One of the most critical challenges in the successful fruition of Molten Salt Reactors (MSR) is the corrosion of structural materials. Liquid salts can be corrosive to materials used for construction of MSR components such as the primary vessel and heat exchangers, particularly at the intended high operation temperatures of MSR. The consequences of corrosion are two-fold. First, corrosion can gradually lead to wall thinning and eventual loss of structural strength below safe values. Second, the introduction of corrosion products in molten salts can adversely change the thermal and optical properties of the liquid salt. More importantly, these dissolved corrosion products can potentially deposit on the relatively cooler sections of the system causing clogging in components such as heat exchangers. The choice of salt and its purity will therefore determine the allowable materials of construction.
In this project, we are focusing on fluoride salts which are known for their high temperature stability, superior neutronics, and excellent heat transfer characteristics. Therefore, they are being considered as coolants and heat transfer media for both the primary and secondary sides of the MSR. The structural materials used for MSR will be in contact with high temperature molten fluoride salts. The concept of relying on a protective oxide layer for corrosion protection as is the case in most high temperature environments is not applicable to molten fluoride salts because of the high instability of oxides in this environment. The oxide layer is readily fluxed away into the salt, an effect that is exacerbated by the inevitable presence of even small amount of cationic or anionic impurities. Therefore the selection of appropriate structural materials becomes particularly important for molten fluoride salts.