Nuclear Plant Building ExplodesCount me as among that faction of people not freaked out by radiation -- it is dangerous, but a lot less dangerous than coal smoke or tsunamis. From what I know about Japanese nuclear technology, it strikes me as very unlikely that there will be a reactor explosion or other major failure. But according to Japanese news reports, Cesium has already been detected in the air around one of the stricken plants, and that means a leak from the reactor itself rather than just the cooling system. Even the best systems can fail in extreme circumstances, and a magnitude 8.9 earthquake certainly qualifies as extreme. So a major leak of radioactive material can't be ruled out.
This might be a good time to point out that we now have the technology to produce nuclear plants that can't explode, under any circumstances. In these designs the fuel is distributed so that it will not overheat even with no coolant at all. We have not pursued this technology for a couple of reasons. It is more expensive and less efficient, so countries with strong nuclear programs like France and Japan are not interested, and in anti-nuclear countries like the US and Germany people don't want anything to do with nuclear power of any kind. But it can be done, and if we are really serious about limiting carbon emissions I think it will have to be done.