Gas breakthrough in compacted Avonlea bentonite

Thumbnail Image
Hume, Harold B.
Journal Title
Journal ISSN
Volume Title
Several processes may generate gas in the proposed Canadian nuclear fuel waste disposal vault. Since a bentonite-based buffer is one of the proposed vault sealing materials, an understanding of gas migration through clay is important. Using specially-designed systems and five-minute pressure increments, the pressures required to pass gas through compacted Avonlea bentonite specimens (the gas-breakthrough pressures) were studied. Tests were conducted on saturated and unsaturated plugs with dry densities $(\rho\rm\sb{c})$ between 0.6 and 1.5 Mg/m$\sp3$. The breakthrough pressure $\rm(p\sb{b})$ of unsaturated specimens was measured with a low-pressure apparatus; $\rm p\sb{b}$ increased with increasing degree of saturation $\rm(S\sb{r}),$ provided that $\rm S\sb{r}$ was greater than about 80%. In specimens with a high degree of saturation, only inconsistent breakthrough was obtained at $\rm\rho\sb{c} > 0.60\ Mg/m\sp3$ before the 50 MPa limit of the equipment was reached. The agreement between results from saturated clay and predictions made using models based on a capillary-pressure theory of gas breakthrough is poor. Tests were also conducted on saturated specimens at constant pressure between $\rm\rho\sb{c} = 0.8$ and 1.4 Mg/m$\sp3,$ and the time to breakthrough $\rm(t\sb{b})$ was measured. Breakthrough occurred in all of these tests, but often after a much longer time than the duration of the increasing-pressure tests. This shows that a model based on capillarity alone is insufficient to describe gas breakthrough. At 1.00 Mg/m$\sp3$ and $\rm p\sb{c} = 0.3$ to 2.8 MPa, an inverse linear relationship exists between $\rm t\sb{b}$ and $\rm p\sb{c}.$ Models of the breakthrough process that assume a hydraulic-conductivity mechanism also suggest that $\rm t\sb{b}$ and $\rm p\sb{c}$ should be inversely related. Furthermore, excellent agreement was obtained between one of the models (the Kozeny-Carman time model) and the experimental data. (Abstract shortened by UMI.)