The Fukushima reactors WERE scrammed. The problem is that this ends only the Uranium fission chain reaction, but still leaves you with a lot of fission products

The Fukushima reactors WERE scrammed. The problem is that this ends only the Uranium fission chain reaction, but still leaves you with a lot of fission products that are themselves unstable with various half-lives. These keep producing a gradually diminishing amount of heat for weeks and months. Which is why you have the waste pools. And no, you CAN'T scram those because it's not a chain reaction going on but simply radioactive decay - there is no known force in the universe which can stop an unstable atom core from decaying.