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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.

As one of the studies on “yoyushindo disposal” whose concept is similar to an intermediate disposal, the development of a disposal container has been conducted by the Federation of Electric Power Companies of Japan. To assess a drop event of a waste package in which stored the radioactive wastes from nuclear power plants, the toughness of the disposal container was evaluated by drop tests using three specimens which have actual dimensions, drop analysis, fracture mechanics assessment and macroscopic tests. The three specimens for drop tests were manufactured in consideration of the design specifications and the manufacture operations in nuclear power plants. The lid plates of the specimens were welded to the body plates without pre- and post-weld heat treatment by using a remote automated welding machine. The drop tests showed that no penetration cracks or splash of its content occurred in the disposal container under conservative conditions such as the maximum weight and height in the handling. Drop analysis and the fracture mechanics assessment indicate that the strain induced by the drop impact did not exceed the fracture strain and an unstable fracture did not occur. And macroscopic tests showed that penetration cracks did not occur at 8m drop events. These tests and evaluations confirmed that the disposal container had sufficient toughness.