XAFS Study of Iron and Nickel Speciation in Complex Sodium Aluminophosphate Based Glasses
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XAFS Study of Iron and Nickel Speciation in Complex Sodium Aluminophosphate Based Glasses S.V. Stefanovsky,1 V.Y. Murzin,2 M.B. Remizov,3 B.F. Myasoedov1,4 1 A.N. Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences, Moscow, Russia 2 National Research Center “Kurchatov Institute”, Moscow, Russia 3 Production Association “Mayak”, Ozersk, Chelyabinsk reg., Russia 4 Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences, Moscow, Russia ABSTRACT Iron and nickel oxidation state and coordination in complex sodium aluminophosphate based glasses suggested as potential matrices for immobilization of legacy high level waste currently stored in stainless steel tanks at PA «Mayak» (Ural reg., Russia) were determined by X-ray absorption fine structure spectroscopy (XAFS: XANES/EXAFS). The glasses containing (wt.%) 20-30 Na2O, 6-12 Al2O3, 40-52 P2O5, 2-5 Fe2O3, 1-3 NiO, 0-6 B2O3, 10-15 other waste oxides produced by quenching of their melts were fully amorphous or contained minor Fe and Ni free phases. Fe in the glasses was found to be predominantly trivalent with an average Fe-O distance and a coordination number (CN) in the first shell of 1.94 to 1.97 Å and 5.2 to 5.8, respectively, mostly in octahedral oxygen environment. Ni is divalent in all the glasses and has in the first shell an average Ni-O distance and CN of 1.97 to 2.03 Å and 4.9 to 5.6, respectively. The first shell of both Fe and Ni is somewhat distorted. The second and further coordination shells are weakly appeared exhibiting no clustering and homogeneous distribution of Fe and Ni ions in the glass network. The data on Fe obtained are in good agreement with those from Mössbauer study of same glasses. After annealing glasses were partly devitrified and interpretation of XAFS data is strongly complicated due to Fe and Ni partitioning among crystalline and vitreous phases. INTRODUCTION Currently legacy liquid high level waste (HLW) from former defense activity is stored in stainless steel tanks at Production Association (PA) Mayak [1]. This HLW is planned to be vitrified jointly with HLW from spent nuclear fuel (SNF) reprocessing in a EP-500 liquid-fed Joule heated ceramic melter yielding aluminophosphate based glass or separately in a new cold crucible based unit with production of borosilicate glass [1]. However, as compared to HLW from SNF reprocessing, the legacy HLW contains elevated concentrations of transition elements (Fe, Ni, Cr, Mn) as well as sulfates and uranium. These components may affect glass properties such as viscosity, electric resistivity, resistance to devitrification, and chemical durability. Therefore a study of the effect of their concentrations and speciation on the glass structure is a key factor for evaluation of suitability of glasses containing these elements for legacy HLW immobilization. One of the most appropriate technique for investigation of elemental speciation in solids is X-ray absorption fine structure (XAFS) spectroscopy in near-edge (XANES) and
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