Development of a diffusion model on an integrated cathode of an electrolytic reduction process

The Advanced Spent Fuel Conditioning Process (ACP) has been developed in KAERI since 1997 to cope with a situation that the number of spent fuels increases with electricity generation. The central purpose of ACP is to reduce the volume, the radiotoxicity, and the heat load of oxide spent fuels from the current fleet of commercial PWRs and create a waste form more stable than the original irradiated fuel for storage and/or reuse. In the concept of ACP, the oxide SF is transformed metal form and high heat-generating elements such as Cs and Sr are selectively separated from the reduced metal. Metallization by electrolytic reduction of ACP is devised to utilize electrochemical properties of a stable molten salt and eliminate the use of reducing agents such as Li and Ca which are difficult to handle because of their high reactivity. An integrated cathode composed of a porous magnesia membrane in which the oxide SF is loaded as powder form and a solid electricity conductor placed in the middle of the membrane and inconsumable anodes made of platinum are electrically connected in a Li2O dissolved LiCl molten salt cell. As electric charges pass the cell, the dissolved lithium ions from Li2O are reduced to lithium metals on the cathode and then the lithium metals react with metal oxides in the cathode producing lithium oxides which travel to the molten salt phase leaving behind the metals reduced from the metal oxides in the cathode. Meantime, oxygen gas is formed at the platinum anodes and dissipates into the cell's inert atmosphere. Finally, inert argon gas continuously conveys the oxygen gas out of the reactor to keep the oxygen partial pressure which affects the anode potential from rising in the cell. Mass transfers of oxygen ions released from the metal oxides and some constituents of oxide SF desirable to be dissolved in the molten salt are studied in this study. An analytical diffusion model is proposed for a cylindrical cathode and experimental data are correlated with the model. The derived model is based on a physical diffusion theory and expected to expand its applicability readily when including electrochemical considerations