Corrosion Behavior of Monel 400 in Molten LiCl-Li2O-Li
Authors: Phillips William , Merwin Augustus , Chidambaram Dev 
Published: February 2020

Introduction

Closing the nuclear fuel cycle is an important step to improve the safety, economics, and environmental impact of nuclear power. To integrate the oxide fuels that are used in the current reactor fleet into a metallic fuel cycle applicable to some proposed advanced reactors, an electrolytic oxide reduction process will need to be performed (1). Currently, this process is conducted in a LiCl-Li2O electrolyte at 650°C (2). As the process proceeds, metallic Li is produced and dissolves into the electrolyte, resulting in a tertiary system composed of LiCl, Li2O, and Li (3, 4). The materials used to construct the containers and equipment in direct contact with the electrolyte have been shown to suffer degradation due to exposure to the resulting LiCl-Li2O-Li electrolyte in previous studies performed by this research group. This work investigates Monel 400 as an alternative material for construction of equipment used for the electrolytic reduction of used nuclear fuel due to this alloy’s high corrosion resistance that is not dependent on the development of protective chromium oxide surface films.

Experimental

Experiments were conducted in an Ar filled glovebox located at the University of Nevada, Reno. Exposure testing was performed by submerging Monel 400 samples in LiCl-2wt%Li2O with varying concentrations of Li at 650°C for 20 hours. Post exposure analysis of the samples was performed using X-ray diffraction, field emission scanning electron microscopy, optical microscopy, micro-Vickers hardness testing, Raman spectroscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma – optical emission spectroscopy.

Results

Figure 1 shows the corrosion rate of Monel 400 exposed to LiCl-2wt%Li2O-Li observed in this study. Minimal degradation of this material was observed by the analytical techniques used to characterize the samples following exposure. The corrosion resistance of Monel 400 in LiCl-2wt%Li2O-Li system was shown to be excellent over a broad range of metallic Li concentrations.

Figure 1: Average corrosion rate for M400 exposed to LiCl-2wt%Li2O-Li for various concentrations of Li. The calculated corrosion rate varied between 0.007mm/year and 0.039mm/year, showing that Monel 400 is highly resistant to corrosion in the LiCl-Li2O-Li system at 650°C.

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, NRC-HQ-10-G-38-0027, NRC-HQ-13-G-38-0027. W.P. and A.M acknowledge the Fellowship Award from the USNRC. 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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Figure 1