Dissolution Mechanisms of the Perovskite and Hollandite Phases in the Synroc Assemblage
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DISSOLUTION MECHANISMS OF THE PEROVSKITE AND HOLLANDITE PHASES IN THE SYNROC ASSEMBLAGE
S. NYHRA,
D.K.
PHAM,
R.ST.C. SMART* AND P.S. TURNER
Nathan, Qid. Division of Science and Technology, Griffith University, 4111, Australia. *School of Chemical Technology, South Australian Institute of Technology Ingle Farm, S.A. 5098, Australia ABSTRACT Perovskite (CaTiO3 )and hollandite
([BaCs,][(Ti,At)
3
+
2x+y
Ti
4
+
]016)
0-2x-y
are two of the three major phases of the Synroc titanate mineral assemblage. The chemical durabilities of these two phases have been analysis and electron analysis, surface by solution investigated While a detailed model of the durability of perovskite has microscopy. evolved from previous work, much less is known about the mechanisms which The present investigations show affect the durability of hollandite. that, for single-phase hollandite, selective extraction by ion exchange 2 On the other hand, of Cs+ and Ba * does not play a significant role. total dissolution of the network does seem to occur as a result of Also, complexation and precipitation of At base-catalysed hydrolysis. and TI species in solution has the effect of driving the system to low pH. These results can be correlated with the more complete model for dissolution of perovskite in order to infer the overall effects of aqueous attack on the Synroc HLW solid. INTRODUCTION The Synroc concept for disposal of high level radioactive waste was The Synroc waste form is a multiphase originally proposed in 1978 [I]. Two of the major phases are perovskite and titanate ceramic ensemble. primary hosts for Sr- and Cs-isotopes, which are the hollandite, respectively. The chemical durabillties, and the mechanisms which affect these, of the two phases are therefore relevant to the role of Synroc as While a phenomenological approach has been adequate for an IILW solid. a comparative evaluation of Synroc under standard conditions [2,3], predictive model will be required in order to demonstrate the performance repository actual and hypothetical various of the waste form in The present approach has been environments over geological time spans. to investigate the major phases separately with a view to a final for the Synroc integration of the results into an overall model To this end we have found it necessary to dispense with the assemblage. customary battery of MCC type tests (4] in favour of methodologies which are tailored to the relevant kinetic and thermodynamic regimes, and which the bulk solution, can probe the total system (the bulk solid, liquid/solid interface and the near-interface surface layers). A variety in order to provide the of experimental techniques have been used, X-ray (solution analysis, information complementary necessary Transmission and Secondary Electron Photoelectron Spectroscopy (XPS), Microscopy (TEN and SEN) with Energy Dispersive X-Ray Spectroscopy (EDXS)). This work has been underway in our laboratory for several years. (5,6]. reported elsewhere results have been the early Some of Comprehensive description
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