LWR (water cooled) reactors require high pressure to prevent water converting to steam. High-pressure containment building and systems are required. That includes a huge reinforced concrete building, to contain steam containing radioactive material; manual construction and size makes this one of the biggest expenses of building a LWR.

Radiation shielding for gamma rays is a few inches of lead, or comparable mass of stone or concrete; radiation shielding for alpha and beta particles requires much less. The huge building around LWR is not for radiation shielding, but to contain high-pressure steam if a pipe breaks.

LFTRs (like all Molten Salt Reactors) are cooled by molten salt far below its boiling point, so they operate at atmospheric pressure. LFTRs have no high pressure that could explode; high pressure containment is unnecessary.

Fukushima reactors (an older design of light water reactor) had hydrogen explosions that damaged the buildings and reactors, and spewed radioactive material into the atmosphere. LFTRs have no steam or hydrogen.

There are a few sodium-cooled reactors in use. Sodium reacts violently to water. A broken pipe weld in a heat transfer unit heavily damaged the Monju Nuclear Power Plant. LFTRs have only chemically stable coolant.

The containment building for a Molten Salt Reactor would have radiation shielding, would catch any leaks or spills of molten salt, and would protect the reactor from external forces (depending on the site, might include car bombs, hurricanes, earthquake). But the huge expensive steam containment building of a LWR is not needed. Build the reactor underground and protection from external forces is done by a few meters of dirt.

Eliminating the steam containment building and high-pressure pipes and safety systems of LWR makes all Molten Salt Reactors far easier to build and install, at much lower cost than LWR. They can be factory assembled, shipped in standard trucks, installed wherever 100MW to 1000MW is needed.