| Description |
Pyrochemical reprocessing in molten chloride is a leading technology option for the reprocessing of used nuclear fuel. This process includes several high temperature unit operations, including electrorefining and oxide reduction. Steels are commonly used in currently operating electrorefining and oxide reduction systems, but other metal alloys such as Inconel 600, Hastelloy C 276, and tantalum are also viable candidates for structural materials. The molten salts can contain oxidizing impurities formed from reactions with residual water and/or oxygen. The oxidation of metal alloys leads to contamination of the molten salt and/or degradation of vessel structures. The goals of this research project were to understand the underlying causes that can lead to metal corrosion in molten salts used for pyroprocessing and develop electrochemical monitoring methods. Anodic polarization of manganese was investigated to determine characteristics such as diffusion coefficient, activity coefficient, and exchange current density. These values needed to be measured so that simulations of the electrorefiner (ERAD) could be performed. The model now predicts that running the electrorefiner for 12 hours or more will essentially eliminate the MnCl2 from the salt that is introduced via anodic polarization of stainless steel anode baskets. Electrochemical experiments were performed with pure samples of Ni and Fe in contact with LiCl-KCl or LiCl-KCl-UCl3, while varying concentrations of H2O vapor were bubbled into the salt. Open circuit potentiometry and cyclic voltammetry were used iv to monitor the reactions occurring in the molten salt in real time and near real time, respectively. There is evidence that gas phase of H2O in concentrations as low as 95 ppm bubbled into LiCl-KCl can induce corrosion. Alloys were tested for corrosion caused by the presence of oxygen in LiCl-Li2O using the zero resistance ammeter (ZRA) and Tafel methods. This study demonstrates that ZRA with a properly selected counter electrode can be an effective way to monitor corrosion in real time and obtain more corrosion rate data over a shorter period of time than possible with conventional. ZRA data were used to determine the corrosion resistance sequence of the alloys of interest: tantalum < SS-316 < Inconel 600 < Haynes C276 |
