Localized Corrosion Susceptibility of Alloy 22 as a Waste Package Container Material
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Localized Corrosion Susceptibility of Alloy 22 as a Waste Package Container Material Gustavo A. Cragnolino, Darrell S. Dunn, and Yi-Ming Pan Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute 6220 Culebra Road San Antonio, TX 78238-5166, U.S.A. ABSTRACT Alloy 22 is the material preferred by the U.S. Department of Energy for the waste package outer container for geological disposal of high-level radioactive waste at the proposed repository site in Yucca Mountain, Nevada. The susceptibility of Alloy 22 to localized corrosion is an important consideration in the evaluation of the waste package behavior and the assessment of the overall performance of the proposed repository. The effects of the environment chemical composition and temperature on localized corrosion susceptibility were examined by measuring the repassivation potential for crevice corrosion in chloride-containing solutions at temperatures ranging from 80 to 150 EC. The effect of potentially inhibiting anionic species, such as nitrate, was also determined. In addition to the mill annealed material, tests were conducted on both welded and thermally aged material to evaluate microstructural effects related to container fabrication processes. The resistance of Alloy 22 to localized corrosion decreased with increasing temperature and chloride concentration. Welding and thermal aging also decreased the localized corrosion resistance of the alloy. INTRODUCTION One of the principal factors in the U.S. Department of Energy (DOE) Repository Safety Strategy for the proposed repository at Yucca Mountain (YM) is the long-term performance of the waste package (WP) [1]. It is expected that the integrity of the WPs, predicated on the use of a highly corrosion-resistant alloy as an outer container material, will prevent exposure of the radioactive waste (spent nuclear fuel and vitrified reprocessed waste) to the environment for times longer than the regulatory performance period of 10,000 years. During this period, the environment in contact with the WPs, initially hot dry-air and later humid air, may become an aggressive aqueous solution as a result of dripping of groundwater, altered in its chemical composition by interactions with the host rock and evaporative concentration due to radioactive decay heating. The currently proposed DOE WP design consists of a 2-cm thick Alloy 22 (58Ni-22Cr13Mo-3W-4Fe) outer container enclosing a 5-cm thick inner container made of type 316 nuclear grade (NG) stainless steel. The main purpose of the inner container is to provide structural strength to the WP. Alloy 22 became the material preferred by the DOE for the outer container because it is extremely resistant to localized corrosion. Nevertheless, Alloy 22 is susceptible to localized corrosion in the form of crevice corrosion within certain ranges of chloride concentration, redox potential, and temperature as are other Ni-Cr-Mo alloys previously considered by the DOE, such as Alloys 825 and 625 [2, 3]. The objective of this paper is to provide a summary of the
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