The Supercontainer is the Belgian reference concept proposed by ONDRAF/NIRAS for the final disposal of heat emitting waste. It is based on a multiple barrier system whereby every component of the Supercontainer plays a specific safety function or role requirement. In this reference concept, the vitrified HLW and Spent Fuel (SF) assemblies are encapsulated in a watertight carbon steel overpack surrounded by a cylindrical concrete buffer for disposal in a deep geological clay layer. The Supercontainer is the first engineered barrier on which the safety of the repository depends and also provides a radiation shield, which minimizes handling operations in the underground repository and guarantees optimum protection for the operators. Therefore, understanding the stress-strain behaviour of Supercontainers from early stages up to several years of exploitation is of significant and strategic importance. Currently, half scale tests are feasibility tests performed at the surface on scaled models of a Supercontainer to study the construction feasibility and thermo-mechanical behaviour of the concrete materials, including concrete cracking and propagation.
In the framework of an experimental testing program to evaluate the construction feasibility of the Supercontainer, Craeye (2009) studied the early stage thermo-mechanical behaviour of the concrete buffer using the numerical model MLS-HEAT and validated the study using results from the first half-scale test performed in 2009 and a second test carried out in 2013. The results of both tests showed the absence of surface cracks in the buffer during the first construction phase, while cracks developed in both tests during the second phase when exposed to heating to simulate the temperature generated by radioactive waste. In both cases, these studies failed to account for the development of cracks observed during the heating phase. The modelling work performed by Craeye in 2009 and 2013 essentially provides a sensitivity analysis of the thermo-mechanical parameters to identify cracking potential in the concrete buffer.
It is therefore apparent that:
- The existing approach to capture the potential for crack formation is incomplete.
- There is no provision to predict initiation and propagation of cracks.
- There is no provision to capture the effects of morphological features of the material such as the presence of aggregates.
In fact, the problem at hand is very challenging and involves complex coupling between thermal, hydraulic and mechanical processes (THM). In particular, the moisture field plays a crucial role in the stress-strain behaviour of the material under thermal transients. Inclusion of the moisture field invokes several important processes such as autogenous and drying shrinkage and creep induced by moisture transport, vapour transport, conduction, convection, latent heat of vapourization, etc. Another important consideration is to model these processes explicitly at a lower scale (mesoscale), which consists of the cement paste, the interfacial transition zone and the aggregates. In this way, the THM behaviour of the different components of concrete can be individually captured, which can then be upscaled to evaluate the macroscopic THM behaviour of the material and hence the potential for crack formation.
The second half-scale test comprised an extensive monitoring program, including AE, DIC and various types of embedded strain gauges and optical fibres installed both on the surface and embedded in the concrete. They also used TDR moisture probes in the concrete to measure the evolution of moisture during the test. These results provide an extensive data base with the necessary input parameters for validation of the THM model.