Phase Compositions and Elements Partitioning in Two-Phase Hosts for Immobilization of a Rare Earth-Actinide High-Level W
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S.V. STEFANOVSKY*, S.V. YUDINTSEV**, B.S. NIKONOV**, A.V. OCHKIN***, S.V. CHIZHEVSKAYA***, N.E. CHERNIAVSKAYA*
* SIA Radon, 7h Rostovskii per., 2/14, Moscow 119121 RUSSIA, [email protected] **Institute of Geology of Ore Deposits, Russian Academy of Sciences, Staromonetni 35, Moscow RUSSIA *** D. Mendeleev University of Chemical Technology, Miusskaya, 9, Moscow RUSSIA
ABSTRACT In two-phase matrices based on pyrochlore-oxide, perovskite-oxide, and pyrochlore-zirconolite assemblages as much as 40 wt.% of the product can consist of an incorporated rare earth - actinide fraction of high level wastes (HLW). In zirconolite, with a nominal stoichiometry of AVI..BVICV-v'20 7 the actmide and rare earth ions occupy VIII- and VII-fold coordinated sites. Charge compensation is achieved by replacement of Ti4+ with lower valence ions such as AI3 , Fe 2+/3+. Mg 2+, etc, that have similar radii. The highest lanthanide incorporation (45 wt.% YREE 20 3) was exhibited by a calcium free zirconolite containing 8 wt.% A120 3 or 0.7 formula units (f.u.). In pyrochlore (AV1 " 2BvI2O 7 .x) actinides and rare earths occupy A-sites. Their incorporation does not require simultaneous ionic substitution in the B-sites, for example A131 or Fe2 11 for TI4 , to achieve charge compensation; this simplifies the required composition of the system. Pyrochlore ceramics are suitable for immobilization of wastes with relative elevated actinide content. For a high zirconia, waste ZrO2 -based or two-phase pyrochlore-dioxide, perovskite-dioxide or pyrochlore-zirconolite ceramics are preferable. All these wasteforms may be produced via melting. INTRODUCTION High level wastes (HLW) contain numerous chemical elements with different properties that create problems when the waste is processed to obtain a waste form for dispodal. To facilitate treatment, HLW partitioning has been proposed in which the HLW would be partitioned to a short-lived Cs/Sr fraction and a long-lived REE-actinide rare earth element (REE)-actinide fraction that contains mainly lanthanides (LaGd), actinides (U-Cm), and zirconium [1]. The REE-actinide fraction also can be partitioned to separate the actinides from the stable rare earth elements [2]. Compositions of the REE-actinide fraction produced in reprocessing of spent fuel from commercial and defense reactors are (in wt.%) 50-60 YLn 20 3 (Nd>Ce> LanPr>Sm>Eu>Gd), 20-30 ZrO2, 10-20 ZAnO 2 (U>Np>PunAm>Cm) [3] and 55 ZAnO 2, 40 YLn20 3, 5 ZrO 2 [2], respectively. Since actinide elements have long half-lives, they must be stored for period up to millions of years. This requires waste forms with excellent long-term stability. The best-known materials for actinide immobilization are zirconia- and titanate-based ceramics [4-14]. Zirconia-based solid solution with a fluorite-type lattice (space group Fm3m) is stabilized by heavy lanthanides (Gd-Lu) and yttrium [7], which are totally miscible in the system (Zr,Ln,Y)O2.-- PuO 2 and other systems with actinide oxides with the same crystal structure. If light lanthanides, which predominate in HLW
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