Generating Tafel Parameters in Support of Electrorefining Modeling for Uranium Recovery from Scrap U-Mo Foils

A model is under development to simulate operation of an electrorefiner for the recovery of uranium from scrap U-Mo foils. U-Mo foils are proposed as a low enriched uranium (LEU) fuel alternative, for research and test reactors currently using highly enriched fuels, as part of the Reduced Enrichment for Research and Test Reactors program. The conversion to LEU fuels will decrease the risk of proliferation at research and test reactors. The model being developed will be used to design and optimize the operation of electrorefining for scrap uranium recovery from the LEU fuel manufacture, thereby improving efficiency of the manufacturing process.

Electrorefining for uranium recovery from metal fuel scrap is traditionally conducted in a molten eutectic LiCl/KCl salt electrolyte with a dilute amount of UCl₃ in the salt at 500°C. The uranium is then dissolved at the anode forming U³⁺ ions and deposited at the cathode as metallic uranium in response to an applied potential. The advantageous choice of operating potential ensures uranium is the only species being transported out of the scrap fuel material. The uranium collected at the cathode is then available for recycle to fuel manufacturing.

The model implements classic corrosion mathematics to determine polarization at the anode and cathode. Polarization at the anode is complicated by formation of a passive layer. Polarization at the cathode is complicated by mixed polarization, caused by the low concentration of uranium in the electrolyte.  Transport across the cell is driven by electronic migration and diffusion. The concentration at each electrode is calculated using the diffusion layer thickness. The model is constructed in MatLab with a user interface and simulation results are exported to Excel for analysis.

The electrolyte, LiCl/KCl 5-10 weight percent UCl₃, used in electrorefining of uranium has not been studied extensively via corrosion research techniques. Therefore, the Tafel constants and exchange current densities are currently unavailable for uranium in this electrolyte. Experiments have been conducted to generate these parameters for materials of interest during the electrorefining of U-Mo foils. 

A three electrode cell with a pseudo-reference tungsten electrode was used to scan potentiodynamically across both anodic and cathodic overpotentials. The potentiodynamic data were normalized to zero overpotential at reaction. The normalized data were examined in the ±4 to 10 mV of overpotential range for Tafel constants, both anodic and cathodic. This overpotential range was chosen in accordance with the standard method for analyzing potentiodynamic data outlined by Scully [1]. The exchange current density was found using the intersection of Tafel lines constructed from the same region of overpotential.

The generation of these parameters will contribute significantly to the basic chemistry knowledge of the electrorefiner system for these materials in this electrolyte. It will also contribute significantly to the development of the mathematical model. Simulations of electrorefining of several types of metallic LEU fuels using the model are proposed to validate the mathematical methods used, including the Tafel parameters.

References

[1]     Scully, John R. “Chapter 7: Electrochemical Tests” of Baboian, Robert. Corrosion Tests and Standards: Application and Interpretation. ASTM International. 2005.

Acknowledgements

Work supported by the U.S. Department of Energy, National Nuclear Security Administration's (NNSA's) Office of Defense Nuclear Nonproliferation, under Contract DE-AC02-06CH11357. Argonne National Laboratory is operated for the U.S. Department of Energy by UChicago Argonne, LLC.

Government license notice

The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.