The design of copper-coating overpack for the high-level radioactive waste disposal concept in Japan

For the geological disposal system in Japan, a vitrified waste will be contained in a metal overpack, which, in turn, will be surrounded by a thick bentonite buffer. The overpack is aiming to prevent the contact of groundwater to vitrified waste during the high radioactivity and heat generation period of the first 1,000 years at least after emplacement. Within the Japanese program, consideration for overpack candidate materials has included carbon steel, copper-steel composite, and titanium-steel composite. Within the extensive safety assessment conducted in 2000, steel was selected. This selection was partly based on the manufacturability of carbon steel, as well as its well-understood corrosion behavior. However, the understanding of copper corrosion and welding/manufacturing technologies have greatly progressed over the past two decades. In this study, we focus on the copper-coated container developed by the Nuclear Waste Management Organization, because this technology is seemingly effective to maintain very long-term containment of hundreds of thousands of years, with a low cost of manufacturing within the Canadian program. We are investigating the applicability of the copper-coating technologies in terms of the corrosion allowance and mechanical design.