In order to incorporate oxide based nuclear fuels from the current light water reactor fleet into a pyrometallurgical based fuel cycle, it is necessary to reduce the oxide fuel to a metallic form. This process is typically conducted electrolytically in a LiCl-Li2O electrolyte at 650°C. Due to the close reduction potentials of UO2 and Li2O, metallic Li is generated at the cathode and subsequently dissolves into the electrolyte, forming a tertiary LiCl-Li2O-Li solution. This study examines the long term corrosion behavior of Inconel 625 when exposed to LiCl-Li2O-Li solutions for 500 and 1000hr.
Experimental
Similar to our previous work on SS316, exposure testing was performed in LiCl with 1 or 2 wt%Li2O and 0, 0.3, 0.6 and 1.0wt%Li at 650°C for 500 and 1000 hours. I625 samples were polished to a 0.05μm surface finish prior to exposure. The salt solutions were replaced every 96 hours to make up for evaporation and reaction of Li and/or Li2O to ensure consistent solution chemistry. Following exposure, the samples were rinsed with methanol and analyzed using Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and micro-Vickers hardness testing.
Results
The primary corrosion products detected via Raman spectroscopy, XPS, and EDS on samples exposed to LiCl-Li2O solutions in the absence of Li were lithium chromite, nickel-iron-chromium spinel oxides, and lithium chromate. XRD confirmed the presence of lithium chromite on these samples. In the presence of Li, these surface layers were destabilized and the surface of the samples were free of any significant oxide film, while titanium oxynitride films of nm thickness were observed via XPS. The surface morphology of the samples was observed via SEM to undergo increased degradation with higher concentrations of Li.
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.