Structure of Rhenium‐Containing Sodium Borosilicate Glass

Abstract

A series of sodium borosilicate glasses were synthesized with KReO 4 or Re 2 O 7 , to 10,000 ppm (1 mass%) target Re, to assess effects of large concentrations of rhenium on glass structure and fto estimate solubility of 99 Tc, a radioactive component in low active waste nuclear glasses. Rhenium was used as a surrogate for 99 Tc for laboratory testing, due to similarities in chemistry, ionic size, and redox. Magic angle spinning nuclear magnetic resonance, Fourier transform infrared spectroscopy, and Raman spectroscopy were performed to characterize the glasses. Si was coordinated in Q 2 and Q 3 units, Al was four-coordinated, and B was mostly three-coordinated. The rhenium additions did not have significant effects on the glass structure up to approximately 3000 ppm Re by mass, the maximum concentration that remained dissolved in glass. Rhenium likely exists in isolated ReO À 4 anions in the interstices of the glass network, as evidenced by polarized Raman spectrum of the Re glass in the absence of sulfate. Analogous to SO 2À 4 in similar glasses, ReO À 4 is a network modifier and above solubility forms alkali salt phases on the surface and in the bulk. Comparisons of phase separation and crystallization in MoO 2À 4 containing borosilicate glasses can also be made to ReO À 4 containing glasses.

Raman analysis of perrhenate and pertechnetate in alkali salts and borosilicate glasses

“…The solubility of Re in glass was previously determined to be~3000 ppm Re, [4] so samples at higher concentrations contained crystalline inclusions of Re salts in the bulk and/or on the surface. [3] Glasses examined here with Raman microscopy were those above the solubility limit of Re, namely, 4000 ppm Re, 6415 ppm Re, 6415 ppm Re (no sulfate [7] ), and 10 000 ppm Re. Technetium concentrations in glasses varied from 500 to 6000 ppm Tc, with some glasses made under slightly reducing conditions.…”

“…The solubility of Re in these glasses has been shown to be 3000 ppm, [4] and the 4000 ppm glass has shown no evidence of crystallization in the bulk glass (by XRD) and no visual evidence of salt formation on the surface. [7] Unlike bulk Raman spectroscopy, which showed no evidence of crystallinity on the 4000 ppm Re glass, [7] the Raman microscopy suggested KReO 4 crystals, and optical microscopy confirmed the presence of white crystals decorating cracks in the glass produced by cooling. As the glass cooled in the ampoule, thermal stresses caused it to crack, while the salt was still molten, and it was pulled into the cracks where it crystallized.…”

“…[5,6] The incorporation of ReO 4 À in alumino-borosilicate glasses was previously measured by conventional Raman spectroscopy, X-ray absorption near edge structure spectroscopy, nuclear magnetic resonance spectrometry, and X-ray diffraction (XRD) analysis. [4,7] A subset of these measurements were performed on analogous Tc-containing glasses, and results are presented elsewhere. [8] This work presents data from high spatial-resolution and high spectral-resolution Raman microscopy, which probed the local structure within the glasses as well as crystalline phases on the bulk glasses.…”

Reactions during melting of low-activity waste glasses and their effects on the retention of rhenium as a surrogate for technetium-99

“…Given the projected concentration of 99 Tc in LAW glass at Hanford is~3 ppm mass on average [26]; the solubility of 99 Tc is not a factor in its retention in LAW glass. In addition, the structural role of Re and Tc in glass network was discussed in Goel et al 2013 [27] and Gassman et al 2014 [28].…”

Formation of Technetium Salts in Hanford Low‐Activity Waste Glass

“…Apparently, when a pertechnetate salt migrates to the surface, it carries other ionic compounds with it, compounds which would not leave the glass on their own. Migration of such salts to the surface of the glass during the melt has been previously reported . Molten sulfate‐containing salts form at much lower values of SO 3 (0.16 wt% of unspiked glass) than typical sulfur solubility (1.6 wt% but depends greatly on alkali concentration) when in the presence of other anions.…”