Electrochemical Behaviors Of Gadolinium And Holmium In Melt

Pyrochemical reprocessing, which will play a central role in future advanced fuel cycles of the Generation IV reactors, has received widespread attention. Gadolinium and holmium, the typical fission products elements, which can largely hamper nuclear reaction due to its high thermal neutron absorption cross-section. Therefore, it is quite necessary to remove gadolinium and holmium by the pyrochemical reprocessing. So it is very important to study the electrochemical behavior of gadolinium and holmiumin the melt. This work presents a study of electrochemical properties of Gd³⁺ and Ho³⁺on the W and the liquid Zn electrodes in Li Cl-KCl melt.It was found that Gd³⁺ and Ho³⁺ reduction on a tungsten cathode occurs in a single step pattern according to square wave voltammetry（SWV）. The major thermodynamic of Gd³⁺/Gd redox reaction was obtained by open-circuit chronopotentiometry（OCP） techniques. The apparent standard enthalpy and entropy for the Gd Cl3 formation was calculated to be-1030.475 k J/mol and-198.2 J/K in Li Cl-KCl eutectic, respectively. Both the diffusion coefficients of Gd³⁺ and Ho³⁺on the W electrode in Li Cl-KCl eutectic were calculated by semi-integral technique. And the standard rate constants for the charge transfer of Gd³⁺ and Ho³⁺ on the W electrode were also investigated by Nicholson method and semi-integral techniques, respectively. And revealing the redox reactions of Gd³⁺ to Gd and Ho³⁺ to Ho are quasi-reversible based on Matsuda-Ayabe criteria for practical concept of electrochemical reversibility.The reductions of Gd³⁺ and Ho³⁺at a liquid zinc electrode in Li Cl-KCl melt were also studied, respectively. The diffusion coefficients of Gd³⁺ and Ho³⁺ at the liquid zinc electrode were obtained by semi-integral and semi-differential technique, respecrively. The standard rate constants for the charge transfer of Gd³⁺at the liquid zinc electrode was also studied by Nicholson method, revealing the redox reaction of Gd³⁺ to Gd is quasi-reversible based on Matsuda-Ayabe criteria for practical concept of electrochemical reversibility.The signal of GdxZny and HoxZny intermetallic compounds formation were observed by CV and OCP technique on the W electrode. In addition, the compositions of HoxZny were estimated by the accumulated coulomb numbers of the relative anodic peaks on the CV curve. The co-reduction behavior of Zn2+ and Ho³⁺ was also explored in Li Cl-KCl melt on the W electrode by square wave voltammetry.The co-deposition of Zn²⁺ with Gd³⁺ and Ho³⁺ were carried out on the tungsten electrode by potentiostatic electrolysis. Electrodeposition of Gd³⁺ and Ho³⁺ were also carried out on the liquid zinc electrode through potentiostatic electrolysis.The X-ray diffraction（XRD） results showed that intermetallic compounds Gd Zn12, Gd2Zn17, Gd13Zn58, Gd Zn, Ho2Zn17 and Ho Zn3 were obtained on the W electrode through co-reduction, whilst intermetallic compound Gd Zn12 and Ho2Zn17 were gained at the liquid zinc electrode by under-deposition. The morphological properties of all the intermetallic compounds were characterized by scanning electron microscopy（SEM） coupled with energy dispersive spectrometry（EDS）.