Effect of Inhibiting Oxyanions on the Localized Corrosion Susceptibility of Waste Package Container Materials
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Effect of Inhibiting Oxyanions on the Localized Corrosion Susceptibility of Waste Package Container Materials D.S. Dunn, L. Yang, C. Wu, and G.A. Cragnolino [email protected] Center for Nuclear Waste Regulatory Analyses (CNWRA) Southwest Research Institute 6220 Culebra Road, San Antonio, TX 78238-5166, USA ABSTRACT The DOE is currently preparing a license application for the permanent disposal of high level radioactive waste at Yucca Mountain, Nevada. The proposed design of waste packages for the disposal of high level radioactive waste consists of an outer container made of Alloy 22, a corrosion resistant Ni-Cr-Mo-W alloy, surrounding an inner container made of Type 316 nuclear grade stainless steel. Under conditions where passivity is maintained, the uniform corrosion rate of Alloy 22 is slow and long waste package lifetimes are projected. However, the initiation of localized corrosion such as pitting or crevice corrosion may decrease waste package lifetimes. In this study the crevice corrosion susceptibility of Alloy 22 was determined in chloride solutions with additions of oxyanions that are present in the groundwater at the potential repository site. When present in sufficient concentrations relative to chloride, nitrate, carbonate, bicarbonate, and sulfate inhibited pitting and crevice corrosion of Alloy 22. INTRODUCTION The U.S. Nuclear Regulatory Commission (NRC) is preparing to review a license application to be submitted by the U.S. Department of Energy (DOE) for the construction and operation of a potential repository for the permanent disposal of the nation’s high-level radioactive waste (HLW) at Yucca Mountain, Nevada. Several attributes of the possible repository design and the engineered barriers such as the long life of the waste packages and the location of the repository horizon in the unsaturated zone that has a low water infiltration rate, contribute to the isolation of the HLW [1]. The current proposed waste package design is intended to provide containment of the radionuclides for thousands of years and consists of an Alloy 22 (56Ni-22Cr-13.5Mo-3W-4Fe) outer container surrounding a Type 316 nuclear grade (NG) stainless steel (SS) inner container. The proposed selection of Alloy 22 as the outer container material for the waste package is based on the well known corrosion resistance of this alloy particularly in chloride containing environments. Formation of topologically close packed (TCP) phases as a result of exposures to temperatures in the range of 600 to 900 °C [1,112 to 1,652 °F] and welding processes, [2] increases susceptibility of Alloy 22 to localized corrosion [3]. Recent investigations have shown that the formation of thin grain boundary TCP phase precipitates [4] following short exposures at elevated temperatures {870 °C [1,598 °F] for 5 minutes} can significantly increase the crevice corrosion susceptibility [5,6]. Localized corrosion of mill annealed, welded, and thermally aged Alloy 22 can be inhibited in chloride solutions with low nitrate concentrations [6]. In addition t
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