Evaporative Evolution of Carbonate-Rich Brines from Synthetic Topopah Spring Tuff Pore Water, Yucca Mountain, NV
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Evaporative Evolution of Carbonate-Rich Brines from Synthetic Topopah Spring Tuff Pore Water, Yucca Mountain, NV Mark Sutton1*, Maureen Alai2 and Susan Carroll2 1
Chemistry & Material Science, 2Energy & Environmental Science Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550
ABSTRACT The evaporation of a range of synthetic pore water solutions representative of the potential highlevel-nuclear-waste repository at Yucca Mountain, NV is being investigated. The motivation of this work is to understand and predict the range of brine compositions that may contact the waste containers from evaporation of pore waters, because these brines could form corrosive thin films on the containers and impact their long-term integrity. A relatively complex synthetic Topopah Spring Tuff pore water was progressively concentrated by evaporation in a closed vessel, heated to 95ºC in a series of sequential experiments. Periodic samples of the evaporating solution were taken to determine the evolving water chemistry. According to chemical divide theory at 25ºC and 95ºC our starting solution should evolve towards a high pH carbonate brine. Results at 95ºC show that this solution evolves towards a complex brine that contains about 99 mol% Na+ for the cations, and 71 mol% Cl-, 18 mol% ΣCO2(aq), 9 mol% SO42- for the anions. Initial modeling of the evaporating solution indicates precipitation of aragonite, halite, silica, sulfate and fluoride phases. The experiments have been used to benchmark the use of the EQ3/6 geochemical code in predicting the evolution of carbonate-rich brines during evaporation. INTRODUCTION Yucca Mountain, NV is the designated site for a permanent geologic repository for high-level nuclear waste. The current waste package design consists of a double walled container with an inner barrier of stainless steel, an outer barrier of highly resistant nickel-chromium-molybdenum alloy, and a titanium drip-shield that covers the containers. Corrosion resistance and long-term integrity of the metal containers and shields are important for the safe disposal of the waste. If the site is licensed, the waste packages will be placed in tunnels several hundred meters below the ground surface in partially saturated volcanic tuff. Once the waste packages are in place, the repository will heat up due to the thermal energy of the nuclear waste. Although the waste packages will be above the groundwater table, pore water present in rock formations within (Topopah Spring Tuff) and above (Paintbrush) the repository may come in contact with the metal containers and shields. The chemical divide theory generally describes the chemical evolution of dilute waters upon evaporation in terms of their equivalent calcium, sulfate and bicarbonate mole ratios [1]. The chemical evolution of the water after significant evaporation is controlled by the high solubility of salt minerals (for example halite) relative to the moderate solubility of calcium sulfate and low solubility of calcium carbonate minerals. An alkaline p
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