Long-Term Integrity of Waste Package Final Closure for HLW Geological Disposal, (VI) Consistency of the Structural Integrity Evaluation Model for the Weld Joint

Abstract

Overpack, a container of high-level radioactive waste for Japan's geological disposal program, is required to prevent the sealed vitrified waste from coming into contact with groundwater for 1,000 years. This means that the same duration and function arerequired for the weld joint between the body and lid of the overpack. The authors have initiated studies on a methodology evaluating the long-term integrity of the lid closure, including welding tests and non-destructive examination tests, and proposed a structural integrity evaluation model for the lid closure as a basis for fracture mechanics assessment. The model makes two main assumptions consistent with six prerequisites. Thisarticle examines the consistency of the mechanical strength of the lid closure (one of the two assumptions) with its prerequisites, through a review of the test results of corrosion behavior and numerical calculations of neutron irradiation embrittlement for weld joints. Furthermore, a comparison of the critical crack lengths at different penetration depths was conducted, and an estimation of the maximum tolerable flaw size, which serves as a form of an acceptance criterion after adding a safety factor for the size of the weld flaw, was made. Based on these results, the consistency of the proposed evaluation model was discussed and identified.