Mountain-Scale Coupled Thermal-Hydrological-Chemical Processes Around the Potential Nuclear Waste Repository at Yucca Mountain

Abstract

Contact : Eric S onnenthal, (5 10)48 6-5 8 66, elsonnen thal @ lbl. gov Research ObjectivesThe objectives of this study were to evaluate the thermal-hydrological-chemical (THC) effects on flow and geochemistry in the unsaturated zone (UZ) at Yucca Mountain at a mountain scale. The major THC processes important in the UZ are (1) mineral precipitatiorddissolution affecting flow and transport to and from the potential repository, and ( 2 ) changes in the compositions of gas and liquid that may seep into drifts. ApproachThe conceptual model developed for THC processes provides the basis for modeling mineral-water-gas reactions, as they influence the chemistry of water and gas and associated changes in mineralogy and hydrologic properties. Data incorporated in the model include hydrologic and thermal properties, geologic layering from the UZ 3-D flow and transport model, geochemical data (fracture and matrix mineralogy, water and gas geochemistry), and thermodynamic and kinetic data. Simulations included coupling among heat, water, and vapor flow; aqueous and gaseous species transport; kinetic and equilibrium mineral-water reactions; and feedback of mineral precipitatiorddissolution on porosity, permeability, and capillary pressure for a dual permeability (fracture-matrix) system. Simulations were performed using TOUGHREACT V2.3.The effects of coupled THC processes on the evolution of flow fields and water and gas chemistry in the UZ were evaluated for an above-boiling thermal operating mode. A cross section was chosen from the UZ 3-D flow and transport model that follows a north-south trend through the potential repository. Minerals include calcite, silica phases, feldspars, zeolites, clays, gypsum, fluorite, and iron oxides with the relevant aqueous species. The gas phase consists of air, water vapor, and CO2. The composition of the initial and infiltrating water was derived from the matrix pore water extracted from the Topopah Spring welded tuff near the ongoing Drift Scale Test. AccomplishmentsThe simulations revealed that by 5,00Oyears, most of the region above and directly below the potential repository undergoes a small reduction in fracture porosity of less than 1 percent (Figure 1). Above the northern edge, a region of slight porosity increase persists, owing to enhanced vapor convection and condensation. The areas that show the most effect as a result of heating are in the zeolitic units below the potential repository, where calcic zeolites dissolve to form alkali feldspars, leading to increased aqueous calcium concentrations and increased porosity. The predicted range in pH from 7 to 9 is linked to changes in gas-phase CO2 concentrations induced by heating of pore waters.

Impacts of Mineral Reaction Kinetics and Regional Groundwater Flow on Long-Term CO2 Fate at Sleipner

“… c Power term n with respect to H + from Palandri and Kharaka d Reactive specific surface area (cm 2 /g) based on Sonnenthal and Spycher, which was reduced by 1 order of magnitude. e From Steefel f Data for chalcedony precipitation but the dissolution kinetics data were assumed to be the same as those for quartz. …”

“… d Reactive specific surface area (cm 2 /g) based on Sonnenthal and Spycher, which was reduced by 1 order of magnitude. …”

“…Generally, a specific surface area calculated from grain size may be a poor estimate of the hydrologically accessible mineral surface area because of many factors, such as coating and water flow channeling . Reactive specific surface areas employed here were based on Sonnenthal and Spycher, who calculated the reactive specific surface area assuming a cubic array of truncated spheres that make up the framework of the rock. However, the data of Sonnenthal and Spycher were arbitrarily reduced by 1 order of magnitude in the present simulations to account for effects such as coating and water flow channeling.…”

“…Reactive specific surface areas employed here were based on Sonnenthal and Spycher, who calculated the reactive specific surface area assuming a cubic array of truncated spheres that make up the framework of the rock. However, the data of Sonnenthal and Spycher were arbitrarily reduced by 1 order of magnitude in the present simulations to account for effects such as coating and water flow channeling. The reactive specific surface areas used in the present study (Table ) are comparable with those used in Xu et al and Audigane et al…”