32 research outputs found
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Modeling coupled thermal-hydrological-chemical processes in theunsaturated fractured rock of Yucca Mountain, Nevada: Heterogeneity andseepage
An understanding of processes affecting seepage intoemplacement tunnels is needed for correctly predicting the performance ofunderground radioactive waste repositories. It has been previouslyestimated that the capillary and vaporization barriers in the unsaturatedfractured rock of Yucca Mountain are enough to prevent seepage underpresent day infiltration conditions. It has also been thought that asubstantially elevated infiltration flux will be required to causeseepage after the thermal period is over. While coupledthermal-hydrological-chemical (THC) changes in Yucca Mountain host rockdue to repository heating has been previously investigated, those THCmodels did not incorporate elements of the seepage model. In this paper,we combine the THC processes in unsaturated fractured rock with theprocesses affecting seepage. We observe that the THC processes alter thehydrological properties of the fractured rock through mineralprecipitation and dissolution. We show that such alteration in thehydrological properties of the rock often leads to local flow channeling.We conclude that such local flow channeling may result in seepage undercertain conditions, even with nonelevated infiltrationfluxes
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A reaction-transport approach for assessing infiltration rates in unsaturated fractured rock from stable isotope compositions
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Implications of the Drift Scale Heater Test at Yucca Mountain for Epithermal Mineralization
An 8-year long, drift scale heater test (DST) is currently underway at the underground Exploratory Studies Facility at Yucca Mountain in Nevada. The host rock for the DST is a highly fractured, welded tuff. The rock has {approx}10% matrix porosity 90% filled with water. After a little more than two years of heating, the temperature at the drift wall reached {approx}200 C and has been maintained at that temperature for the past {approx}1.5 years. Gas and water (both vapor and liquid) have been collected from monitoring boreholes since the test began. The CO{sub 2} concentration of the gas and the isotopic compositions of the water and CO{sub 2} are measured. These data are used to constrain numerical models of coupled thermal, hydrological, and chemical processes occurring in the system. Despite obvious differences from epithermal systems (e.g., the DST is being conducted in an unsaturated system), the trends observed in the isotopic compositions of the water and CO{sub 2} have interesting implications for natural systems. In areas below boiling, the isotope ratios of the water are near that of the ambient pore water ({delta}{sup 18}O about -12{per_thousand}). Where significant amounts of vapor condensate occur (above the boiling front above the drift and in fracture zones to the sides of the drift), the {delta}{sup 18}O values of the water are lower than the pore water, reflecting addition of low-{delta}{sup 18}O steam condensate. Conversely, in boiling zones, the {delta}{sup 18}O values of the water become progressively higher, representing Rayleigh fractionation of the pore water as it is vaporized. As the temperature approaches boiling, the gas phase becomes dominated by water vapor. The remainder of the gas phase consists of air with elevated CO{sub 2} (up to 15%). The source of the CO, is primarily dissolved inorganic carbon (DIC) in the pore water. As the temperature increases, the {delta}{sup 13}C values of the CO{sub 2} shift from approximate equilibrium with the pore water DIC (-15{per_thousand}) to much higher values (>0{per_thousand}). Dissolution of calcite in fractures is also a significant source of CO{sub 2} in regions with drainage of vapor condensate. Isotopic data from several Mexican epithermal vein systems will be discussed in light of these findings
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Modeling of coupled heat transfer and reactive transport processes in porous media: Application to seepage studies at Yucca Mountain, Nevad a
When hot radioactive waste is placed in subsurface tunnels, a series of complex changes occurs in the surrounding medium. The water in the pore space of the medium undergoes vaporization and boiling. Subsequently, vapor migrates out of the matrix pore space, moving away from the tunnel through the permeable fracture network. This migration is propelled by buoyancy, by the increased vapor pressure caused by heating and boiling, and through local convection. In cooler regions, the vapor condenses on fracture walls, where it drains through the fracture network. Slow imbibition of water thereafter leads to gradual rewetting of the rock matrix. These thermal and hydrological processes also bring about chemical changes in the medium. Amorphous silica precipitates from boiling and evaporation, and calcite from heating and CO2 volatilization. The precipitation of amorphous silica, and to a much lesser extent calcite, results in long-term permeability reduction. Evaporative concentration also results in the precipitation of gypsum (or anhydrite), halite, fluorite and other salts. These evaporative minerals eventually redissolve after the boiling period is over, however, their precipitation results in a significant temporary decrease in permeability. Reduction of permeability is also associated with changes in fracture capillary characteristics. In short, the coupled thermal-hydrological-chemical (THC) processes dynamically alter the hydrological properties of the rock. A model based on the TOUGHREACT reactive transport software is presented here to investigate the impact of THC processes on flow near an emplacement tunnel at Yucca Mountain, Nevada. We show how transient changes in hydrological properties caused by THC processes often lead to local flow channeling and saturation increases above the tunnel. For models that include only permeability changes to fractures, such local flow channeling may lead to seepage relative to models where THC effects are ignored. However, coupled THC seepage models that include both permeability and capillary changes to fractures may not show this additional seepage
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