Uranium dendrites in molten salt electrorefining

Pyrochemical reprocessing is an attractive technique for the separation of uranium and plutonium from used nuclear fuel. Uranium electrorefining in molten salts is a key step in pyrochemical reprocessing that warrants additional study due to the tendency to form branched, dendritic uranium electrodeposits. Uranium dendrite formation has been widely reported in molten salt systems and is associated with several issues inhibiting continuous operations in pyrochemical reprocessing cells. Phenomena controlling electrodeposit nucleation and growth are explored, including mass transport, nucleation, electron transfer, diffusion-limited tip growth of electrodeposits, and the tip effect. Six experimental factors that affect dendrite formation are discussed: temperature, applied overpotential, the presence of other ions in the molten salt, inner- and outer-coordination sphere effects, uranium concentration, and electrode material, geometry, and orientation. The progress in understanding the complex interplay of these factors over the past 30 years through electrochemistry, spectroscopy, microscopy, and phase-field simulations is summarized in this chapter. Implementation of spectroelectrochemical analysis, more rigorous techniques and more detailed descriptions of experimental methods in diffusion parameter measurements, and further development of phase-field modeling will improve understanding of uranium dendrite formation in molten salt systems.