Nuclear Technology / Volume 171 / Number 1 / July 2010 / Pages 74-87
Technical Paper / Radioactive Waste Management and Disposal
A coupled thermal-hydrologic-airflow model is developed, solving for the transport processes within a waste emplacement drift and the surrounding rock mass together at the proposed nuclear waste repository at Yucca Mountain. Natural, convective airflow as well as heat and mass transport in a representative emplacement drift, embedded in a three-dimensional, mountain-scale rock mass with edge cooling, are explicitly simulated for the first time in the literature, using the MULTIFLUX model. The conjugate, thermal-hydrologic transport processes in the rock mass are solved with the TOUGH2 porous-media simulator in a coupled way to the in-drift processes. The new simulation results show that large-eddy turbulent flow, as opposed to small-eddy flow, dominates the drift airspace for at least 5000 years following waste emplacement. The size of the largest, longitudinal eddy is equal to half of the drift length, providing a strong axial heat and moisture transport mechanism from the hot drift sections to the cold drift sections. The in-drift results are compared to those from simplified models using a surrogate, dispersive model with an equivalent dispersion coefficient for heat and moisture transport. Results from the explicit, convective velocity simulation model provide higher axial heat and moisture fluxes than those estimated from the previously published, simpler, equivalent dispersion models, in addition to showing differences in temperature, humidity, and condensation rate distributions along the drift length. A new dispersive model is also formulated for comparison, giving a time- and location-variable function that runs generally about ten times higher in value than the highest dispersion coefficient currently used in the Yucca Mountain Project.