Discrete-fracture modeling of thermal-hydrological processes at G-tunnel and Yucca Mountain

The U.S. Department of Energy is investigating the suitability of Yucca Mountain, Nevada, as a potential site for high-level nuclear waste storage. Decay heat can significantly improve or degrade repository performance by affecting the waste-package (WP) environment, and it can move substantial water vapor across various length scales. Thermally driven moisture movement has been modeled in the past using either equivalent-continuum (ECM) or (less commonly) discrete-fracture (DFM) models. The ECM assumes local thermodynamic equilibrium between the fractures and matrix within a composite representative elementary volume. It is, therefore, a single-continuum model with composite characteristic curves given as functions of bulk-averaged saturation. The DFM, on the other hand, separates the fractures and matrix into distinct domains, which, can represent disequilibrium between matrix and fractures. The DFM is well suited to investigating the dynamics of decay-heat-generated condensate, on how much water imbibes into the matrix by matrix suction or drains down fractures. However, the ECM is much more computationally efficient, so it is important to understand conditions under which the ECM is appropriate