The Microstructure of Synroc
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D.R.COUSENS, S.MYHRA, J.PENROSE, R.L.SEGALL, R.St.C. SMART AND P.S. TURNER School of Science, Griffith University, Nathan, Queensland 4111, Australia
INTRODUCTION The Synroc concept was introduced by Ringwood in
19781 and, as is
well known,
the material now proposed for the disposal of hiah-level civilian nuclear waste consists primarily of hollandite (BaAl zirconolite
There is ceramics.
2 Ti 6 0 1 6
), perovskite
(CaTiO3 ) and
(CaZrTi 2 0 7 ). limited theoretical understanding of the dissolution kinetics of
The known excellent leach resistance of rutile does not of itself
provide an explanation for the leach resistance of a mixture of titanates such as Synroc. Moreover,
variable dissolution rates have been reported depending on 2
the precise conditions of manufacture '
3
and clearly the microstructure will be
important in determining dissolution rates. For example, the presence of minor phases4 will be significant if
important elements such as Cs and Sr segregate
into them and become more readily leached. More generally,
studies of simple
ionic5 and semiconducting oxides6,7 have shown that structural parameters are important in dissolution.
Another aspect of any proposed high-level waste
matrix which must be understood is
the effect of irradiation by energetic par-
ticles. Again microstructural information is
essential for a detailed analysis.
Factors like phase size, grain size, phase distribution, the presence of minor phases and their location are all significant in interpreting radiation damage studies.
Finally, and perhaps most significantly, the routine production of a
reproducible ceramic waste form requires a knowledge of the processes of manufacture under real, non-equilibrium conditions.
Microstructural
studies are
a major source of information on the details of the evolution of the final product from the complex
The work is
mixture of starting materials.
primarily on Synroc B (the material without radwaste) which shoed
have the same structure as Synroc C (the material containing simulated waste) since the waste elements are supposed to partition themselves between the perovskite, described.
8
hollandite and zirconolite .
However,
work on Synroc C is
also
310 EXPERIMENTAL The research has been done on Synroc prepared at the Australian Atomic Energy Commission Research Establishment via the oxide route.
Fine powders of TiO2 ,
A12 0 3 , ZrO2 , BaCO3 and CaCO3 are ball milled and the final product is made by hot pressing 9 . The Synroc C studied had a waste loading of 10 wt percent. Specimens have been examined by conventional transmission electron microscopy at 100kV, by high resolution transmission electron microscopy using the JEOL 200CX operating at 200kV, by high voltage transmission electron microscopy at 1MeV using the EM7, by conventional scanning electron microscopy and by scanning transmission electron microscopy with energy dispersive analysis.
In addition
mechanically polished and chemically or thermally etched specimens have been examined by optical
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