MSRs have negative temperature reactivity coefficient and outlet temps around 700C at atm pressure. PWR is at measly 300C and 150 Bar.

If all control is lost, the salt expands as it heats up pushing the expanded volume out from the reactor core. The fission stops once the fuel is leaves the core region where the moderator is. Reverse is also true: you pull heat off from the loop, so the fuel-salt becomes denser, increasing reactivity. MSRs can naturally “follow” the load, if done right.