Text

theses

ETD graduate

The vitrification of high Fe, Ni, Mn, and Cr containing nuclear waste faces the problem of the crystallization of spinel in the melter cold cap that poses a threat to the melter's efficiency and brings down the volumetric efficiency of the vitrified waste. Understanding the mechanisms driving the nucleation and crystal growth of spinels in HLW glasses is a vital subject in the vitrification of nuclear waste that is fostered by several environmental and economic reasons. The research focuses on understanding the relationship between alkali/alkaline-earth cations with iron redox, glass structure, and spinel crystallization tendency in the glass compositions of the xLi2O(or CaO)-(25-x)Na2O-9.12B2O3-6.4Al2O3-51.25SiO2-7.22Fe2O3-0.38MnO-0.08Cr2O3-0.55NiO system. Glass compositions of the system mentioned above with varying Li2O/Na2O and CaO/Na2O ratios were synthesized using two different approaches - (1) quenching the melt from 1450 °C between two copper plates in order to understand the structure and crystallization behavior of melts in the HLW melter during operation at 1150 °C, (2) allowing the melt at 1450 °C to cool down to 950 °C followed by isothermal heating at 950 °C for 48 h in order to emulate melter idling. While the modifier cation field strength is reported to affect the NBO distribution substantially, the cation charge and size are also found to affect the Al and B differently. X-ray diffraction, infrared spectroscopy, density measurement, differential scanning calorimetry, and vibration sample magnetometry were used along with optical basicity and Mössbauer spectroscopy to explore the effects of the cation with higher-field strength on the possible evolution of NBOs in the structure, the tendency of iron to enter the crystallization field, the phases formed, etc., Differential scanning calorimetry along with Rietveld analysis was used to understand the crystalline phases. Spinel crystallization is found to decrease with higher basicity and higher Fe3+ concentration of the system. With the introduction of cations with higher field strength, the optical basicity reduces Fe2+ increases, and the tendency to crystallize spinel increases. Vibration sample magnetometry, scanning electron microscopy, and elemental dispersive spectroscopy have been used to understand the size, shape, nature, and distribution of the crystallized phases.

ETD_11307

doi:10.7282/t3-k9p5-yx80

M.S.

Includes bibliographical references

Includes vita

The author owns the copyright to this work.