Evaluation of long-term interaction between cement and bentonite for geological disposal, (5) chemical alteration analysis of bentonite in consideration of the hydraulic/mechanical properties

Document available in extended abstract form only. In order to carry out long-term safety evaluation of the artificial barrier of a TRU waste repository, we are examining the analysis methods. Our purpose is to reduce the uncertainty in safety evaluations. We analyzed the chemical alteration of the bentonite engineered barrier material in a previous study, but have not considered the hydraulic/mechanical modeling of the materials. The bentonite in the geological repositories must maintain very low hydraulic conductivity by swelling, and the mechanical and hydraulic properties of bentonite swelling may affect the evaluation of chemical alteration. Therefore, in this paper, we add the effect of mechanical and hydraulics properties to the safety evaluation of conventional chemical alteration. In particular, we consider the effect of weak coupling by use of a coupled geochemical reaction/mass-transport code and hydraulic/mechanical code. Normally the mechanical action and a chemical alteration of bentonite take place simultaneously. However, a fully coupled analysis method is very difficult to solve simultaneously. In a coupled analysis, the governing equation is expected to become nonlinear and its solution for the analysis cannot be guaranteed. Moreover, even if the equation can be solved, the calculation convergence time with error is very long. Therefore, for this study, we determined to use a weakly coupled analysis in order to allow the following: 1) shortened analysis time, despite the lower accuracy of the solution, and 2) development of separate chemical and mechanical analysis tools. The geochemical code used here was PHREEQCTRANS and the hydraulic/mechanical code was DACSAR-MP. Although Iteration A of the coupling method has a stronger effect, the computation time is very long. Therefore, in this study we considered the effect on the evaluation of long term alteration caused by use of Iteration C. Common parameters are the amount of montmorillonite (content), Ca exchange rate, percentage of void, and dry density. However, percentage of void and dry density cannot be passed directly from the chemical analysis, and the mechanical analysis does not take the quantity of the solid phase changes into consideration (because there is no concept). We therefore decided to pass the montmorillonite content and Ca exchange rate that were obtained from the chemical alteration analysis to the mechanical analysis. Also, the distribution of dry density from the mechanical analysis was reflected in the chemical analysis. For the model, we used one-dimensional horizontal cross-section of the repository. We first performed a hydraulic/mechanical analysis from unsaturated moisture conditions to saturation. The hydraulic/mechanical analysis that was conducted until re-saturation showed that dry density has a heterogeneous distribution at the saturation point. We set this distribution as the initial condition for the chemical analysis. As a reference case, we also conducted another analysis that set a homogeneous distribution for the initial conditions. The analysis periods for these two analyses were set to 10,000 years. Data was passed to the chemical analysis and mechanical analysis at 1,000, 2,000, 3,000, and 5,000 years. Examination of the results showed the following: - The coupled analysis showed that the dissolution of the montmorillonite in the section that touches the cement material is suppressed. - The equivalent hydraulic conductivity of bentonite showed virtually no change. However, inspection of the entire mesh for the bentonite section showed that the effect from the coupling varied by location. - At the time of moisture saturation, there was almost no change in the state of chemical alteration for either the homogenous density distribution or the heterogeneous density distribution. (authors)