Process Development for Fabrication of Zircaloy-4 Dissolver Assembly for Reprocessing of Spent Nuclear Fuel

摘要

Spent fuel reprocessing for Fast Breeder Reactor (FBR) requires a dissolver made of a material which is resistant to corrosion as the process involves nitric acid as the process medium. The focus was on the use of advanced materials with high performance in high concentrations (greater than 8 N) and high temperatures of nitric acid employed in the dissolver. The commercially available materials like SS3161 , titanium, Ti-5% Ta, and Ti-5% Ta-1.8% Nb were tried and the corrosion behavior of these materials was studied in detail. It was also decided to try out Zircaloy-4 as the material of construction due to its excellent corrosion resistance in nitric acid environment. The specifications were stringent and the geometrical configurations of the assembly were very intricate. On accepting the challenge of fabricating the dissolver, NFC made different fixtures for electron beam (EB) welding and TIG welding. Trials were carried out for optimization of various operating parameter for EB welding & TIG welding processes, which were qualified by radiography, liquid dye penetrant testing, and metallographic, mechanical and chemical analyses. Finally the dissolver was subjected to helium leak test. The tests conducted at IGCAR with Zircaloy dissolver assembly revealed that Zircaloy-4 exhibited the superior corrosion resistance over all other materials tried out so far for this purpose. The results clearly indicates that Zircaloy-4 is a near-zero corrosion material in comparison to the other materials. It is thus demonstrated that the dissolver assembly for spent fuel reprocessing for Fast Breeder Reactor (FBR) can be manufactured from Zircaloy-4 on an industrial scale, with long life expectancy with near zero corrosion, employing EB & TIG welding processes.