Corrosion Processes Affecting the Performance of Alloy 22 as a High-Level Radioactive Waste Container Material
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Corrosion Processes Affecting the Performance of Alloy 22 as a High-Level Radioactive Waste Container Material G.A. Cragnolino, D.S. Dunn, Y.-M. Pan and O. Pensado Center for Nuclear Waste Regulatory Analyses (CNWRA), Southwest Research Institute 6220 Culebra Road, San Antonio, TX, 78238-5166, USA Email: [email protected] 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 site in Yucca Mountain, Nevada. Alloy 22 is considered to be extremely resistant to various modes of aqueous corrosion over broad ranges of temperature, pH, and concentration of anionic and oxidizing species. Uniform corrosion under passive dissolution conditions, localized corrosion in the form of crevice corrosion, and stress corrosion cracking are discussed on the basis of experimental results obtained with mill annealed, thermally treated, and welded specimens using electrochemical techniques. The approach developed for long-term performance prediction, including the use of empirically derived parameters for assessing localized corrosion and the modeling of the passive dissolution behavior, is described. 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) [1] is the performance of the waste package (WP) barriers. 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, 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-22Cr-13Mo-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 integrity to the WP. Alloy 22 became the material preferred by the DOE for the outer container because it is more resistant to localized corrosion in the predicted WP environment than other Ni-Cr-Mo alloys previously considered, such as Alloys 825 and 625 [2, 3]. The objective of this paper is to provide a summary of the results of our experimental investigations on the various corrosion processes (passive corrosion, crevice corrosion, and stress corrosion cracking (SCC)) that are expected to affect the long-term performance of the Alloy 22 containers in the repository. Information previously reported [4, 5] is updated with new results and the modeling of long-term passive dissolution [6] is briefly described.
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