Here is a major issue with people believing what they are reading about this. Many of the people "discussing" Fukushima have no fricking clue about radiation or how nuclear reactors work, including terminology.
Example: (This is the first result on the search for Fukushima cores I got)
3 Fukushima reactor cores melted into the earth, and are still missing
This guy has no fricking clue.
"They could go critical (atom-bomb style)." - This is proof he has no idea what he is talking about because "critical" in nuclear power is good. It means the dang thing is on and working. It is not generally bad. (Although it, admittedly, would not be good for Fukushima, but it also wouldn't be likely either.) And it is in no way "atom-bomb style". Heck, I heard "the reactor is critical" so often during my time on the ship that I had to eventually force myself to listen for so I could record the time in my logs when it happened, and it was "hey, we'll have steam soon, good, more work". "Supercritical" is the bad one when it comes to nuclear reactors.
Nuclear reactor physics - Wikipedia, the free encyclopedia