Abstract
Knowledge of how nuclear waste migrates
and spreads in low-permeability fractured rocks is central to the
successful disposal of nuclear waste and creation of a safe environment.
The mobility and spreading of decaying solute transport are
significantly affected by matrix diffusion in low-permeability hard
rocks. Thus, it
is important to understand their
influence on changes in effective matrix diffusion coefficients of rock
matrix for solutes with a range of half-life periods. For this purpose,
a numerical model is developed. Spatial moment analysis suggests that
the reduction in solute velocity is inversely proportional to the solute
decay rate, and solute spreading follows a skewed Gaussian profile. For
higher effective matrix diffusion coefficients, time-averaged temporal
moment analysis for the fracture-matrix coupled system results in a
constant solute front velocity and macro-dispersion coefficient along
the fracture, and indicates its spatial independency. The profiles of
relative solute mass retention in the fracture indicate that it is
difficult to distinguish individual effects of matrix diffusion and
first-order decay, while the effective decay rate is found to increase
with the effective matrix diffusion coefficient and the first-order
radioactive decay constant.
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