Modelling the interaction of hyperalkaline fluids with simplified rock mineral assemblages

Abstract

Designs for geological disposal facilities for radioactive wastes often envisage the extensive use of cementitious materials. After closure, the repository will saturate with groundwater and cement porewater will migrate into the geosphere to form an alkaline (pH 12.5–13) plume. This plume will react with the components of the surrounding host rock leading to mineralogical, chemical and physical changes which will be complex in nature. Coupled computer models of geochemistry and hydrogeological transport will be needed to scope these changes. In the recent past, a series of laboratory column experiments were carried out by the British Geological Survey in order to test the capabilities of coupled models to predict the evolution of outflow fluid compositions and product solids. These experiments reacted single minerals (i.e., quartz, albite, calcite and muscovite/quartz) and potential host rock lithologies (i.e., Borrowdale Volcanic Group fault rock, Äspö granite and Wellenberg marl) with cement pore fluids. The objectives of the present work were to develop the understanding of these experiments and to improve simulations of the data by computer models. To this end, a systematic approach was adopted in which dissolution rates for individual minerals were calibrated to the results of the single mineral columns and the resulting parameter values applied to the more complex columns. It was found that for the systems studied, reasonable agreement between the experimental data and the calculated results could be obtained. However, the calibrated dissolution rate constants used in the models first vary between experiments by up to half an order of magnitude and secondly differ from literature values by up to one order of magnitude, with the calibrated values being generally slower than those found in the literature.