Reactive-transport modelling of the LCS in situ experiment

Cementitious materials are commonly included as encapsulants and backfill or barrier materials in geological disposal facilities for radioactive wastes and an understanding of the long-term behaviour of cement-based materials is therefore required to provide confidence in their safety functions.  The Long Term Cement Studies (LCS) project was an international collaboration with the overall aim of increasing understanding of high-pH cement interaction effects in the repository near field and geosphere, in order to make confident, robust and safety-relevant predictions of future system behaviour, irrespective of host rock, EBS and waste type.  The experiment involved the emplacement of hardened Portland cement through a borehole into a fully-saturated, water-conducting shear zone (fracture) in granite for a period of six years, to explore cement leaching/degradation and the interaction of highly-alkaline fluids with the rock.

The results of the work have been published in a paper co-authored by Claire Watson and James Wilson, along with colleagues from Savage Earth Associates Ltd, and RWM.  Reactive transport models of the cement present in the experiment illustrate that it is possible to simulate the behaviour of Portland cement interacting with a low ionic strength groundwater, namely leaching of the cement (dissolution of primary solids, especially portlandite) and precipitation of carbonate minerals and secondary aluminosilicates, using a complex 3D model geometry.  However, the model results highlight uncertainties surrounding cement solid dissolution rates and rates of secondary mineral formation, both of which could be explored in future research.  The study illustrates the importance of modelling large-scale experimental systems which, along with natural/industrial analogue data, can be used to build confidence in the long-term behaviour of engineered barriers in radioactive waste disposal systems.