Optimising glass-ceramic compositions for zirconolite-based actinide immobilisation

• Al/Ca/Ti additions to borosilicate glass achieved phase-purity in zirconolite glass-ceramics. • Phase-pure Ce-bearing zirconolite glass-ceramics with varying glass contents were created. • Glass addition lowered the sintering temperature (1320°C to 1270°C) to immobilise Ce. Zirconolite is a candidate wasteform for actinide immobilisation. The addition of glass to form a glass-ceramic (GC) is also under consideration as GC materials provide flexibility to immobilise heterogeneous actinide wastes and simplify processing requirements. However, a major challenge in the design of zirconolite GCs is control of the phase assemblage to minimise unwanted phase formation, particularly at high glass contents where zirconolite can be destabilised in the glass melt during consolidation. In the current research, an optimal glass composition was developed to minimise unwanted secondary phases. Initially, GCs targeting zirconolite (CaZrTi 2 O 7 ) with varying amounts (0-100 wt%) of glass addition (NaAl 0.5 B 0.5 Si 2 O 6 ) were fabricated using a pre-synthesis route. X-ray diffraction (XRD) analysis of these baseline formulations showed that undesired phases (e.g., zircon) became more apparent at higher glass contents (e.g., 75 wt%). Following this, the additions of Al 2 O 3 , CaO and TiO 2 to the glass composition minimised unwanted phase formation in the GCs, including those formulations with high glass contents. The optimal glass composition was determined to be NaAl 1.5 Ca 0.7 Ti 0.2 B 0.5 Si 2 O 8.6 . Ce-bearing zirconolite GCs (Ca 0.8 Ce 0.2 ZrTi 1.6 Al 0.4 O 7 ; Ce as actinide surrogate) with varying amounts (0-100 vol%) of the tailored glass design (NaAl 1.5 Ca 0.7 Ti 0.2 B 0.5 Si 2 O 8.6 ) were then fabricated using an in-situ crystallisation route. XRD and scanning electron microscopy (SEM) analyses showed that near phase-pure microstructures were achieved across all glass contents. Furthermore, the addition of glass lowered the sintering temperature (1320°C to 1270°C) needed to immobilise CeO 2 in zirconolite.