Thermal cycling of sodium system and possibility of metal dusting: an analysis

Liquid sodium has been widely recognized as an efficient coolant, but when used at high temperature may encounter thermal cycling during operation, for example, as normally encountered in pool-type fast reactors in service. Under these conditions the behaviour of carbon dissolved in sodium needs special attention. Carbon – sodium chemistry is very complex because of the existence of multiple carbon-bearing species. In addition carbon exists both in the dissolved (‘active’) and undissolved (‘inactive’) forms. Under thermal cycling conditions, and as the temperature is lowered, carbon may precipitate as sodium acetylide. The equilibrium carbon activity imparted by this species is high enough to cause the precipitation of iron carbide (Fe3C) in ferrous alloys. The Fe3C may become destabilized at lower activities of carbon in the environment (when the temperature is increased) and may decompose to a fine dispersion of metal and graphite. This phenomenon of ‘metal dusting’ is extremely detrimental to the components of sodium systems. This paper analyses the possibility of metal dusting in an operating sodium system.