Rare Earth-Bearing Murataite Ceramics
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0985-NN04-10
Rare Earth-Bearing Murataite Ceramics Sergey Stefanovsky1, Sergey Yudintsev2, Boris Nikonov2, and Olga Stefanovsky1 1 SIA Radon, Moscow, 119121, Russian Federation 2 IGEM RAS, Moscow, 119017, Russian Federation
ABSTRACT Phase composition of the murataite-based ceramics containing 10 wt.% lanthanum, cerium, neodymium, europium, gadolinium, yttrium, zirconium oxides was studied. The ceramics were prepared by melting of oxide mixtures in 20 mL glass-carbon crucibles in air at ~1500 0C. They are composed of predominant murataite-type phases and minor extra phases: rutile, crichtonite, perovskite, ilmenite/pyrophanite, and zirconolite (in the Zr-bearing sample only). Three murataite-related phases with five- (5C), eight- (8C), and three-fold (3C) elementary fluorite unit cell are normally present in all the ceramics. These phases form core, intermediate zone, and rim of the murataite grains, respectively. They are predominant host phases for the rare earth elements whose concentrations are reduced in a row: 5C>8C>3C. Appreciate fraction of La and Ce may enter the perovskite phase. INTRODUCTION HLW partitioning provides for separation of a long lived rare earth/actinide (REE/An) or actinide (An) fraction to be stored after conditioning for a period of up to 1 million years from a relatively short-lived cesium/strontium (Cs/Sr) fraction which must be stored for several thousands years. The REE/An fraction contains major lanthanum and lanthanides (from Ce to Gd), actinides (U, transuranic elements – Np, Am, Cm, residual Pu) and other elements, in particular zirconium [1]. Ceramic waste forms are the most suitable for a long-term storage of this fraction because stability of glass currently applied for HLW storage cannot be guaranteed for a period of hundreds of thousands to one million years. Among crystalline phases capable to incorporate actinides and rare earths (La, Ln, Zr) the phases with fluorite-derived structure – pyrochlore and zirconolite as well as zirconia-based cubic solid solution (tazheranite or fianite) are well-known [2]. We have proposed one more cubic phase having fluorite-derived structure (space group F⎯43m or Fm3 or F432) whose natural analogue is murataite [3]. Similar phase was found in the ceramic designed for Savannah River Plant defense waste immobilization [4] but due to low actinide contents it didn’t take in close consideration. We have demonstrated that this phase may incorporate of up to 30 wt.% actinides and lanthanides in total. At that their partitioning over the grains may be zoned with the highest concentration in the core and the lowest in the rim creating an additional barrier preventing leaching [2]. Zoned partitioning of actinides and rare earths is typical of murataite ceramics produced by melt crystallization in particular using an inductive cold crucible melting whereas for the cold pressed and sintered ceramics elemental distribution over the murataite grains is more uniform. As it has been shown [5] there is a family (polysomatic series) of polytypes with variable
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