Evolution of Chemistry and Its Effects on the Corrosion of Engineered Barrier Materials
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Evolution of Chemistry and Its Effects on the Corrosion of Engineered Barrier Materials Darrell Dunn1, Yi-Ming Pan2, Xihua He2, Lietai Yang2, and Roberto Pabalan2 1 Mechanical and Materials Engineering, Southwest Research Institute, San Antonio, TX, 78238 2 Center for Nuclear Waste Regulatory Analyses, San Antonio, TX, 78238 ABSTRACT The evolution of environmental conditions within the emplacement drifts of a potential highlevel waste repository at Yucca Mountain, Nevada, may be influenced by several factors, including the temperature and relative humidity within the emplacement drifts and the composition of seepage water. The performance of the waste package and the drip shield may be affected by the evolution of the environmental conditions within the emplacement drifts. In this study, tests evaluated the evolution of environmental conditions on the waste package surfaces and in the surrounding host rock. The tests were designed to (i) simulate the conditions expected within the emplacement drifts; (ii) measure the changes in near-field chemistry; and (iii) determine environmental influence on the performance of the engineered barrier materials. Results of tests conducted in this study indicate the composition of salt deposits was consistent with the initial dilute water chemistry. Salts and possibly concentrated calcium chloride brines may be more aggressive than either neutral or alkaline brines. INTRODUCTION The U.S. Nuclear Regulatory Commission (NRC) is preparing to review a license application for a potential high-level radioactive waste repository. Waste packages for the potential repository may consist 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 use of a corrosionresistant waste package outer container material, the installation of a titanium alloy drip shield to protect the waste packages, and the location of the emplacement drifts in the unsaturated zone have been identified by the U.S Department of Energy (DOE) as key attributes of the repository design that are expected to contribute to waste isolation [1,2]. Degradation of the Alloy 22 waste package outer containers may occur by uniform corrosion, pitting or crevice corrosion, and stress corrosion cracking. Work has been conducted by DOE and by NRC and the Center for Nuclear Waste Regulatory Analyses (CNWRA) to characterize the conditions under which the various degradation mechanisms may occur [3,4]. Uniform corrosion rates are typically less than 10−4 mm/yr [4 ◊ 10-3 mpy] under conditions where the passive film on Alloy 22 is maintained. However, at temperatures above 120 ∞C [248 ∞F], accelerated uniform corrosion of Alloy 22 may be possible if the oxide film has poor stability. Previous studies [5,6] indicated that localized corrosion is possible only at elevated temperatures in oxidizing environments with high chloride concentrations and low concentrations of inhibiting anions such as nitrate, carbonate, bicarbonate, and sulfate. Addition
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