Phase Relations In The Systems Based On Yttrium, Gadolinium, Manganese, Titanium, and Uranium Oxides Doped with Calcium,
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PHASE RELATIONS IN THE SYSTEMS BASED ON YTTRIUM, GADOLINIUM, MANGANESE, TITANIUM, AND URANIUM OXIDES DOPED WITH CALCIUM, ALUMINUM AND IRON OXIDES Sergey V. Stefanovsky1, Olga I. Stefanovsky1, Sergey V. Yudintsev2, Boris S. Nikonov2 1
SIA Radon, 7th Rostovskii lane 2/14, Moscow 119121 RUSSIA, [email protected] 2 Institute of Geology of Ore Deposits RAS, Staromonetnii lane 35, Moscow 109017 RUSSIA, ABSTRACT Titanate-based ceramics with yttrium, gadolinium, manganese, uranium, calcium, aluminum, and iron oxides being of interest as matrices for radioactive wastes containing lanthanides, actinides and corrosion products were synthesized using cold pressing and sintering and melting routes. Major constituents of the ceramics are fluorite structure phases (pyrochlore, “murataite”, cubic oxide with fluorite-related lattice) as well as brannerite. Incorporation of aluminum and iron oxides (corrosion products) forms second phases with perovskite or/and crichtonite structure. INTRODUCTION Titanate ceramics based on the phases with fluorite-derived structure (zirconia-urania based cubic solid solution, yttria-stabilized zirconia, pyrochlore, murataite, zirconolite) are considered as promising matrices for immobilization of lanthanide and actinides of high level waste (HLW) [1]. Waste corrosion products (mainly iron group elements and aluminum) may be incorporated in ceramics within either some of above-listed or additional second (perovskite, hibonite/loveringite, crichtonite) phases. Polyphase ceramics in which complex phases capable of accumulating actinides and/or lanthanides and corrosion products simultaneously have been formed under various conditions within certain compositional ranges. These retaining high chemical durability and long-term stability are of particular interest [2,3]. Systems in which formation of the fluorite-related phases (pyrochlore, murataite and modular pyrochlore-murataite polytypes) occur under some conditions and e.g. pyrochloreperovskite, pyrochlore-crichtonite assemblages form others based on titanium, manganese and Y-group lanthanides oxides containing some calcium, actinide (Th, U, Np, Pu), zirconium, iron and aluminum oxides [3-5]. Such ceramics depending on chemical composition and synthesis conditions are normally composed of several phases of the pyrochlore-murataite series with two(pyrochlore), seven- (M-7C), five (M-5C), eight- (M-8C) and three-fold (M-3C or murataite) repeatability of the elementary fluorite unit cell. In this series concentration of actinides and lanthanides in the phases decreases but concentration of iron group elements and aluminum increases [3-6]. In the present work the effect of some HLW oxides (U, Fe, Al) on phase formation in the yttrium- and gadolinium-manganese-titanate systems is studied. EXPERIMENTAL Chemical composition and synthesis conditions of the samples are given in Tables I and II, respectively. The samples were synthesized from oxides milled in a planetary mill AGO-2U (made in Russia) for 5 or 10 min at acceleration of 40g and operation bodies (
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