Fukushima. Can anyone shed some light?

I've been seeing many articles recently that the structure housing fuel rods in the Fukushima reactor complex is highly unstable. The articles propose that if it collapsed it could wipe out half the earths population. My bullshit detector goes off when I hear stuff like this, but I'm looking for an educated opinion, as my knowledge of physics doesn't go too far past a high school level. Any insight?