Long-Term Integrity of Waste Package Final Closure for HLW Geological Disposal, (III) Applicability of Electron Beam Welding to Overpack Final Closure

Abstract

Overpack, a high-level radioactive waste package for Japan's geological disposal program, is required to isolate the sealed vitrified waste from contact with groundwater for 1,000 years. In this study, EBW (Electron Beam Welding), a typical application of high energy density beams widely used in various industries, was examined for its applicability to sealing a carbon steel overpack lid with a thickness of 190 mm. Welding conditions and welding parameters were examined for single-pass welding in a narrow area for three different penetration depths. Weld joint tests including macro- and microstructure, and mechanical properties were conducted and 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. The induced void and cold shut inside the weld joint and surface roughness were also discussed for their improvement and evaluation, which need to be established to assure the long-term integrity of overpack lid closure.