With Molten Salt Reactors, a catastrophe like Fukushima cannot happen. It began with a magnitude 9.0 earthquake not far from the Fukushima 6 Nuclear reactor complex. The impact was a magnitude 6.8 earthquake and the operators immediately scrammed the safety rods to stop all the reactors. This succeeded! The reactors were designed with earthquakes in mind, and they passed the test. The backup power started up successfully so the cooling pumps could operate. There was one major problem though. The earthquake was so bad that the water in the spent fuel holding tanks splashed out and exposed the spent fuel rods to air, releasing enough radioactivity to make entering the buildings impossible.
The water pumps worked for a while, but then came the tsunami. All the reactors were inside a tsunami wall, so far, so good. But the fuel storage tanks for the fuel for the backup power generators were outside the tsunami wall and were washed away. The batteries were only supposed to last until backup power was established, and with complete power loss and water circulation ended the meltdown started. This disaster was even bigger than Chernobyl and contamination is still spreading.
In a molten salt Thorium reactor, when power is cut off in an emergency, only gravity is needed for a safe shutdown, and gravity hasn’t failed us yet.