Thoughts on nuclear power and disasters [long, rant/tangenty]
Reactors cannot be just stopped: after shutdown they are still producing lot of heat.
If you plug that bottleneck then what you created is a (dirty) bomb. The temperature and pressure will rise and...
Ps.: the other point is, that to be sure that nothing is leaking and everything is under control you have to be able to reach and check the whole site. It's a bit harder underground, especially with the bottleneck plugged.
In Fukushima it's also hard, but at least they have choices other than keep it plugged and praying.