¹³³Cs and ²³Na MAS NMR Spectroscopy of Molybdate Crystallization in Model Nuclear Glasses

Abstract

Sodium borosilicate base glasses modeled on French nuclear waste materials were prepared to test the dependence of crystallization product quantity and distribution on cesium‐ and molybdenum‐loading and glass cooling rate. Scanning electron microscopy shows the presence of micrometer‐sized domains of Mo‐rich crystalline precipitates. X‐ray diffraction patterns are complex but reveal the presence of sodium molybdates and CsNaMoO4·2H2O. 133Cs and 23Na magic‐angle spinning NMR spectroscopy exhibit distinct peaks for glassy and crystalline phases which can be quantified to obtain the identities of the individual compounds that are formed as well as the fractions of these nuclei in particular crystalline phases. In these model systems, 1 mol% Mo can be entirely incorporated into the glassy network whereas 2.5 and 5 mol% Mo produce significant quantities of crystalline phases, with little dependence on cooling rate. Cesium content appears to have a weak influence on crystallization behavior. Sodium molybdate and sodium‐cesium molybdate hydrate are the dominant devitrification phases in all cases.

NMR Spectroscopy in Glass Science: A Review of the Elements

“…Interestingly, many of these selenide glasses have fairly low T g values, so variable temperature 77 Se NMR has been used to follow network dynamics, correlating different T-dependent line shapes with structural and viscous relaxation [85]. Cesium NMR ( 133 Cs (spin-7/2; 100%)), which represents another alkali oxide modifier, has been used especially to understand the cesium environments and crystallization in glasses for nuclear waste containment [86]. 133 Cs NMR peaks from glasses are broad and give rise to significant spinning sideband intensities, as can be seen in the 133 Cs MAS NMR data of Figure 13.…”

Effect of V2O5 on crystallization tendency and chemical durability of Mo-bearing aluminoborosilicate glass

“…On account of the solubility of Na 2 MoO 4 in water, the yellow phase can severely decrease the chemical durability of nuclear waste forms. In addition, the yellow phase would also corrode melter liners and impede the vitrification process [10][11][12].…”

Crystallization study of rare earth and molybdenum containing nuclear waste glass ceramics

“…The importance of RE for the stabilization of Mo in the glass matrix has been well‐described, along with the effect of removing RE from the glass by crystallization of oxyapatite . At the same time, when Mo does precipitate in a crystalline phase, it is desirable to avoid the formation of water‐soluble alkali molybdates above the Mo solubility limit, by controlling the distribution of Na and Cs, to force the more chemically durable calcium molybdate (powellite) phase to form preferentially …”

“…There is a trade‐off between simplifying a complex glass in order to better understand it and over‐simplifying it, such that behavior becomes substantially different than the analogous complex glass. For instance, it is known that some of the alkali behave differently in these glass‐ceramic systems, in that Cs tends to partition to a soluble molybdate or stay in the glassy phase (depending on details of the glass composition), Li goes normally to the glass, and Na can go to the glass, powellite, or oxyapatite phase . Cesium (and even more so rubidium) is expected to be a small fraction of the alkali on a molar basis .…”

“…[13][14][15][16][17] At the same time, when Mo does precipitate in a crystalline phase, it is desirable to avoid the formation of water-soluble alkali molybdates above the Mo solubility limit, by controlling the distribution of Na and Cs, to force the more chemically durable calcium molybdate (powellite) phase to form preferentially. [18][19][20][21][22] A key barrier to maturation and exploitation of glass-ceramic technologies for nuclear waste forms is the gap in fundamental understanding of the molecular-scale mechanisms of phase separation and crystallization leading to the development of the desired phase assemblage and microstructure, ultimately determining long-term product performance. 1,23 The challenge is in predictably achieving the targeted phase assemblage and microstructure, requiring a detailed understanding of the transformation process as a function of both cooling rate and melt chemistry.…”