Modeling acid-gas generation from boiling chloride brines

Simple calculations are first carried out to evaluate condensate pH as a function of HCl gas fugacity and condensed water fraction for a vapor equilibrated with saturated calcium chloride brine at 50-150°C and 1 bar. The distillation of a calcium-chloride-dominated brine is then simulated with a reactive transport model using a brine composition representative of partially evaporated calcium-rich pore waters at Yucca Mountain. Results show a significant increase in boiling temperature from evaporative concentration, as well as low pH in condensates, particularly for dynamic systems where partial condensation takes place, which result in enrichment of HCl in condensates. These results are in qualitative agreement with experimental data from other studies.The combination of reactive transport with multicomponent brine chemistry to study evaporation, boiling, and the potential for acid gas generation at the proposed Yucca Mountain repository is seen as an improvement relative to previously applied simpler batch evaporation models. This approach allows the evaluation of thermal, hydrological, and chemical (THC) processes in a coupled manner, and modeling of settings much more relevant to actual field conditions than the distillation experiment considered. The actual and modeled distillation experiments do not represent expected conditions in an emplacement drift, but nevertheless illustrate the potential for acid-gas generation at moderate temperatures (<150°C).This study was conducted as part of investigations related to the long-term safety assessment of the proposed high-level nuclear waste repository at Yucca Mountain, Nevada. The heat released by the spent nuclear fuel is expected to induce boiling at near-atmospheric pressure of pore water (moisture) present in the unsaturated rock around waste-emplacement tunnels [1-5]. This boiling, as well as the deliquescence of naturally occurring salts in dust, could lead to the formation of residual brines [6,7] at temperat