Flow analysis of electrode reaction in pyrochemical reprocessing. Estimation of the a diffusion layer thickness in oxide electrowinning method

A pyrochemical reprocessing system using a molten salt electrolysis has been studied as one of the feasible concepts for the FBR fuel reprocessing system. In order to calculate the amount of deposits of the salvaged material in molten salt electrolysis process in high accuracy, it is necessary to evaluate appropriately the concentration diffusion layer thickness of a deposit ingredient near the electrode surface. However, measurement of diffusion layer thickness is extremely difficult. Therefore, it has been a subject to develop the evaluation methods of the diffusion layer thickness. In this report, we have developed the two-dimensional analysis code, in order for numerical analysis to estimate the diffusion layer thickness near the electrode surface. This code can solve simultaneously a deposit by electrode reaction of Nernst's equation, concentration distribution of reactant near the electrode surface, and the natural convection resulting from the density change by concentration change. By using the code, we carried out analysis of the oxide electrolysis experiment in Russia, and evaluated the characteristic of diffusion layer thickness and the polarization curve. As a result, although indeterminacy remained in the physical-properties value, the polarization curve in analysis agreed with the experiment result in general. By the analysis of a molten salt electrolysis, it was clarified that the diffusion layer is formed with hundreds micrometers thickness along the natural convection on the cathode surface, and the diffusion layer thickness changes corresponding to the velocity boundary layer of the natural convection flow which is driven by mass transfer at the electrode surface. In addition, Ito's mass transfer correlation for aqueous solutions shows good agreement with the molten salt calculation within a reasonable accuracy. Therefore, it is prospected that the diffusion layer thickness in the molten salt can be evaluated from Ito's correlation. (author)