Effect of temperature on formation of murataite and murataite-pyrochlore ceramics
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Effect of temperature on formation of murataite and murataite-pyrochlore ceramics O.I. Kirjanova,1 S.V. Stefanovsky,1 S.V. Yudintsev2 SIA Radon, 7 Rostovskii per. 2/14, Moscow 119121 RUSSIA 2 IGEM RAS, Staromonetnii per. 35, Moscow 109017 RUSSIA 1
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ABSTRACT Processes of phases formation in the ceramic mixtures with basic compositions (wt.%) 10 CaO, 10 MnO, 5 Al2O3, 5 Fe2O3, 55 TiO2, 5 ZrO2, 10 UO2 (M1) and 8 CaO, 8 MnO, 4 Al2O3, 4 Fe2O3, 20 Gd2O3, 44 TiO2, 4 ZrO2, 8 UO2 (M4) were studied using X-ray diffraction, scanning and transmission electron microscopy. The batches were milled, compacted in pellets at 200 MPa, and heat-treated in a resistive furnace at 1100 0C, 1300 0C, 1400 0C, and 1500 0C as well as melted in a cold crucible at ~1600 0C. Reactions in the mixtures heat-treated at 1100 0C were not completed and samples contained significant amount of unreacted and intermediate (altered rutile, cubic oxide solid solution, perovskite) phases. Within the temperature range 1100-1300 0C reactions are mainly completed and ceramics sintered at 1300 0 C are composed of major murataite and minor rutile (M1) or major murataite and pyrochlore and minor zirconolite and perovskite (M4). However full homogenization at 1300 0C has not been reached yet and to obtain the ceramics with uniform compositions of the phases sintering at 1400 0C or melting at 1500-1600 0 C were required. In the ceramic sample M1 two murataite varieties with five- (murataite-5C) and eight-fold (murataite-8C) fluorite-type unit cells were found. The sample M4 is composed of pyrochlore, murataite-8C and zirconolite-3O. In the sample M1 murataite-5C is enriched with U and Ca and depleted with iron group elements as compared to murataite-8C. Fraction of murataite-5C concentrates about 80% of total U and about 70% of Mn+Fe (corrosion products). Waste elements partitioning among the phases in the M4 sample depends significantly on temperature of heat-treatment. INTRODUCTION Pyrochlore A2B2O7-x (A = Ca, REE3+, An3+/4+; B = Ti, Zr) is well-known host phase for actinide (An) and rare earth elements (REE) – see for example, [1,2]. These elements are present in high level waste (HLW) which is suggested to be partitioned with separation of a long-lived actinide-REE fraction to be incorporated in long-term stable ceramics. The other sources of actinides are An-bearing residues and wastes from conversion of excess weapons plutonium to MOX-fuel [3]. Murataite is a new phase proposed for incorporation of An and REEs [4]. Its approximate formula is A4B2C7O22-x [4] or A3B2C6O19x [5] (0≤x≤1, A = An, REE, Zr, Ca, Na; B = Mn, Fe; C = Ti, Al, Fe). In the present work we will follow the latter formula. The main advantage of this phase is ability to accommodate iron group elements (corrosion products) in its structure providing opportunity to immobilize An-REE fraction containing residual corrosion and fission products [6]. We believe two-phase pyrochlore -murataite ceramics are the most promising for immobilization of complex An-REE-bearing wastes. Typical met
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