2010:12 An Evaluation of Models of Bentonite Pore Water Evolution

The pore-water composition of a KBS-3 bentonite buffer may gradually change as a result of reaction between groundwater and buffer minerals. The rather minor occurrences of the most rapidly reacting minerals in the buffer will have the most immediate and observable influences, but contributions from slow reactions of the main bulk clay phases cannot be ruled out. The performance implication of such bentonite chemical reaction is that radionuclide solubility and sorption depend on the buffer chemical conditions. Moreover, pore-fluid composition at the interface between the buffer and the surrounding bedrock is important for the modelling and assessment of potential buffer loss scenarios. Any process with the potential to affect this composition needs to be identified and analysed.

The purpose of this project is to assess to smectite hydrolysis as a potential contribution to long-term geochemical processes in a KBS-3 bentonite buffer.

The outcome of this project is a suggested modelling approach for handling the influences of smectite clay transformations in the performance evaluation of a KBS-3 bentonite buffer. In the more traditional buffer geochemical modelling approach, the focus has been on more reactive trace constituents such as calcite, gypsum and pyrite, but the potential bulk-transformation of the main constituent cannot be ignored without justification. In absolute quantitative terms, several uncertainties remain, such as the validity of thermodynamic estimates for smectite properties, the validity of the buffer porosity model, the influence of reaction kinetics and the nature of precipitating new minerals. It is concluded in this study that smectite hydrolysis may (based on model predictions) be significant for the future geochemical state of a buffer, but that the time-scale of smectite hydrolysis is too long for experimental verification. The utilized experimental data could therefore not determine whether or not the process has performance implications for safety assessment. Potential performance significances of such process are as described above related to buffer pH variations and pore-fluid cation composition.