Structure and Dynamics of NaCl/KCl/CaCl2-EuCln (n = 2, 3) Molten Salts

Modern molten salt reactor design and the techniques of electrorefining spent nuclear fuels require a better understanding of the chemical and physical behavior of lanthanide/actinide ions with different oxidation states dissolved in various solvent salts. The molecular structures and dynamics that are driven by the short-range interactions between solute cations and anions and long-range solute and solvent cations are still unclear. In order to study the structural change of solute cations caused by different solvent salts, we performed first-principles molecular dynamics simulations in molten salts and extended X-ray absorption fine structure (EXAFS) measurements for the cooled molten salt samples to identify the local coordination environment of Eu²⁺ and Eu³⁺ ions in CaCl₂, NaCl, and KCl. The simulations reveal that with the increasing polarizing the outer sphere cations from K⁺ to Na⁺ to Ca²⁺, the coordination number (CN) of Cl^- in the first solvation shell increases from 5.6 (Eu²⁺) and 5.9 (Eu³⁺) in KCl to 6.9 (Eu²⁺) and 7.0 (Eu³⁺) in CaCl₂. This coordination change is validated by the EXAFS measurements, in which the CN of Cl^- around Eu increases from 5 in KCl to 7 in CaCl₂. Our simulation shows that the fewer Cl^- ions coordinated to Eu leads to a more rigid first coordination shell with longer lifetime. Furthermore, the diffusivities of Eu²⁺/Eu³⁺ are related to the rigidity of their first coordination shell of Cl^-: the more rigid the first coordination shell is, the slower the solute cations diffuse.

Mechanisms of Plutonium Redox Reactions in Nitric Acid Solutions

Plutonium (Pu) exhibits a complex redox behavior in aqueous solutions. This is due to the ability of the element to adapt a wide range of oxidation states typically from +3 to +6 and the tendency for dynamic interconversion between the oxidation states that primarily depend upon acid concentration and presence of coordinating ligands. This work interrogates the Pu redox behavior in aqueous nitric acid via a combination of voltammetry and in situ vis-NIR spectroelectrochemistry under controlled potentials to map the interconversion between the various Pu oxidation states. The NIR-spectroelectrochemistry studies used to complement the visible spectroscopy bring a new and more complete perspective into the plutonium redox transformations. This allows elucidation of the mechanisms of the involved redox reactions facilitating an in-depth understanding of the relative stability of the Pu oxidation states as a function of redox potentials and nitric acid concentrations. It is observed that oxidation of Pu(III) results in generation of Pu(IV) and Pu(VI) (the latter as PuO₂²⁺), bypassing the Pu(V) oxidation state. Further, with increasing acid concentrations, the formation of the Pu(VI) species progressively decreases so that the dynamic equilibrium between the Pu(III) and Pu(IV) oxidation states dominates. These findings have significant implications for developing separation processes for used nuclear fuel reprocessing and treatment.

Development and testing of a novel micro-Raman probe and application of calibration method for the quantitative analysis of microfluidic nitric acid streams

To simplify and improve the safety of reprocessing nuclear fuel, the Micro-Raman technique was applied at the microfluidic scale with a view toward the on-line spectroscopic measurement of radioactive solutions.

Incorporating spectroscopic on-line monitoring as a method of detection for a Lewis cell setup

A Lewis cell was designed and constructed for investigating solvent extraction systems by spectrophotometrically monitoring both the organic and aqueous phases in real time. This new Lewis cell was tested and shown to perform well compared to other previously reported Lewis cell designs. The advantage of the new design is that the spectroscopic measurement allows determination of not only metal ion concentrations, but also information regarding chemical speciation - information not available with previous Lewis cell designs. For convenience, the new Lewis cell design was dubbed COSMOFLEX (COntinuous Spectroscopic MOnitoring of Forrest's Liquid-liquid EXtraction cell). After construction performance testing was done for establishing the ideal stir speed range, UV-Vis measured concentration and D value determination. Each one of these tests was satisfactorily passed.