In the Belgian reference design for geological disposal of high-level radioactive waste in a poorly indurated clay formation, the waste forms are encapsulated in non-reinforced concrete buffers (called supercontainers), which are then placed in disposal galleries. Mechanical stability of such galleries is ensured via construction of concrete liners to prevent their convergence due to overburden pressure. Furthermore, cementitious backfills are introduced in the annulus between the concrete buffer and gallery liner. This implies that significant volumes of cementitious materials would be inevitable. Therefore, the study of long term durability of such materials are of vital importance to predict the behavior of the disposal system. In particular, a thorough understanding of time dependent deformation of the material that includes creep and shrinkage are crucial. This is not only to understand stability issues during the operational phase but also the post-closure phase of the disposal system.
Significant work has already been carried out in the field of creep and shrinkage of cementitious materials at normal as well as slightly elevated temperatures. However, the problem at hand is rather unique. The main challenge arises from the gradual release of heat due to radioactive decay that can exceed 80 °C by far (depending on the type of waste) at the waste canister and buffer interface during initial years after emplacement. These conditions last for several hundreds to thousands of years until the temperature drops back to normal geological conditions. In case of concrete liners, the peak temperature may even reach 60 °C and requires similar time frame for the temperature to drop back to geological conditions. Furthermore, specific to the operational phase, the shrinkage behaviour may be affected by the prevailing relative humidity (RH) conditions.
The objective of the PhD is therefore to explore these time dependent deformation processes at elevated temperature and varying RH conditions from a phenomenological perspective via experimental and numerical investigations. The main deliverable of the thesis will be a new constitutive law that can predict multi-decade creep and shrinkage strains at the relevant elevated temperatures.