Speciation and spectroscopy of the uranyl and tetravalent plutonium nitrate systems: Fundamental studies and applications to used fuel reprocessing

This dissertation explores the use of UV-Visible spectroscopy and Time Resolved Laser Induced Fluorescence spectroscopy as near real time process monitors of uranium and plutonium concentrations in aqueous reprocessing trains. The molar absorptivities and linear ranges of these metals were investigated under total nitrate and acid concentrations similar to those found in current reprocessing systems. Concurrent to this, a new multiple wavelength monitor was derived that is capable of determining the total nitrate concentration spectroscopically. This method uses the uranium absorbance spectrum to calculate the nitrate concentration in solution. When used as part of an Advanced Safeguard suite, this technique can provide information on the process chemistry in use.;The fundamental chemistry of the uranyl nitrate system was investigated to add to the thermodynamic data set. A combination of spectroscopic measurements, Density Functional Theory calculations, Extended X-ray Absorption Fine Structure spectroscopy, and observations of solvent extraction studies were used to theorize a new model of uranyl-nitrate speciation. In this model, the dominant species at low nitrate concentrations is UO2(NO3) 2 and the UO2NO3+ species is de-emphasized. Spectrophotometric titrations of the uranyl system were used to determine the log β2,1 values for this system at multiple ionic strengths and the zero ionic strength stability constants were calculated using the Specific Ion Interaction Theory.;The UV-Visible spectroscopy of the tetravalent plutonium nitrate system was investigated as a function of nitric acid concentration. Two pseudo-isobestic points were identified in the spectra which can be used to determine the total PuIV concentration. Factor analysis was then used to investigate the speciation of the system; a total of 5 species exist between 2 and 10 M HNO3. This information can be used to focus future studies.