Thermal Loading Studies Using the Unsaturated Zone Model

Abstract

Several factors will affect Thermal-Hydrological (TH) response of the Unsaturated Zone (UZ) to thermal load at the potential repository. These factors include small and large-scale heterogeneity, the thermal load within the repository drifts and presence of lithophysal cavities. The objective of this study is to quantify these effects. Numerical modeling was used to investigate the effects of heat on UZ flow, temperature and liquid saturation on two spatial scales, for a range of potential repository operating modes. The TH simulations were conducted on two dual-permeability numerical grids. The first grid is a refined North-South Mountain-scale 2D model with layer-wise constant fracture permeability. The second grid is a refined half-drift (1-m grid near drift) 2D model with several realizations of spatially variable fracture permeability in the Topopah Spring welded unit (TSw). In the TSw lithophysal units, the thermal capacity and thermal conductivity of the lithophysal units were scaled using the lithophysal porosity. The second model includes both small-scale (less than 1-meter correlation length) heterogeneity in the fracture permeability and discrete high permeability fractures. Monte Carlo methods were used to generate several realizations of spatially variable fracture permeability (up to 4 orders of magnitude) in the TSw, based on the measured distribution of fracture permeability within the exploration drifts. Above boiling and below boiling repository operating modes are investigated by varying the initial thermal load and the amount of heat removed by ventilation. The simulations of coupled heat and mass flow were conducted using TOUGH2 (EOS3 module) over a simulated period of 100,000 years.