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.

Overpack, a high-level radioactive waste package for Japan’s geological disposal program, is required for preventing sealed vitrified waste from contact with groundwater for at least 1,000 years. The weld joint between the body and lid must also meet this requirement. Certain welding methods were examined for applicability through full-scale welding tests using various welding depths up to 190 mm and two different lid structures. Results show that generation of welding flaws must be considered unavoidable. Therefore, ultrasonic testing (UT) must be conducted on the assumption that weld flaws will be present. Such UT systems must be designed for natural defects. Several types of UT must be evaluated for detection and size estimation capability at depths ranging from the surface to the bottom of the weld joint. Certain UT methods were examined for their ability to detect natural defects that were created on the surface of and inside a 190-mm thick carbon steel specimen. Probability of detection (POD) of each UT method was calculated by comparing the results of UT and destructive examination. In consideration of the preferred range of scanning depth for each UT method, a concept that combines UT methods was proposed as a practical UT system for the overpack weld joint.