Zirconolite-Rich Ceramics for Actinide Wastes
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ZIRCONOLITE-RICH CERAMICS FOR ACTINIDE WASTES ERIC R.VANCE, B.D.BEGG, R.A.DAY and C.J.BALL Advanced Materials Program, ANSTO, Menai, NSW 2234, AUSTRALIA.
ABSTRACT New X-ray diffraction and scanning electron microscopy data are given for the incorporation of Np and Pu in zirconolite, at levels of tens of percent. The actinide valences and the cations they replace are deduced from the microanalysis of the zirconolite compositions, and X-ray absorption data are used to obtain more direct information on the valences of Ce and Nd, which are used as simulants of Pu and trivalent actinides respectively. Trivalent rare earths and actinides have extensive solid solubility in zirconolite, mainly but not exclusively in the Ca site. Tetravalent rare earths and actinides have considerable solid solubility in the Zr site of zirconolite, and some solubility in the Ca site, but the strong tendency of zirconolite with ions substituted in the Zr site to undergo phase separation complicates structural interpretation. In zirconolite-rich Synroc-type ceramics designed to immobilise waste actinides, the target actinide waste loading has been set at 20 wt% and early leach results indicate the durability is at least as good as that of Synroc-C.
INTRODUCTION Zirconolite, CaZrTi 20 7 , is the most durable phase in Synroc-C[l]. As zirconolite can accommodate appreciable amounts of actinide ions in both the Ca and Zr sites, it is an ideal host for actinide-rich high-level wastes. These wastes can arise from chemical partitioning of highlevel waste, which is under study in Japan and France. U.S. and Russian weapons-grade Pu could also be immobilised in zirconolite. Our present target with these wastes is a titanate ceramic containing 80 wt% of zirconolite,
together with small amounts of perovskite, hollandite and rutile which incorporate residual heatgenerating fission products, plus metal alloys if noble metal fission products are also present. This ceramic can be made by substantially the same methods as Synroc-C and is designed to have enough chemical flexibility to deal with inexact waste/precursor ratios, changes in waste chemistry, imperfect mixing, and metastable phase formation due to finite-time consolidation sequences. In a multi-cation crystalline host such as zirconolite, the inclusion of guest ions proceeds by substitution for a given host ion, rather than via a simple additive mechanism. Charge compensation must also be considered when the guest ion and the host ion being replaced have different valences. Hence a major feature of the phase design of actinide-containing zirconoliterich titanate ceramics is the valence state that the actinide will assume when it is present on a given site in zirconolite. Another important aspect is the solid solution limit of each actinide. Although many data exist on the limits of redox stability of individual actinide oxides , crystalchemical stabilisation forces will be present when actinides are incorporated in other crystalline structures. For example, although U0 2 is highly unstable with
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