Behavior of uranium and zirconium in direct transport tests with irradiated EBR-II fuel

A theoretical model is used to analyze the transport of U and Zr in electrorefining of irradiated binary Experimental Breeder Reactor-II fuel. A limiting-current hypothesis is advanced to explain the observed dissolution of Zr in the presence of U at high, intermediate, and low cell voltages. The internal diffusion model predicts the existence of a critical current and a critical voltage for Zr oxidation. Experimental results are presented for a test designed and run based on optimum conditions determined from the model to dissolve U expediently while retaining Zr in the anode baskets. A simple model of kinetic exchange reactions between salt-phase U and Cd-phase Zr is formulated to explain the measured electrodeposition of Zr on the solid cathode. It is shown that the Zr content of the deposit is overpredicted if the pool is considered isolated and grossly underpredicted if the salt phase is equilibrated instantaneously with the Cd pool. Finally, the aspects of anodic current efficiency and cathodic collection efficiency are discussed taking into account shorting between the dissolution baskets and the Cd pool, multiple oxidation states of Zr, and the exchange reactions between the fuel and UCl3 prior to electrotransport