Quintessa

Cement in underground vaults

The long-term behaviour of cementitious engineered barriers is an important process to consider when modelling the stability and safety of geological repositories for nuclear waste. Many geological disposal concepts incorporate cement either to encapsulate the waste forms, to provide a chemical buffer, or to provide structural integrity for the underground system of deposition tunnels. In the presence of invasive groundwater, the chemical and physical properties of cement, such as its pH buffer capacity, resistance to flow, and its mechanical properties, can evolve with time. A variety of transport, chemical, and mechanical processes play an important role in the degradation of concrete and must be better understood in order to quantify such effects.

The modelling of cement is complicated by the fact that the cement is dominated by the behaviour of calcium silicate hydrate (CSH) gel, which is a complex solid exhibiting incongruent dissolution behaviour. Quintessa has carried out a range of modelling studies for this system using the Raiden reactive geochemistry and transport code.

For example, it has been shown that it is possible to couple various conceptual models for the evolution of physical properties of concrete with a solid solution model for cement degradation in a fully coupled geochemical transport model to describe the interaction of cement/concrete engineered barriers with groundwater. The results show that changes to the conceptual models and flow rates can give rise to very different evolutions. Most Simulations have been carried out at a reduced 'experimental' scale and at the full repository scale.