To clarify: molten fueled reactors (like LFTRs) trivially can't have their fuel accidentally melt, because their fuel is already molten. (And there are some nice options for dumping their fuel into a subcritical passively-cooled configuration in case of an emergency.) This is different from TRISO fuel, which isn't supposed to be molten in normal operation but which structurally limits the spread of fuel and fission products at high temperatures.