Modeled Near-Field Environment Porosity Modification due to Coupled Thermohydrologic and Geochemical Processes
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PREVIOUS WORK Extensive TH modeling of the potential high-level nuclear waste repository at Yucca Mountain [5, and references therein] has consistently demonstrated that a region of above-boiling temperatures around waste-emplacement drifts will develop. The extent, magnitude, and duration of these effects depend on thermal-loading strategies. Independently, numerous geochemical studies [7] have documented that changes in the mineralogy will occur under repository conditions. Although some effort has been made to examine the consequences of coupling these THC effects [8], these studies have been limited because the computational ability has not been available to simulate TH evolution under conditions where permeability is evolving. MODEL DESCRIPTION Physical Model The model domain contains a single drift within a two-dimensional (2-D) columnar section of Yucca Mountain stratigraphy, which extends vertically from the water table to the ground surface. A dual-permeability model (DKM; see below) was used to model the 2-D fracturematrix system. The mineralogical system assumed that the rock consisted of a specified fraction of cristobalite and that the remainder of the rock being treated was inert. Phases also allowed to participate in the evolution of the system were quartz and amorphous silica. We discuss here only those simulations considering the cristobalite-quartz system. Mass action expressions were developed for dissolution and precipitation of these minerals and for the aqueous species present in this system. This system, albeit simplified, considers the volumetrically dominant phases expected to participate in reactive transport in the vicinity of the potential repository. Simulations involving a wide range of other mineral species are currently underway. Although numerous scenarios were simulated, we present here two thermal loading scenarios to illustrate significant effects. In Case 1 ("Point-Load Scenario"), nominal design characteristics include an approximately 5 m, end-to-end spacing between waste containers, a 28 m spacing between emplacement drifts, no backfill, and an infiltration flux of 16 mm/yr. In Case 2 ("LineLoad Scenario"), the conditions were the same, except that the end-to-end spacing between containers was reduced to 10 cm and the spacing between emplacement drifts was doubled (to 56 m). Other conditions examined included variation in the infiltration flux and variation in the effective surface area of the dissolving phase. Computational Model All of the calculations described here have been accomplished using a modified version of the NUFT code [9]. The model represents both heat flow and multiphase (gas- and aqueous-phase) flow and transport of chemical species, including air and water. It explicitly accounts for thermal conduction in the bulk rock and in drift materials (including the dependence of rock thermal conductivity Kth on liquid-phase saturation Sliq); liquid-phase convection for saturated and unsaturated conditions; thermal radiation between all surfaces in the emplacement dr
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