Investigating microstructural evolution during the electroreduction of UO₂ to U in LiCl-KCl eutectic using focused ion beam tomography

Reprocessing of spent nuclear fuels using molten salt media is an attractive alternative to liquid-liquid extraction techniques. Pyroelectrochemical processing utilizes direct, selective, electrochemical reduction of uranium dioxide, followed by selective electroplating of a uranium metal. Thermodynamic prediction of the electrochemical reduction of UO₂ to U in LiCl-KCl eutectic has shown to be a function of the oxide ion activity. The pO²⁻ of the salt may be affected by the microstructure of the UO₂ electrode. A uranium dioxide filled "micro-bucket" electrode has been partially electroreduced to uranium metal in molten lithium chloride-potassium chloride eutectic. This partial electroreduction resulted in two distinct microstructures: a dense UO₂ and a porous U metal structure were characterised by energy dispersive X-ray spectroscopy. Focused ion beam tomography was performed on five regions of this electrode which revealed an overall porosity ranging from 17.36% at the outer edge to 3.91% towards the centre, commensurate with the expected extent of reaction in each location. The pore connectivity was also seen to reduce from 88.32% to 17.86% in the same regions and the tortuosity through the sample was modelled along the axis of propagation of the electroreduction, which was seen to increase from a value of 4.42 to a value of infinity (disconnected pores). These microstructural characteristics could impede the transport of O²⁻ ions resulting in a change in the local pO²⁻ which could result in the inability to perform the electroreduction. - Highlights: • The microstructural evolution of the reduction from UO₂ to U has been investigated. • The porosity and tortuosity is seen to change significantly during electroreduction. • Low porosity and high tortuosity associated with the UO₂ phase may impede O²⁻ transport.