Investigation on gas migration behaviours in saturated compacted bentonite under rigid boundary conditions

To ensure the long-term safety and performance efficiency of a deep geological repository for disposal of high-level radioactive waste, understanding and assessment of gas migration behaviour in its engineered barrier system are of significant importance. In this study, a coupled gas transport model is utilised to simulate and to analyse a series of gas injection/breakthrough experiments on saturated bentonite under rigid boundary or constant volume conditions. To explain the laboratory gas migration and breakthrough results, a diffusion and solubility-controlled gas transport mechanism, instead of controversial visco-capillary flow or dilatancy-controlled flow mechanisms, is implemented in the model. The aim is to examine the extent to which this mechanism can describe helium migration and breakthrough behaviours in rigidly confined, saturated bentonite specimens. The predicted results are found to be in good agreement, both qualitatively and quantitatively, with the observed experimental results indicating the adequacy of this mechanism to describe the transport processes. The model represents the gas breakthrough phenomenon as a function of gas solubility. When the dissolved concentration of the injected gas reaches the maximum soluble concentration in the entire porewater domain, gas breakthrough occurs. Since, the system reaches a steady state and no further gas can be dissolved in the rigidly confined specimens, any injected/dissolved gas must be equated by the amount dissipated to comply with the principle of mass conservation. The maximum solution concentrations of helium are predicted to be 2.01 x 10(-5), 7.75 x 10(-5), 1.07 x 10(-4) mol/L for GMZ bentonite specimens with dry densities of 1.3, 1.5 and 1.7 g/cm(3), respectively. The analysis of the injection pressure effects on gas migration behaviour revealed that, if sufficient time is permitted, gas breakthrough may occur at pressures lower than the laboratory observed injection pressures.