Effect of supplementary cementitious materials on chemical degradation of cement-based barriers in radioactive waste repository using reactive transport modeling

Abstract

This study analyzed the chemical degradation behavior and the effect of supplementary cementitious materials (SCMs) replacement on cement-based disposal vaults for radioactive waste in a surface disposal facility using a reactive transport model. The reactive transport model was developed by combining pure thermodynamic models with water and solute transport simulations. Pure thermodynamic calculations were performed using the PHREEQC code and the THERMOCHIMIE database, while water and solute transport calculations were conducted using the COMSOL code. Simulations were carried out by varying the SCMs content to reflect the key factors that are expected to cause the chemical degradation of the cement-based disposal vault walls, mainly precipitation and the cement-SCMs mixing ratio. The modeling results showed that the chemical degradation of the cement-based barrier was most significantly influenced by the replacement of silica fume among other SCMs, which was found to promote the pozzolanic reaction. The durability of the barriers against chemical degradation was highest when the silica fume content was around 20%. This was attributed to the influence of the initial formation of calcium silicate hydrate (CSH) compounds. The reactive transport model was used to predict degradation in the structural integrity of the disposal vault over time by examining the relationship between porosity and compressive strength. Furthermore, to manage the uncertainty in the degradation model of the cement-based barrier, boundary conditions related to climate change were developed. It was found that the model was highly sensitive to factors such as temperature and effective diffusion coefficient. Based on the calculated durability of the cement-based vault walls, the barrier is predicted to last for at least several thousand years.