Long-Term Integrity of Waste Package Final Closure for HLW Geological Disposal, (V) Applicability of MAG Welding Method to Overpack Final Closure

Abstract

Overpack, a high-level radioactive waste package for Japan's geological disposal program, is required for preventing the sealed vitrified waste from contact with groundwater for 1,000 years. In this study, metal active gas (MAG) welding, a typical arc welding method, was evaluated for its applicability in sealing a carbon steel overpack lid with a thickness of 190 mm. Welding conditions and parameters were examined with multilayer welding for three different groove depths. Welded joint tests including the observation of macro- and microstructures were conducted, and mechanical properties were within tentatively applied criteria. Measurements and numerical calculations for residual stress were also conducted, and residual stress distribution tendencies were discussed. These test results were compared with those previously reported for tungsten inert gas welding (TIG) and electron beam welding (EBW). MAG welding possesses the potential to complete overpack lid closure with a maximum groove depth of 190 mm, but the applicability of MAG welding to overpack final closure should be discussed from the viewpoint of the presence of slag inclusions possibly induced in the weld metal.