Right now, the "explosions" aren't too surprising. Zirconium can generate a great deal of hydrogen at the temperatures that are likely given the state of the reactors right now, and it's trivial for that stuff to explode once it escapes. Reading the commentary from some atomic energy experts (not the talking heads on TV), I suspect this isn't going to be nearly as bad as Chernobyl. Totally different reactor designs for one, and I'm pretty confident that the Japanese technicians are doing everything they possibly can to get the reactors under control.
From what I've read, the zirconium cladding comes apart and melts at about 1200F. The fuel rods melt at about 5000F. The steel reactor vessel is supposedly 6" steel, and even if that is melted down, there's the containment building itself that's designed (hopefully) to contain anything that comes out, preventing it from exiting the structure and entering ground water. There seems to also be a number of things they can do in the event that happens--though many first responders would die, and the risk of atmospheric contamination would be grave. Right now, as long as they can cool the fuel rods, I don't think there'll be anything other than hydrogen out gassing.
Take a look at
this picture showing the containment building. The top part that blew off was a tin structure that appears to only shield the containment building from the elements. I've read from some sources that it's basically designed to be destroyed in the event of an out gassing like what has happened, hopefully preventing rebound pressure waves from damaging the reactor itself.
Course, none of this stops the anti-nuclear crowd from surfacing. Never mind that Japan had a 9.0 earthquake followed by a massive tsunami that swept away the reactor's support equipment, backup generators, and flooded many (all?) of the battery rooms. If the Japanese can cool the reactors down and get them under control, then I think we have much to learn from what they've done.
Though, I guess we do have a lot more to learn from the Japanese. Considering New Orleans after Katrina and the relatively small scale bit of destruction that wrecked and the general behavior of our population following that disaster, I think I'd rather be in Japan if I had to choose a location to be in during such an event.