Thermal loading studies using the Yucca Mountain unsaturated zone model

Abstract

A systematic modeling study of the effect of thermal loading on moisture, gas, and heat flow in the unsaturated zone of Yucca Mountain has been carried out. The study is based on a two-dimensional (2-D) north–south vertical cross-section, using both the effective continuum and the dual-permeability modeling approaches. A study was also conducted on a three-dimensional (3-D) model using the effective continuum approach. The 2-D model conducted in 1996 uses a average uniform infiltration rate of 4.4 mm/year and a thermal load of 83 kW/acre. The 3-D model uses an spatially varying infiltration rate and a revised average thermal load of 87.6 kW/acre. For the rock properties used in the Topopah Springs (TSw) hydrogeological unit, heat pipe conditions develop above and below the repository in 10–100 years in both models. The average temperature of the boiling zone is about 96°C. This boiling zone is confined to the TSw hydrogeological unit and lasts about 1000 years. At the top of the CHn (vitric/zeolitic interface), predicted maximum temperature is about 70–75°C after about 2000 years. The model predicts a temperature increase of approximately 30°C at the water table. The results show that thermal loading at the repository also results in significant changes in the moisture distributions at the repository horizon and the zone directly above and below it. A large increase in liquid and gas flux, several orders of magnitude above ambient conditions, is predicted near the repository. This study indicates the ECM and dual- k modeling approaches provide similar simulation results, in terms of temperature and moisture flow and distribution. The only difference is that at early times, the ECM model predicts more extensive boiling conditions. Localized dry-out is predicted in areas with low infiltration flux within the central part of the repository. However, because coarse grids were used, average saturation in the matrix blocks representing the repository indicates will remain high even when the regions near repository are completely dry. Analysis of the flux pattern at the top and bottom boundaries of the repository shows that the liquid always flows into the repository for most the thermal loading period (up to 10,000 years).