For performance assessments of geological disposal of high-level radioactive
waste, activation energies for the diffusion of strontium ions and the basal
spacings of compacted sodium montmorillonite in the water-saturated state
were determined.
Basal spacings determined by XRD indicated changes in the interlamellar
space from a three-water layer hydrate state to a two-water layer hydrate
state as the dry density of the montmorillonite increased from 1.0 to 1.8 Mg
m-3. Activation energies from 17.3 to 30.8 kJ mol-1
for the apparent diffusion coefficients of strontium ions were obtained. The
lower activation energies than for diffusion of strontium ions in free water
were determined for montmorillonite specimens of lower dry density (1.2 Mg
m-3 and below), while the higher activation energies were at
higher dry densities (1.4 Mg m-3 and above).
These findings cannot be explained by changes in only the geometric
parameters, which the pore water diffusion model is based upon. Possible
explanations for the dry density dependence of the activation energy are the
changes of the temperature dependence of the distribution coefficients
and/or of the diffusion process with increasing dry density