Performance assessment of a potential repository at Yucca Mountain includes
flow and transport modeling of the unsaturated zone as the critical
predictive component, and involves a series of model calculations that
provide predictions of the migration of important radionuclides in the
inventory to the water table. The modeling requires the relevant properties
of both the natural environment and the engineered systems. The Unsaturated
Flow Apparatus, UFA, was used to directly measure the unsaturated and
saturated transport properties of whole rock tuff cores and candidate
barrier materials to provide real input parameters to the models. These
properties included hydraulic conductivity, matric potential, air
permeability, and diffusion coefficient, all of which are strong functions
of the volumetric water content. Results show that for all recharges above
0.1 mm/yr, fractures will be partially saturated and conducting at that
recharge rate. Whenever the thermal conditions relax enough to allow
rewetting of the host rock, there will be dripping from the drift ceiling.
Unsaturated transport of colloids through fractured cores of Topopah Spring
and Prow Pass tuffs was also investigated and found to depend primarily upon
colloid charge, and not size, for the rock cores investigated.