(Invited) Understanding the Corrosion of SS316L in Molten LiCl-Li2O-Li Using 3D Transmission X-Ray Microscopy and 3D XANES
Authors: Moon Jeremy , Phillips William C. , Chidambaram Dev 
Published: December 2020

Pyrochemical reprocessing (pyroprocessing) of used nuclear fuel (UNF) is a promising pathway towards closing the nuclear fuel cycle and providing the means to generate durable wasteforms for portions of the UNF that are not able to be recycled. To incorporate commercial UNF (UO2) into the pyroprocessing scheme, the ceramic UNF must be converted to a metallic form at the head end of the process, which can be accomplished via electrolytic reduction of the UNF in LiCl-Li2O molten salt electrolyte. During the electrolytic reduction of UO2 at the cathode, metallic Li is inevitably co-reduced from the electrolyte due to the proximity of the reduction potential of the U4+ in UO2 and Li+ in the electrolyte. Metallic Li disperses into the LiCl electrolyte and can then interact with the process vessel and other apparatus in contact with the electrolyte, altering the rate and mechanism of the degradation of those materials.

Our group has previously shown that the presence of metallic Li in the electrolyte creates a reducing condition where the surface oxide layer of SS316 is removed, and extensive intergranular corrosion takes place. To further the understanding of the mechanism and morphology of the intergranular corrosion, transmission X-ray microscopy (TXM) at Brookhaven National Laboratory’s National Synchrotron Light Source II (NSLSII) beamline 18-ID was used to examine ~20 μm diameter focused-ion beam (FIB) milled pillars of the cross-section of SS316L exposed to LiCl-Li2O-Li with 0 wt.%, 0.3 wt.%, 0.6 wt.% and 1 wt.% Li. Computed Tomographic (CT) images were obtained at X-ray energies above and below the absorption edges of alloying elements of interest, providing insight into the distribution of alloying elements after exposure to the molten salt. 3D X-ray absorption near edge spectroscopy (XANES) tomography at the Cr K-edge was also attempted. This analysis provided additional evidence of the formation of Cr-rich intermetallic phases and 3D CT images of the void structure of the exposed samples, strengthening the evidence that the sensitization of SS316L at process temperatures and the subsequent dissolution of the Cr carbide precipitates into the electrolyte is the primary driver of the deep intergranular corrosion of SS316L in LiCl-Li2O-Li. To our knowledge, this is the first application of this FIB milling technique and TXM imaging of the FIB-milled pillars to corrosion samples and lessons learned regarding the sample preparation and analysis will be presented.

Acknowledgements: The authors thank Zachary Karmiol, research scientist at Materials Characterization Nevada, for technical assistance with the FIB sample preparation. This work was performed under the auspices of the Department of Energy (DOE) under contracts DE- NE0008236, and the US Nuclear Regulatory Commission (USNRC) under contract NRC-HQ-13-G-38-0027. Dr. Kenny Osborne and Ms. Nancy Hebron-Isreal serve as the program managers for the DOE and NRC awards, respectively. This research used beamline 18-ID of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. This work was supported as part of the Molten Salts in Extreme Environments Energy Frontier Research Center, funded by the U.S. Department of Energy (US-DOE), Office of Science, Basic Energy Sciences, at Idaho National Laboratory under contract DE-AC07-05ID14517. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1447692. One of the authors, JM, acknowledges the Graduate Research Fellowship from the US National Science Foundation.