A STUDY OF THE EFFECTS OF AN ALKALINE PLUME FROM A CEMENTITIOUS REPOSITORY ON GEOLOGICAL-MATERIALS

Current plans for the disposal of intermediate-level radioactive waste in the U.K. include a cementitious backfill within the disposal caver ns. Nuclides migrating from the repository into the rock will be prece ded by an alkaline plume derived from the cement pore water. The alkal ine plume will interact with the rock around the repository and may ch ange the mineralogical and transport properties of the rock. An experi mental and modelling study of alkali interaction with rock is presente d. In the experiments, an alkaline solution was passed through a sampl e 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. Afte r completion of the experiment, the sample was examined using SEM. The se observations identified quartz and potassium feldspar dissolution, and precipitation of CSH phases. As a first stage in the interpretatio n of this behaviour, a very simple model was used. Successive refineme nts were made to the model to examine the effect of individual phenome na. Agreement with the steady-state Ph and Ca concentration was obtain ed with a model involving kinetically-controlled quartz dissolution an d equilibrium precipitation of CSH phases. An initial period of severa l pore volumes, where the pH and Ca are buffered, can be explained qua litatively assuming sorption onto Fe phases. However, the amount of bu ffering predicted is significantly lower than that observed.