The Influence of Repository Thermal Load on Multiphase Flow and Heat Transfer in the Unsaturated Zone of Yucca Mountain

The 500-700 m thick Yucca Mountain unsaturated zone (UZ) is under extensive investigation as a subsurface repository for the permanent disposal of high-level nuclear wastes. The site characterization has been mostly carried out for analyzing unsaturated flow and radionuclide transport under ambient, isothermal conditions. However, significant research effort has also been devoted to understand the nature of flow and transport processes under non-isothermal conditions. In particular, substantial repository heating from radioactive waste decay has motivated investigations of the coupled thermo-hydrologic (TH) behavior of the UZ under repository heating and its potential impact on repository performance. Significant progress has been made in quantitative coupled TH studies in the last decade. Despite the significant advances made so far in modeling and understanding TH processes, the previous studies have been in general limited to modeling in 1-D and 2-D (instead of the full 3-D representation), and/or small spatial and temporal scale analysis. In addition to these limited modeling exercises, multidimensional modeling has been carried out for large-scale (at the scale of the entire mountain) TH analyses. However, these previous large, mountain-scale TH models utilized the effective continuum model (ECM), rather than the more rigorous dual-continuum model (DKM). This is primarily due to numerical difficulties and computational burden involved with simulating highly non-linear coupled two-phase fluid flow and heat transfer in the fractured unsaturated rock with over one hundred thousand grid blocks (required for mountain-scale simulations). In general, 3-D, mountain-scale, DKM investigations of coupled TH processes in the fractured rock of Yucca Mountain is lacking in the literature. In parallel to the TH modeling studies, significant progress has also been made in site characterization of UZ flow and transport processes. For example, field and modeling studies conducted over the past few years have updated and enhanced our understanding, and revealed many new insights into how the UZ system works under the natural, ambient conditions. As a result, both geological and conceptual models have been updated by model calibration and verification efforts, and fracture-matrix properties and model parameters are better estimated. In addition, the repository design and drift layout plan, which are different from the ones used in previous modeling studies, are also revised. These advances in site characterization, data collection and parameter estimates motivate this work for updated TH modeling efforts. This paper presents the results of our latest effort to develop a representative 3-D, mountain-scale TH model to investigate the coupled TH processes for the repository under thermal load. More specifically, the TH model implements the current geological framework and hydrogeological UZ flow conceptual models, and incorporates the most updated, best-estimated input parameters from the 3-D model calibration (Wu et. al., 2003). Using the more rigorous DKM approach, the TH model consists of (1) a 2-D north-south cross section modeling studies with refined meshes near and around the repository block and (2) a full 3-D representation of the repository and UZ system, which explicitly includes every waste emplacement drift of the repository. For better description of the ambient geothermal condition of the UZ system, the TH model is first calibrated against measured borehole temperature data. The temperature calibration provides the needed surface and water table boundary and initial conditions for the TH model