Corrosion behavior of the weld zone of carbon steel overpack for HLW final disposal

Yokoyama, Yutaka*; Mitsui, Hiroyuki*; Takahashi, Rieko*; Otsuki, Akiyoshi*; Asano, Hidekazu*; Taniguchi, Naoki ; Yui, Mikazu

In Japan, carbon steel is one of the candidate materials for the disposal container (overpack) for high-level radioactive waste (HLW). Overpack seals vitrified waste and is required to isolate it from contact with groundwater for 1,000 years in Japan's waste management program. After overpack is placed in a deep underground repository, it is presumed that it will be exposed to environmental conditions which include lithospheric pressure from bedrock, hydrostatic pressure from groundwater and swelling pressure of the buffer material as mechanical forces and contact with groundwater as chemical conditions that may induce corrosion. Therefore, the wall thickness of overpack is designed with consideration for mechanical strength and corrosion property so that it can maintain its integrity for 1000 years. On the other hand, the overpack is enclosed by welding. With regard to the assessment of the applicability of welding techniques to the overpack, it is important to confirm the long-term integrity of the weld joint from the aspects of mechanical strength and corrosion property, too. In this study, the corrosion behavior of weld joints, which were provided by TIG (GTAW), MAG (GMAW) and EBW (Electron Beam Welding), was investigated by immersion tests under the aerobic conditions considering environment at the early stage of the repository.