I'm only aware of what I've heard about from a podcast (.Net Rocks!), so I'm not trying to argue viability/feasibility. From what I remember, the whole benefit of using a liquid salt as your fissile material is to reduce (eliminate) the risk of a steam explosion, which is (citation needed) the leading cause of meltdown.

edit: And the whole point of using Thorium is because it's more plentiful, and doesn't contribute to nuclear weapons.