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Understanding the Structure and Speciation of Lanthanides Dissolved in LiCl-KCl Eutectic Molten Salt Using Absorption Spectroscopy
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Moon_unr_0139D_13871.pdf
Date
2022Type
DissertationDepartment
Materials Science and Engineering
Degree Level
Doctorate Degree
Abstract
A comprehensive knowledge of the coordination, molecular bonding, and speciation of components in molten salt mixtures is necessary to understand and predict the chemical, thermochemical, and thermophysical properties of the salt. Such knowledge is needed to design and implement the next generation of molten salt nuclear reactors and to recycle used nuclear fuel using pyrochemical reprocessing. Absorption spectroscopy can yield information about the chemistry and bonding of species of interest in alkali halide molten salt mixtures that are used in both of those applications by revealing information about the electronic structure and transitions of those species. For this dissertation research, the electronic structures of the trivalent lanthanides Nd3+, Sm3+, and Dy3+ were studied using ultraviolet (UV), visible (Vis), and near-infrared (NIR) absorption spectroscopy. These species are present in molten salt reactors and in pyrochemical reprocessing as fission products and make up a non-trivial portion of the melt. Molar absorbance coefficients were calculated for NIR transitions of Sm3+ and Dy3+ absorbance features for the first time. These molar absorbance features had not previously been studied and have advantages over absorbance features in the ultraviolet and visible range for concentration analysis. A weak NIR absorbance feature of Nd3+ that had only been theoretically predicted was experimentally verified for the first time in this work and compared to the predicted electronic transitions. The dependence of the intensity, position, and lineshape of hypersensitive and non-hypersensitive absorbance features on temperature and on anion ligand field is also presented. This dissertation also presents the results observed in absorbance when the anion (Cl-) was substituted with either a smaller (F-) or a larger (I-) anion. The substitution of I- for Cl- produced very little alteration in absorbance feature position or lineshapes. Quantum-mechanics molecular dynamics (QM-MD) simulations suggest that the I anions do not tend to be involved in the lanthanide coordination shells at the relevant concentrations in this work. On the other hand, QM-MD reveals that F- tends to prefer to complex with lanthanides in the melt and was confirmed experimentally by the large spectral alterations that are related to the effect of F- anions in the first coordination. For the first time, Gaussian peak fitting was used to show that the lineshape of the absorbance features for the mixed Cl-/F melts are combinations of a set of absorbance bands from complexes with only Cl- ligands and a set of absorbance bands from complexes with Cl- and F- ligands. Finally, the reduction of the trivalent lanthanides was observed using absorption spectroscopy. Sm2+ was successfully produced in detectable concentrations and the decrease in concentration of Dy3+ during the reduction to Dy0 was observed using NIR spectroscopy. This study extends our knowledge of the use of absorption spectroscopy to detect, quantify, and understand the absorption spectra of trivalent lanthanides to study their coordination and speciation and for use in engineering contexts. In all, this dissertation research provides a promising pathway to utilize absorption spectroscopy for in operando, online monitoring of molten salts in not only nuclear reactors and pyroprocessing of used nuclear fuels, but also in other applications such as solar thermal power plants.
Permanent link
http://hdl.handle.net/11714/8311Additional Information
Committee Member | Carlson, Krista; Mushongera, Leslie; Tsoulfanidis, Nicholas; Vahidi, Ehsan |
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