Modeling Corrosion Processes for Alloy 22 Waste Packages
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Modeling Corrosion Processes for Alloy 22 Waste Packages D.S. Dunn, O. Pensado, Y.-M. Pan, L.T. Yang, and X. He [email protected] Center for Nuclear Waste Regulatory Analyses (CNWRA) Southwest Research Institute® 6220 Culebra Road, San Antonio, TX 78238-5166, USA ABSTRACT The proposed waste package design for the disposal of high-level radioactive waste at the potential repository at Yucca Mountain, Nevada, 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. Models to assess the influence of waste package degradation modes on the overall system performance consider uniform and localized corrosion processes. Based on measurements of passive anodic current density, the uniform aqueous corrosion rate of Alloy 22 is estimated to be slow, and long waste package lifetimes are projected in the absence of conditions that promote accelerated degradation processes. The initiation of localized corrosion is possible in chloride-containing waters with low concentrations of inhibiting oxyanions such as nitrate. Although propagation rates for localized corrosion are typically orders of magnitude greater than the passive uniform corrosion rates, the maximum penetration depth of localized attack may be limited to depths significantly less than the container thickness as a result of stifling and arrest of localized corrosion. INTRODUCTION The U.S. Nuclear Regulatory Commission (NRC) is preparing for the review of a license application for a potential high-level radioactive waste repository. Waste packages for the potential repository at Yucca Mountain, Nevada, consists of an Alloy 22 (56Ni-22Cr-13.5Mo3W-4Fe) outer container surrounding a Type 316 nuclear grade (NG) stainless steel (SS) inner container. The use of a corrosion resistant waste package container material, the installation of a titanium alloy drip shield, 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 the isolation of the high-level waste [1]. The Center for Nuclear Waste Regulatory Analyses (CNWRA) and NRC have developed the Total-system Performance Assessment (TPA) code to evaluate the overall performance of the potential repository [2]. In the TPA code, passive corrosion is assumed for environments that do not promote crevice corrosion, and the corrosion rates are modeled using a distribution that encompasses the measured variability as a result of environmental conditions and the effects of fabrication processes. Under environmental conditions where passivity is sustained, the uniform aqueous corrosion rate of Alloy 22 is low, not strongly dependent on solution composition, and follows an Arrhenius dependence on temperature with an activation energy in the range of 33.5 to 49.6 kJ/mol [8.0 to 11.9 kcal/mol] [3]. This approach is conservative because the passive dissolution rate tends to decrease with ti
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