Alkali-activated materials for radionuclide immobilisation and the effect of precursor composition on Cs/Sr retention

Abstract

One of the major challenges for the nuclear industry is the safe and sustainable immobilisation of radioactive wastes (RAW). Currently, the most commonly used immobilisation matrices for low and intermediate level wastes are based on ordinary Portland cement. For the more difficult to immobilise nuclides, such as caesium (Cs+) and strontium (Sr2+), researchers have been studying alternative immobilisation matrices, of which alkali-activated materials (AAM) are a very promising option. However, the differences in precursor compositions and the use of different types of activating solutions make it difficult to fully understand the effects of precursor composition on the immobilisation of introduced nuclides. Therefore, six different compositions of laboratory-synthesised Ca-Si-Al slags were developed to serve as precursors for low-alkaline AAMs to study their immobilisation behaviour. Immobilisation capacities up to 97.6% Cs+ and 99.9% Sr2+ were achieved with 1 wt% and 0.1 wt% waste loading, respectively, when leaching for 7 days at 20 °C in Milli Q water. Cs+ immobilisation is higher at lower Si/Al and Ca/(Si + Al) ratios. Immobilisation of Sr2+ is higher at a lower Ca/(Si + Al) ratio and independent of Si/Al ratio. The results of this study offer a deeper understanding of the immobilisation behaviour of AAMs and can encourage further research and application of AAMs for RAW immobilisation.