Year
2010
Abstract
UV-Visible spectroscopy is utilized in an online fashion to directly measure concentration and speciation of special nuclear materials, such as uranium and plutonium, allowing real-time accountability and tracking for the solvent extraction processes. The implementation of UV- Visible spectroscopy for concentration measurements in flowing systems has been demonstrated at labscale, and this initiative boasts great potential to simultaneously and directly monitor chemical process conditions and metal concentrations through an analysis of fundamental chemical speciation. By evaluating the impact of process conditions, such as acid concentration and flow rate, on the sensitivity of the UV-Visible detection system, the process-monitoring concept can progress from a common use of instrumentation to an advanced application of fundamental spectroscopy. Ultimately, the experiments conducted pursuant to these objectives will quantitatively describe the relationship between certain UV-Visible spectra and their process conditions through the exploitation of chemical speciation. Understanding this relationship is complex due to the undefined uranyl nitrate speciation, thus groundwork tasks focused on characterizing the system encompassing 0.01-1.26 M U and 0.01-8 M HNO3. Results suggest dominant speciation changes from low (0.01 M) to high (>6 M) HNO3, and peak shifts in the high (>1 M) uranyl system similarly imply an ingrowth of uranyl nitrate species not present at lower uranyl concentrations. Trends in uranyl molar absorptivity dependence on HNO3 concentration across wavelengths provide a complimentary approach for investigating uranyl nitrate system behavior. EXAFS spectroscopy has been employed to elucidate uranyl nitrate speciation changes in solution, and extend nitrate solution studies to the plutonyl system. Density functional theory (DFT) calculations of the geometric and electronic structures of the lowest- energy uranyl nitrate complexes indicate that the kinetic stability of the tetranitrato species is significantly lower than for other uranyl nitrate complexes, supporting experimental results in which the tetranitrato species is absent. Current efforts focus on integrating a fiber optic dip probe for UV-Visible spectroscopy into a bank of centrifugal contactors to demonstrate the online process monitoring concept. Further, the sensitivity of the UV-Visible system to nitrate and uranyl conditions, and expected extrapolation to plutonyl conditions, confirm the acute potential for this method of online process monitoring.