Corrosion of Candidate Alloys in Molten LiCl-Li2O-Li for Pyroprocessing of Used Nuclear Fuel

Abstract

Pyrochemical reprocessing of used nuclear fuel (UNF) requires large structural process vessels and functional components fabricated from stainless steel 316L (SS316L) or another high-temperature molten salt-compatible material like Inconel 625 (I625). The unique chemistry of the molten salt electrolyte during processing of UNF leads to complex interactions between container materials and the electrolyte that have been well documented by our group. In the head-end process to incorporate oxide UNF into the pyrochemical reprocessing scheme, LiCl with 1-2 wt. % Li2O is utilized as an electrolyte to facilitate the electrolytic reduction of UO2 via the Fray-Farthing-Chem (FFC) process. The Li+ reduction potential is quite close to the UO2 reduction potential and it is generally recognized that lithium metal will be produced at the cathode during the reduction process. While much of the metallic lithium further facilitates the reduction of UO2 via a metallothermic pathway, forming Li2O, a fraction of the metallic lithium dissolves or otherwise disperses into the molten salt. The metallic lithium then diffuses throughout the salt and reacts with process vessels and other components. To further understand corrosion in these conditions, SS316L and I625 coupons were exposed to LiCl-1 wt. % Li2O-Li and LiCl-2 wt. % Li­2O-Li with lithium metal concentrations of 0, 0.3, 0.6 or 1.0 wt. %. These samples were then studied using focused ion beam (FIB) milling, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS). Results from these studies will be discussed. Acknowledgements: The authors thank Zachary Karmiol, research scientist at Materials Characterization Nevada, for technical input. One of the authors, JM, acknowledges the Graduate Research Fellowship from the US National Science Foundation. This work was supported by the United States Department of Energy (DOE) under contracts, DE-NE0008262, DE-NE0008236, and DE-NE0008572, and the United States Nuclear Regulatory Commission (NRC) under Contract NRC-HQ-13-G-38-0027. Dr. Kenny Osborne and Ms. Nancy Hebron-Israel serve as the DOE and NRC award program managers, respectively. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1447692.