Molten salt reactors is a nightmare for material science. You effectively have half of Mendeleev's table on your hands and it's borderline impossible to design

Molten salt reactors is a nightmare for material science. You effectively have half of Mendeleev's table on your hands and it's borderline impossible to design a material which will have enough corrosion resistance (reactors have to serve at least several decades) and don't forget about high levels of radiation as well, which also contributes to material damage. Plus online fuel processing is much more difficult than the fuel processing required for fast breeder reactors, and it's one of the main stumbling blocks for breeders right now (well, judging by Russian's experience).