Thermo-chemo-mechanical behaviour of cemented waste forms at the drum scale - ASR case study 1

Ordinary Portland cement (OPC)-based conditioning is widely applied to radioactive waste because it provides a practicable and economical route for immobilizing heterogeneous waste streams into packages that can satisfy requirements for handling, interim storage, transport and eventual disposal (Uras et al., 2021). During the predisposal phase, however, package acceptability depends not only on initial strength and radionuclide retention, but also on controlling formulation-dependent reactions that may cause swelling, cracking or loss of containment during storage. The origin of these durability issues is thermo-chemo-mechanical perturbations. For example, the substantial heat released by OPC hydration, particularly at high binder or waste loadings and in large packages, can raise internal temperatures sufficiently to suppress early ettringite stability and create conditions for subsequent delayed ettringite formation (DEF), expansion and cracking (Danfour et al., 2024). Yet another relevant example is the mechanical consequences of alkali-silica reaction (ASR) in cemented waste forms. ASR may develop when reactive siliceous waste or aggregate constituents interact with the highly alkaline pore solution, producing an expansive gel and progressive mechanical damage. OPC-based conditioning must therefore be designed and qualified through control of waste compatibility, alkali and sulfate availability, aggregate reactivity, hydration heat, package dimensions and curing conditions, supported by monitoring capable of detecting incipient ASR-DEF damage during predisposal storage. Currently, SCK CEN is carrying out live drum scale experiments for these two processes to understand the evolution of the OPC based simulated waste forms. Thermal, strain and humidity variables have been continuously monitored, with post characterization of these experiments planned for the coming years.

For this Master’s thesis, the main objectives are

1.    Apply an existing thermodynamic model to determine hydration heat evolution of the cemented waste form. This step involves calibrating the isothermal calorimetric data with a thermodynamic model of cement hydration.
2.    Apply an existing DEF model coupled with the above hydration heat model to predict the potential DEF in the drum and if data becomes available to validate it.