Also, a big reason LWRs use so much reinforced concrete is that they have huge containment domes. The reason they're so big is that the water is pressurized at around 100 atmospheres, and if there's a pipe break it'll flash into steam at a thousand times the volume. You need plenty of space for the steam to expand inside containment.

A molten salt reactor is at atmospheric pressure and has nothing to cause a chemical explosion. You still need containment, but you don't need to contain a large empty space.