Trying to understand a boiling water reactor schematic diagram, to begin to understand Japan's situation

If you're like me, i.e. NOT a nuclear physicist, all the coverage of the quickly-deteriorating nuclear situation in Fukushima has gone a bit over your head. Not that you're not a compassionate, intelligent person, but man, can those experts on the radio and television talk fast and loose with terms like "partial-meltdown"--which it turns out has no actual definition and is a terrible way to describe anything, especially because it's probably way milder than an actual meltdown would and could be. Not to mention the other terms that scientists use that I can't even recall. Words I don't use a lot.

So what is the deal, and what are the dangers, and what do all the diagnoses really add up to? Those are all really hard for anyone to say right now. But more importantly for my understanding, how does it all work in the first place? How does it function under regular circumstances? I have looked at a few helpful diagrams of nuclear electricity plants, and how they function, which basically looks very confusing even in simple-colored-coded-drawing form. Every one I've seen has been a bit overwhelming.

There have also been at least three (by my count according to news via NPR and Β Twitter news reports from a couple sources) explosions, which no one has really explained--at least that I had come across. Hydrogen was involved, but it's been awhile since I studied anything along the lines of nuclear chemistry, or regular chemistry, and all I could remember was that you could potentially have an enormous explosion from hydrogen too. But since no one was saying this, there had to be more to it than I knew or could comprehend.

POPSCI's article "How Nuclear Reactors Work, and How They Fail" (by Dan Nosowitz) gets all my kudos, for basically explaining in an approachable and even conversational way how the reactors work, whether they have an "off switch" (they do! it's just not your momma's off-switch), why and how they do all these double- and triple-back ups for power and cooling, and what a "dreaded meltdown" means--in real terms, like damage and that irksome "partial meltdown" that has been used by Japanese officials to describe the situation thus far.

Definitely read it; it's the most helpful ten minutes I've spent thus far learning about a situation that has lots of people all over the world curious, cautious, and downright worried about the whole issue of nuclear energy. I am a more informed citizen of the world now. But, no, no plans to work further on my nuclear physics.

In case you did not click on the link yet, here's a taste. Full disclosure: a nuclear meltdown sounds horrifying, even in raw chemical talk. This is the scariest bit of the article. The rest is far milder.

What people mean when they say "meltdown" can refer to several different things, all likely coming after a hydrogen explosion. A "full meltdown" has a more generally accepted definition than, say, a "partial meltdown." A full meltdown is a worst-case scenario: The zirconium alloy fuel rods and the fuel itself, along with whatever machinery is left in the nuclear core, will melt into a lava-like material known as corium. Corium is deeply nasty stuff, capable of burning right through the concrete containment vessel thanks to its prodigious heat and chemical force, and when all that supercharged nuclear matter gets together, it can actually restart the fission process, except at a totally uncontrollable rate. A breach of the containment vessel could lead to the release of all the awful radioactive junk the containment vessel was built to contain in the first place, which could lead to your basic Chernobyl-style destruction.