Use of UV-visible Spectroscopy to Determine Solution Chemistry Under Used Nuclear Fuel Reprocessing Conditions

UV-visible spectroscopy is utilized in an online fashion to directlymeasure concentration and speciation of special nuclear materials,such as uranium and plutonium, allowing real-time accountabilityand tracking for the solvent extraction processes. The implementationof UV-visible spectroscopy for concentration measurementsin flowing systems has been demonstrated at labscale, andthis initiative boasts great potential to simultaneously and directlymonitor chemical process conditions and metal concentrationsthrough an analysis of fundamental chemical speciation. Byevaluating the impact of process conditions, such as acid concentrationand flow rate, on the sensitivity of the UV-visible detectionsystem, the process-monitoring concept can progress froma common use of instrumentation to an advanced applicationof fundamental spectroscopy. Ultimately, the experiments conductedpursuant to these objectives will quantitatively describethe relationship between certain UV-visible spectra and their processconditions through the exploitation of chemical speciation.Understanding this relationship is complex due to the undefineduranyl nitrate speciation, thus groundwork tasks focused on characterizingthe system encompassing 0.01-1.26 M U and 0.01-8M 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 nitratespecies not present at lower uranyl concentrations. Trends inuranyl molar absorptivity dependence on HNO3 concentrationacross wavelengths provide a complimentary approach for investigatinguranyl nitrate system behavior. Extended X-ray AbsorptionFire Structure spectroscopy has been employed to elucidateuranyl nitrate speciation changes in solution, and extend nitratesolution studies to the plutonyl system. Density functional theory(DFT) calculations of the geometric and electronic structuresof the lowest energy uranyl nitrate complexes indicate that thekinetic stability of the tetranitrato species is significantly lowerthan for other uranyl nitrate complexes, supporting experimentalresults in which the tetranitrato species is absent. Current effortsfocus on integrating a fiber optic dip probe for UV-visible spectroscopyinto a bank of centrifugal contactors to demonstrate theonline process monitoring concept. Further, the sensitivity of theUV-visible system to nitrate and uranyl conditions, and expectedextrapolation toplutonyl conditions, confirm the acute potentialfor this method of online process monitoring.