ACTINIDE AND RARE-EARTH INCORPORATION INTO ZIRCONOLITE

Nd3+ and Ce3+ can substitute for about 65% of the Ca in monoclinic zirconolite, using Al as a charge compensator in a Ti site. Further substitution up to 85% of Nd3+ and Ce3+ produces an orthorhombic structure, while more than 85% substitution produces additional phases. Substitution of Ce4+ in the Zr site appeared to be quite limited. Incorporation of U4+ into the Ca and Zr sites in zirconolite gave results which were similar to those observed by others. Both trivalent and tetravalent Np and Pu can be substituted in the Ca and Zr sites, respectively, under oxygen partial pressures of 0.2-1x10(-5) atm, provided appropriate charge compensators are present. The implications of these results for formulating actinide-bearing zirconolite-rich ceramics are discussed briefly.