Gas breakthrough pressure of FEBEX bentonite compacted under different conditions

The internationally preferred option for the final disposal of High Level Nuclear Waste (HLNW) is the Deep Geological Repository. This solution in some cases, like the Spanish one, involves the use of an engineering barrier composed of compacted bentonite. The generation and accumulation of gases are a significant concern for the long-term performance of the clay-based barrier.

The FEBEX bentonite is the Spanish reference barrier material for the Engineering Barrier System (EBS). This material is a granulated bentonite (GB), composed predominantly of montmorillonite (>90%) with a maximum grain size of 5 mm.

The main aim of this study was to determine the gas breakthrough (BT) pressure on saturated samples under different conditions of compaction: dry density (1.5, 1.6 and 1.7 Mg·m^-3), water content (14%, 22% and 26%), grain size distribution, and length/diameter (L/D) ratio of the cell (diameter 38 and 50 mm, length 20 mm).

Two types of custom-built equipment were used to generate de BT episodes and the detailed pressure-time data series. For the lower dry density values (1.5 and 1.6 Mg·m^-3) gas-flow was calculated from dynamic fall-out tests (with variable injection and backpressures). For the highest dry density (1.7 Mg·m^-3), gas flow was directly measured by mass-flowmeters in a high-pressure steady-state gas permeability unit (with steady injection pressure and atmospheric backpressure).

Before gas injection in each phase, samples were saturated and their hydraulic conductivity was measured. Average values of hydraulic conductivity, before and after the gas injection phase, were similar (0.1 – 8.5) 10^-21 m², indicating no major effect of gas injection on this property. After gas testing the samples were resaturated and the BT testing was repeated.

The BT pressure increased with higher dry density of the samples, higher water content at compaction and the decrease in the L/D ratio. Overall, there was a systematic repetition of the values of BT pressure in the same sample after resaturation, but the shape of the pressure-time series was different depending on the real BT value versus the injection gas pressure.

To study the effect of gas transport on macro-mesostructure (>7 nm) mercury intrusion porosimetry (MIP) analyses were performed after testing to compare with similar untested samples and study the evolution of pore size distribution during BT. Subsamples were taken in each bentonite sample close to the gas inlet and outlet zones, but no significant differences were observed between them.