Phase-Field Modelling and Morphological Classification of Uranium Dendrites for the Electrorefining of Used Nuclear Fuel

Abstract

Electrorefining is an important unit operation for the pyroprocessing of used nuclear fuel; however, the uncontrolled growth of uranium dendrites on the cathode is hindering its engineering application. In this study, the phase-field modelling is applied to the study of the growth of uranium dendrites using the finite element method, and the fractal dimension and the perimeter-to-area ratio are employed to classify quantitatively the morphologies of uranium dendrites. It is shown that uranium dendrites can form sprout-like, fishbone-like, and tree-like morphologies, and the effects of anisotropic strength, symmetry index, overpotential, and temperature to the morphologies of uranium dendrites are discussed. It is concluded that the diffusion of uranium cations (diffusion rate-controlling) in molten salt and the electrode kinetics (kinetic rate-controlling) are the two rate-controlling steps for the electrodeposition of uranium, and the diffusion rate-controlling mechanism is responsible for the growth of complicated dendritic morphologies.