Comment by Merad

They're easy to shut down, but when the reactor is first turned off the core is full of unstable isotopes that are byproducts of the fission reaction. Those isotopes continue decaying even with the reactor "off" until the become stable, and they put out enough heat to cause a meltdown on their own. This means the reactor needs active cooling for a couple of weeks until it reaches a safe state. That's what happened at Fukashima; the reactors were shut down when the earthquake hit, but then the tsunami disabled the backup generators that were powering the cooling pumps, and they weren't able to restore cooling before the core melted.