Electrochemical and thermodynamic properties of Pu(III) ions at the Mo electrode in LiCl-KCl eutectics

To enhance the extraction of remaining fissile nuclides from spent nuclear fuel through pyrochemical reprocessing, the operational lifetime of such fuels can be extended, ultimately leading to increased cost efficiency and a reduction in the amount of radioactively contaminated waste generated by fast-neutron reactors. Understanding the electrochemical behavior of fissile nuclides in LiCl-KCl is pivotal to the success of molten salt electrorefining pyrochemical reprocessing. In this pursuit, a comprehensive study of Pu(III) salt was conducted to comprehend its electrochemical characteristics within molten chloride salt mixtures. To achieve this, PuCl₃ was meticulously prepared by reacting PuO₂ with HCl in a LiCl-KCl mixture. Subsequently, we investigated the reduction mechanism, the diffusion coefficient of Pu(III) (D_Pu(III)), and the apparent standard reduction potential of Pu(III)/Pu(0) (E^0*Pu_(III)/Pu(0)) in situ, using a Mo working cathode. Our findings revealed that Pu(III) undergoes a single-step reduction to Pu(0), involving the exchange of three electrons. Furthermore, the rate of diffusion governs the reduction of Pu(III) at the Mo cathode. The relationship between the diffusion coefficient and temperature was described by lnD = - 5.51 to 4244.2/T, with an activation energy of 35.28 kJ/mol. Additionally, we examined the temperature-dependent variations of E^0*_{Pu(III)/Pu(0)} and the Gibbs free energy of formation for PuCl₃ (ΔG_PuCl3). These dependencies were found to be E^0*_{Pu(III)/Pu(0)} = - 3.194 + 6.4 × 10^-4 T and ΔG_PuCl3 = - 924.5 + 0.185 T, respectively. (author)