Investigation and characterisation of material degradation mechanisms across extremely hot interfaces in molten salts

The molten salt reactor (MSR) is a Generation – IV nuclear reactor, designed to use molten salts as a carrier fuel for nuclear material and as a heat transfer fluid. However, many the molten salt combination proposed for use in the MSR are highly corrosive to many of the proposed materials, including Ni-based alloys. There is still a limited understanding of the corrosion behaviour of Ni-based alloys in molten salt environment, particularly in those containing oxygen as a contaminant. Therefore, the synergy of oxidation and corrosion in molten salt environments was the focus of this research.
A high temperature furnace was used to conduct isothermal high temperature oxidation and molten salt corrosion experiments on coupons of Hastelloy C276 under conditions similar to those of a molten salt reactor environment. These included exposing coupons to temperatures of 700 °C, in dry air environments with and without molten FLiNaK salt. Results from the experimental research of high temperature oxidation were used to validate empirical and mechanistic predictive models for high temperature oxidation of Ni-based alloys before the mechanistic model was developed further to provide predictions for molten salt corrosion of Ni-based alloys in FLiNaK salt.
For the high temperature oxidation of Hastelloy C276, parabolic oxidation kinetics were observed experimentally, with a parabolic rate constant of 2.57x10-7 mg2/cm4/s being calculated to reflect the high temperature oxidation behaviour of Hastelloy C276 in the time period of 0 to 200 hours. In the same time period and in a molten salt environment, Hastelloy C276 was observed to exhibit high corrosion rates, in excess of 0.3 mmpy. Material loss from Hastelloy C276 was observed to follow a linear relationship, indicating that any corrosion product layers were offering no protection to the underlying substrate from the corrosive medium.