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. 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.
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