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Abstract
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, based on the structure of carbon steel overpack presented in JNCs H12 Report, the basic requirements for welding and non-destructive examination methods presumed to be necessary to seal the overpack lid and to examine the weld, were investigated and categorized into two separate groups of technical and theoretical issues. Through discussions about these issues from the viewpoint of the long-term integrity required of the overpack, an incremental approach was identified consisting of (1) experimental study on applicability of welding and NDE methods, (2) introduction of evaluation methods for the obtained data, and (3) Presentation of promising methods for overpack final closure including lid structure, appropriate welding and NDE methods. This led to the provision of a methodology that would be able to ensure the duration of containment for the overpack final closure weld and is discussed in the conclusion of this paper, as the 1st report.