Evaporative Evolution of Brines from Synthetic Topopah Spring Tuff Pore Water, Yucca Mountain, NV
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Evaporative Evolution of Brines from Synthetic Topopah Spring Tuff Pore Water, Yucca Mountain, NV Maureen Alai and Susan Carroll Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, U.S.A. ABSTRACT We are investigating the evaporation of pore water representative of the designated high-levelnuclear-waste repository at Yucca Mountain, NV to predict the range of brine compositions that may contact waste containers. These brines could form potentially corrosive thin films on the containers and impact their long-term integrity. Here we report the geochemistry of a relatively complex synthetic Topopah Spring Tuff pore water that was progressively evaporated in a series of experiments. The experiments were conducted in a vented vessel in which HEPA filtered air flowed over the 95oC solution. Samples of the evaporating solution and the condensed vapor were taken and analyzed to determine the evolving water chemistry and gas volatility. The final solid was analyzed by X-ray diffraction. The synthetic Topopah Spring Tuff water evolved towards a complex brine that contained about 45 mol % Cl, 7 mol% NO3, 43 mol% Na, 4 mol % K, and less than 1 mol % each of SO4, Ca, Mg, HCO3 and Si. Trends in the solution data and identification of CaSO4 solids suggest that fluorite, carbonate, sulfate, and Mg-silicate precipitation minimize the corrosion potential of “sulfate type pore water” by removing F, Ca, and Mg during the early stages of evaporation.
INTRODUCTION Evaporative concentration experiments reported here investigate the evolution of the pore water found in rock formations within (Topopah Spring Tuff) and above (Paintbrush) the designated high-level-nuclear-waste repository at Yucca Mountain, NV. The primary goal of our experiments is to provide constraints on water chemistries and salt compositions that are likely to contact and react with the high-level waste canisters and any engineered canister shields. The chemical divide theory describes the chemical evolution of dilute waters upon evaporation in terms of their equivalent calcium, sulfate and bicarbonate ratios and is shown in Figure 1A. The chemical evolution of evaporating waters is controlled by the high solubility of salt minerals relative to the moderate solubility of calcium sulfate and low solubility of calcium carbonate minerals. An alkaline pH, carbonate brine (Na-K-CO3-Cl-SO4-NO3) forms from dilute waters with dissolved calcium that is less than the dissolved carbonate (2Ca < HCO3+2CO3, equivalent %). A near neutral pH, sulfate brine (Na-K-Mg-Cl-SO4-NO3) forms from dilute waters with dissolved calcium that is greater than the dissolved carbonate, but less than the combined dissolved sulfate and carbonate (2Ca < 2SO4+HCO3, equivalent %). A Ca-chloride brine with near neutral pH (Na-K-Ca-Mg-Cl-NO3) forms from dilute waters with dissolved calcium that is greater than the combined dissolved sulfate and carbonate (2Ca > 2SO4+HCO3, equivalent %).
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A.
Possible Pore Water Types Ca++
Ca-Chloride Brine (Na-K-Ca-Mg-Cl-NO3)
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