Formation of oxide films on titanium alloys under the conditions of the primary circuit of light-water nuclear reactors (a review)

Abstract

The paper presents a review of the structure of oxide films, the processes of radioactive deposits’ formation and methods of their removal from the surfaces of titanium alloys used in the primary circuit of nuclear reactors with water coolant. Upon these conditions a solid protective oxide layer is formed on the surface of construction materials, consisting of titanium dioxide – rutile and anatase. On this layer, there are individual crystallites of titanium dioxide and iron-titanium mixed oxides, mainly ilmenite or ulvospinel. Radioactive contamination of titanium alloys’ surface occurs due to co-precipitation of corresponding isotopes with titanium compounds from coolant and by the sorption. For effective removal of radioactive deposits – decontamination – the development of special compositions is necessary. They should ensure the dissolution of titanium dioxide and ilmenite. Concentrated sulfuric and hydrochloric acids explored in metallurgy cannot be used to decontaminate surfaces of construction materials of the primary circuit due to their corrosion damage. The application of organic acids’ solutions – ascorbic and oxalic – can be an alternative option, as they dissolve effectively both titanium dioxide and ilmenite. The rate and efficiency of their dissolution greatly increase upon the addition of hydrogen peroxide. An additional advantage of these decontaminating compounds is their ability to dissolve fuel fragments if they enter the circuit from leaky fuel rods. At the same time, the application of these compounds can result in corrosion damage to the surface of titanium alloys. To prevent this effect, aliphatic and aromatic amines can be used as corrosion inhibitors.