Estimated Effects of Temperature-Relative Humidity Variations on the Composition of InDrift Water in the Potential Nucle
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Estimated Effects of Temperature-Relative Humidity Variations on the Composition of InDrift Water in the Potential Nuclear Waste Repository at Yucca Mountain, Nevada 1
Lauren Browning, 1Randall Fedors, 1Lietai Yang, 1Osvaldo Pensado, 1Roberto Pabalan, 1 Chandrika Manepally, and 2Bret Leslie. 1Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, San Antonio, TX; 2U.S. Nuclear Regulatory Commission, Rockville, MD ABSTRACT We define four distinct thermohydrochemical environments for drip shield and waste package corrosion in the potential nuclear waste repository, referred to here as the Dry, Seepage + Evaporation, Seepage + Condensation + Evaporation, and the Seepage + Condensation environments. These environments are bounded by temperature and relative humidity conditions at drift wall and drip shield/waste package surfaces judged most likely to initiate fundamental changes in the quantity and/or chemistry of in-drift waters. The duration in which different environments might exist is evaluated by comparing simulated, time-dependent temperature and relative humidity curves for three different locations within repository drift 25. In-drift conditions and processes postulated to cause drip shield/waste package corrosion are evaluated within the context of the thermohydrochemical environments by various means, including analytical calculations and geochemical simulations. Of the four environments considered here, the Seepage + Evaporation environment presents the most significant potential for aqueous corrosion of drip shield and waste package materials, and may persist for approximately 500 years in center drift locations. The likelihood for corrosion in other thermohydrochemical environments is significantly lower, but may increase with the acquisition of new data or the demonstration of extenuating circumstances. INTRODUCTION Estimated lifetimes for drip shield (DS) and waste package (WP) materials in the potential underground repository for high-level nuclear waste at Yucca Mountain (YM), Nevada, are strongly dependent on estimates of the quantity and chemistry of repository waters [1]. There are three potential sources of in-drift waters — seepage into the drifts from fractures in the overlying rock, condensation on repository surfaces from in-drift cold trap processes, and the deliquescence of salts. The initial composition of seepage and condensed waters will probably differ significantly, and then evolve over time as a result of chemical mixing and interactions with rocks and engineered materials, evaporation, and salt formation. Deliquescent salts sequester moisture from the atmosphere and may generate brine compositions that initiate WP corrosion. Deliquescence occurs under specific temperature and relative humidity conditions that are strongly dependent on salt compositions, and may occur in association with dust particles that are physically emplaced on DS/WP surfaces or salts that formed in situ from evaporated in-drift waters. Equilibrium brine compositions and deliquescen
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