Isomorphic Capacity of Synthetic Sphene With Respect to Gd and U
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per. 35, Moscow 109017 RUSSIA
ABSTRACT Phase relations in the system: CaO-TiO 2-Si0 2-(Na 2O,AM 20 3,Gd 2O 3,UO 2) were studied. This system is of interest due to the formation of sphene, perovskite,. and other phases potentially suitable for immobilization of high level waste (111LW) elements. Along with sphene, other phases found in the samples were rutile, chevkinite, anorthite, crystobalite, and pyrochlore-structured phases. Sphene is able to incorporate up to 21.5 wt.% Gd20 3 and 9.3 wt.% U0 2 or, in formula units: 0.25 Gd3 + and 0.07 U4+. INTRODUCTION Currently high level wastes (HLW) are incorporated in aluminophosphate and borosilicate glasses [1,21 for long-term storage. Due to the thermodynamic instability of the vitreous state, glasses can not guarantee safe isolation of long-lived radionuclides, such as actinides, for the periods necessary to reduce their activity to natural background levels. Therefore more stable matrices are required for the long-term storage of actinide waste. Promising matrices for this purpose are sphene-based glass ceramics [3-5]. These can be produced using existing vitrification technologies, which are able to incorporate various waste elements (fission products, corrosion products, and fuel components), distributing them among vitreous and crystalline phases, and which have high stability under the conditions of underground disposal. However, an uncertainty remains with respect to the limiting amounts of actinides and rare earths in the sphene structure because the sphene composition with respect to these elements was investigated by an indirect route - on the basis of comparison of powder X-ray diffraction (XRD) patterns for the samples prepared with differing dopant concentrations. The number of direct determinations of the sphene compositions are limited to a few analyses and, as a rule, data on the content of individual elements in the mineral rather than complete chemical compositions were reported. The reason is that the extremely fine grain size of the synthetic sphene makes their investigation very complicated. At the same time such data on solid solution limits (isomorphic capacity) for elements in the sphene structure is necessary to specify the composition for the initial batch to be used to provide the required phase relations. This work describes compositions of the synthetic sphene obtained from crystallization of melts in the systems CaO-TiO 2Si0 2-(Na2O,AI20 3,Gd 2O 3,UO 2) and relationships between the sphene and other phases in these systems. Natural sphene is a common accessory mineral of magmatic and methamorphic rocks. Its maximum its amount (about 0.3%) is observed in intrusive rocks intermediate in silica content - granodiorites [6]. In most cases the sphene compositions are close to ideal stoichiometry. The most typical impurities are rare earths, aluminum, and iron [7]. Maximum amounts of total yttrium and rare earths in the natural sphene reach 3-4 wt.% [7,8]. Actinide (uranium and thorium) content is lower by one order of magnitude and does not e
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