Corrosion behavior of weld zone of carbon steel overpack for HLW geological disposal

Yokoyama, Yutaka*; Mitsui, Hiroyuki*; Takahashi, Rieko*; Asano, Hidekazu*; Taniguchi, Naoki ; Naito, Morimasa 

In Japan, carbon steel is one of the candidate materials for the disposal container (overpack) for high-level radioactive waste (HLW). Overpack seals vitrified waste and is required to isolate it from contact with groundwater for 1,000 years in Japan's waste management program. Its integrity for 1,000 years is evaluated in consideration of mechanical strength and corrosion property. The corrosion property of base metal is evaluated along with the corrosion scenarios of carbon steel overpack presented by JNC. However, the corrosion behavior of the weld zone of overpack under repository conditions has not been investigated enough to evaluate its long term integrity. The weld zone may have a different microstructure and chemical composition from the base metal because of the influence of welding. Therefore, the corrosion property of the weld zone may differ from that of the base metal. In this study, to evaluate the corrosion property of the weld zone produced by TIG, MAG and EBW, the corrosion modes which had become the main concerns in the weld zone were extracted from the corrosion scenario of base metal, and four tests: passivation behavior, corrosion property under aerobic conditions, susceptibility to stress corrosion cracking, and corrosion property and possibility of hydrogen embrittlement under anaerobic conditions, were examined.