Evaluation of the Corrosivity of Dust Deposited on Waste Packages at Yucca Mountain, Nevada
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Evaluation of the Corrosivity of Dust Deposited on Waste Packages at Yucca Mountain, Nevada Charles Bryan1, Russell Jarek1, Thomas Wolery2, David Shields3, Mark Sutton2, Ernest Hardin3, Deborah Barr4 1 Sandia National Laboratories, Albuquerque, NM, USA. 2 Lawrence Livermore National Laboratory, Livermore, CA, USA. 3 Bechtel-SAIC Co., Las Vegas, NV, USA 4 U.S. Department of Energy, Office of Repository Development, Las Vegas, NV, USA ABSTRACT Potentially corrosive brines can form during post-closure by deliquescence of salt minerals in dust deposited on the surface of waste packages at Yucca Mountain during operations and the pre-closure ventilation period. Although thermodynamic modeling and experimental studies of brine deliquescence indicates that brines are likely to form, they will be nitrate-rich and noncorrosive. Processes that modify the brines following deliquescence are beneficial with respect to inhibition of corrosion. For example, acid degassing (HCl, HNO3) could dry out brines, but kinetic limitations are likely to limit the effect to increasing their passivity by raising the pH and increasing the NO3/Cl ratio. Predicted dust quantities and maximum brine volumes on the waste package surface are small, and physical isolation of salt minerals in the dust may inhibit formation of eutectic brines and decrease brine volumes. If brines do contact the WP surface, small droplet volumes and layer thicknesses do not support development of diffusive gradients necessary for formation on separate anodic-cathodic zones required for localized corrosion. Finally, should localized corrosion initiate, corrosion product buildup will stifle corrosion, by limiting oxygen access to the metal surface, by capillary retention of brine in corrosion product porosity, or by consumption of brine components (Cl–). INTRODUCTION: Small amounts of dust will be deposited on the surfaces of waste packages in drifts at Yucca Mountain during the operational and the preclosure ventilation periods. Salts present in the dust will deliquesce as the waste packages cool and relative humidity in the drifts increases. In this paper, we evaluate the potential for brines formed by dust deliquescence to initiate and sustain localized corrosion that could result in failure of the waste package outer barrier. Multiple arguments are used to show that waste package failure due to dust deliquescence-induced localized or crevice corrosion of the outer barrier (Alloy 22) is insignificant with respect to repository performance. Measured atmospheric and underground dust compositions are the basis of thermodynamic modeling and experimental studies to evaluate the likelihood of brine formation and persistence, the volume of brines that may form, and the relative corrosivity of the initial deliquescent brines and of brines modified by processes on the waste package surface. In addition, we evaluate several mechanisms that could inhibit or stifle localized corrosion should it initiate. Arguments are developed using a logical framework approach, similar to that de
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