Characterization of coatings on materials for components in fission and fusion reactors

With regard to applied materials, fission and fusion reactors show corresponding properties: iron- and nickel-based alloys resistant to high temperatures are used as well as graphites, and in both systems special precautions have to be taken to minimize uncontrolled tritium release; in fission reactors, especially in high temperature gas-cooled reactors, corrosion of the metallic components plays a life-limiting role whereas in fusion reactors erosion, by chemical and physical processes, is of similar or still higher importance. A possible way to overcome some of the materials problems arising in both fission reactors and fusion reactors is to use materials coated with protecting layers optimized for the special problem. The authors discuss an example to highlight the application of coated materials in reactor technology for the reduction of hydrogen isotope permeation through metallic walls. For this purpose, a coating process is used for the production of the layers by which the hydrogen isotope permeation is reduced. In this case, the alloys are subjected to an oxidation process - either by special pre-oxidation or in situ. Thus a surface oxide layer is formed. This layer is suitable for the reduction of tritium permeation when its structure and chemical composition fulfils certain conditions. The structural and chemical characterization of the oxide layer is discussed, and the conditions under which a convenient surface oxide layer can grow are described. The permeation-impeding factors achieved are specified for the different types of oxide scale. (Auth.)