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

Overpack, a high-level radioactive waste package for geological disposal, seals vitrified waste and in line with Japan's waste management program is required to isolate it from contact with groundwater for 1,000 years. In this study, TIG (Tungsten Inert Gas) welding method, a typical arc welding method and widely used in various industries, was examined for its applicability to seal a carbon steel overpack lid with a thickness of 190 mm. Welding conditions and welding parameters were examined for multi-layer welding in a narrow gap for four different groove depths. Weld joint tests were conducted and weld flaws, macro- and microstructure, and mechanical properties were assessed within tentatively applied criteria for weld joints. Measurement and numerical calculation for residual stress were also conducted and the tendency of residual stress distribution was discussed. These test results were compared with the basic requirements of the welding method for overpack which were pointed out in our first report. It is assessed that the TIG welding method has the potential to provide the necessary requirements to complete the final closure of overpack with a maximum thickness of 190 mm.