RADWASTE INCORPORATION IN SYNROC. E. R. Vance, B. D. Begg, R. A. Day, C. J. Ball, Materials Division, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia

Synroc is a multiphase titanate ceramic designed to immobilize high-level radioactive waste. It is largely composed of hollandite (Ba(AlTi)₂Ti₆O₁₆), zirconolite (CaZrTi₂O₇), perovskite (CaTiO₃) and rutile (TiO₂), and incorporates the radwaste ions in dilute solid solution. Early work on Synroc was concerned with waste from reprocessing of spent fuel from commercial power reactors. Recently attention has turned to a variety of other waste compositions, such as defense wastes and actinide-rich wastes arising from chemical partitioning of commercial waste. Defense wastes differ from commercial waste in that they contain large amounts of processing contaminants, notably sodium and aluminium. A modified Synroc formulation can accommodate these contaminants.

Investigations with single phases, mainly zirconolite, have sought to determine the limits of solubility of different elements in different sites. Preliminary work with Ce, Nd and Y established the probable behaviour of Np and Pu in zirconolite, and this has been confirmed at levels of tens of percent. Trivalent rare earths and actinides have extensive solid solubility in the Ca site of zirconolite if sufficient Al, for example, is present to provide charge compensation by substituting for Ti. The solubility in the Zr site depends on ionic size. Tetravalent rare earths and actinides can also substitute in the Ca and Zr sites. The limit of such substitution in the Zr site is 0.15 formula units, and at least 0.2 formula units can also be substituted on the Ca site. X-ray absorption spectroscopy confirms the valency of these ions, but the precise charge compensation mechanism is yet to be determined.