Abstract
Borosilicate nuclear waste glasses develop complex altered
layers as a result of coupled processes such as hydrolysis of network
species, condensation of Si species, and diffusion. However, diffusion
has often been overlooked in Monte Carlo models of the aqueous corrosion
of borosilicate glasses. Therefore, three different models for dissolved
Si diffusion in the altered layer were implemented in a Monte Carlo model
and evaluated for glasses in the compositional range (75-x) mol% SiO2
(12.5+x/2) mol% B2O3 and (12.5+x/2) mol% Na2O, where 0 ≤ x ≤ 20%, and
corroded in static conditions at a surface-to-volume ratio of 1000 m-1.
The three models considered instantaneous homogenization (M1), linear
concentration gradients (M2), and concentration profiles determined by
solving Fick's 2nd law using a finite difference method (M3). Model M3
revealed that concentration profiles in the altered layer are not linear
and show changes in shape and magnitude as corrosion progresses, unlike
those assumed in model M2. Furthermore, model M3 showed that, for
borosilicate glasses with a high forward dissolution rate compared to the
diffusion rate, the gradual polymerization and densification of the
altered layer is significantly delayed compared to models M1 and M2.
Models M1 and M2 were found to be appropriate models only for glasses
with high release rates such as simple borosilicate glasses with low ZrO2
content.
