Electrodeposition Of UO₂ And Effect Of Al³⁺ On Its Deposition In LiCl-KCl Molten Salt System

Effective management of spent fuel is a key problem restricting the development of nuclear energy,and molten salt electrochemical separation technology is the most promising dry reprocessing technology for spent fuel at present.In order to recover uranium（U）from spent oxide fuel efficiently and improve the recovery rate of UO₂,it is necessary to study the electrodeposition process of molten salt UO₂ and the influence of fuel element components and cladding materials on the deposition of UO₂.Thus,in order to study the effect of cladding material Al on the electrodeposition of UO₂,the electrochemical behaviors and electrodeposition process of Al³⁺and UO₂²⁺were studied in LiCl-KCl molten salt.Meanwhile,the effect of Al³⁺on the electrodeposition process of UO₂ was also discussed.The main contents are as follows:The electrochemical behavior of Al³⁺in LiCl-KCl melt was studied by different electrochemical testing methods.Al³⁺was reduced to metal Al by one-step three-electron transfer in the melt.The Al³⁺/Al electrode reaction was found to be quasi-reversible and controlled by diffusion.The diffusion coefficient of Al³⁺in the melt is 3.14×10^-5 cm² s^-1 at 723K,and the activation energy was 10.45 k J mol^-1.The nucleation mode of Al on W electrode was determined to be instantaneous nucleation by chronoamperometry,and the nucleation number density and crystal out growth rate increased with the increase of applied potential.Al was electrodeposited from LiCl-KCl melt by constant potential electrolysis at the overpotential of50 m V,150 m V and 250 m V,respectively.It was found that randomly oriented Al crystals existed in the products,and the cathodic current efficiencies were calculated to be 76.3%,72.0%and 70.3%.When Al is deposited by constant current electrolysis at 70 m A cm^-2 and 40 m A cm^-2,it is found that Al crystals were preferentially oriented along the（111）crystal face,along（111）crystal face and（200）crystal face,respectively.The crystal morphologies were dendritic,and the cathodic current efficiencies were calculated to be 81.4%and 85.9%.In order to improve the dendrite morphology and current efficiency,pulse electrolysis was used at these two current densities.It was found that the Al crystals were all along the preferred orientation of（200）crystal face,the crystal morphology were granular,and the current efficiency increased to 89.6%and 91.2%,respectively.The electrochemical behavior of UO₂²⁺in LiCl-KCl melt was studied by different electrochemical testing methods.UO₂²⁺was reduced to UO₂ by two-step single electron transfer,and the intermediate product UO₂⁺could undergo disproportionation reaction.The reduction process of UO₂²⁺/UO₂⁺was controlled by diffusion,and the diffusion coefficient of UO₂²⁺at 723 K was estimated to be 2.21×10^-5 cm² s^-1.The dynamic properties of the electrode reaction for UO₂⁺/UO₂ were estimated,and activation energies were 33.57 k J mol^-1 and 26.08k J mol^-1 by Tafel method and LP method,respectively.The nucleation mode of UO₂ on W electrode was found to be continuous nucleation by by chronoamperometry.Furthermore,UO₂crystals were deposited by constant potential electrolysis at-0.8 V and-0.4 V,and the crystal size of UO₂ decreased with the increase of potential.Then pulsed electrolysis was performed at the same potential.It was found that the morphology of UO₂ gradually changed from dendrite to massive with the change of potential and duty ratio,and finally the regular octahedral structure of UO₂ was obtained.The effect of Al³⁺on the electrodeposition of UO₂ was studied.The p O^2--T diagram of Al³⁺,U（Ⅳ）and UO₂²⁺in LiCl-KCl molten salt was drawn by thermodynamic calculation.It is found that the p O^2-of Al³⁺reaction with oxygen ion is the largest.It is speculated that Al³⁺would be easy to exchange oxygen with high valent uranium oxide.Combined with the results of electrochemical testing and characterization of deposition products,the influence of Al³⁺on UO₂ deposition was determined:the addition of Al³⁺could promote the electrodeposition of UO₂,but a small amount of Al₂O₃ was doped in the product of UO₂.