The weld joint of the carbon steel overpack is required to isolate the sealed vitrified waste from coming into contact with groundwater for a duration equal to the base metal, which is 1000 years. The authors have proposed a structural integrity evaluation model of the lid closure, which is based on fracture mechanics assessment. Corrosion phenomena at the weld joint and sizing capability of non-destructive examination for weld flaws are key issues in this evaluation model. In this paper, reviewing the most recent test results on the susceptibility of stress corrosion cracking at the weld joint and the estimation of the probability of detection for ultrasonic testing methods, consistency of the evaluation model was discussed.