Deliquescence Behavior of Multicomponent Salts: Effects on the Drip Shield and Waste Package Chemical Environment of the
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Deliquescence Behavior of Multicomponent Salts: Effects on the Drip Shield and Waste Package Chemical Environment of the Proposed Nuclear Waste Repository at Yucca Mountain, Nevada Roberto T. Pabalan, Lietai Yang, Lauren Browning Center for Nuclear Waste Regulatory Analyses, San Antonio, TX 78238-5166, U.S.A. ABSTRACT Thermodynamic calculations were conducted to determine the deliquescence behavior of salt mixtures and to simulate the evaporation of Yucca Mountain groundwaters. The results are consistent with published experimental data that show the deliquescence points of salt mixtures are lower than that of individual salts. For mixtures of NaCl and KCl salts, the deliquescence point of pure NaNO3 salt is an appropriate lower bound. However, mixtures containing magnesium and calcium salts have much lower deliquescence points than pure NaNO3. If magnesium and calcium salts are deposited on waste package and drip shield surfaces, it could lead to earlier initiation of aqueous corrosion than assumed by the DOE in its performance assessment abstractions. Such salt mixtures can be formed by evaporation of waters with compositions similar to some Yucca Mountain porewaters and would be characterized by low deliquescence relative humidity, high chloride concentration, and low concentrations of anions such as free (uncomplexed) nitrate and sulfate that could mitigate against the chloride-enhanced corrosion of the waste package. Evaporation of Yucca Mountain groundwaters also could lead to fluoride concentrations that are above the threshold for accelerated corrosion of the titanium drip shield. INTRODUCTION A key attribute of the U.S. Department of Energy (DOE) safety strategy for its proposed high-level nuclear waste repository at Yucca Mountain, Nevada, is long-lived waste packages and drip shields. The waste packages will consist of an outer container of a highly corrosion-resistant Ni-Cr-Mo alloy (Alloy 22) over an inner container made of Type 316 nuclear grade stainless steel. Extending over the length of the emplacement drifts and covering the waste packages will be an arched drip shield made of a Ti-Pd alloy (Ti Grade 7). In the absence of mechanical disruption, aqueous corrosion is expected to be the primary degradation process limiting the life of the waste package and drip shield [1]. The mode and rate of corrosion of these materials will depend on the chemistry of water contacting them and on temperature. Coupled thermal-hydrological-chemical processes could significantly alter the chemistry of groundwater that seeps into the repository drift and contacts the waste package and drip shield. Within a hot repository setting, evaporation processes could lead to formation of corrosive brines and to accumulation of dry, multicomponent salt deposits on drip shield/waste package surfaces. Additional salts, such as soluble chlorides, nitrates, and sulfates of sodium, potassium, calcium, and magnesium, that are present as aerosols in atmospheric air [2] and entrained in ventilation air introduced into the repository drif
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