Coolant Compatibility Studies for Fusion and Fusion-Fission Hybrid Reactor Concepts: Corrosion of Oxide Dispersion Strengthened Iron-Chromium Steels and Tantalum in High Temperature Molten Fluoride Salts

Abstract

Alloys such as 12YWT & 14YWT have exceptional high-temperature strength at temperatures greater than 550 C. This class of materials has also demonstrated relatively little radiation induced swelling at damage levels of at least 75 dpa in sodium-cooled fast reactors. However, corrosion of oxide dispersion strengthened (ODS) steels in high temperature molten fluoride salts may limit the life of advanced reactor systems, including some fusion and fusionfission hybrid systems that are now under consideration. This paper reports corrosion studies of ODS steel in molten fluoride salts at temperatures ranging from 600 to 900 C. Electrochemical impedance spectroscopy (EIS) was used to measure the temperature dependence of charge transfer kinetics in situ, while an environmental electron microscope (ESEM) equipped with energy dispersive spectroscopy (EDS) was used for postexposure examination of test samples. ODS steel experienced corrosion in the molten fluoride salts at 550 to 900 C, even in carefully controlled glove-box environments with very low levels of oxygen and moisture. The observed rate of attack was found to accelerate dramatically at temperatures above 800 C. Tantalum and tantalum-based alloys such as Ta-1W and Ta-10W have exceptional high temperature strength, far better than ODS steels. Unlike ODS steels, tantalum has been found to exhibit some immunity to corrosive attack by molten fluoride salts at temperatures as high as 900 C, though there is some indication that grain boundary attack may have occurred. Unfortunately, tantalum alloys are known to become brittle during irradiation and exposure to hydrogen, both of which are important in fusion applications.