Chemical Species Transformation during the Dissolution Process of U3O8 and UO3 in the LiCl–KCl–AlCl3 Molten Salt

Dynamics of an Excess Electron in Molten LiF, KF, MgF2, and BeF2

Ab initio molecular dynamics simulations suggest that the dynamics of an excess electron in different types of molten salts are not always the same. In molten LiF, KF, and MgF2, the excess electron localizes in the cavity as a solvated electron for 10 ps, which agrees with the widely accepted theory of Pikaev. In molten BeF2, the excess electron shows a different localization pattern: it mostly exists in localized states but also occurs in many delocalized states. This "localize-delocalize" pattern originates from the high viscosity of BeF2 (16 000 cP at 900 °C), which will lead to slow ionic motion and finally result in slow solvent relaxation. Besides, the species formed by the localization of the excess electron in these four melts are also different. The spectral feature (broad peak in the vis-IR region) of the localized electron in molten alkaline halides was also observed in LiF, KF, MgF2, and BeF2. Both an excess electron and electrons in the bulk liquid could contribute to the spectra, but the excitation of the excess electron makes a bigger contribution to the broad vis-IR peak. Our predicted spectrum of molten LiF/KF qualitatively reproduces the major feature of the experimental spectrum, which partially validates our simulations.

Molten salts for efficient removal of radioactive contaminants from stainless steel surface: Mechanisms and applications

Here, we have demonstrated an innovative decontamination strategy using molten salts as a solvent to clean stubborn uranium contaminants on stainless steel surfaces. The aim of this work was to investigate the evolutionary path of contaminants in molten salts to reveal the decontamination mechanism, thus providing a basis for the practical application of the method. Thermodynamic analysis revealed that alkali metal hydroxides, carbonates, chlorides and nitrates can react with uranium oxides (UO3 and U3O8) to form various uranates. Notably, the decontamination mechanism was elucidated by analyzing the chemical composition of the contaminants in the molten salts and the surface morphology of the specimens considering NaOH-Na2CO3-NaCl melt as the decontaminant. The decontamination process involved two stages: a rapid decontamination stage dominated by the thermal effect of molten salt, and a stable decontamination stage governed by the chemical reactions and diffusion of molten salt. Subsequently, a multiple decontamination strategy was implemented to achieve high decontamination rates and low residual radioactivity. Within the actual cleaning time of 30 min, the decontamination efficiency (DE) of UO3-contaminated specimens reached 97.8% and 93.0% for U3O8-contaminated specimens. Simultaneously, the radioactivity levels of all specimens were reduced to below the control level for reuse in the nuclear domain. Particularly, the actual radioactive waste from the nuclear industry reached a reusable level of radioactivity after decontamination. The NaOH-Na2CO3-NaCl melt outperforms conventional chemical solvents and may be one of the most rapid and efficient decontaminants for stubborn uranium contamination of metal surfaces, which provides insights in regard to handling nuclear waste.

Preparation and Electrochemical Dissolution of a Soluble Uranium Oxycarbide Anode

A soluble uranium oxycarbide (UC0.4O0.6) anode was synthesized by a carbothermal reduction process, using U3O8, UO2, and graphite as raw materials, in a vacuum environment of 0.1 MPa at 1750 °C. The sintered UC0.4O0.6 exhibited excellent conductivity and stability in LiCl-KCl molten salt. The dissolution process of UC0.4O0.6 in molten salt was analyzed by linear sweep voltammetry (LSV), and the initial dissolution potential, rapid dissolution potential, and passivation potential were determined. The oxidation/reduction process of dissolved uranium ions in LiCl-KCl molten salt was studied by cyclic voltammetry (CV) and square wave voltammetry (SWV). The experimental results showed that the addition of Li2O significantly reduced the volatilization of U(VI) and increased the concentration of UO22+ in molten salt. Finally, the product was conducted by electrolysis at 1.0 V and was confirmed to be UO2 by XRD and SEM-EDS analysis.