The chemical behavior of ternary Ca–UO₂–CO₃ complexes was investigated by using time-resolved laser fluorescence spectroscopy (TRLFS) in combination with EDTA complexation at pH 7–9. A novel TRLFS revealed two distinct fluorescence lifetimes of 12.7 ± 0.2 ns and 29.2 ± 0.4 ns for uranyl complexes which were formed increasingly dependent upon the calcium ion concentration, even though nearly indistinguishable fluorescence peak shapes and positions were measured for both Ca–UO₂–CO₃ complexes. For identifying the stoichiometric number of complexed calcium ions, slope analysis in terms of relative fluorescence intensity versus calcium concentration was employed in a combination with the complexation reaction of CaEDTA²⁻ by adding EDTA. The formation of CaUO₂(CO₃)₃ ²⁻ and Ca₂UO₂(CO₃)₃(aq) was identified under given conditions and their formation constants were determined at I = 0.1 M Na/HClO₄ medium, and extrapolated to infinitely dilute solution using specific ion interaction theory (SIT). As a result, the formation constants for CaUO₂(CO₃)₃ ²⁻ and Ca₂UO₂(CO₃)₃(aq) were found to be log β⁰ ₁₁₃ = 27.27 ± 0.14 and log β⁰ ₂₁₃ = 29.81 ± 0.19, respectively, providing that the ternary Ca–UO₂–CO₃ complexes were predominant uranium(VI) species at neutral to weakly alkaline pH in the presence of Ca²⁺ and CO₃ ²⁻ ions.

The formation of ternary Mg-UO₂-CO₃ complexes under weakly alkaline pH conditions was investigated by time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fine structure (EXAFS) and compared to Ca-UO₂-CO₃ complexes. The presence of two different Mg-UO₂-CO₃ complexes was identified by means of two distinct fluorescence lifetimes of 17±2 ns and 51±2 ns derived from the multi-exponential decay of the fluorescence signal. Slope analysis in terms of fluorescence intensity coupled with fluorescence intensity factor as a function of log [Mg(II)] was conducted for the identification of the Mg-UO₂-CO₃ complexes forming. For the first time, the formation of both MgUO₂(CO₃)₃ ²⁻ and Mg₂UO₂(CO₃)₃(aq) species was confirmed and the corresponding equilibrium constants were determined as log β⁰ ₁₁₃=25.8±0.3 and log β⁰ ₂₁₃=27.1±0.6, respectively. Complementarily, fundamental structural information for both Ca-UO₂-CO₃ and Mg-UO₂-CO₃ complexes was gained by extended EXAFS revealing very similar structures between these two species, except for the clearly shorter U-Mg distance (3.83 Å) compared with U-Ca distance (4.15 Å). These results confirmed the inner-sphere character of the Ca/Mg-UO₂-CO₃ complexes. The formation constants determined for MgUO₂(CO₃)₃ ²⁻ and Mg₂UO₂(CO₃)₃(aq) species indicate that ternary Mg-UO₂-CO₃ complexes contribute to the relevant uranium species in carbonate saturated solutions under neutral to weakly alkaline pH conditions in the presence of Mg(II) ions, which will induce notable influences on the U(VI) chemical species under seawater conditions.

Electrochemical and spectroscopic properties of Tb(III) in molten LiCl–KCl eutectic at high temperature were investigated by cyclic voltammetry and time-resolved laser-induced fluorescence spectroscopy (TRLFS). The diffusion coefficient of Tb(III) and the formal standard potential of Tb(III)/Tb 0 were determined to be 2.06 ± 0.4 × 10 − 5 cm 2 s − 1 and − 2.83 ± 0.03 V vs. Cl 2 /Cl − at 887 K, respectively. Additionally, visible fluorescence of Tb(III) due to the electronic transitions from 5 D 3 and 5 D 4 to 7 F J was observed and measured by TRLFS for the first time. These results provide the first fluorescence spectroscopic evidence for a direct in situ quantification of Tb(III) in the high temperature molten salt system. Keywords Terbium Molten chlorides Fluorescence Cyclic voltammetry In situ analysis 1 Introduction For recycling spent nuclear fuel and thus reducing the amount of high-level radioactive waste, different pyrochemical processes are under development. They are based primarily on electrochemical methods such as electrolysis or electrorefining, or electroreductive extraction in a molten salt solvent at high temperature. It enables to beneficially reuse uranium and plutonium by the recovery of group actinide (U, Pu, and minor actinide (Np, Am, and Cm)) from the fission products present in spent nuclear fuel according to the different standard free energy of formation of chloride [1] . Knowledge about the electrochemical behaviors of elements in the pyrochemical process of spent nuclear fuel is a major concern. For the purpose, electrochemical properties, such as the diffusion coefficient, the activity coefficient, and the formal standard potential, have been intensively investigated by conventional transient electrochemical techniques, such as cyclic voltammetry and chronopotentiometry. For the quantification and structural analysis, the absorption spectroscopic properties of the most of lanthanide elements were characterized in LiCl–KCl molten salt at high temperatures [2–6] . Raman spectroscopic studies for Sm(III), La(III), Nd(III), and Gd(III) were also performed in the lanthanide chloride–alkali chloride binary melt system [7,8] . Recently, the fluorescence measurements on the lanthanide elements in molten salt were reported at room temperature [9,10] . However, there is still no direct fluorescence measurement of elements dissolved in the high-temperature molten salt system. This work focuses on the fluorescence spectroscopic behaviors of Tb(III) in the molten LiCl–KCl eutectic. For the first time, we demonstrate the direct in situ quantification of Tb(III) in the chloride molten salt at high temperature using TRLFS. Besides, the diffusion coefficient of Tb(III) and the formal potential of Tb(III)/Tb 0 were also determined by cyclic voltammetry. 2 Experimental 2.1 Apparatus Electrochemical and fluorescence experiments were performed in a glove box under high-purity argon gas atmosphere (99.999% Ar, H 2 O and O 2 < 1 ppm). A temperature controlled furnace system was installed leak-proof in the lower part of the glove box and designed for fluorescence measurements, as shown in Fig. 1 . A 1 cm rectangular quartz cell with a long-necked cylindrical quartz tube was placed at the center of the electric furnace with optically accessible windows. The operating temperature was monitored by a thermocouple placed in contact with the sample quartz cell. Cyclic voltammetry was carried out in an electrochemical cell having a three-electrode system with a Princeton Applied Research VersaSTAT3-200 potentiostat. The molybdenum (Sigma-Aldrich 99.99% purity) wire with a diameter of 1 mm served as both working electrode (WE) and counter electrode (CE). The Ag/AgCl electrode (99.99% Ag wire with 1 mm in diameter) used as reference electrode (RE) was encased in a thin-end pyrex glass tube, in which the LiCl–KCl eutectic salt contains 1.0 wt.% AgCl (Sigma-Aldrich 99.998% purity). Prior to measurements, all electrodes were polished with a sand paper and cleaned with nitric acid (25% dilution), ultrapure water using ultrasonic bath, and ethanol sequentially. To measure the fluorescence of Tb(III) ions in molten LiCl–KCl eutectic, a quadrupled wavelength (266 nm) of a Nd:YAG laser (Continuum, Surelite-20) was used as an excitation source. The laser energy was monitored by a low energy detector (Gentec, XLE4). Fluorescence spectra were recorded with an intensified charge coupled device (ICCD, Andor Technology, iStar) coupled with a Czerny-Turner spectrometer (Andor Technology, shamrock sr-303i). Fluorescence was transferred into the entrance slit of the spectrometer through optic fiber bundle perpendicular to the laser beam direction. All fluorescence spectra were measured at 813 K. 2.2 Chemicals The concentration of Tb(III) ions in the chloride form (Sigma-Aldrich 99.99% TbCl 3 powder) varied from 0.05 wt.% to 0.48 wt.% by dissolution in polarographic-grade anhydrous LiCl–KCl eutectic salt (Sigma-Aldrich 99.99% purity, 44 wt.% LiCl). The amount of Tb(III) ion concentrations was determined by inductively coupled plasma-atomic emission spectrometer (ICP-AES). 3 Results and discussion 3.1 Electrochemical behavior Fig. 2 shows typical cyclic voltammogram of TbCl 3 in the molten LiCl–KCl eutectic on the molybdenum electrode at 887 K. The electrochemical window corresponds to the reduction of Li(I) and the oxidation of Cl(− I). The potential was applied beginning from 0.8 V to − 2.65 V and in a reverse way from − 2.65 V to 0.8 V with a scan rate of 0.1 V s − 1 . The cathodic and anodic peaks observed in the electrochemical window range indicate the reduction of Tb(III) to Tb 0 and re-oxidation of Tb 0 to Tb(III), respectively. The absence of crossover between the cathodic and anodic parts implies that no complication occurs during the metal nucleation onto the working...

The isotope analysis of iron was performed using laser ablation molecular isotopic spectrometry (LAMIS) for the detection of radioactive hotspots in nuclear power plant (NPPs). Using an optical double-pulse configuration, the emission bands from FeO (D⁵Δ_i → X⁵Δ_i) and FeO₂ (l⁵B₁ → l⁵B₂) transitions were investigated and characterized. The isotopic shifts of ⁵⁴FeO/⁵⁶FeO and ⁵⁴FeO₂/⁵⁶FeO₂ molecular emissions were identified at each rotational branch line. The quantitative analysis for isotopically enriched iron samples was performed by partial least squares regression. The cross-validation revealed superior results with root mean squares errors of prediction of about 0.07%. Subsequently, the detection of iron isotopes was carried out on the surface of structural materials of stainless steel-304, Inconel-690, and Portland cement to verify the feasibility for the application to NPPs. The surface density of ⁵⁴Fe particulates on each matrix was evaluated, and the results were comparatively analyzed according to the matrix characteristics. The spatial distribution of ⁵⁴Fe particulates on the stainless steel was identified by resolving the emission responses with focused spots. The present study not only provided characteristic optical information on the iron oxide emission, but also demonstrated the feasibility of LAMIS for the detection of iron isotopes in structural materials of NPPs.