[zyztem]

The usual problem is not shutting down the reactor: both TMI and Fukushima was properly shutdown. Important task is cooling down the core (and SFP) after that.

Left on its own, halted thermal reactor generates hundreds MW of heat after shutdown. That heat destroys cladding of fuel, generates explosive Hydrogen and causes meltdowns of former core.

It is believed that ATF, core catchers and electricity-independent passive cooling will reduce risk of this types of incidents, commonly called LOCA - Loss of coolant

[cameldrv]

Fukishima unit 1 had an Isolation Condenser, which is the sort of passive safety system you're talking about. It required no electricity, merely that some valves be open. Supposedly after the earthquake and before the tsunami, the operators opted to close at least one of those valves for reasons that are a little hazy, but might have had something to do with not cooling the reactor too fast. After the tsunami, they couldn't electrically control the valves, and for some reason there was ambiguity about the state of the valves, and no one went to physically check them and open them until it was too late and the fuel had melted.