Precipitation of Mixed-Alkali Molybdates During HLW Vitrification
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1265-AA03-03
Precipitation of Mixed-Alkali Molybdates During HLW Vitrification Scott Kroeker,1* Carolyn S. Higman,1 Vladimir K. Michaelis,1 Nicholas B. Svenda and Sophie Schuller2 1 2
Department of Chemistry, University of Manitoba, Winnipeg, MB R3T 2N2, Canada CEA, DEN, DTCD, SECM, LDMC, F-30207 Bagnols/sur/Cèze, France
ABSTRACT Crystalline precipitates from molybdenum-containing nuclear waste glasses are complex, often containing multiple cations which confound routine structural techniques. A simplified mixed-alkali borosilicate model glass was found to have minor crystalline phases which could not be identified by x-ray diffraction. Multinuclear magnetic resonance (NMR) spectroscopy revealed sharp peaks characteristic of crystallinity superimposed on the broader glass signals, but were unattributable to any known molybdate phases. When a comprehensive range of cesium molybdates failed to reveal any matches with the observed 133Cs magic-angle spinning (MAS) NMR peaks in the composite glass/crystalline material, a series of mixed-alkali sodium-cesium molybdate phases was synthesized. 23Na, 133Cs and 95Mo MAS NMR revealed the formation of two mixed-cation molybdates which correlate with the observed NMR peaks for the phaseseparated model glass. This work highlights the prominence of multiple crystalline phases in Mo-bearing nuclear waste glasses, and demonstrates the unique utility of solid-state NMR as a fingerprinting approach to identifying complex phases, especially where x-ray diffraction is limited by multiple phases, low concentrations or substitutionally disordered precipitates. INTRODUCTION Vitrification of high-level waste (HLW) is a mature and reliable technology, but optimization of glassy wasteforms for efficiency without compromising long-term chemical durability is an ongoing process. One challenge is the incorporation of poorly soluble fission products, such as molybdenum. At high levels, Mo is known to cause macroscopic phase separation and crystallization of molybdate phases undesirable in HLW borosilicate glass.1 The current pragmatic strategy is to limit waste loading of Mo-bearing streams, thereby maintaining Mo at safe levels. However, research is being done to develop new glass matrices to allow higher loading levels within macroscopically homogeneous glasses, the quality of which are not compromised by partial crystallization at the microscopic scale (see, for example, reference 2 and references therein). Finding such alternate approaches which do not decrease disposal efficiency will be aided by further information about the nature of the precipitates. Such phases are difficult to characterize because they cannot be cleanly separated and must be analyzed in the bulk composite material. Furthermore, the separated phases are typically complex, incorporating multiple ions from the base glass and waste cocktail, poorly crystalline and present in low concentration levels, all factors which make conventional x-ray diffraction of limited utility. Nuclear magnetic resonance (NMR) spectroscopy is
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