Phase and Chemical Stability of Murataite Containing Uranium, Plutonium and Rare Earths
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Phase and Chemical Stability of Murataite Containing Uranium, Plutonium and Rare Earths S.V. Yudintsev1, S.V. Stefanovsky2, B.S. Nikonov1 and B.I. Omelianenko1 1 Institute of Geology of Ore Deposits, Staromonetny 35, Moscow 109017, Russia 2 SIA "Radon", 7-th Rostovskii per., 2/14, Moscow 119121, Russia ABSTRACT Murataite – a complex actinide (An)- and rare earth element (REE)-bearing oxide with a cubic fluorite-related lattice – is one of the promising host-phases for immobilization of Pucontaining waste. The murataite phase composition corresponds to the empirical formula: A4B2C7O22-x, where the A-sites are occupied by Ca, Mn, REE, and An (U); B sites – by Mn, Ti, Zr, and An (U); and C sites – by Ti, Al, and Fe. The total amount of the actinides (U) and REE (Ce, Gd) in the murataite may exceed 20 wt%. In contrast to the other prospective hosts for actinide waste immobilization (cubic zirconia and pyrochlore), murataite accommodates higher amounts of corrosion products (Al, Fe) along with the actinides. The authors compared murataite-based ceramics having similar compositions and produced by melting in a hightemperature resistance furnace or via inductive melting in a cold crucible. Eight samples of the murataite-based ceramics were produced and investigated in detail. Murataite was found to be the major phase in four of the samples – with a basic composition, in wt%, of: 5.0 Al2O3, 10.0 CaO, 55.0 TiO2, 10.0 MnO, 5.0 Fe2O3, 5.0 ZrO2, and 10.0 UO2. These samples were produced by melting in a resistive furnace and in the cold crucible and included Gd-bearing samples and one Pu-bearing sample. The extra phases were other titanates: (from more to less typical) rutile, pyrochlore, zirconolite, crichtonite, pseudobrookite, and perovskite (in the Pudoped samples only). Three varieties of the murataite, with 3-, 5-, and 8-fold fluorite-type lattices, were observed. Addition of uranium and rare earth oxides stabilizes pyrochlore as the major phase, whereas addition of zirconia yields zirconolite. Plutonium stabilizes the perovskitetype phase, probably due to the formation of Pu3+. The maximum waste oxide content in the murataite for the elements studied was found to be 10% ZrO2, 12% CeO2, 13% Gd2O3, and 14% UO2. Waste element partitioning among the murataite and all the other phases with similar fluorite-related structure (pyrochlore and zirconolite) was analyzed. The uranium leach rate for the sample with maximum murataite content was measured using a procedure similar to MCC-3. This leach rate was close to 10-5 g/(m2*day) in a ½-day test and decreased by more than one order of magnitude in 28-day tests. Investigation of the stability of the murataite structure after irradiation by heavy ions is in progress. INTRODUCTION Many countries are searching for materials suitable to immobilize actinide wastes. The most promising matrices are ceramics based on complex fluorite-structured oxides such as zirconolite and pyrochlore [1,2]. Recently, we have proposed murataite for this purpose as one more phase of this type [3]. Syntheti
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