Published online by Cambridge University Press: 10 February 2011
In two-phase matrices based on pyrochlore-oxide, perovskite-oxide, and pyrochlore-zirconolite assemblages as much as 40 wt.% of the product can consist of an incorporated rare earth - actinide fraction of high level wastes (HLW). In zirconolite, with a nominal stoichiometry of AVIIIBVICV-VI2O7 the actmide and rare earth ions occupy VIII- and VII-fold coordinated sites. Charge compensation is achieved by replacement of Ti4+ with lower valence ions such as Al3+, Fe2+/3+. Mg2+, etc, that have similar radii. The highest lanthanide incorporation (45 wt.% σREE2O3) was exhibited by a calcium free zirconolite containing 8 wt.% A12O3 or 0.7 formula units (f.u.). In pyrochlore (AVIII2BVI2O7−x) actinides and rare earths occupy A-sites. Their incorporation does not require simultaneous ionic substitution in the B-sites, for example Al3+ or Fe 2+/3+ for TI4+, to achieve charge compensation; this simplifies the required composition of the system. Pyrochlore ceramics are suitable for immobilization of wastes with relative elevated actinide content. For a high zirconia, waste ZrO2-based or two-phase pyrochlore-dioxide, perovskite-dioxide or pyrochlore-zirconolite ceramics are preferable. All these wasteforms may be produced via melting