Cover gas selection considerations for molten salt systems

Abstract

An investigation was performed to identify and evaluate cover gas selection criteria for molten salt systems. The scope of this study is limited to nitrate, chloride, and fluoride salts ((NO3)-, Cl−, and F−) in a temperature range of 100–1500 °C. Three cover gases typically used with molten salts are examined: argon, helium, and nitrogen (Ar, He, and N2). A literature review was performed to gather broader insights, including an important report on cover gas selection for the Fast Flux Test Facility (FFTF) by Dulin, along with some minor calculations. Major avenues of inquiry include chemistry, heat transfer, radiological concerns, and cost. Chemical compatibility is investigated by considering the reactivity of the gases with the salts, and with the containment materials. Gas solubility in molten salts is touched upon. Heat transfer performance of cover gases is examined to assess their relative performance at different temperatures, as cover gases frequently serve a secondary purpose as cooling gases for auxiliary equipment. Radiological concerns are studied by simulating activation of the candidate gases under thermal and fast flux neutron irradiation, followed by decay. Cost is addressed in a miscellaneous section. No cover gas is found to be “ideal.” Rather, the optimal choice depends heavily on the specifics of the system being considered (temperature, salt, radiation environment, etc.). Conclusions drawn from the study are compiled into a reference table, in the conclusions section. The goal of this study is to provide insight early in the design process, providing a starting point. It is not intended to be used as the sole decision-making resource.