molten salt systems can be fail safe by having the coolant drain from parts of the system that can’t operate with solidified salt in them, even if they do, you can sends someone with a heating element to remelt the system at critical points before turning it on. It’s not like water where the coolant will physically expand and burst things when it freezes, water’s actually pretty weird in that regard, most things take up less volume when they freeze.

I don’t know why they couldn’t do the same for an alpha class, but I suspect it’s because running the reactor without coolant in it would have caused a melt down, and if any coolant was left inside it when turned off, the control rods would have been frozen in place preventing it from being restarted. Perhaps in such a tightly sealed system, the shrinkage caused by cooling could have caused things to break as well.