Influence of Transition Metal Charge Compensation Species on Phase Assemblage in Zirconolite Ceramics for Pu Immobilisation

Abstract

Immobilisation of Pu in a zirconolite matrix (CaZrTi₂O₇) is a viable pathway to disposition. A-site substitution, in which Pu⁴⁺ is accommodated into the Ca²⁺ site in zirconolite, coupled with sufficient trivalent M³⁺/Ti⁴⁺ substitution (where M³⁺ = Fe, Al, Cr), has been systematically evaluated using Ce⁴⁺ as a structural analogue for Pu⁴⁺. A broadly similar phase assemblage of zirconolite-2M and minor perovskite was observed when targeting low levels of Ce incorporation. As the targeted Ce fraction was elevated, secondary phase formation was influenced by choice of M³⁺ species. Co-incorporation of Ce/Fe resulted in the stabilisation of a minor Ce-containing perovskite phase at high wasteloading, whereas considerable phase segregation was observed for Cr³⁺ incorporation. The most favourable substitution approach appeared to be achieved with the use of Al³⁺, as no perovskite or free CeO₂ was observed. However, high temperature treatments of Al containing specimens resulted in the formation of a secondary Ce-containing hibonite phase.