Glass structure and crystallization in boro-alumino-silicate glasses containing rare earth and transition metal cations: a US-UK collaborative program

Abstract

Nuclear wastes generated from reprocessing of used nuclear fuel tend to contain a large fraction of rare earth (RE, e.g., Nd3+), transition (TM, e.g., Mo6+, Zr4+), alkali (A, e.g., Cs+), and alkaline earth cations (AE, e.g., Ba2+, Sr2+). Various strategies have been considered for immobilizing such waste streams, varying from nominally crystal-free glass to glass-ceramic to multi-phase ceramic waste forms. For glass and glass-ceramic waste forms, the added glass-forming system is generally alkali-alkaline earth-aluminoborosilicate (i.e., Na-Ca-Al-B-Si oxide). In a US-UK collaborative project, summarized here, we investigated the glass structure and crystallization dependence on compositional changes in simulated nuclear waste glasses and glass-ceramics. Compositions ranged in complexity from five – to – eight oxides. Specifically, the roles of Mo and rare earths are investigated, since a proposed glass-ceramic waste form contains crystalline phases such as powellite [(AE,A,RE)MoO4] and oxyapatite [(RE,AE,A)10Si6O26], and the precipitation of molybdenum phases is known to be affected by the rare earth concentration in the glass. Additionally, the effects of other chemical additions have been systematically investigated, including Zr, Ru, P, and Ti. A series of studies were also undertaken to ascertain the effect of the RE size on glass structure and on partitioning to crystal phases, investigating similarities and differences in glasses containing single RE oxides of Sc, Y, La, Ce, Nd, Sm, Er, Yb, or Lu. Finally, the effect of charge compensation was investigated by considering not only the commonly assessed peralkaline glass but also metaluminous and peraluminous compositions. Glass structure and crystallization studies were conducted by spectroscopic methods (i.e., Raman, X-ray absorption, nuclear magnetic resonance (NMR), optical absorption, photoluminescence, photoluminescence excitation, X-ray photoelectron spectroscopy), microscopy (i.e., scanning electron microscopy, transmission electron microscopy, electron probe microanalysis), scattering (i.e., X-ray and neutron diffraction, small angle measurements), and physical characterization (i.e., differential thermal analysis, liquidus, viscosity, density). This paper will give an overview of the research program and some example unpublished results on glass-ceramic crystallization kinetics, microstructure, and Raman spectra, as well as some examples of the effects of rare earths on the absorption, luminescence, and NMR spectra of starting glasses. The formal collaboration described here has resulted in the generation of a large number of results, some of which are still in the process of being published as separate studies.

Partitioning of rare earths in multiphase nuclear waste glass‐ceramics

“…Oxyapatites were only observed in Dy and Ho samples. In our previous work on peralkaline compositions, 29,32 oxyapatite—Ca 2+x RE 8‐x (SiO 4 ) 6 O 2‐y —formed preferentially for all but the smallest RE cations tested (ie, Yb, Lu) in these low RE 2 O 3 (~5 mol.%) low B 2 O 3 (~11 mol.%) glasses. Though the XRD suggested a vacancy apatite structure, the EPMA for the Ho sample (the Dy sample was not investigated by EPMA) suggested some Ca in the structure.…”

“…In light of previous work, an 8‐oxide composition, SiO 2 ‐CaO‐Na 2 O‐Al 2 O 3 ‐ZrO 2 ‐B 2 O 3 ‐MoO 3 ‐RE 2 O 3 was proposed as the baseline glass in this work, following on from other studies investigating GC formation 9,11,29,32 . On the basis of the investigated composition, peralkaline and peraluminous glasses were defined by the concentration of [Na 2 O excess ] = [Na 2 O]‐[Al 2 O 3 ]‐[ZrO 2 ] (mol.%).…”

“…When [Na 2 O excess ] >0, the glass is peralkaline, when [Na 2 O excess ] <0, the glass is peraluminous. Note that CaO was ignored for the purposes of this definition, as was any participation of four‐coordinated boron which would require charge compensation from alkali 32 . Other relations have been suggested in the literature to denote systems as peralkaline or peraluminous; notably Gasnier et al 33 define the peralkaline/peraluminous ratio as R p = ([M’O]+[M 2 O])/([M’O]+[M 2 O]+[Al 2 O 3 ]), where M’ are alkaline earth and M are alkali metals.…”

“…Therefore, a series of peraluminous borosilicate GCs containing alkali (Na), alkaline earth (Ca), RE (Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu), and transition metal (Zr and Mo) were formulated to test the effects of RE cation size on the crystallization of RE containing phases in these simulated nuclear waste GCs. The rationale for the specific glass composition has been previously discussed 32 . Briefly, peralkaline compositions are generally well‐understood, so peraluminous compositions were selected to represent higher waste loading glasses with more RE.…”

“…Here, boron oxide levels were increased, leaving RE 2 O 3 levels high, to move the composition into the regime predicted to form the RE borosilicate (RE 3 BSi 2 O 10 ) phase, as we desired to define compositional constraints for formation of this phase, as discussed previously 32 . Note that the purpose of the current study was not necessarily to optimize a set of GCs for high waste loading of a particular waste, but rather to determine the effects of RE size on the crystalline phase formation of a peraluminous glass as described.…”