Long-Term Integrity of Waste Package Final Closure for HLW Geological Disposal, (IV) Influence of Welding and Prediction of Long-Term Integrity of Weld Joint

Overpack, a high-level radioactive waste container for geological disposal in the Japanese waste management program, is required to isolate the sealed vitrified waste from contact with groundwater for 1,000 years. This means that the weld joint between the body and the lid of the overpack is also required to endure for this period of time. The authors have already started studies on a methodology guaranteeing the long-term integrity of the lid closure, the applicability of several welding and non-destructive examination methods for the weld joint and the corrosion properties of the weld joint. In this paper, through review of the obtained test results, the influences of welding were summarized. Considering the presence of weld flaws, crack growth phenomenon was estimated by fracture mechanics assessment, while the critical crack length and the maximum tolerable flaw size were discussed. Moreover, referring to the Fitness for Service for nuclear power plant components, a method guaranteeing the long-term integrity of the lid closure was discussed and proposed with consideration to the physical and chemical environmental conditions peculiar to the overpack.