The electroreduction of used oxide nuclear fuel in a LiCl-Li2O electrolyte is a necessary step for incorporating light water reactor fuel into a pyroprocessing based fuel cycle.1 During the electrolytic reduction of UO2, significant quantities of metallic Li are generated at the cathode due to the close reduction potentials of UO2 and Li2O, and the overpotential necessary to ensure a complete reduction of UO2.2 Due to the non-zero solubility limit of Li in LiCl, some of the Li thus generated dissolves into the electrolyte.3 The effects of this tertiary LiCl-Li2O-Li electrolyte on the degradation of structural materials have not been thoroughly studied for long exposure periods. Thus this work focuses on the long-term corrosion behavior of Stainless Steel 316 (SS316) in this system.
Exposure testing of SS316 coupons in LiCl containing either 1 or 2 wt%Li2O and 0, 0.3, 0.6 and 1.0wt%Li were performed for 500 and 1000 hour exposure periods at each salt composition, for a total of 16 unique conditions. Due to the depletion of Li over time caused by both evaporation and reaction, the salt in which the samples were submerged in was replaced every 96 hours. Post exposure surface analysis was performed using Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, micro-Vickers hardness testing, and optical microscopy. Inductively coupled plasma – optical emission spectrometry was used to quantify the rate at which alloying elements were dissolved into the molten LiCl-Li2O-Li solution.
The presence of lithium chromite surface films was detected on SS316 samples exposed to molten LiCl-Li2O solutions containing no metallic Li. However, samples exposed to LiCl-Li2O containing Li showed significantly thinner or no surface oxide, indicating that protective chromium oxide surface films are not stable under the reducing conditions caused by metallic Li. Additionally, significant degradation of the Ni crucibles used to contain the molten solutions was observed when metallic Li was present, while minimal degradation was observed when no metallic Li was present in the molten salt. These observations indicate that there is a marked change in corrosion mechanism when metallic Li is added to the LiCl-Li2O system.
Acknowledgements: This work was performed under the auspices of the Department of Energy (DOE) under contracts DE-NE0008262 and DE-NE0008236, and the US Nuclear Regulatory Commission (NRC) under contracts NRCHQ-11-G-38-0039 and NRC-HQ-13-G-38-0027. W.P. acknowledges the Fellowship Award from the NRC. Dr. Kenny Osborne serves as the program manager for the DOE award and Ms. Nancy Hebron-Isreal serves as the grants program officer for the NRC awards.
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3 Dworkin, A. S., Bronstein, H. R. & Bredig, M. A. Miscibility of Metals with Salts. VI. Lithium-Lithium Halide Systems. The Journal of Physical Chemistry 66, 572-573 (1962).