Long-Term Extrapolation of Passive Behavior of Alloy 22
- PDF / 99,533 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 65 Downloads / 191 Views
II4.2.1
Long-Term Extrapolation of Passive Behavior of Alloy 22 Osvaldo Pensado, Darrell S. Dunn, and Gustavo A. Cragnolino Center for Nuclear Waste Regulatory Analyses (CNWRA) 6220 Culebra Road, San Antonio TX, U.S.A. ABSTRACT Common assumptions to extrapolate the lifetime of proposed high-level waste disposal containers made of Ni-Cr-Mo alloys, in the absence of environmental and electrochemical conditions leading to localized corrosion and stress corrosion cracking, are evaluated based on a mechanistic model for passive dissolution. The predominant charge conduction mechanism through the oxide film formed on Ni-Cr-Mo alloys is hypothesized to be interstitial transport of metal cations. Dissolution of the alloy and conduction of interstitial species through the film create vacancies in the alloy. The anodic current density under potentiostatic control decreases as a function of time, and potentiostatic decays in the current density are rationalized on the basis of vacancy accumulation at the metal-oxide interface. It is concluded that the dissolution process is regulated by vacancy-enhanced diffusion of the elements in the alloy. Long-term stoichiometric dissolution arises if the diffusion coefficients of the alloying elements are similar. No credible scenario is envisioned by which catastrophic failure may occur as a result of longterm passive dissolution. INTRODUCTION One of the key attributes of the overall system performance in the U.S. Department of Energy (DOE) Repository Safety Strategy for the proposed repository at Yucca Mountain, Nevada U.S.A., is the estimated long life of waste packages as the main engineered barrier to radionuclide release [1]. The DOE currently plans that the waste package will be composed of two concentric cylindrical containers emplaced horizontally in a drift. The outer container is proposed to be made of a nickel-chromium-molybdenum corrosion-resistant alloy, Alloy 22 (Ni22Cr-13Mo-3W-3Fe). The inner container is proposed to be made of Type 316 nuclear grade stainless steel, intended to provide structural strength to the waste package. The estimated long life of the outer container is dictated by the resistance of Alloy 22 to various modes of aqueous corrosion throughout broad ranges of temperature, pH, and concentrations of anionic and oxidizing species. In an aqueous environment, assumed to be established on the waste package surface when the relative humidity achieves a critical value, and in the absence of conditions leading to localized corrosion, Alloy 22 will dissolve exhibiting passive behavior. The slow, passive dissolution rate (approximately hundreds of nanometers per year) is estimated to corrode the outer container in tens of thousands of years. The long-term passive behavior of Alloy 22 as a container material has been the subject of a recent international workshop with the participation of many corrosion experts [2]. Additional insights on the issue of long-term extrapolation of passive behavior are provided in this paper.
II4.2.2
MECHANISTIC MODEL FOR PASSIVE D
Data Loading...
