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The
long-term performance of the potential repository at Yucca
Mountain will be affected by the coupling of thermal, hydrological,
and chemical processes (THC) in the rock around the emplacement
tunnels and potentially in other geologic units that undergo
heating. The major driving forces for THC processes are
the repository heat load due to radioactive decay and over
long time periods the rate of water percolation. The effect
of the transport of heat, fluid, and vapor will be mineral
dissolution and precipitation that may lead to permanent
changes in porosity and permeability and in the water and
gas chemistry. Studies have shown that several important
considerations are required to account for the major THC
processes that are likely to take place over the life of
the potential repository. These include:
(a) CO2 transport in the gas phase and equilibration
with the liquid phase;
(b)
chemical, mineralogical, and hydrological differences between
fractures and the rock matrix must be captured;
(c)
multidimensional (minimum 2-D, to account for gas phase
convection and fluid flow in heterogeneous media);
(d)
kinetics of mineral-fluid reactions to account for very
slow reaction rates; and
(e)
consideration of aluminosilicate mineral reactions (for
example, feldspars, clays, and zeolites) in addition to
silica phases and calcite.
Work
in the Earth Sciences Division has been on the computer
simulation of these processes at a range of spatial scales,
from the mountain-scale (tens of meters to kilometers) to
the drift-scale (centimeters to meters). Examples of computer
simulations at different spatial scales are shown in the
accompanying diagrams.
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