In a conventional nuclear reactor the amount of neutrons 'flying around' is maintained with control rods and water is usually used as a moderator. From a certain point of view the control rods keep the nuclear fuel just critical enough.
In a molten salt reactor where the fuel is dissolved in the salt irrelevant of this is thorium, plutonium or uranium. And in a reactor where the nuclear fuel is solved into the salt :
How is it possible to maintain the mass critical enough in a molten salt reactor?
I can understand that they use control rods in the reactor itself, but if the salt is dropped into the safety reservoir below the reactor isn't the mass getting uncontrolled supercritical?