31 Key - No Containment Building
One of the major expenses in the design and building of a nuclear plant is the containment building.  The structure must be designed to contain the steam that is released if a major breach of the water coolant occurs.  The structure is therefor large , strong and expensive.  Since a Molten Salt Reactor works at 1 ATM Pressure and has no water that comes in contact with the core there is no need for this structure.  A simple concrete structure is all that is required.