EFFECT OF ALKALI FLUORIDE SALT ON THE OXIDATION BEHAVIOUR OF GRAPHITE UNDER AIR ATMOSPHERE

抄録

Air ingress into the core of molten salt reactor (MSR) in an accident scenario can cause serious safety problems and especially damages to molten salt infiltrated graphite components. However, the effect of air exposure on the oxidation behaviour of molten salt infiltrated graphite remains elusive. In this study, the effect of alkali metal fluorides, i.e., LiF, NaF, KF, and ternary salt FLiNaK, on the oxidation behaviour of graphite powder was systematically investigated under air atmosphere. Oxidation tests show that the molten-salt-infiltrated graphite samples oxidized at 600–900 °C in air exhibit increased weight loss compared with the virgin sample, and the weight loss follows the order of KF-G > NaF-G > LiF-G. The fluoride salts also reduce the activation energy of the air-exposed graphite at elevated temperatures, and the catalytic oxidation efficiency of the alkali metal fluorides follows the order of KF > NaF > LiF. Scanning electron microscopy reveals that the fluoride salts induce active sites on the surface of graphite plane, which can accelerate the oxidation reactions of graphite in air. It is explained that the alkali metal atoms can be adsorbed on the surface of the graphite planes to activate the carbon atoms to show a high affinity for binding of the oxygen atoms and, thus, to accelerate the graphite oxidation. The results have gained deeper insight into the air oxidation mechanism of fluoride-saltinfiltrated graphite and are meaningful for improving the fabrication technology of nuclear graphite to ensure the operation safety of MSR during air ingress accidents.