A study of the effects of an alkaline plume from a cementitious repository on geological materials

Abstract

Current plans for the disposal of intermediate-level radioactive waste in the U.K. include a cementitious backfill within the disposal caverns. Nuclides migrating from the repository into the rock will be preceded by an alkaline plume derived from the cement pore water. The alkaline plume will interact with the rock around the repository and may change the mineralogical and transport properties of the rock. An experimental and modelling study of alkali interaction with rock is presented. In the experiments, an alkaline solution was passed through a sample of Clashach Sandstone, which has a well-characterised mineralogy and is relatively homogeneous. The pH and ionic composition of the eluate were measured and suggested a complex set of chemical reactions. After completion of the experiment, the sample was examined using SEM. These observations identified quartz and potassium feldspar dissolution, and precipitation of CSH phases. As a first stage in the interpretation of this behaviour, a very simple model was used. Successive refinements were made to the model to examine the effect of individual phenomena. Agreement with the steady-state pH and Ca concentration was obtained with a model involving kinetically-controlled quartz dissolution and equilibrium precipitation of CSH phases. An initial period of several pore volumes, where the pH and Ca are buffered, can be explained qualitatively assuming sorption onto Fe phases. However, the amount of buffering predicted is significantly lower than that observed.