As I type this, there are at least two nuclear reactors in serious danger of meltdown in Japan. I think they are already in partial meltdown. I don't know, because nuclear reactors are tough things to get inside, and Japan's nuclear regulatory agency is spending most of its time trying to avert disaster rather than speculating on what might be happening (this doesn't stop lots of pundits, myself included, from weighing in). For the latest, check this page from the IAEA. But despite the headlines, we must remember Douglas Adams's advice: "Don't Panic." (Always, always bring a towel.)
Reactor cooling efforts over the weekend included flooding the reactors with seawater and boric acid, after several explosions in containment buildings around reactors. A third explosion in a containment building was reported today. Explosions at nuclear plants often include a release of radioactive material, but it's debatable how much impact any given release is. This doesn't look good -- but it's not the end of the world, either. On the bright side, the term "meltdown" isn't quite as bad as most people think. It means there's something seriously wrong with the cooling system in a nuclear reactor, and continued failure of the cooling system leads to overheating, which can lead to bad things like explosions. Meltdowns can be contained (indeed the reactors are designed to contain them), or they can turn catastrophic -- and the meltdowns in Japan appear to be containable. In other words, even if there are two or three meltdowns in these reactors, we aren't necessarily talking about two or three Chernobyl-level events. It's hard to say what the eventual fallout (apologies for the pun) will be, but I'm guessing Japan has three Three Mile Islands on its hands. And, in case you don't know, the TMI-2 reactor was (more or less) cleaned up, albeit at great expense (nearly $1 billion) and over a shockingly long time (roughly 14 years). TMI is its own story, and that story is told in a documentary I've embedded below.
As the tragedy unfolds in Japan, lots of people are wondering: what's a meltdown? How does a nuclear reactor work? Weirdly enough, I studied this topic in college, although my degree is in Library and Information Science. I took a series of courses covering major disasters, with the notion being that if you could understand how a disaster occurred, perhaps the things you built would not themselves be so disaster-prone. (I went on to build computer systems and software -- fortunately none that were "mission critical.") On Saturday, fellow _flosser Maggie Koerth-Baker posted Nuclear energy 101: Inside the "black box" of power plants, a good overview of the technology involved. Read up, then come back for the documentary I've posted below: Meltdown at Three Mile Island.
The common pattern in a disaster is the confluence of at least one technical or environmental problem (for example, an earthquake knocking around some nuclear power plants), leading to secondary technical problems (the tsunami apparently disabling backup diesel-powered cooling systems at those plants), and then human behavior that interacts with those problems in unexpected ways (human attempts to shut down reactors can sometimes be destructive by themselves, although they're always well-intentioned -- currently Japanese nuclear plant workers have a makeshift cooling system rigged up to replace the failed diesel backup pumps, but it's not perfect). The operators of nuclear reactors often have limited information about what's happening inside the reactor itself, and workers under pressure to make decisions on the fly. Sometimes these high-pressure decisions turn out to be correct -- other times, not so much.
Here's a representative quote from the TMI documentary: "If the operators had not intervened in that accident at Three Mile Island and shut off the pumps, the plant would have saved itself. They had thought of absolutely everything! Except: what would happen if the operators intervened anyway?" -Mike Gray, author of The Warning: Accident at Three Mile Island. (See also: Normal Accidents: Living with High-Risk Technologies, a classic text by Charles Perrow.)
The rest is after the jump.
Another good resource: Mr. Reid's explanation of the situation (Reid is a Physics teacher).