Uranium and Rare Earth Partitioning in Synroc
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URANIUM AND RARE EARTH PARTITIONING IN SYNROC K. L. SMITH, G. R. LUMPKIN and M. G. BLACKFORD Advanced Materials Program, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, N.S.W. 2234, Australia. ABSTRACT Improved AEM techniques were used to investigate three Synrocs containing 10 wt% simulated HLW and a fourth sample with -18 wt% simulated HLW. One of the 10 wt.% loaded Synrocs also contained an addition of 1.0 wt% Na20 and another contained an addition of 2.0 wt% Fe20 3 . This work is part of a larger study initiated with the objective of determining if the bulk composition of Synroc affects the partitioning of elements between individual phases. Results from the four samples in this study show that, as expected, elemental partitioning is mainly controlled by the ionic radius criterion, with smaller Y, Gd, and U ions having a preference for zirconolite and the larger Ce and Nd ions favouring perovskite. Additions of Na and Fe lead to the formation of CAT and loveringite at the expense of rutile or Magneli phases, but only have minor effects on partitioning coefficients. Partitioning coefficients, D7 /P, for REE, Y, and U in the four Synrocs are the same (within experimental error). INTRODUCTION Synroc is a dense titanate ceramic designed to immobilise high-level nuclear waste (HLW). In standard Synroc-Cl the constituent elements in HLW are predominantly incorporated in zirconolite, perovskite, and hollandite. High quality Synroc with 10 wt% simulated HLW consists of approximately 30 vol%
hollandite, 25 vol% zirconolite, 20 vol% perovskite, and 20 vol% rutile + Magneli phases. 2 The remaining 5 vol% consists of intermetallic alloys and a selection of minor phases including aluminium oxide, hibonite, pseudobrookite, loveringite, and an incompletely characterized calcium aluminium titanate phase (CAT3 ). The durability of Synroc when exposed to aqueous solutions is well established. 1 The relative stability of Synroc phases upon exposure to leachants decreases in the order zirconolite > hollandite > perovskite > intermetallic and minor phases. 2 ,4 Scanning and transmission electron microscopy (SEM and AEM respectively) of Synroc exposed to deionised water at 150'C for 532d, showed that zirconolite remains virtually unaltered, hollandite suffers minor alteration to anatase, perovskite alters to anatase to a maximum depth of 0.2 pIm and some of the minor phases showed alteration to slightly greater depths (< 0.5 gm). Secondary phases, including monazite, form on the surface of Synroc as a result of alteration. In order to predict the amounts of waste elements that go into solution, are incorporated by alteration phases, or remain locked within Synroc host phases, it is essential that we make an assessment of their partitioning behaviour. Previous authors have provided information on ACT and REE partitioning in Synroc and related waste forms 5 -9 and on the solid solution limits of ACTs and REEs in individual phases 10,11. Eighteen Synrocs containing various amounts of a single impurity (Na20, P2 0
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