Immobilization of molten salt residue using alkali-activated and cement-based materials

Some radioactively contaminated solid organic waste streams are problematic to dispose of by direct immobilization in known matrices (e.g. cementitious binders), as they can potentially degrade after disposal, possibly resulting in the release of radionuclides. Within work package 6 of the PREDIS project, techniques for the treatment and disposal of these waste streams are investigated [1]. One of the possible ways of treatment is the molten salt oxidation (MSO) process, in which the bulk of the organic waste can be oxidized at a high temperature in the presence of a sodium carbonate salt [2]. The residue after the MSO process is a salt containing radionuclides in carbonate forms. The goal of this study is to investigate whether it is possible to immobilize this salt residue in an alkali-activated or blended cementitious matrix resulting in a waste form with good mechanical properties and long-term durability. Previous experiments with direct immobilization of the salt into these matrices has been proven to be ineffective. Therefore, the salt will first be pre-treated to increase the compatibility with its matrix. After ensuring good mechanical properties, thermodynamic modelling of the system will be performed to understand the stability of the salt-related phases. Future work will include carbonation, leaching and resistance to alkali-silica reaction tests to ensure long-term durability of the waste forms.