Solid-state and precipitation-based synthesis routes for zirconolite-rich powders for plutonium immobilisation

Zirconolite (prototypically CaZrTi2O7) is a wasteform system under consideration for the immobilisation of the United Kingdom’s civil inventory of separated PuO2. The laboratory-scale synthesis route for zirconolite typically adopted is a route whereby individual oxide precursor powders are co-milled with PuO2 (or a non-radioactive surrogate such as CeO2). However, translating this sort of approach to an industrial-scale process is likely to be challenging due to the difficulty of feeding multiple feedstocks into a Pu-active plant under stringent constraints. As such, a process route through which a single precursor could be prepared in a separate, upstream inactive facility, prior to being later combined with PuO2, would be strategically advantageous. In this work, ‘single zirconolite-rich precursor’ approaches were investigated for synthesis of Ce-doped zirconolite, with both dry solid-state and wet alkoxide-nitrate precipitation synthesis routes utilised. These approaches were assessed using cold press and sinter trials in this case, with both dry and wet routes found to be capable of producing high density zirconolites. The dry route formed crystalline Ce-bearing zirconolite ceramics containing a relatively minor perovskite secondary phase. The wet-route precursor was significantly structurally disordered and subsequently formed a zirconolite-rich ceramic which also contained a perovskite secondary phase. Based on a limited study under consistent analytical conditions, it was also found that dry route reported a more predictable powder flow behaviour and therefore may offer a better route to future industrialisation. Overall, these results indicate that a single zirconolite-rich precursor may be a credible approach for wasteform synthesis in the context of PuO2 immobilisation.