Feasibility of using MAX phase materials in molten salt reactors

The applicability of MAX phases in molten salt reactors has been investigated via their resistance to corrosion in chloride salts. High purity Ti3AlC2 (95wt.%) was synthesised by milling titanium and graphite for 2 h then sintering at 1350°C for 15 min with 1.0 parts aluminium. The milling time, aluminium fraction and sintering temperature were varied. Samples of Ti3AlC2, Maxthal 312 and TiC were exposed to molten LiCl-KCl (LKE) and KCl-MgCl2 (KME) eutectics under argon with variation of the exposure time (125 h and 250 h), exposure temperature (600°C and 850°C) and salt processing. TiC performed best whereas Ti3AlC2 performed worst. In the absence of salt processing, Ti3AlC2 corroded by dissolution of aluminium and penetration of chlorine into the layers of the material. A Ti-C-Cl phase was observed which appeared to have a crystal structure similar to that of the original MAX phase. When salt processing was implemented, the extent of corrosion was minimal at 600°C but severe at 850°C. At 850°C, the sample exposed to LKE formed an oxide scale containing lithium aluminate and lithium titanate which underwent pitting corrosion, whereas the sample exposed to KME formed a stable magnesium aluminate scale by diffusion of aluminium from the underlying MAX phase. The samples of Maxthal 312 which were exposed to processed salts underwent minimal reaction at 600°C, forming a thin titanium oxide scale. At 850°C, more complex oxides formed such as lithium silicate and lithium titanium silicate in LKE and magnesium titanate in KME. Samples exposed to as-received KME formed magnesium silicate, but also underwent significant reaction with nickel wire which was used to suspend them. Corrosion of TiC was minimal. A thin coating of titanium oxide formed at 600°C, whereas lithium titanate and magnesium titanate formed in LKE and KME respectively at 850°C.