Nuclear Material Accountancy in High Temperature Molten Salt Using In Situ Raman Spectroscopy and Cyclic Voltammetry
AuthorSingh, Vickram Jit
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As the world attempts to transition to more sustainable and environmentally friendly forms of energy, nuclear power has the potential to play a central role in meeting the growing energy needs of the planet. The security concerns associated with special nuclear materials and nuclear waste continue to hinder the growth of nuclear energy. High temperature molten salt systems have been proposed as an advanced, proliferation-resistant media for nuclear energy production, fuel reprocessing, and securing the nuclear weapons complex. A chemical-based sensing system was designed, constructed, and evaluated for nuclear material accountancy in high temperature molten chloride salt. Raman spectroscopy and cyclic voltammetry were utilized as sensing methods to study lanthanide concentration, diffusion kinetics, speciation, and coordination chemistry in the molten LiCl-KCl salt of eutectic composition. In situ Raman spectroscopy was conducted using a custom-built, fiber-based system. A commercial system was modified through an iterative process to address the hands-free and extreme environmental requirements associated with the molten salt studies. The experimental system was designed and constructed with remote control and data acquisition capabilities for the high temperature environment. This system was used to investigate samarium speciation and coordination chemistry in the molten LiCl-KCl at 500 ˚C. Trivalent samarium was confirmed to be present as an octahedral SmCl63- complex, in agreement with the literature. Raman spectra were analyzed for bond-specific vibration modes and fluorescence. The electroanalytical voltammetry system coupled with the fiber-based Raman system was used to study divalent samarium ions. Divalent samarium was produced in situ using both chemical and electrochemical reduction protocols and analyzed using Raman spectroscopy. To our knowledge, this is the first Raman spectrum of divalent samarium in molten LiCl-KCl eutectic. Raman spectra of CeCl3 in molten LiCl-KCl was found to be identical to that obtained from SmCl3 in molten LiCl-KCl except for the fluorescence features. Spectra obtained from multi-analyte environment (mixture of Sm and Ce in molten LiCl-KCl) showed that differentiating mixed analytes is not possible without significant technological and design improvements and development of complex deconvolution protocols. A cyclic voltammetry analysis protocol available in the literature was further developed for more accuracy. Collected data was analyzed using existing electrochemical relationship, the Randles-Sevcik equation, which was originally developed for aqueous, fully-reversible systems. The applicability of this empirical relationship was evaluated over a broad concentration range. Calculated diffusion coefficients were compared and contrasted to the existing literature. The coupled spectroelectrochemical system was then employed to study the nature of samarium oxychloride formation in the molten LiCl-KCl-SmCl3 system. A synthesis method reported in the literature was confirmed in situ, and the synthesized product was confirmed to be samarium oxychloride using solid state characterization methods. In summary, this dissertation reports the construction and successful deployment of a combined Raman spectroscopy and electroanalytical system for nuclear material accountancy in high temperature molten salt. The electroanalytical voltammetry system produced accurate and repeatable data in the low analyte concentration regime which was confirmed with the literature. The in situ Raman spectroscopy system was engineered, constructed and successfully tested for use in high temperature molten salt environments. The system designed in this dissertation can be deployed for use in studying single analyte in molten salt systems immediately.