Synroc: A Suitable Waste Form for Actinides
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strong likelihood of existing for millions of years in hot, wet environments. The compatibilities of the phases mean that their proportions can undergo great changes, thus minimizing rigid constraints on the waste/precursor ratios. The most durable phase in Synroc-C is zirconolite, a ceramic of nominal CaZrTi2O7 stoichiometry. Because it can accommodate appreciable amounts of actinide ions in both the Ca and Zr sites, zirconolite is an ideal host for the actinide-rich, highlevel wastes that would arise in the chemical partitioning strategies being considered by Japan and France. Similarly, U.S. and Russian weapons-grade Pu could be immobilized in zirconolite. Our approach to immobilizing these wastes is to fabricate a titanate ceramic containing approximately 80 wt% of zirconolite, together with small amounts of perovskite, hollandite, and rutile—which incorporate residual heat-generating fission products—plus metal alloys, if noble metal fission products are also present. This zirconolite-rich Synroc can be made by substantially the same methods as Synroc-C and has enough chemical flexibility to deal with inexact waste/precursor ratios, changes in waste chemistry, imperfect mixing, metastable phase formation due to finitetime consolidation sequences, and reduc-
ing atmospheres imposed by the necessity to avoid the formation of leachable pentaand hexavalent actinides. This review will deal mainly with zirconolite. The CaO-ZrO2-TiO2 system has been carefully studied by Coughanour et al.3 Subsequent work by Rossell4 has shown that zirconolite has a range of stoichiometry, with (Zr + Ti) being a constant at 3 formula units, but with the Zr content ranging from 0.85 to 1.15 units. This article is concerned mainly with the incorporation of rare earths and actinides into zirconolite. The incorporation of high levels of rare earths stimulates the formation of a variety of poly types in zirconolite5 but, while these have been studied in detail by transmission electron microscopy (TEM), the precise crystallography and operative charge compensation schemes are still not clear. The rare earths are used to simulate actinides in the present application, but they are also of interest relative to Synroc-C. In a multi-cation crystalline host such as zirconolite, the inclusion of guest (mainly radioactive) 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 host ion being replaced have different valences. Hence, a major feature of the phase design of actinide-containing, zirconolite-rich titanate ceramics is the valence state that the actinide will assume when it is directed by an explicit substitution, toward a given site in zirconolite. Penta- and hexavalent actinides are undesirable on the basis of leachability, so we need to process zirconolite-rich ceramics under reducing conditions. Although many data exist on the occurrences of individual actinide oxides as a function of oxygen fugacity and temperature, differe
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