A Modeling Study Evaluating the Thermal-Hydrological Conditions in and near Waste Emplacement Drifts at Yucca Mountain

Abstract

In heated drifts such as those designated for emplacement of radioactive waste at the proposed geologic repository at Yucca Mountain, temperature gradients cause natural-convection processes that may significantly influence the moisture conditions in the drifts and in the surrounding fractured rock. Large-scale convection cells in the heated drifts would provide an effective mechanism for turbulent mixing and axial transport of vapor generated from evaporation of pore water in the nearby formation. As a result, vapor would be transported from the elevated-temperature sections of the drifts into cool end sections (where no waste is emplaced), would condense there, and subsequently would drain into underlying rock units. To study these processes, we have developed a new simulation method that couples existing tools for simulating thermal-hydrological conditions in the fractured formation with a module that approximates turbulent natural convection in heated emplacement drifts. The new method simultaneously handles (a) the flow and energy transport processes in the fractured rock, (b) the flow and energy transport processes in the cavity, and (c) the heat and mass exchange at the rock-cavity interface. An application is presented studying the future thermal-hydrological conditions within and near a representative waste emplacement drift at Yucca Mountain. Particular focus is on the potential for condensation along the emplacement section, a possible result of heat output differences between individual waste packages.