http://edition.cnn.com/2011/WORLD/asiapcf/03/12/japan.nuclear.quesions/?hpt=C2
The reactors that we are all so worried about are boiling water reactors (I don't know what other kinds of reactors there are, so I don't know if that's good or bad.)the reactor that was the site of the explosion went online in November 1970, so it's an oldie. Of the six reactors at that plant, it is the oldest.
The reactor works through nuclear fission resulting from Uranium 235. It emits a lot of heat, which makes steam, which turns a turbine to make the electricity. (At least that part seems simple.)
3 reactors at the plant were in operation at the time of the quake. They shut down when it hit, just like they were supposed to, switching to diesel backup generators for cooling. They quit after an hour (I don't think that was supposed to happen.) They quit because tsunami damage.
(Supposedly) the blast at the plant was not caused by damage to the reactor, but failure of a pumpin system as they tried to get water into the reactor to cool it. (I posted a link last night though that says that if the reactor had hit 2000 degrees F, it would hvae been expected that there be an explosion due to hydrogen when the water made it in, not so much a malfucntion, but an expected explosion, so...)Once again, they say no damage to the containment chamber.
There are good quotes at the link from a nuclear expert saying essentially the same thing.
Japanese government is saying that the cesium found at the plant was due to the melting of a fuel rod. (That does sound possible and logical, but not necessarily the full truth, IMO). One definition of meltdown provided here is that the liquid coolant boils away, exposing those fuel rods, leading to temps of over 5000 degrees F.
There are two factors that determine whether or not it will melt down completely, those being whether they can get coolant pumping and whether the steel containment chamber can withstand the heat if they can't get coolant flowing.
Once again, some good quotes from the experts regarding that aspect of the issue and comparisons to Three Mile Island, which it turns out was a totally different issue.
I also looked up cesium:
http://en.wikipedia.org/wiki/Caesium
The radioactive isotope caesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Although the element is only mildly toxic, it is a hazardous material as a metal and its radioisotopes present a high health risk in case of radiation leaks. (The rest of the info on that page I don't even understand.)
Health risks include hyperirritability, spasms, and it is highly explosive when it touches water. It creates a lot of hydrogen gas and a great big explosion. It is corrosive, and even corrode glass pretty fast. The one good thing that I'm reading is that it doesn't build up in the body as well as some other radioactive materials. It passes through sweat and urine fairly quickly if it does enter the body. As I understand it cesium is one of the things they are worried about, since it is in high levels at that control room in the plant.