Valence and Local Environment of Molybdenum in Aluminophosphate Glasses for Immobilization of High Level Waste from Uran
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Valence and Local Environment of Molybdenum in Aluminophosphate Glasses for Immobilization of High Level Waste from Uranium-Graphite Reactor Spent Nuclear Fuel Reprocessing Sergey V. Stefanovsky1, Andrey A, Shiryaev1, Michael B. Remizov2, Elena A. Belanova2, Pavel A. Kozlov2, and Boris F. Myasoedov3 1 Frumkin Institute of Physical Chemistry and Electrochemistry RAS, Leninskii av. 31, Bld. 4, Moscow, 119071 Russia. 2 FSUE Production Association “Mayak”, Lenin st. 13, Ozersk Chelyabinsk reg. 456780 Russia 3 Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Kosygin st. 19, Moscow 119071 Russia ABSTRACT Two Mo-bearing glasses considered as candidate forms for high level waste (HLW) a uranium-graphite reactor spent nuclear fuel (SNF) reprocessing were characterized. Incorporation of Mo in sodium aluminophosphate (SAP) glass increases its tendency to devitrification with segregation of orthophosphate phases. Valence state and local environment of Mo in the materials containing ~2 wt.% MoO3 were determined by X-ray absorption fine structure (XAFS) spectroscopy. In the quenched samples composed of major vitreous and minor AlPO4 nearly all Mo is located in the vitreous phase as [Mo6+Ɉ6] units whereas in the annealed samples Mo is partitioned among vitreous and one or two orthophosphate crystalline phases in favor of the vitreous phase. Mo predominantly exists in a hexavalent state in distorted octahedral environment. Four oxygen ions are positioned at a distance of ~1.71-1.73 Å and two - at a distance of 2.022.04 Å. Minor Mo(V) is also present as indicated by a response in EPR spectra with g | 1.9111.915. INTRODUCTION Currently in Russia some SNF compositions such as molybdenum-bearing SNF of uranium-graphite reactors (AMB) are not reprocessed yet but their reprocessing is under consideration now. HLW from AMB SNF reprocessing is suggested to be incorporated in SAP-based glass similarly to different current HLW [1]. Among the AMB fuels there are Mo-bearing varieties U+9%Mo/Mg, U+9%Mo/Ca, and U+3%Mo/Mg which will be reprocessed yielding Mocontaining HLW. Molybdenum is one of the troublesome components of HLW causing liquid/liquid phase separation in borosilicate glasses or devitrification of phosphate glasses thus reducing chemical durability of vitrified waste. Therefore, the effect of Mo solubility, its valence state and speciation on chemical durability of glasses has to be studied. Previous studies demonstrated a negative effect of MoO3 on resistance to devitrification and chemical durability of SAP glasses [2]. There are numerous works on incorporation of Mo-bearing HLW in borosilicate glass (see, for example, [3-5]). Glass properties depend on Mo speciation in glass. Silicate and borosilicate glasses containing Mo(VI) have a very high tendency to liquid-liquid phase separation with formation of so-called “yellow phase” composed mainly of alkali and alkali earth molyddates,
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chromates, sulfates, and chlorides [6-8]. Maximum concentration of these anions is about 1 to 2 mol.% depending on glass compos
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