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HDEHP Activity Coefficients by Vapor Pressure Osmometry


Components of used nuclear fuel remain more radiotoxic than natural uranium ore for over one hundred thousand years, but advanced fuel recycling could reduce that time to a thousand years. Yet several engineering challenges need to be addressed, including the scale up of certain solvent extraction based separations. To do so, an improved thermodynamic understanding of the chemistry is required since data for many of the chemical components is unreliable. The topic of my work, di(2-ethylhexyl) phosphoric acid (HDEHP), is one such example: past studies on HDEHP yield inconsistent thermodynamic data.

The technique of vapor pressure osmometry was used to address that problem, and extensive validation was carried out. The equation relating the vapor pressure osmometry signal to the solution activity was examined in detail, and its accuracy was demonstrated on aqueous solutions. Analysis of VPO data using the Gibbs-Duhem equation yielded activity coefficients for HDEHP in seven different hydrocarbon diluents. The values for HDEHP at 21°C matched two results from past literature, building a consensus. A second analysis was performed using regular solution theory with entropic corrections. That second result provides qualitative values of the activity coefficient in systems where no experimental data is available. The final section of the report analyzes osmometry data collected for HDEHP metal complex and the activity coefficient of HDEHP obtained with slope analysis. The slope analysis data points to two main reasons for discrepancies between the current VPO activity coefficient results and slope analysis data in the literature. Further analysis of the metal complex data may help resolve that discrepancy.

The data on HDEHP presented in this dissertation will improve the accuracy of extraction models, assisting the scale up of advanced separation processes for used nuclear fuel.

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