Fuel, made up of heavy atoms that split when they absorb neutrons, is placed into the reactor vessel (basically a large tank) along with a small neutron source. The neutrons start a chain reaction where each atom that splits releases more neutrons that cause other atoms to split. Each time an atom splits, it releases large amounts of energy in the form of heat. The heat is carried out of the reactor by coolant, which is most commonly just plain water. The coolant heats up and goes off to a turbine to spin a generator or drive shaft. So basically, nuclear reactors are exotic heat sources.
Cyclotrons continue to be efficient accelerators for radio-isotope production. In recent years, developments in the accelerator technology have greatly increased the practical beam current in these machines while also improving the overall system reliability. These developments combined with the development of new isotopes for medicine and industry, and a retiring of older machines indicates a strong future for commercial cyclotrons. For both systems, efficient cooling is required. Nowadays liquid metals are in wide use for this purpose. Liquid metal cooled reactors were first adapted for nuclear submarine use but have also been extensively studied for power generation applications. Liquid metals have safety advantages because they have high heat transfer characteristics, due to high boiling point, no high vapor pressure, and they allow a much higher power density than traditional coolants. Cooling by liquid mercury, sodium, NaK, lead has been studied and used. But due to numerous imperfections of these coolants (such as toxicity, high vapor pressure, not appropriate melting/boiling points, corrosion etc) it is necessary to develop new cooling systems for research and industrial purposes. Thus after choosing the new coolants (liquid metals with desirable properties) for target cooling we will develop the protection coatings for tubes taking into account factors which influence corrosion such as solution pH, oxidizing agent, temperature, velocity, stresses, impurity content. So we can conclude that development of new protective thin films, coatings, claddings are required and mandatory for protection. For its development we propose to use the magnetron sputtering which is a very perspective method for obtaining of thin films. One feature of magnetron sputtering which explains its wide use for the coatings is the low charged particle fluxes reaching a substrate. By this method we can obtain coatings on conductive materials as wellas on nonconductive, also on materials with low melting points. Sputtered films typically have a better adhesion on the substrate than evaporated films. Thereby the producing of protective coatings from liquid metals coolants embrittlement used for nuclear reactors and target cooling in the radiopharmaceutical sphere has been under development within the bounds of this project.