ACTINIDE AND RARE-EARTH INCORPORATION INTO ZIRCONOLITE

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