There's a pretty active discussion on Fark about this, and from what I can gather these things are happening. Disclaimer: I'm not a nuclear scientist, merely a pudgy engaged observer. Consume the appropriate amount of salt. The reactor is a Boiled Water Reactor design, with a metal Reactor Vessel, a concrete containment chamber around that, and possibly a chamber around that designed so that weather events don't erode or compromise the containment chamber. From what I understand, they are still running on batteries (more which are being delivered) since the diesel generators were knocked out in the tsunami. Whatever power source they have (or don't), it's not enough to keep the pumps running continuously to circulate fresh water, hence the pressure buildup. Don's post is somewhat accurate in that they scrammed the reactors after/just before the quake, however that puts them out of OMGWTFBBQ CHERNOBYL! disaster range and more into Three Mile Island range. Reactors don't just shut off, the analogy I'm hearing thrown around the most is they are akin to a stove; you can kill the flame/element, but you'll still burn your hand if you touch the top. What we're seeing now is decay heat, which as long as they can keep that under control they should be out of the woods. Explosions are never good, and I don't know whether the outer shell of the containment structure blew or the containment structure itself. They are talking about releasing pressure (steam) to the atmosphere, the vents of which are filtered so the environmental fallout of the venting would be controlled. One last thing, try not to get too caught up with the media talking about 100x levels this or 1000x levels that. Radiation is measured using a logarithmic, not exponential scale, I'll let some of the mathletes here explain that in further detail.