Solid solution limits in synroc zirconolite

Abstract

In the titanate ceramic high-level wasteform SYNROC, zirconolite (CaZrTi 2O 7) is the primary host for uranium and tetravalent actinides, and the secondary host for rare earths and trivalent actinides. Significant amounts of other fission products and processing contaminants also enter into solid solution in its lattice. Experiments investigating the binary joins between CaZrTi 2O 7 and various real or hypothetical end-members show that zirconolite solid solutions can accept 27 wt% UO 2, 20% ThO 2, at least 29% rare earth oxides, 14% Nb 2O 3, 7% Al 2O 3, 4% MgO, 4% FeO or 5% MnO, but only limited amounts of Na 2O, SrO and NiO. The smaller cations Nb 5+, Al 3+, Mg 2+ and Fe 2+ are partitioned into the Ti-sites. Rare earths are preferentially accommodated in the Ca-site but also occupy the Zr-site. Th 4+ and Mn 2+ are strongly partitioned into the Ca-site. In contrast with natural zirconolites, U 4+ enters the Zr-site, but can be accommodated in both Ca- and Zr-sites via coupled substitution of a small lower valence cation eg. Mg 2+, Fe 2+, Fe 3+, or Al 3+ for Ti 4+ A comparison of natural and synthetic zirconolites indicates that the structure requires at least 50 mole % occupancy of the Ti-sites by Ti 4+ and 70 mole % occupancy of the Zr-site by Zr 4+ The Ca-site accepts a variety of multivalent cations similar in size to Ca 2+. The five distinct cation-acceptor sites in the zirconolite lattice permit numerous simple and multiple cation substitutions and thereby contribute to the capacity of SNYROC to immobilize high-level wastestreams of variable composition.