Structure and crystallization of iron-rich borosilicate-based nuclear waste glasses
Creators: name: Balasubramanya, Nikhila
Publisher: name: No Publisher Supplied
Year: 2023

description: The U.S. Department of Energy (DOE) is building a Tank Waste Treatment and Immobilization Plant at the Hanford site to separately vitrify ~55 million gallons of low activity waste (LAW) and high-level waste (HLW) in borosilicate glass at 1150 °C using Joule-heated ceramic melters (JHCM). Since the cost of vitrifying radioactive waste is directly proportional to the volume of glass to be produced, it is, therefore, desirable to maximize the waste loading in the glass to decrease the overall volume, but without posing unacceptable risk for the melter operation, or long-term performance of the vitrified waste form. One major impediment to the design of glass formulations with enhanced waste loadings for iron-rich HLW waste streams is the precipitation, growth and subsequent accumulation of spinel crystals (Fe, Ni, Mn, Zn, Sn)II(Fe, Cr)III2O4 in the glass discharge riser of the melter during idling (lowering the melter temperature to 850 °C – 900 °C when not in operation). Once formed, spinels are stable to temperatures much higher than the typical JHCM operating temperatures (1150–1200 °C). During the process of idling, when the temperature of the melters are brought down to ~850 °C, this can result in clogging of the melter discharge channel, and interfere with the flow of glass from the melter. As a result, spinel crystallization has a negative impact on waste loading efficiency. Robust understanding of the compositional constraints for the formation of spinel and the kinetics of crystallization is still lacking. Therefore, this study is an in-depth investigation of the chemo structural descriptors controlling the crystallization behavior in these glasses. The study focused on topics of (i) impact of non-framework cation mixing and (ii) impact of mixed-network former effect on the structure and crystallization of glasses in the system Na2O - Al2O3 - B2O3 - SiO2 - Fe2O3 - MnO - NiO -Cr2O3 where Fe2O3 = 9 mol%. A subsidiary investigation on the effect of iron content on the borosilicate matrix is also presented. A suite of characterization techniques such as X-ray diffraction (XRD), vibrating sample magnetometry (VSM), differential scanning calorimetry (DSC), Raman spectroscopy, Mössbauer spectroscopy, high-temperature viscosity measurements and Scanning electron microscopy-electron dispersive spectroscopy (SEM-EDS) has been employed for the investigation. Despite the challenges of performing nuclear magnetic resonance (NMR) on high iron-containing systems, high-resolution high-field 11B, 29Si, 23Na, and 27Al magic angle spinning has been instrumental in probing the short-to-medium range order surrounding paramagnetic iron species. The obtained results are supported by classical molecular dynamics (MD) simulations wherever necessary. Very importantly, the [AlO4]- and [FeO4]- tetrahedral avoidance observed by NMR is supported by partial distribution functions (PDF) derived from simulations. To summarize, the investigation proved that (i) tendency to crystallize increases when Na+ is replaced with Li+or Ca2+ where the latter cations have a higher ionic field strength and significantly modify the iron-rich aluminoborosilicate network that has higher crystallization tendency (ii) in a aluminoborosilicate matrix with only Na+ as the alkali/alkaline earth cation and ~9 mol% Fe2O3, the preferential charge compensation of the Na+ amongst the tetrahedral network forming units (Al, B, Fe) in the order Al &gt; B &gt; Fe has been found to lead to an increase in Fe2+/ΣFe and, (iii) in a similar system, clustering of iron is found to significantly change with composition and is greatest at high Al2O3/SiO2 and B2O3/SiO2 regimes. The extent of clustering has been found to be a determining factor for spinel crystallization, (iv) With preferential linkages around the iron clusters, heterogeneity in the mixing of the network formers in the aluminoborosilicate matrix was observed.