Graduate Thesis Or Dissertation

 

Investigations of Two Chemical Systems Relevant to Pyroprocessing Used Nuclear Fuel Public Deposited

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  • Pyroprocessing is an advanced technology for recycling used nuclear fuel. Pyrochemical processes encompass a wide range of chemical, physical, and electrochemical methods to partition fission products and other components from used nuclear fuel, which allows for the reuse of the actinides in nuclear fuel. This dissertation investigates two chemical systems relevant to pyroprocessing. The first investigation explores the possibility of using molybdate melts containing sodium molybdate (Na₂MoO₄) and molybdenum trioxide (MoO₃) to partition fission products from used nuclear fuel by crystallization. The difference in solubility of the fission product metal oxides compared to the uranium oxide or molybdate in the molybdate melt allows for these separations to occur. Uranium dioxide dissolves in the molybdate at high temperatures, and upon cooling, the uranium precipitates as uranium dioxide or molybdate, whereas the fission product metals remain soluble in the melt. The feasibility of UO₂ purification from the fission products was studied using small-scale experiments with gram quantities of uranium dioxide. The composition of the uranium precipitate as a function of molybdate melt composition was determined through a series of tests. The effectiveness of the partitioning of several fission product surrogates between the uranium precipitate and molybdate melt for various parameters in the process was also studied. A melt consisting of 20 wt% MoO₃- 50 wt% Na₂MoO₄-30 wt% UO₂ heated to 1313 K and cooled to 1123 K for the physical separation of the UO₂ product from the melt, and washed once with Na₂MoO₄ resulted in excellent separation of the UO2 from the surrogate fission products. The second investigation explored the phase equilibria of UCl₃ and NpCl₃ in the LiCl-KCl molten salt electrolyte used in electrorefining used nuclear fuel. The re-evaluation of the LiCl-UCl₃, KCl-UCl₃ and the LiCl-KCl-UCl₃ phase diagrams and the first known evaluation of the KCl-NpCl₃ system were performed. Samples of varying compositions within each of these systems were thermally analyzed by DSC to determine the temperature and types of the phase transitions. Samples were then analyzed by XRD to determine the identity of the phases formed, and ICP-OES or ICP-MS to establish the cation ratio. The LiCl-UCl₃ system displayed a simple eutectic system. The KCl-UCl₃ system displayed two eutectics and the K₂UCl₅ phase, which was identified by DSC and XRD. There was no evidence of a K₃UCl₆ phase. These LiCl-UCl₃ and KCl-UCl₃ phase diagrams were used to produce a portion of the LiCl-KCl-UCl₃ phase diagram relevant to electrorefining. The LiCl-KCl-UCl₃ system displayed two ternary eutectics and was consistent with literature data. The KCl-NpCl₃ system displayed two eutectics and the K2NpCl5 and K₃NpCl₆ phases, which were identified by DSC and XRD. The evaluation of these phase diagrams allows for an improved understanding of the LiCl-KCl-UCl₃ and KCl-NpCl₃ systems and their application to pyroprocessing.
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