Zirconium use for large process components

Austenitic stainless steels display satisfactory behaviour in reprocessing facilities. However, certain failures are occasionally observed and usually ascribed to corrosion in oxidizing nitric media capable of bringing these materials close to or within the transpassive range. Zirconium does not present this drawback and helps to solve corrosion problems in these particularly aggressive conditions for stainless steels. Zirconium generally displays an outstanding general corrosion resistance in nitric acid media. In practice, however, its use in reprocessing facilities leads to the consideration of certain factors that may either restrict its application, or should be closely investigated. The most important among these are: fluoride content of the medium; nitrogen content of the metal; formation of possible insoluble compounds despite very low ionic release rates; stress corrosion cracking observed only in some severe laboratory testings. It was decided to use zirconium in the new facilities at La Hague for most of the weld-fabricated equipment treating boiling nitric solutions and for a number of specific process units (for example, reactors containing sulphides in acidic media). A total of 80 tons of zirconium will be employed, and 5,500 meters of piping circuits are being installed with this material. This was achieved through the development of semi-finished products (plates, pipes, etc.) with properties adapted to the construction of the large weld-fabricated units of nuclear chemistry. Shaping, welding and inspection specifications have been developed. Finally, the choice and development of zirconium were facilitated by the use of zirconium-stainless steel junctions allowing the connection of a zirconium unit to a stainless steel piping system. (author)